Calcium orthophosphate deposits: Preparation

Materials Science and Engineering C 55 (2015) 272–326

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Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec

Review

Calcium orthophosphate deposits: Preparation, properties and biomedical applications Sergey V. Dorozhkin Kudrinskaja sq. 1-155, Moscow 123242, Russia

a r t i c l e

i n f o

Article history: Received 16 December 2014 Received in revised form 21 March 2015 Accepted 8 May 2015 Available online 13 May 2015 Keywords: Calcium orthophosphate Hydroxyapatite Deposits Coating Film Layer Deposition Conversion Surface

a b s t r a c t Since various interactions among cells, surrounding tissues and implanted biomaterials always occur at their interfaces, the surface properties of potential implants appear to be of paramount importance for the clinical success. In view of the fact that a limited amount of materials appear to be tolerated by living organisms, a special discipline called surface engineering was developed to initiate the desirable changes to the exterior properties of various materials but still maintaining their useful bulk performances. In 1975, this approach resulted in the introduction of a special class of artificial bone grafts, composed of various mechanically stable (consequently, suitable for load bearing applications) implantable biomaterials and/or bio-devices covered by calcium orthophosphates (CaPO4) to both improve biocompatibility and provide an adequate bonding to the adjacent bones. Over 5000 publications on this topic were published since then. Therefore, a thorough analysis of the available literature has been performed and about 50 (this number is doubled, if all possible modifications are counted) deposition techniques of CaPO4 have been revealed, systematized and described. These CaPO4 deposits (coatings, films and layers) used to improve the surface properties of various types of artificial implants are the topic of this review. © 2015 Elsevier B.V. All rights reserved.

Contents 1. 2. 3. 4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . General knowledge, terminology and definitions . . . . . . . . . . Brief knowledge on the important pre- and post-deposition treatments Deposited CaPO4 . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Thermal spraying techniques . . . . . . . . . . . . . . . . 4.1.1. Plasma spraying . . . . . . . . . . . . . . . . . . 4.1.2. High velocity oxy-fuel (HVOF) spraying . . . . . . . 4.2. Vapor deposition techniques . . . . . . . . . . . . . . . . 4.2.1. Ion beam assisted deposition (IBAD) . . . . . . . . 4.2.2. Pulsed laser deposition (PLD) . . . . . . . . . . . . 4.2.3. Magnetron sputtering . . . . . . . . . . . . . . . 4.2.4. Electron-cyclotron-resonance (ECR) plasma sputtering 4.2.5. Metalorganic chemical vapor deposition (MOCVD) . . 4.2.6. Molecular precursor and thermal decomposition . . . 4.3. Wet techniques . . . . . . . . . . . . . . . . . . . . . . 4.3.1. Electrophoretic deposition (EPD) . . . . . . . . . . 4.3.2. Electrochemical (ECD) or cathodic deposition . . . . 4.3.3. Sol–gel deposition . . . . . . . . . . . . . . . . . 4.3.4. Wet-chemical and biomimetic deposition . . . . . . 4.3.5. Dip coating . . . . . . . . . . . . . . . . . . . . 4.3.6. Spin coating . . . . . . . . . . . . . . . . . . . 4.3.7. Hydrothermal deposition . . . . . . . . . . . . . . 4.3.8. Thermal substrate deposition . . . . . . . . . . . .

E-mail address: [email protected].

http://dx.doi.org/10.1016/j.msec.2015.05.033 0928-4931/© 2015 Elsevier B.V. All rights reserved.

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4.3.9. Alternate soaking . . . . . . . . . . . . . . . . . . 4.3.10. Micro-arc oxidation (MAO) . . . . . . . . . . . . . 4.4. Other deposition techniques: miscellaneous . . . . . . . . . . 4.4.1. Hot isostatic pressing (HIP) . . . . . . . . . . . . . 4.4.2. A double layered capsule hydrothermal hot pressing . . 4.4.3. Detonation gun spraying . . . . . . . . . . . . . . . 4.4.4. Aerosol–gel deposition . . . . . . . . . . . . . . . 4.4.5. Aerosol deposition (AD) . . . . . . . . . . . . . . . 4.4.6. Cold spraying (CS) . . . . . . . . . . . . . . . . . 4.4.7. Blast coating . . . . . . . . . . . . . . . . . . . . 4.4.8. Direct laser melting . . . . . . . . . . . . . . . . . 4.4.9. Transmission laser coating . . . . . . . . . . . . . . 4.4.10. Laser cladding . . . . . . . . . . . . . . . . . . . 4.4.11. Laser-engineered net shaping (LENS™) . . . . . . . 4.4.12. Matrix assisted pulsed laser evaporation (MAPLE) . . 4.4.13. Liquid phase laser deposition . . . . . . . . . . . . 4.4.14. Electrostatic spray deposition (ESD) . . . . . . . . . 4.4.15. Spray pyrolysis (pyrosol) . . . . . . . . . . . . . . 4.4.16. Drop-on-demand (DOD) micro-dispensing . . . . . . 4.4.17. Mechanochemical synthesis or ball impact technique . 4.4.18. Polymeric deposition route . . . . . . . . . . . . . 4.4.19. Autocatalytic deposition . . . . . . . . . . . . . . 4.4.20. Cyclic electrodeposition . . . . . . . . . . . . . . 4.4.21. Cyclic spin coating . . . . . . . . . . . . . . . . . 4.4.22. Biomediated deposition (biosynthesis) . . . . . . . . 4.4.23. Emulsion route . . . . . . . . . . . . . . . . . . 4.4.24. Slurry processing . . . . . . . . . . . . . . . . . 4.4.25. Slip coating . . . . . . . . . . . . . . . . . . . . 4.4.26. Deposition by solvent evaporation . . . . . . . . . . 4.4.27. Powder mixed electrical discharge machining (PMEDM) 4.4.28. Investment casting . . . . . . . . . . . . . . . . . 4.4.29. Adsorption . . . . . . . . . . . . . . . . . . . . 5. Deposition of ion-substituted CaPO4 and CaPO4-containing biocomposites 6. Conversion-formed CaPO4 deposits . . . . . . . . . . . . . . . . . 7. Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1. Elastic modulus and hardness . . . . . . . . . . . . . . . . 7.2. Fatigue properties . . . . . . . . . . . . . . . . . . . . . . 7.3. Thickness . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4. Adhesion and cohesion . . . . . . . . . . . . . . . . . . . 7.5. Biodegradation . . . . . . . . . . . . . . . . . . . . . . . 7.6. Interaction with cells and tissue responses . . . . . . . . . . . 8. Biomedical applications . . . . . . . . . . . . . . . . . . . . . . 9. Future directions . . . . . . . . . . . . . . . . . . . . . . . . . . 10. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction All existing materials have the specific properties of their own. Namely, some of them are aggressive, corrosive or biologically incompatible, certain ones are sensitive to light, heating or oxidation, others are hydrophilic, transparent or slimy in nature, etc. Depending on the situations and applications, such properties might be either desirable or undesirable. In the latter case, to get rid of the undesired properties, the surface of such materials should be modified. This is the subject of a special sub-discipline of materials science called surface engineering, which implies various surface modifications of the solid matter. In broad terms, surface engineering has applications to chemistry, mechanical engineering and electrical engineering (particularly in relation to semiconductor manufacturing) [1]; however, the latter case is beyond the scope of this review. In general, all available types of the surface modifications can be broadly classified into 3 categories: 1) deposition of materials possessing the desirable functions and properties onto the surface, 2) conversion of the existing surface into more desirable compositions, structures and/or topographies, and 3) partial removal of a material from the existing surface to create specific topographies [2]. As seen

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from the list of the available options, 2 of 3 categories comprise an application of surface coatings, films and layers to solve the problems in a conventional form. In the case of artificial bone grafts, synthetic materials to be used in the biological environments must display an adequacy of both their surface and bulk characteristics in order to fulfill the dual requirements of biocompatibility and suitable mechanical properties for the given application. Otherwise, either fibrous tissues encapsulate the implants made from non-biocompatible materials or mechanically weak grafts do not function properly. Both types of flaws prolong the healing time. Considering that surface is always the first part of any insert that interacts with the host, various types of surface deposits (coatings, films and layers) have been developed to enhance biocompatibility and osteoconductivity of the implants. On the other hand, it is well known that, due to the great chemical similarity to the inorganic part of bones and teeth of mammals, calcium orthophosphates (CaPO4) appear to be very friendly compounds for the in vivo applications [3–6]. The full list of the existing CaPO4 is presented in Table 1. However, since bulk CaPO4 have a ceramic nature, they are mechanically weak (brittle) and cannot be subjected to the physiological loads as encountered in human skeletons, other than compressive ones. Therefore, for many years, the clinical applications of CaPO4

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Table 1 Existing calcium orthophosphates and their major properties [6]. Ca/P molar ratio

Compound

Formula

Solubility at 25 °C, −log(Ks)

Solubility at 25 °C, g/L

pH stability range in aqueous solutions at 25 °C

0.5 0.5 1.0 1.0 1.33 1.5 1.5 1.2–2.2

Monocalcium phosphate monohydrate (MCPM) Monocalcium phosphate anhydrous (MCPA or MCP) Dicalcium phosphate dihydrate (DCPD), mineral brushite Dicalcium phosphate anhydrous (DCPA or DCP), mineral monetite Octacalcium phosphate (OCP) α-Tricalcium phosphate (α-TCP) β-Tricalcium phosphate (β-TCP) Amorphous calcium phosphates (ACP)

1.14 1.14 6.59 6.90 96.6 25.5 28.9

~18 ~17 ~0.088 ~0.048 ~0.0081 ~0.0025 ~0.0005

0.0–2.0

c

c

~5–12d

1.5–1.67

Calcium-deficient hydroxyapatite (CDHA or Ca-def HA)e

~85

~0.0094

6.5–9.5

1.67 1.67 1.67 2.0

Hydroxyapatite (HA, HAp or OHAp) Fluorapatite (FA or FAp) Oxyapatite (OA, OAp or OXA)f, mineral voelckerite Tetracalcium phosphate (TTCP or TetCP), mineral hilgenstockite

Ca(H2PO4)2·H2O Ca(H2PO4)2 CaHPO4·2H2O CaHPO4 Ca8(HPO4)2(PO4)4·5H2O α-Ca3(PO4)2 β-Ca3(PO4)2 CaxHy(PO4)z·nH2O, n = 3–4.5; 15–20% H2O Ca10 − x(HPO4)x(PO4)6 − x(OH)2 − x (0 b x b 1) Ca10(PO4)6(OH)2 Ca10(PO4)6F2 Ca10(PO4)6O Ca4(PO4)2O

116.8 120.0 ~69 38–44

~0.0003 ~0.0002 ~0.087 ~0.0007

9.5–12 7–12

a

2.0–6.0 a

5.5–7.0 b b

b b

a

Stable at temperatures above 100 °C. These compounds cannot be precipitated from aqueous solutions. Cannot be measured precisely. However, the following values were found: 25.7 ± 0.1 (pH = 7.40), 29.9 ± 0.1 (pH = 6.00), 32.7 ± 0.1 (pH = 5.28). The comparative extent of dissolution in acidic buffer is: ACP ≫ α-TCP ≫ β-TCP N CDHA ≫ HA N FA. d Always metastable. e Occasionally, it is called “precipitated HA (PHA)”. f Existence of OA remains questionable. b c

alone have been largely limited to non-load bearing parts of the body. Nevertheless, the research kept going and, attempting to combine the advantages of various materials, investigators started to deposit biocompatible CaPO4 onto the surface of mechanically strong but bioinert or biotolerant materials [7,8]. For example, metallic implants are encountered in endoprosthesis (such as total hip joint replacements) and artificial teeth sockets because the requirements for a sufficient mechanical stability necessitate the use of a metallic body for such devices. Since metals alone do not undergo bone bonding, i.e., they do not form mechanically stable links between the implant and bone tissues, they are covered by CaPO4 to create such bonding. However, the issue of non-osseointegration is not limited to metals. Biodegradable polymers often lack bioactivity either. Therefore, to eliminate this drawback, the surface of such polymers is also covered by CaPO4. After being implanted, the CaPO4 deposits can be replaced by autologous bone because they participate in bone remodeling responses similar to natural bones [7–15]. However, for successful performing the key functions (namely, a bioactive adaptation of biologically inert implants), CaPO4 deposits (coatings, films and layers) should meet a number of requirements. The minimal requirements for HA coatings (Table 2) have first been described in 1992 in the Food and Drug Administration (FDA) guidelines [16], as well as a little bit later in the ISO standards [17]. Afterwards, the FDA guidelines were updated in 1997 [18], while the ISO standards were updated in 2000 [19] and 2008 [20]. In addition, there is the ISO standard of 2002 on determination of HA coating adhesion strength [21]. Briefly, the critical quality specifications for CaPO4 deposits comprise their thickness, phase composition, crystallinity, the Ca/P ratio, Table 2 FDA requirements for HA coatings [16]. Properties

Specification

Thickness Crystallinity Phase purity Ca/P atomic ratio Density Heavy metals Tensile strength Shear strength Abrasion

Not specific 62% minimum 95% minimum 1.67–1.76 2.98 g/cm3 b50 ppm N50.8 MPa N22 MPa Not specific

microstructure, porosity, surface texture and roughness. All these parameters appear to influence the resulting mechanical properties of the CaPO4 deposits, such as cohesive, bond, tensile and shear strength, Young's modulus, residual stress and fatigue life.

2. General knowledge, terminology and definitions According to Wikipedia, the free encyclopedia, “Coating is a covering that is applied to the surface of an object, usually referred to as the substrate. In many cases, coatings are applied to improve surface properties of the substrate, such as appearance, adhesion, wettability, corrosion resistance, wear resistance and scratch resistance. In other cases, in particular in printing processes and semiconductor device fabrication (where the substrate is a wafer), the coating forms an essential part of the finished product” [22]. Obviously, all the aforementioned is also valid for films; however, commonly, films are considered as coverings, which are thinner than coatings. A layer is another important definition. It is determined as a single thickness of some material covering a surface or forming an overlying part or segment. Finally, deposition is defined as an act of applying of a coating, a film or a layer to a surface; therefore, deposits are determined as everything that has been deposited. Historically, deposition of various coverings dates to the metal ages. The known examples comprise protection of the first metallic tools and artifacts (iron, brass, silver) with animal fat, beeswax, gelatin, vegetable oils and various clay minerals. Water repellence, brightness, corrosion and wear protection, as well as lubrication are examples of properties early man sought. A bit later, consider an ancient craft of gold beating and gilding, which has been practiced continuously for, at least, four millennia. Namely, the Egyptians appear to have been the earliest known practitioners of this art. Many magnificent examples of statuary, royal crowns and coffin cases that have survived intact attest to the level of skills achieved. For example, leaf samples from Luxor dating to the Eighteenth Dynasty (1567–1320 B.C.) appears to be ~ 0.3 μm thick. Such leaves were carefully applied and bonded to smoothed wax or resin-coated wood surfaces in a mechanical (cold) gilding process to create the earliest coatings [23]. To return to the subject of this review, to the best of my findings, the earliest research paper on CaPO4 coatings was published in 1965 [24]. Since then, over 5000 publications on various aspects of this topic have been published, which is a good indication of the actuality of the subject.


In spite of the fact, that creation of coatings, films and layers appears to be simultaneously one of the oldest arts and one of the newest sciences, the distinction among these terms is not well established yet and, moreover, it may vary depending on the field of science and/or technology. For example, in food industry, the following statement was published: “An edible coating (EC) is a thin layer of edible material formed as a coating on a food product, while an edible film (EF) is a preformed, thin layer, made of edible material, which once formed can be placed on or between food components [25]. The main difference between these food systems is that the EC are applied in liquid form on the food, usually by immersing the product in a solution-generating substance formed by the structural matrix (carbohydrate, protein, lipid or multicomponent mixture), and EF are first molded as solid sheets, which are then applied as a wrapping on the food product” [26]. Here, one should notice that terms “a coating” and “a film” were defined via the term “a layer”. To clarify this topic further, an extensive search in the scientific databases (Scopus, ISI Web of Knowledge) has been performed and a great number of fixed collocations have been revealed. For example, according to Scopus (as of December 2014), a combination of terms “wear-protecting + coating” in the publication titles is used more frequently, if compared with that of “wear-protecting + film” (92 and 19 publications, respectively). On the contrary, a combination of terms “ferroelectric + film” in the publication titles is used much more frequently, if compared with that of “ferroelectric + coating” (6769 and 28 publications, respectively). Back on the subject of current review, a combination of terms “apatite + coating” was found in the titles of 3254 publications, while those of “apatite + film” and “apatite + layer” were found in the titles of 550 and 452 publications, respectively. A similar correlation is valid for the combinations of terms “calcium + phosphate + coating”, “calcium + phosphate + film” and “calcium + phosphate + layer”: they were found in the titles of [945,167] and [191] publications, respectively. As seen from the figures, all types of CaPO4 deposits are most commonly associated with coatings. However, to offset the terminological effects, in this review a term “CaPO4 deposits” will be used instead of “CaPO4 coatings”, “CaPO4 films” and “CaPO4 layers”. Perhaps, the aforementioned facts might be just a matter of terminology or even a habit for each particular sub-direction of science and technology. Since almost all types of materials can be placed and/or fabricated on both similar and dissimilar substrates, many possibilities are available to classify the available types of coatings, films and layers. For example, they might be classified according to their structural material, such as metallic, polymeric, ceramic or composite coatings, films and layers. Similarly, they might be classified as a function of the precursor state: gas, liquid or solid, as well as by the temperatures utilized for the deposition. Furthermore, they might be classified according to their properties, such as biodegradability, edibility, transparency, reflectivity, conductivity, hardness, porosity, solubility, permeability, as well as by the adhesion strength to the substrates. Besides, using a formation approach, all types of coverings can be divided into two big categories: i) conversion ones, which are formed by reaction products of the base material (for example, formation of an oxide layer by surface oxidation)

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and ii) deposited ones (Fig. 1) [27]. In turn, the deposited ones might be further classified according to the deposition techniques (Table 3) [28–30]. More to the point, since coatings and films may consist of either one or many individually deposited layers, all of them might be divided into the monolayer deposits and the multilayer ones. While the former ones are produced by a single stage, the latter ones are produced by a layer-by-layer deposition. Furthermore, the individual layers of the multilayer deposits might be either indistinguishable (in this case, they behave as a thick monolayer) or distinguishable from each other. In the latter case, there might be an opportunity (sometimes, only hypothetical) to remove one or several individual layers from the surface, making deposits thinner. In addition, the layer-by-layer deposition technique easily allows using variable compositions of the deposited substances, which permits deposition of multilayer structures with graded composition and/or properties. This introduces one more classification into the graded and non-graded deposits. What is more, the deposits themselves may be all-over ones, completely covering the substrate, or they may only cover parts of the substrate, which is still another classification. Finally yet importantly, all types of deposits might be thin or thick. These terms appear to be relative and the distinction between them is not well determined either; furthermore, it depends on the specific application. Nevertheless, in general, researchers consider a thin layer, film or coating as one ranging from fractions of a nanometer to several micrometers in thickness. Therefore, a thick layer, film or coating has thickness exceeding several micrometers. Interestingly, according to the aforementioned scientific databases, all types of deposits are much more often “thin” than “thick”. Namely, according to Scopus (as of December 2014), a combination of terms “thin + coating” in publication titles is used more frequently, if compared with that of “thick + coating” (5480 and 673 publications, respectively). Similarly, a combination of terms “thin + film” in publication titles is used much more commonly, if compared with that of “thick + film” (178,703 and 8021 publications, respectively) and a combination of terms “thin + layer” in publication titles is used much more repeatedly, if compared with that of “thick + layer” (38,588 and 2173 publications, respectively). As seen from the numbers, the combination of terms “thin + film” appears to be the most frequent, while that of “thick + coating” appears to be the least often. Further one should mention on the reasons why people apply coverings to the surface of various materials. There are many of them. Namely: 1. The core contains a material, which is toxic, provokes adverse responses, allergic reactions, etc., or has a bitter taste, an unpleasant odor, etc.; 2. Coverings protect the core material from the surroundings to improve its stability and shelf life; 3. Coverings develop the mechanical integrity, which means that the coated products are more resistant to mishandling (abrasion, attrition, etc.); 4. To modify surface properties of the core, such as biocompatibility, light reflection, electrical conductivity, color, etc.; 5. Decoration (in the cases, when the core alone is inelegant);

Fig. 1. A schematic drawing showing the differences between formation of the conversion coatings and the deposited ones. Reprinted from Ref. [27] with permission.

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Table 3 A mutual comparison of several deposition techniques of calcium orthophosphates coatings, films and layers on various substrates [28–30]. Technique

Thickness

Advantages

Disadvantages

Plasma spraying

30–200 μm

A simple and flexible technique; uniform coatings are produced; high deposition rates; low cost

High-velocity oxy-fuel spraying

30–200 μm

High deposition rates; improved wear and corrosion resistance and biocompatibility

RF magnetron sputtering

0.5–3 μm

Pulsed laser deposition (laser ablation)

0.05–5 μm

Ion beam assisted deposition

0.05–1 μm

Electrostatic spray deposition

10 nm–30 μm

Uniform coating thickness on flat substrates; high purity and adhesion; dense pore-free deposits; excellent coverage of steps and small features; ability to coat heat-sensitive substrates Coatings with crystalline and amorphous phases; dense and porous coatings; high adhesive strength; ability to produce wide range of multilayer coatings from different materials Uniform coating thickness; high reproducibility and reliability; dense; high adhesion; wide atomic intermix zone at the coating/substrate interface Low cost; easy set-up; ambient conditions; a wide choice of both precursors (dissolved salts, suspensions, sols) and substrates

Line of sight technique; high temperatures induce partial decomposition and formation of non-stoichiometric and amorphous compounds; simultaneous incorporation of biological agents is impossible; rapid cooling produces cracks Line of sight technique; high temperatures induce partial decomposition and formation of non-stoichiometric and amorphous compounds; simultaneous incorporation of biological agents is impossible; rapid cooling produces cracks Line of sight technique; expensive; low deposition rates; produces amorphous coatings; high temperatures prevent from simultaneous incorporation of biological agents Line of sight technique; expensive; high temperatures prevent from simultaneous incorporation of biological agents; lack of uniformity Line of sight technique; expensive; produces amorphous coatings

Dip coating

2 μm–0.5 mm

Spin coating

2 μm–0.5 mm

Sol–gel technique

b1 μm

Electrophoretic deposition

0.1–2.0 mm

Electrochemical (cathodic) deposition Biomimetic process

0.05–0.5 mm

b30 μm 0.2–2.0 μm

Easy set-up; low cost; coatings applied quickly; can coat complex substrates Easy set-up; low cost; coatings applied quickly

Can coat complex shapes; low processing temperatures; thin coatings; inexpensive process; can incorporate biological molecules Uniform coating thickness; rapid deposition rates; simple setup; low cost; can coat complex substrates; can incorporate biological molecules Good shape conformity; room temperature process; uniform coating thickness; short processing times; can incorporate biological molecules Low processing temperatures; can form bonelike apatite; can coat complex shapes; can incorporate biological molecules Coatings are crystalline; can coat complex shapes

Hydrothermal deposition Thermal substrate deposition Hot isostatic pressing

0.2–2.0 μm

Micro-arc oxidation

3–30 μm

Simple, economical and environmentally friendly technique, suitable for coating of complex geometries

Dynamic mixing method

0.05–1.3 μm

High adhesive strength

0.2–2.0 μm

Deposition is enhanced by heat and current; different CaPO4 phases can be formed Produces dense coatings; homogeneous structure; high uniformity; high precision; no dimensional or shape limitations

6. The core contains a material, which migrates easily to stain hands, clothes and other objects; 7. To modify the release profile of active components, e.g., drugs, from the core. Reasons Nos. 1, 2, 3, 4 and 7 appear to be applicable to the biomedical field in general, while reasons Nos. 1, 2 and 4 are relevant to the subject of this review. To conclude this section, one should note that, in a certain sense, all types of coated materials resemble the functionally graded ones but with an extremely high gradient in both the composition and properties at the core/coating interface. 3. Brief knowledge on the important pre- and post-deposition treatments Due to the excellent biomedical but unfavorable mechanical properties of bulk CaPO4 bioceramics, an extensive research has been focused on their spreading over the surfaces of materials possessing a better mechanical behavior. Various deposition techniques have been already

Line of sight technique; problems to coat large surfaces; low flow rates; requires high temperatures to decompose the precursor solvents and salts Requires high sintering temperatures; possible thermal expansion mismatch; crack appearance Requires high sintering temperatures; possible thermal expansion mismatch; crack appearance; cannot coat complex substrates Some processes require controlled atmosphere processing; expensive raw materials; high permeability; low wear resistance; hard to control the porosity Difficult to produce crack-free coatings; requires high sintering temperatures Sometimes stressed coatings are produced, leading to their poor adhesion with substrate; requires good control of electrolyte parameters Very low deposition rates; requires replenishment and a pH constancy of the simulating solutions (HBSS, SBF, etc.) High pressure and temperatures are required Less common technique; coatings of diverse crystallinities are produced Cannot coat complex substrates; high temperature required; thermal expansion mismatch; elastic property differences; expensive; removal/interaction of encapsulation material; high temperatures prevent from simultaneous incorporation of biological agents Unless the proper electrolytes are used, the procedure rather should be considered as a pre-deposition technique onto which CaPO4 are deposited by other methods Line of sight technique; expensive; produces amorphous coatings

developed as the result (see below). However, in the vast majority of the cases, prior to be coated, the surface of substrates needs to be prepared. The preparation normally consists of cleaning and/or degreasing to remove any sort of surface contamination arising from manufacturing and/or storage. Depending on both the nature and the properties of the substrates, they might be cleaned in solvents, such as acetone, alcohol, trichloroethylene, mixtures thereof, and/or distilled water [31–42]. Furthermore, various types of mechanical modifications of the surface are commonly used to increase the mechanical anchoring between the CaPO4 deposits and substrates. The examples comprise sand- [43,44] or grit-blasting [31,32,35,36,45,46], abrading [37], polishing [39,40,42,47] and grinding [38]. Besides, the surface of substrates might be chemically treated, modified and/or functionalized to promote chemical bonding between the deposits and substrates [48]. The examples comprise chemical activation [36,49], acid [31–33,36,37, 39,41,49,50] and/or alkaline [33–37,39,51–57] treatments, anodizing [38,40,41,58], etching [33,35,36,49,50,57,59], chemically or electrochemically polishing [39,41], oxidizing [36,39,41,55–57,60], passivating and/or tarnishing, phosphorylation [61–63], grafting [64,65], etc. All these treatments appear to be of a special importance if CaPO4 needs


to be deposited onto the chemically inert surfaces, such as carbon nanotubes [66] and graphene nanosheets [67]. In the case of chemically active metals, such as Mg and its alloys, treatment in boiling water might be employed to form a layer of Mg hydroxides on their surface [68]. The majority of such treatments are performed by dipping, spraying, rinsing and/or soaking, depending on both the quality requirements and the limitations of a substrate to be coated. In addition, there are physical treatments, resulting in various types of surface modifications. The examples comprise heat [36,37], glowdischarge [69,70], ultraviolet [47,70,71] and high energy low current DC electron beam [72,73] irradiations, as well as laser [74–76] treatments, which result in oxidation and/or partial decomposition of the very thin surface layer of the substrates. More to the point, prior deposition of CaPO4, the surface of substrates might be coated by an interlayer of another compound [32,64,77–84], including special coupling agents [85–87]. For example, an organophosphate polymer was chemically bound onto a chemically inert polyethylene by surface graft polymerization of a phosphate-containing monomer [64]. For the same purpose, collagen was pre-deposited and immobilized as well [79]. Furthermore, pre-deposition of Ti powder onto the surface of Ti–6Al–4V alloy substrates was found to reduce the residual stresses in the deposited HA and enhance the interface adhesive strength (Fig. 2); thus, Ti appeared to be a bond coat between a Ti–6Al–4V alloy and HA [88]. In other studies, Ti substrates were wet blasted by mixed HA/Ti powders to create a bond coat by embedding the HAp/Ti powders onto them [89–91]. Besides, to eliminate chemical interactions between the substrate surface and CaPO4 deposits, inert compounds could be pre-deposited. Namely, to prevent the chemical reactions between ZrO2 and HA during sintering, an intermediate FA layer was introduced [92,93]. Further, the surfaces of substrates might be modified by an ion implantation of calcium and phosphorus, which might lead to a partial formation of CaPO4 [94–97], as well as either calcium or orthophosphate ions could be pre-adsorbed or incorporated [50,57, 98]. Finally, pre-immobilization of some biologically active compounds (biomolecules), such as enzyme alkaline phosphatase [99–101], on the substrate surface might be used to promote CaPO4 deposition. As seen from the aforementioned citations, several successive treatments of the substrate surfaces are commonly used. Additional details and examples on this topic are also given in Section 4.3. Wet techniques. Besides, after CaPO4 deposits have been produced, diverse types of post-deposition treatments might be applied to provide crystallization/recrystallization of various phases, as well as to both improve their fixation and evaporate traces of solvent(s) trapped within the deposits. For example, post-deposition heat-treatment (annealing) leads to conversion of the deposited amorphous (ACP) and nonapatite phases into more stable compounds, such as HA, with simultaneous increasing of their crystallinity, enhancing corrosion resistance, as well as reducing the residual stress [32,43,46,102–111]. Laser remelting might be used as well [112]. Furthermore, the presence of

Fig. 2. A cross-sectional SEM image of plasma sprayed HA put down over a deposited Ti bond coat on the surface of a Ti–6Al–4V alloy substrate. Reprinted from Ref. [88] with permission.

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water during the post-deposition heat treatment also plays an important role in this conversion [113–117]. Namely, in comparison to heat treatments at 450 °C in dry conditions, the presence of water vapor resulted in a significant increase in coating crystallinity [115,116]. A self-healing of the microstructural defects was also detected [118,119]. In addition, for the same purposes, chemical treatment of the CaPO4coated samples in aqueous alkaline [42,120,121] or F-containing [122] solutions might be used. Similarly, the coated samples might be either kept in boiling water or hydrothermally treated [10,60,123–127]. Immersion into water at 37 °C might be used as well [43]. More to the point, physical treatments, such as electron irradiation might be employed [36,128]. Some results of such treatments are shown in Figs. 3 and 4. One can see that besides the phase and compositional modifications (Fig. 3), the post-deposition treatments caused changes in both the surface morphology and roughness of the deposited CaPO4 (Fig. 4). Finally, other compounds, such as stearic acid [129], sodium bisphosphonates [130] or bioactive glass [131], might be either adsorbed onto or deposited over the CaPO4 coverings to form biocomposites. 4. Deposited CaPO4 In general, since most deposition processes are non-equilibrium in nature, the majority of the deposits differs from the initial materials used for deposition. Namely, the composition of the deposited coatings, films and layers is not always constrained by the phase diagrams. In addition, various properties, such as crystallinity and/or amorphization degrees, microstructure, surface morphology, etc. appear to depend on both the deposition process and application or non-application of pre- and post-deposition treatments. Moreover, various technologies produce deposits of various morphologies, structures and properties. This comes from the specific formation mechanisms, the details of the vast majority of which remain unknown. Furthermore, almost any deposition process of CaPO4 can be performed under simultaneous application of the additional physical forces, such as a magnetic field [132,133], an ultraviolet radiation [134] or an ultrasonic treatment [122,135,136]. All these processing variables introduce additional

Fig. 3. The XRD patterns of (a) — an initial HA powder, (b) — as sprayed HA coatings, (c) — air heat-treated at 600 °C HA coatings (AH600) and (d) — hydrothermally-treated at 150 °C HA coatings (HT150). It is easily seen that both the crystallinity and phase purity of poorly crystalline HA coatings increased after both heat and hydrothermal treatments. Reprinted from Ref. [124] with permission.

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Fig. 4. The representative surface morphologies of the sputtered CaPO4 coatings on glass substrates: (A) as-sputtered surface; (B) after heat treatment at 350 °C for 1 h in absence of water vapor; (C) after heat treatment at 350 °C for 1 h in the presence of water vapor. Reprinted from Ref. [28] with permission.

dissimilarities into the composition, structure, orientation and other properties of the CaPO4 deposits. Both a brief description and the major characteristics of currently available deposition techniques are given below. 4.1. Thermal spraying techniques Thermal spraying is the process in which heat-melted or heatsoftened materials are sprayed onto a colder surface to be deposited on it. A feedstock with either a coating material or its precursor might be heated by various ways, such as a high temperature flame or a plasma jet, by means of which thermal spraying is classified into flame

spraying and plasma spraying, respectively. The maximum temperature achievable is the principal difference between them. In any case, coating materials are fed into a jet, where they are either melted or heatsoftened and the obtained droplets flatten and propel toward a substrate. Since the temperature of the jet rapidly decreases as a function of the distance, the droplets rapidly solidify and form deposits. Commonly, the deposits consist of a multitude of overlapping pancake-like lamellae called “splats”, formed by flattening of the liquid or softened droplets [137]. These splats remain adherent to the substrate and represent the building blocks of the deposits. Due to very high processing temperatures, thermal spraying techniques are always characterized by a high kinetic energy of the flux components, uncertainty of the flux molecular composition ranging from single atoms to droplets (the droplets might also contain the remainders of the crystalline phase) even when the elemental composition is quite definite, as well as by presence of ionized components. In addition, a very fast (up to ~ 106 K/s) cooling often yields formation of metastable, amorphous, morphologically and structurally inhomogeneous phases, as well as deviations from the elemental composition of the initial powders caused by the loss of volatile components. Therefore, for thermal spraying techniques, a high degree of non-equilibrium is predetermined. In view of this, the growth mechanisms of thermally sprayed deposits are not established. In general, the quality of the thermally sprayed deposits is increased with increasing of particle velocities. As the feedstock typically consist of powders with sizes from several micrometers to ~1 mm, the lamellae have thickness in the micrometer range and lateral dimension from several to hundreds of micrometers. That is why, thermal spraying always provides thick (from ~20 μm to several mm, depending on the process and feedstock) deposits over a large area at high deposition rate [138], as compared to other processes listed in Table 3. To the best of my findings, the idea of using thermal spraying to deposit CaPO4 was first proposed in Japan. Namely, on April 3, 1979, a patent describing “an improved implant for a bone, a joint and tooth root comprising a metallic base material and a coating layer of HA which is formed by thermally sprayed HA powder or a mixture of HA powder and ceramic powder around the outer surface of the metallic base material, optionally a layer of a bonding agent and further a layer of ceramics are formed between the metallic base material and the layer of HA” was issued [139]. Since the patent application was filed on December 29, 1976, we can consider this date as the earliest mentioning on thermal spraying of CaPO4. Since thermal spraying occurs at very high temperatures, the deposited hot droplets always heat up the substrates. In some cases, this might result in phase transformation and re-crystallization of the near surface zones. For example, a martensitic transformation and recrystallization was found to occur in near surface of a low-modulus Ti–24Nb– 4Zr–7.9Sn alloy substrate after application of a plasma sprayed HA coating. Both phenomena were attributed to the combination of temperature with cooling process [140]. Certainly, such phenomena introduce additional ambiguities to the both mechanical and adhesive properties of the deposits. In addition, the substrate temperature was found to play an important role in the splat morphology (Fig. 5), which also influences the properties of the deposited CaPO4. Therefore, in many cases, an appropriate cooling technique should be used to keep the substrates at the optimal temperatures [141]. 4.1.1. Plasma spraying Plasma is often referred to as the fourth state of matter, as it differs from solid, liquid and gaseous states, and does not obey the classical physical and thermodynamic laws. In industry, plasmas are used in many different processing techniques, for example, for modification and activation of various surfaces [142]. According to Ref. [8], a plasma spraying technique was discovered accidentally in 1970 by a student, who used the equipment to study melted and rapidly solidified aluminum oxide coatings on a metal substrate [143]. It is a technique in


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Fig. 5. Typical topographical morphology of the single flame sprayed CaPO4 splats deposited onto Ti substrates preheated to various temperatures: a — no preheating (ambient conditions), b — preheated to 100 °C and c — preheated to 300 °C. Reprinted from Ref. [141] with permission.

which a direct current (DC) electric arc (synonyms: plasma torch, plasma gun) is struck between two electrodes, while a stream of gasses (usually, Ar; however, He, H2 or N2 might be used as well) passes through this arc. A high-voltage discharge between the cathode and anode inside the arc turns these gasses into an ionized mixture (plasma) of a very high (up to ~20,000 K) temperature and with a high speed of up to ~400 m/s. A material to be deposited (feedstock) — typically as a powder, sometimes as a liquid, suspension or wire — is introduced into a plasma jet. The temperature of the plasma jet rapidly decreases as a function of distance and just 6 cm outside the electrodes, typical temperatures hover around 2000–3000 K. It decreases further with the distance increasing. Although the injected particles are exposed to the very high temperatures for a very short time (~ 10− 3–10−4 s), most of them are sufficiently heated to become molten, semi-molten or, at least, softened, which is important for adhesion. Therefore, each type of the surface (metallic, ceramic or even organic materials such as paper) that is placed into the jet becomes coated. One pass of a plasma gun can produce a layer of about 5–15 lamellae thick. Once a layer has been applied to the whole substrate, the gun returns to the initial position and another layer is applied [144]. Thus, the plasma sprayed deposits usually consist of several layers. By choosing the optimum relation between particle dimensions (a molten layer of a given thickness on a large particle occupies a smaller relative volume than that on a smaller one), type of gas (the heat content of a plasma, and thus the ability to increase the temperature of the particles, depends strongly on the gas used) and its pressure, plasma speed (the longer the particles reside in a plasma, the higher their temperature) and cooling process of the substrates, one obtains deposits with the desired composition and crystallinity. Typical current values that are used for spraying HA

coatings range from 350 A [114] to 1000 A [145]. Good schematic setups of the plasma spraying process are available in literature [27,30, 144–148]. A typical image of a plasma sprayed HA coating is shown in Fig. 6 [147]. Depending on the experimental conditions, various sub-modifications of plasma spraying technique have been outlined. Namely, there are atmospheric [149–151], vacuum and/or low-pressure [152,153], powder

Fig. 6. A scanning electron microscopy of a typical plasma sprayed HA coating on titanium implants. As seen in the image, the coating morphology has a rough relief caused by the presence of drops of different sizes in the flux. Bar is 10 μm. Reprinted from Ref. [147] with permission.

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[10,123], suspension [154–160], liquid and/or solution [156], gas [161, 162], gas tunnel type [163–165], radio frequency [166,167], low energy [168], etc. plasma spraying techniques and all of them are used to deposit CaPO 4. Such modifications have some specific advantages, e.g., they allow obtaining thinner coatings of 5–50 μm, which are a few times thinner than those obtained by dry powder processing [148]. In addition, there is a microplasma spraying technique [169–174], which is characterized by small dimensions, a low level (25–50 dB) of noise and hardly any dust, as well as a low power consumption (e.g., 1–4 kW, cf. 10–40 kW or higher). All of these make it possible to operate under normal workroom conditions. The process provides CaPO4 deposition on small-sized parts and components, including those with fine sections, this being unachievable with any other methods. Due to a low heat input of the microplasma jet, overheating is reduced for both the deposited materials and the substrates. In addition, the microplasma spraying generally reduces formation of admixture and/or amorphous phases, provides higher degrees of phase purity and crystallinity (e.g., N80%, cf. ~70%) and induces a higher degree of porosity (e.g., ~ 20%, cf. ~ 2–10%) that facilitates bony tissues in-growth. The mechanical properties of the deposits are generally good [169–174]. This makes it possible to widen the application scales of plasma spraying and produce different functional coatings. It is important to stress that among the deposited lamellae there are small voids, such as pores [175], cracks and regions of incomplete bonding. Due to such inhomogeneous structure, the plasma sprayed deposits can have properties significantly different from the initial bulk materials [144]. In addition, due to very high processing temperatures (leading to an incongruent melting coupled with a partial dehydroxylation of HA, as well as a partial decomposition of any other material) followed by a rapid solidification, various admixtures and metastable phases are always present in the deposits. For example, in the case of plasma spraying of CaPO4, complicated mixtures and/or solid solutions of various phases (high temperature ACP, α-TCP, β-TCP, HA, OA, TTCP) with other compounds, such as calcium pyrophosphates, calcium metaphosphates, and CaO are obtained [43,44,46,176–187]. Among them, the presence of CaO causes the most serious problems since hydration reactions occurring during storage or after implantation in vivo transform CaO into Ca(OH)2 with ~50% volume increasing, resulting in considerable internal strains and cracks, especially if CaO was at high amounts in the deposits [177]. Thus, the chemical and phase compositions of the final deposits are also dependent on the thermal history of the powder particles. This leads to a variable solubility of the plasma sprayed deposits, dictated by the amounts of more soluble phases, such as ACP and CaO. Furthermore, the distribution of by-products, amorphous and metastable phases appears to be inhomogeneous. For example, the coating crystallinity was reported to be lower at the interface with the Ti substrate than at the surface of the coating. This happened because metals had a higher rate of thermal diffusivity than CaPO4 and, thus, the cooling rate of the first layer was faster [181]. In addition, the influence of the presence of amorphous and metastable phases on the properties of the entire deposits depends on their location. Namely, being located in the surface layer, such phases promote growth of bone tissues due to a higher bioresorption, while near the coating/substrate interface, a fast dissolution of the amorphous and metastable phases leads to the adhesive strength decreasing and peeling off the coatings before the bone tissue has formed. Besides, residual stresses (they can be either compressive or tensile; both of them are expected to arise from the different thermal expansion coefficients of the substrates and deposited CaPO4) in the plasma sprayed CaPO4 were found and measured. Since the thermal expansion coefficients of metals exceed those of CaPO4, the compressive residual stresses were observed for metallic substrates covered by CaPO4. Depending on the experimental conditions and the application or non-application of pre- or post-deposition techniques, the numerical values appeared to vary from − 140 to − 10 MPa [88, 173,188–192]. Furthermore, immersion in simulated body fluid (SBF) was found to reduce the compressive residual stress until zero and

even redistribute it from compressive to tensile [88]. Therefore, with the general regularities being the same, the quantitative estimates of the properties might be different in different studies. There are a large number of technological parameters influencing the interaction of the deposited particles with both the plasma jet and the substrate and, therefore, the properties of the final deposits. These parameters include feedstock type, plasma gas composition, flow rate, energy input, torch offset distance, and substrate cooling [193,194]. Ideally, only a thin outer layer of each powder particle should be heated to the molten plastic state, which unavoidably undergoes both chemical transformations and phase transitions. This plastic state is necessary to ensure dense and adhesive coatings but it should comprise just a negligible volume fraction of the particles. By choosing the optimum relations among the processing variables, one can deposit CaPO4 with the desired thickness and crystallinity [176,182,183,195]. Furthermore, the dimensions of CaPO4 particles were found to affect their melting characteristics within the plasma flame. Namely, larger particles undergo a lesser degree of melting in the plasma flame than smaller particles [196–198]. For example, during spraying of HA particles with dimensions exceeding ~ 55 μm they were found to remain crystalline and showed little or no melting during plasma spraying. HA particles with dimensions within 30–55 μm were partially melted and consisted of mixtures of crystalline and amorphous phases, while HA particles less than ~ 30 μm were fully melted and contained large amounts of ACP and traces of CaO [196]. Similar results were obtained in another study [198]. In still another study, plasma sprayed HA particles of 20–45 μm in size were found to produce denser lamellar deposits than the deposits obtained by plasma spraying of 45–75 and 75–125 μm HA particles. Deposits formed from 20 to 45 μm-sized HA particles did not show the presence of cavities but contained a flatter smoother surface profile as a result of neatly stacked disk-like splats, while those formed from 45 to 75 and from 75 to 125 μm-sized HA particles contained numerous un-melted particles, cavities and macropores [197]. Interestingly the coating's roughness might be used as a measure of the melting degree of particles. Namely, when the deposited particles reach a more fluid state within the plasma flame they become less viscous and, thus, can be spread out to a greater degree over the substrate. A smoother coating will result in this case. Partially melted particles cannot flatten easily on the surface. This situation leads to large undulations and rough deposits [199]. A diagram for the formation of various phases during plasma spraying of HA coatings is presented in Fig. 7 [200]. According to the authors, if the outside skin of an HA particle is molten and the core remains un-molten, insufficient heat is transferred to melt the particle completely. This model is modified to a totally molten hydroxyl-rich core (with the stoichiometry of HA) with further changes depending on the heat transfer to the particle. The first condition depicts a molten droplet with a hydroxyl-depleted skin. The center containing the hydroxylrich molten material will crystallize upon deposition to form HA. The dehydroxylated region, which is exposed to the substrate upon droplet spreading, will form an ACP phase, but the area distant from the substrate will crystallize to form OA. OA requires smaller atomic rearrangements to occur for crystallization from a viscous melt, and, therefore, crystallizes in preference to a mixture of TCP and TTCP. Growth of HA will begin in the hydroxyl-rich core and will finally change to OA in response to the depleted hydroxyl concentration at the top of the lamellae (Fig. 7, case (i)). If the molten particle flattens to an extent where the cooling rate is increased, then the entire particle becomes amorphous. Both TCP and TTCP are observed in greater quantities when a higher heat transfer to the particle prevails. If the heat dissipation is slow through the already-solidified amorphous and crystalline layers of the coating, TCP and TTCP can be nucleated at the top surface of the lamellae (Fig. 7, case (ii)). The growth of TCP and TTCP may delay the growth of OA with the latent heat of fusion. With a high level of dehydroxylation in the molten particle, lesser amounts of HA or OA will form and so the large volume of dehydroxylated material will then mostly contain


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Fig. 7. A proposed model for phase formation in the plasma sprayed HA coatings. The process stage depicts the various melt chemistries as a function of particle temperature. The microstructure depicts the different phases that can be formed in a lamella. Reprinted from Ref. [200] with permission.

TCP and TTCP. The growth mechanism may begin within the droplet, since a more fluid droplet facilitates faster diffusion. CaO is observed when even higher heating conditions are employed. In addition to being hydroxyl-deficient, the outer shell of the molten particle also becomes phosphate-deficient (Fig. 7, case (iii)) [30]. In addition, a numerical simulation model of HA powder behavior in plasma jet was suggested [201]. Within that model, the authors created temperature fields and estimated the phase composition inside a single HA particle during plasma spraying (Fig. 8). After the hot particles impinge at the substrate surface to be coated, a relatively clear phase separation shown in Fig. 8 becomes lost (Fig. 9) [183]. The result is the aforementioned inhomogeneous mixture of various compounds (ACP, α-TCP, β-TCP, HA, OA, TTCP, calcium pyrophosphates, calcium metaphosphates, CaO, etc.) are interspersed on a microcrystalline scale [201]. Further details on this point might be found in a topical review [187]. To conclude this section, plasma spayed CaPO4 (mainly, HA) deposits on various implants are commercially produced. The examples comprise PureFix™ HA (Stryker Howmedica Osteonics, NJ, USA), Duofix® HA (DePuy Orthopedics, NJ, USA), Osteotite® Bone Screws with HA coating (Orthofix, TX, USA), Landos Corail threaded cups (Landanger, France), Endosteal dental screw implants (Interpore International), Margron femoral stem (Portland Orthopaedics, Australia), AQB Implant System (Advance Co. Ltd., Tokyo, Japan). In addition, Cam Bioceramics (Leiden, Netherlands), Plasma Biotal (Buxton,

Derbyshire, UK), Eurocoating (Trento, Italy) and MedicalGroup (Velin, France) are the contract manufacturers, who, among other types of CaPO4-based business, also use plasma spaying to deposit CaPO4 (mainly, HA) on the customers' specific implants. The substantial number of the industrial manufactures clearly indicates to a great popularity of the plasma spaying deposition technique. 4.1.2. High velocity oxy-fuel (HVOF) spraying In the 1990s, a new class of thermal spray processes called HVOF spraying was developed [202,203]. A mixture of either gaseous (H2, CH4, propane, propylene, acetylene, natural gas, etc.) or liquid (kerosene, etc.) fuel with oxygen is fed into a combustion chamber, where it is ignited and combusted continuously. The resultant hot (~3000 °C as the maximum) gas at a pressure ~ 1 MPa emanates through a converging–diverging nozzle and travels through a straight section. The jet velocity (N 1000 m/s) at the exit of the barrel exceeds the speed of sound. A powder feed stock is injected into the gas stream, which accelerates the powder particles up to ~800 m/s. The stream of hot gas and powder is directed toward the surface to be coated. The powder particles are partially melted and then deposited upon substrates as splats. The resulting CaPO4 deposits have both a low porosity and a high adhesion strength [202–209]. Sometimes, this deposition process is also called thermal printing [210]. Due to the high processing temperatures, the deposited CaPO4 also represent complicated mixtures of various

Fig. 8. The temperatures and assumed phase composition inside a single plasma sprayed HA powder particle. Reprinted from Ref. [201] with permission.

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Fig. 9. A schematic diagram showing the development of a CaPO4 coating during thermal spray deposition of HA. The dashed line separates the amorphous (outer) and recrystallized (inner) phases. Reprinted from Ref. [183] with permission. Another good illustration of this process is available in literature [187].

phases (high temperature ACP, α-TCP, β-TCP, HA, OA, TTCP) with other compounds, such as calcium pyrophosphates, calcium metaphosphates, CaO, etc. [184]. Similar to the aforementioned results on plasma spraying, in the case of HVOF spraying, larger particles were also found to undergo a lesser degree of melting than smaller particles [205,206]. Namely, cross-sectional SEM investigations of the sprayed HA particles of 50 ± 10 μm revealed that they were melt only partially from the surface, while those for HA particles of 30 ± 10 μm revealed that they were melt almost completely. Thus, the melt fraction could be from ~ 20% up to 100% depending on the particle dimensions [206]. The coating

morphology shown in Fig. 10 further reveals the influence of the melt state on the grain size of the deposits. It clearly demonstrates an interface zone between the melted and un-melted parts within a HA splat. It is noted that the HA grains located in un-melted part are of far larger size than those in melted part, which states the influence of rapid cooling on grain growth during coating formation [205]. Furthermore, Raman spectroscopy qualitative inspection on the HVOF sprayed HA particles (partially melted) revealed that a thermal decomposition of HA occurred within the melted part rather than the un-melted zone [206]. Therefore, to both achieve high crystallinity of the deposits and reduce the amount of admixture phases, the appropriate powder size together with the apt HVOF spray parameters must be carefully selected. To finalize this section, one should mention on existence of a highvelocity suspension flame spraying (HVSFS) process developed at the Institute for Manufacturing Technologies of Ceramic Components and Composites (University of Stuttgart, Germany), which appears to be a modification of HVOF spraying. As the name implies, CaPO4-based suspensions are sprayed [211–213]. 4.2. Vapor deposition techniques

Fig. 10. TEM image of as sprayed HA coating showing the interface between melted and un-melted parts within a HA splat and different grain size. Reprinted from Ref. [205] with permission.

First, one should mention, that all available vapor deposition techniques can be broadly divided into two main groups: physical and chemical vapor deposition (abbreviated as PVD and CVD, respectively). Among them, all types of the PVD techniques can be further classified into another set of two groups: (1) those involving thermal evaporation, where a material is heated until its vapor pressure becomes greater than the ambient pressure, and (2) those involving ionic sputtering, where highly energetic beams of ions and/or electrons strike a solid target and knock atoms off from its surface [30,146]. Usually, PVD occurs in vacuum; however, it also might be performed in presence of some gasses. The target is the source material (in our case, CaPO4). Substrates


are placed into a chamber and they are pumped down to a prescribed pressure. Sputtering is driven by momentum exchange between the ions and atoms of the target due to collisions. Afterwards, the dislodged atoms or molecules are deposited on a substrate, which is also placed into the same vacuum chamber. An important advantage of the sputter deposition is that even materials with very high melting points are easily sputtered. For the efficient momentum transfer, the atomic weight of the sputtering gas should be close to the atomic weight of the target, so Ne or Ar are preferable for sputtering of light elements, while Kr or Xe are used for heavy elements [214]. However, for deposition of CaPO4 oxygen might be used as well. It has a number of features and a better stoichiometry with respect to HA of the deposited coatings, films and layers is one of them [215]. To sputter materials, several types of the techniques are used, such as: ion beam [216–226], radio-frequency (RF) magnetron [227–244], pulsed laser [58,75,232,245–265], electron-cyclotron-resonance plasma [266–268], diode, direct current and reactive sputtering or deposition [269,270]. However, since CaPO4 belong to the electrically insulating materials, the last three techniques cannot be used for their spattering. The physical and aggregate states of a CaPO4 source might influence the deposition kinetics. For example, the deposition rate of HA was found to be much higher in a solid plate target than in a powder lump target owing to the difference of apparent density ~75% and ~18%, respectively [234]. Depending on the type of sputtering system and the processing parameters, the structure and chemical composition of the deposited CaPO4 may be quite different from that of the initial material used for sputtering. The differences in Ca/P ratios between the initial CaPO4 and that in the sputtered ones were suggested to be attributed to the preferential sputtering of calcium, probably due to a possibility of orthophosphate ions being pumped away before they are deposited at the substrate [271]. It was also suggested that orthophosphate ions might be weakly bound to the growing deposits and, therefore, they are sputtered away by incoming ions or electrons [228]. Nevertheless, all sputtering techniques have the advantage of putting down of thin deposits with strong adhesion and compact microstructure. 4.2.1. Ion beam assisted deposition (IBAD) IBAD is a PVD technique performed in vacuum, in which ions of a material to be deposited (in our case, CaPO4) are generated from a source by collisions with bombarding electrons. Both one and two target source systems are possible. In the former case, a single electron gun is used to evaporate a CaPO4 source [272], while, in the latter case, two independent Ca-containing (e.g., CaO) and P-containing (e.g., P2O5) sources are evaporated simultaneously by two electron guns (a dual ion beam system). The latter case is called “simultaneous vapor deposition” and by choosing the optimum parameters, this technique allows performing a fine adjustment of the Ca/P ratio in the deposits [273,274]. Nevertheless, in any case, the detached ions are then accelerated by an electric field emanating from a grid toward a target. As the ions leave the source, they are neutralized by electrons from the second external filament and form neutral atoms. A pressure gradient between the ion source and a sample chamber is generated by placing a gas inlet at the source and shooting through a tube in into the sample chamber [275]. Therefore, a typical deposition system consists of two main parts: electron or ion beam vaporizing a CaPO4 bulk target to produce an elemental cloud toward the surface of a substrate and a source for simultaneous irradiation of a substrate with highly energetic inert + (e.g., Ar+) or reactive (e.g., O+ 2 ) gas ions to assist the deposition. If Ca ions are used for irradiation, the technique is called “ion beam dynamic mixing” [276,277]. Good illustrations of the IBAD technique are presented in literature [30,146,148]. In IBAD, first thin (a few hundred atomic layers thick) and amorphous (ACP) deposits are put down. Then an ion implantation tech+ nique, with ions such as Ar+, N+ 2 and O2 , is used to make them crystalline [216–226]. A high bond strength associated with IBAD

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technique appears to be a consequence of an atomic intermixing interfacial layer, which can be up to a few microns thick. Studies revealed alterations in the chemical composition of the ion beam deposited coatings, layers or films. For example, CaPO4 were deposited on silicon wafers by electron beam evaporation of β-TCP both with and without simultaneous Ar+ ion beam bombardments. It was observed that a simultaneous bombardment with Ar+ had a significant effect on both the morphology (Fig. 11) and composition of the deposits. Namely, the deposits formed without Ar+ bombardment were found to have a Ca/P ratio of ~ 0.76 and reacted immediately with the moisture in the air as soon as it is removed from the chamber. In contrast, the deposits formed with Ar+ bombardment had a Ca/P ratio of ~0.80 with smooth and featureless surface morphology [220]. In another study, the effect of Ar+ ion beam current on the bond strength and dissolution of the coating in a physiological solution was studied. The bond strength between the coating and the substrate increased with increasing current, whereas the dissolution rate in physiological solution decreased remarkably [218]. Still in another study, CaPO4 were deposited on silicon substrates by using a simultaneous dual ion beam vapor deposition. The method comprised an electron beam heater and a resistance heater vaporizing CaO and P2O5, respectively, while an Ar+ ion beam was focused onto substrates to assist the deposition. All deposits appeared to be amorphous (ACP), regardless of the current density level of the ion beam. Therefore, a post-heat treatment was applied to crystallize them. The effects of ion beam current density on the phase composition of the crystallized CaPO4 are shown in Fig. 12. The Ca/P ratio was found to increase with increasing ion beam current density presumably due to the high sputtering rate of P2O5 compared to that of CaO from the layer being coated. As seen in Fig. 12, biphasic (HA + TCP) formulations were found when the ion beam was either not used or used at current density

Fig. 11. Optical micrographs of a CaPO4 layer deposited on a Si wafer: (a) without ion beam bombardments and (b) with Ar+ ion beam bombardments (120 V, 0.8 A). Reprinted from Ref. [220] with permission.

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layers, distinguishable by their crystalline features corresponding to various substrate temperatures during deposition [225]. Further details on the IBAD technique are available in the aforementioned references.

Fig. 12. XRD patterns of fully crystallized (after a heat-treatment at 1200 °C) CaPO4 coatings sputtered at 3 different values of ion beam current density. Reprinted from Ref. [274] with permission.

of 180 mA/cm2, while at ion beam current density of 260 mA/cm2 HA peaks were observed only [274]. In one more study, the X-ray photoelectron spectroscopy (XPS) analysis of the deposited FA on titanium revealed several distinct zones: (i) the ambient-exposed surface exhibited elevated concentrations of carbon due to atmospheric contamination; (ii) the bulk zone contained relatively constant concentrations of Ca, O, P and F indicating the chemistry for calcium fluoride (CaF2) and FA formation; and (iii) while the underlaying zone exhibited elevated Ti and O photoelectron peaks suggesting the coexistence of CaPO4 within titanium oxides. Furthermore, the substrate was shown to be identical to a passivated Ti surface prior to deposition [217]. A similar zone structure was also discovered by other researchers [219]. In addition, a cross-section of functionally graded thin HA deposits on silicon substrate obtained by a dual IBAD and simultaneous heat treatment was investigated and the microstructural analysis of the deposits revealed a gradual decrease of the grain size and crystallinity toward the surface, leading to nano-scale grains and eventually amorphous layer at the surface [223]. In addition, the substrate temperature appears to be important. Namely, a series of multilayered CaPO4 deposits were put down onto Ti substrates heated to various temperatures: 650, 550 and 450 °C. As can be seen in Fig. 13, the CaPO4 deposits consisted of three different

4.2.2. Pulsed laser deposition (PLD) Shortly after the discovery of a laser in the end of the 1950s [278], researchers began focusing their beams at materials to observe the interactions. Therefore, PLD (synonym: laser ablation deposition) technique for producing thin films became increasingly popular in the 1970s due to the advent of lasers delivering nanosecond pulses [279]. However, the earliest papers on PLD of CaPO4 were published in 1992 [245,246], in which an excimer and a ruby laser were used, respectively. PLD is another example of a PVD technique, in which a high power laser provides the energy source to melt and vaporize materials from a target (in our case, CaPO4). Owing to the high power density of a focalized pulsed laser, the ablated material is thermally decomposed and forms a plasma plume consisting of a gaseous cloud of highly excited molecules, molecular clusters, atoms, ions, electrons and, in some cases, droplets and target fragments [260]. The thorough investigation of a plasma plume expansion during an ArF laser ablation of HA is well described elsewhere [280]. Rotation of the target prevents crater formation, fast erosion of its surface and deviations of the plasma plume. Therefore, to achieve the stable ablation rates, a target is rotated during deposition. For the sufficiently high laser energy density, each laser pulse vaporizes or ablates a small amount of the material, which is ejected from the target in a highly forward-directed plume. The plume provides the material flux, which is then deposited on a substrate. Usually, the substrates are positioned at a certain distance parallel to the target on a heated substrate holder. One of the main advantages of the PLD technique is its ability to retain the stoichiometry of the target in the deposits. This is due to the high ablation rates causing all compounds or elements to evaporate simultaneously. Good schematic illustrations of the experimental setup of the PLD technique are available in literature [30,146,148,270]. It essentially consists of a laser source, an ultrahigh vacuum deposition chamber equipped with a rotating target and a fixed substrate holder, as well as pumping systems. Mostly, the substrates are attached to the surface parallel to the target surface at a target-to-substrate distance of 2– 10 cm. For ablation, an ultraviolet (UV) excimer lasers with pulses of ~10 ns duration and power densities in the order of 10–500 MW/cm2 are usually used. A two-laser beam technique (so-called, laser-assisted laser ablation method) is used as well. In the latter technique, one laser beam from KrF laser (an ablation laser) is used for ablation of a

Fig. 13. TEM images of the cross-section of CaPO4 deposits on Ti substrates heated to various temperatures: 650, 550 and 450 °C. A-1 is the image of cross-section including both Ti and coating; A-2 is a higher-magnification image of the area, showing the interface and crystalline layer only. The layer nearest to the substrate with thickness of ~190 nm was deposited at 650 °C. It shows a columnar grain structure with grains of 4–30 nm wide aligned perpendicular to the interface. The middle semi-crystalline layer with thickness of ~226 nm was deposited at 550 °C and the top amorphous layer with thickness of ~278 nm was deposited at 450 °C. Reprinted from Ref. [225] with permission.


HA target. The other beam from ArF laser (an assist laser) is used to irradiate a Ti substrate surface during deposition. The assist laser plays an important role in both the formation of crystalline HA deposits and their adhesion strength improvement to the Ti substrate [281]. The PLD process can be performed in both ultra high vacuum and presence of background gasses, such as oxygen [265], which is commonly used when depositing oxides to fully oxygenate the deposits. Ar [282] and water vapor [283] can be used as well. In addition, various types of lasers with different wavelengths, energy fluences and pulse repetition rates might be used. All these processing variables were found to influence the structure, composition, properties, crystallinity, crystal orientation, morphology and roughness of the deposited CaPO4. For instance, striking differences were found between CaPO4 deposits put down by a Nd:YAG laser and a KrF laser: the KrF deposits had a columnar structure, while Nd:YAG ones were granular [249, 250]. This example clearly demonstrates that the mechanism of a laser ablation of CaPO4 appears to be quite complex; its description is available in literature [284]. The PLD method is characterized by a high particle flow density (up to ~1020 cm−2 s−1) and, hence, high condensation rate (0.01–1.0 Å per pulse) and a kinetic energy of particles in the flux within 10–200 eV [256]. Furthermore, due to a laser vaporization mechanism, according to which an accelerated (N106 cm/s) flux is formed from plasma arising upon laser beam interaction with the surface of the target, the local temperature could reach ~104 K. These features predetermine the structure and properties of the deposits: presence of amorphous phases (ACP), high hardness and adhesive strength, as well as reproduction of the initial elemental composition of the target in the deposits. The substance transfer with retention of the composition is due to high heating rates of the target in the laser spot area and thermal ablation [284]. A PLD process is used to put down thin (0.05–5 μm) CaPO4 deposits onto various substrates [58,75,232,245–265]. The process involves ablation of a CaPO4 (usually, HA) target using a pulsed (usually, pulses of 30 ns and 120 mJ at a repetition of 10 Hz) KrF excimer laser beam (λ = 248 nm) in 0.3 Torr/H2O atmosphere and deposition of the ejected HA material on a heated (400–800 °C) substrate. The deposition rate of PLD is about 0.02–0.05 nm per laser shot [8]. An investigation into the effects of high laser fluence (between 2.4 J/cm2 and 29.2 J/cm2) on the properties of CaPO4 deposits was performed. The films deposited at 2.4 J/cm2 were found to be partially amorphous and had rough surfaces with many droplets, while higher laser fluences showed a higher level of crytallinity and lower surface roughness. Furthermore, higher laser fluences also decreased the Ca/P ratio in as-deposited films and, probably, increased their density [261]. The substrate heating is necessary to ensure the formation of a highly crystalline and phase pure deposits. In addition, to prepare deposits with the desired fine texture and roughness, the substrate temperature could be varied [125,263,285]. As PLD is usually carried out at high substrate temperatures, a thin oxide layer might be formed on the substrate surface prior CaPO4 deposition and, thereby, it influences its adherence to the substrate [248]. Similar to the aforementioned thermal spraying techniques, the PLD of CaPO4 results in formation of a complicated mixture of various compounds, including admixtures (such as CaO, calcium pyrophosphates, etc.). In addition, the deposits also contain both amorphous and crystalline phases [247,286]. Interestingly both calcined bones [287] and biphasic formulations, such as HA + TTCP [288], might be deposited as well. Furthermore, TTCP in the deposits was not formed by a partial conversion of previously deposited HA. Instead, it was produced by nucleation and growth of TTCP itself from the ablation products of the HA target or by accretion of TTCP grains formed during ablation [288]. Besides, the PLD deposited CaPO4 might be of various morphologies (e.g., granular and columnar), which have different resistances to delamination [247]. More to the point, various types of oriented textures might be fabricated [254,257,289]. Finally, one should mention that various modifications, such as PLD assisted by in situ ultraviolet radiation

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[134], electric discharge assisted PLD [290] and PLD combined with radio frequency plasma [291] were introduced as well. Further details on the PLD of CaPO4 might be found in literature [148, 256,270]. 4.2.3. Magnetron sputtering Magnetron sputtering is the third example of a PVD technique. To the best of my findings, the idea of using magnetrons to deposit CaPO4 was first proposed in the USA. Namely, on November 11, 1975, a patent (an application was filed on May 13, 1974) describing “a system of coating prostheses with ground bone particles” was issued [292]. According to the abstract: “Prostheses made of various metals and other substances are coated using RF sputtering techniques to form a film which adheres to the device, stimulates living bone attachment thereto and which is ultimately replaced by new bone”. Although that patent described deposition of bone particles, needless to explain that they contain CaPO4 of a biological origin as the major constituent. However, it was in 1992, when sputtering was for the first time used to deposit CaPO4 [227]. A magnetron is a high-powered vacuum tube that generates microwaves using the interactions of a stream of electrons with a magnetic field. Magnetron sputtering was emerged in the mid of the 1960s [293] and was considered as a high-rate vacuum coating technique for depositing metals, alloys and compounds onto a wide range of materials with thickness up to ~5 μm (Table 3). The process is operated at room temperature. A sputtering system consists of an evacuated chamber, a wave generator, a magnetron, a cooling system, as well as it contains a target and a substrate. It works on the principle of applying a specially shaped magnetic field to a sputtering target. Once the substrate is placed into the vacuum chamber, air is removed and the target material (in our case, CaPO4) is released into the chamber in the form of a gas. Powerful magnets ionize particles of the target material. Then, the negatively charged target material lines up on the substrate to form deposits [294]. In addition, a deposition system containing two opposing magnetrons was proposed. The authors verified that when two opposing magnetrons were placed such that their magnetic fields were added, the right-angle geometry could simultaneously achieve both a low substrate back sputtering and high deposition rates. Using this system, one could produce as sputtered HA thin films with desirable stoichiometry and crystallinity without the need for annealing [295]. Besides, a multi-target system was proposed to put down CaPO4 deposits of variable Ca/P ratio. The authors prepared 3 independent CaPO4 targets, consisting of compressed HA, α-TCP and DCPA powders respectively, and put them in various combinations into a cluster of three high vacuum sputtering sources, each of which was mounted at 75° to the substrate surface normal at a distance of 100 mm from the substrate holder. As the result, CaPO4 deposits with Ca/P ratios of 1.82, 1.72, 1.47, 1.43 and 1.0 were prepared after annealing [242]. The preparation details of HA targets are available elsewhere [296]. In principle, magnetron sputtering can be done in either DC (direct current) or RF modes; however, since the DC mode might be done with conducting materials only, the RF mode is solely used to deposit CaPO4. Typically, RF magnetron sputtering employs a sinusoidal wave generator operating at 13.56, 5.28 or 1.78 MHz. The parameters that directly affect the quality and integrity of CaPO4 deposits include discharge power, gas flow rate, working pressure, substrate temperature, deposition time, post-heat treatment and negative substrate bias [148]. For example, the deposition rate was found to increase with increasing Ar gas pressure up to 2 Pa but decreased significantly as the pressure increased up to 5 Pa, while the Ca/P ratios of as-deposited coatings decreased significantly at the higher Ar gas pressures [297]. Variations of negative substrate bias, deposition time and RF power were found to lead to variations in the Ca/P ratio from 1.53 to 3.88 and either crystalline or amorphous structure of the deposits [237]. A good schematic setup of a magnetron sputtering system is available in literature [30,146,148].

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Currently, magnetron sputtering is a convenient method for deposition of CaPO4 on various substrates [227–244]. The advantages of this technique over other deposition processes comprise a high deposition rate, an excellent adhesiveness and an ability to coat implants with difficult surface geometries (Table 3). Namely, the deposition rate of 0.29–1.75 μm/h was determined in various studies [237]. Still, several issues, such as the endurance and the Ca/P ratio, have to be solved before magnetron sputtering can be applied to deposit, on a routine basis, pure and crystalline CaPO4 on implants. For example, both microstructure and mechanical properties of HA thin films, grown on Ti–5Al–2.5Fe alloys by RF magnetron sputtering, were investigated [229]. The deposition was performed from pure HA target in low pressure Ar or Ar + O2 mixtures at substrate temperatures ranging from 70 to 550 °C. Smooth (an average roughness of ~ 50 nm) and uniform CaPO4 deposits were fabricated. It was observed that the films grown at the substrate temperatures below ~ 300 °C were prevalently amorphous (ACP) and contained a small amount of crystalline phases. On the contrary, the films obtained at a substrate temperature of 550 °C or those grown at room temperature followed by annealing at 550 °C consisted of HA [229]. Interestingly regardless of the type of CaPO4 source used (HA, α-TCP and DCPA were tested), the as spattered deposits consisted of ACP, which after a post-deposition annealing at 500 °C were found to be transformed into apatite-like structures with the Ca/P ratios of 2.14, 1.84 and 1.57, respectively [239]. One can see that Ca/P ratios in the deposits were always higher than those in the initial targets. The chemical composition of the deposited CaPO4 might be modified by varying the RF sputtering power density [238]. Namely, when the power density was increased by 240%, the Ca/P ratio increased from ~ 1.51 to ~ 1.82. X-ray diffraction (XRD) studies revealed the phase pure HA except for the samples prepared at the highest power density values, in which the presence of CaO and TCP was also detected. Interestingly deviations from the stoichiometric HA resulted in reduction of the elastic modulus. Namely, for Ca/P ~ 1.51, the elastic modulus dropped by ~15%, which was attributed to Ca vacancies in the CDHA lattice, while for Ca/P ~ 1.82, the average elastic modulus decreases by ~10% due to formation of additional phases [238]. Various types of calcium phosphates were magnetron sputtered from TTCP, HA, β-TCP, β-calcium pyrophosphate (CPP) and β-calcium metaphosphate (CMP) powder targets [235]. The composition of the deposits was changed depending on the target materials, while the Ca/P molar ratios of the deposits varied from 0.74 to 2.54, increasing with the Ca/P molar ratio of the target. Interestingly the deposition rate of the aforementioned calcium phosphates was established as: TTCP ≈ β-CMP N β-TCP N β-CPP ≈ HA, which correlated well to the solubility order: TTCP ≈ β-CMP N β-TCP N β-CPP ≈ HA [235]. To conclude, RF magnetron sputtering might be combined with other deposition techniques. For example, plasma-assisted RF magnetron co-sputtering deposition was used to put down CaPO4 on Ti–6Al– 4V orthopedic alloy [298,299]. One should mention that the magnetron spattered CaPO4 deposits on implants are commercially available, for example, BioComp HAVD (Hydroxy Apatite Vapour Deposition) implants (BioComp Industries BV, Netherlands). Further details on this technique are available in other reviews [148,300,301].

4.2.4. Electron-cyclotron-resonance (ECR) plasma sputtering ECR plasma sputtering is the fourth example of a PVD technique; however, initially it was considered as a CVD process [302]. It is an offaxis low temperature deposition method using a raw material supply by sputtering using a microwave ECR excited plasma, while a plasma stream extraction onto the specimen is performed by a divergent magnetic field. A microwave ECR technique is used for highly ionized plasma generation and plasma stream extraction at low gas pressures of 0.001 to 0.1 Pa. Gas molecules and particles sputtered by ions in the plasma stream are ionized and transported to the specimen substrate with energies of 10 to 30 eV without the need for substrate heating. Fully

Fig. 14. (a) A schematic sketch of an ECR plasma sputtering apparatus; (b) magnetic-field strength as a function of distance from the end of plasma source. Reprinted from Ref. [303] with permission.

oxidized, dense and high quality deposits are obtained at room temperature [302]. A schematic sketch of an ECR plasma sputtering apparatus is shown in Fig. 14a [303]. Briefly, 2.45 GHz microwaves (500 W power) were divided into two waves of equivalent magnitude and transmitted into a plasma source through two face-to-face silica glass windows. An ECR plasma was generated by introducing Ar or Xe gas (flow rate was 8 cm3/min) into the plasma source with simultaneous activating of a magnetic field. The plasma density was the highest around the “ECR point” (Fig. 14b). The plasma was then transported to a cylindrical HA target along the disperse magnetic field (magnetic current was 14 A). When RF voltage (500 W) was applied to the target, Ar or Xe ions in the plasma stroke an inner cylindrical surface of the HA target and the sputtered atoms and clusters were deposited onto the substrate surface that was placed downstream. Si(100) wafers were used as substrates, which were not intentionally heated from the backside, but their illuminating with an ECR plasma stream raised the surface temperature up to 70 °C. The background pressure in the deposition chamber was about 3 × 10−5 Pa, which was mainly due to residual water molecules. Since the plasma source was located remotely from the HA target, a low plasma damage was ensured in exchange for a low deposition rate [303]. To fabricate a cylindrical HA target, HA powders were dispersed in a polyvinyl alcohol binder, shaped into cylindrical form and sintered at a temperature within 400 and 800 °C. Once the HA target was installed in the sputtering chamber, its background pressure increased from 3 × 10−5 to ~10−3 Pa. It took more than one month for the background pressure to settle at pressures lower than 5 × 10−5 Pa, which was very long. A stainless bottle filled with distilled water was connected to the deposition chamber through a variable leak valve. To generate PO34 − and OH− functional groups, water vapor was supplied during sputtering through a variable leak valve to supplement oxygen. The water pressure range was between 10−4 and 10− 1 Pa. A deposition rate was ~ 1.3 nm/min. Crack-free ACP deposits of 300–500 nm thick with smooth morphologies were manufactured at room temperature, which were transformed into HA deposits after annealing in presence


of oxygen [266]. However, crack-free HA deposits of high hardness and the specific crystallographic orientations were manufactured when Xe gas was used and Si(100) substrates were heated to 450–500 °C from their rear [267]. Similar results were obtained for sapphire substrates [268]. Up to date, nothing has been reported on the biomedical properties of such deposits. To conclude, since the productivity of ECR plasma sputtering appears to be very low, this technique might be feasible for specific applications, in which epitaxial deposition of thin HA coatings, films and layers with the specific crystallographic orientations is sufficient. Afterwards, this technique might be followed by further deposition using a more productive method. 4.2.5. Metalorganic chemical vapor deposition (MOCVD) As seen from the name, MOCVD is a CVD technique. Historically, this technique was developed to put down deposits in the semiconductor industry and it demonstrated several benefits, such as an ability to modulate precursor concentrations during deposition to create functionally graded deposits. In the MOCVD process, a heated gaseous stream of a carrier gas + metalorganic precursors containing reactive constituents for the desired coating material are directed onto a heated substrate, where a reaction takes place to form solid deposits. Reaction byproducts are then pumped out. The process is terminated when a desired thickness is reached [304]. For the first time, CaPO4 were deposited by MOCVD in 1996 [305], followed by other studies [306–311]. To perform deposition, Ca- and Pcontaining precursors are necessary and, in many cases, Ca-containing metalorganic precursors have rather complicated compositions. The examples comprise, Ca(tmhd)2 (where, tmhd = 2,2,6,6,-tetramethylheptane-3,5-dione) [305], Ca(hfpd)2(tetraglyme) (where, hfpd = 1,1,1,5,5,5,-hexafluro-2,4-pentadione) [306], calcium-dibenzoylmethane [307], Ca(dpm)2 (where, dpm = bis-dipivaloylmethanato) [308,310]; however, simpler compounds, such as calcium lactate [309,311] might be used as well. P-containing precursors comprise P2O5 [305], trimethylphosphate [307,309,311], and triphenylphosphate [308,310]. Obviously, the precursors must be volatile at relatively low temperatures (b~600 °C) and pressures between 1 Torr and atmospheric pressure. To perform MOCVD, both Ca- and P-containing precursors were evaporated by heating and, by means of a carrier gas (e.g., Ar), forwarded into a reactor, in which a heated substrate is located [305–311]. For better oxidation of the precursors, oxygen might be separately introduced into the reactor, where it is mixed with the precursor vapors. The substrate is heated to 600–800 °C. Depending on the Ca/P ratio, α-TCP, HA or biphasic formulations such as α-TCP + HA could be deposited [308]. The deposited CaPO4 were found to be dense and homogeneous in appearance, while the grain sizes of the deposits were found to decrease as the substrate temperature decreased. Therefore, the grain growth of CaPO4 was faster at higher temperatures. Such temperature dependence of the microstructure makes it possible to deposit CaPO4 with controlled surface characteristics. The main disadvantage of the MOCVD process comes from the precursors. Namely, due to using metalorganic precursors, carbon contamination can become a problem. Additional details on MOCVD might be found in the aforementioned references. 4.2.6. Molecular precursor and thermal decomposition A molecular precursor and a thermal decomposition appear to be similar; therefore, they are combined in this section. Both techniques belong to CVD and have many similarities with the previously mentioned MOCVD. Namely, molecular precursor and thermal decomposition techniques also are based on deposition of a mixture of the Caand P-containing organic compounds (molecular precursors) with the desired Ca/P ratio onto substrates, followed by drying, calcining and/ or sintering. High temperatures result in burning out and elimination

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of the organic components, while the remaining oxides of Ca and P are combined into solid CaPO4 deposits [312–317]. The difference between these techniques and the aforementioned MOCVD lies in a substrate temperature during deposition: in these techniques the substrates are coated at ambient conditions, followed by drying and sintering to decompose the Ca- and P-containing precursors, while in MOCVD the substrates are heated; therefore, a thermal decomposition of the precursors and formation of CaPO4 deposits occurs in situ during deposition. Therefore, to deposit HA, calcium 2-ethyl hexanoate was stoichiometrically mixed with bis(2-ethyhexyl) phosphite in ethanol. The mixture was stirred for 2 days at ambient conditions, followed by dip coating procedure. Afterwards, the coated substrates were air dried and calcined at 1000 °C in air [312]. Similarly, to deposit carbonatecontaining CDHA, a solid dibutylammonium metaphosphate salt was added to an ethanol solution of Ca-ethylenediamine-N,N,N′,N′tetraacetic acid/amine complex in the amounts to get Ca/P ratio 1.67 [313–317]. Afterwards, the prepared molecular precursor solution was dropped on the Ti surface to cover the entire area of the substrates. Then the substrates were dried at 60 °C for 20 min followed by firing at temperatures exceeding 500 °C for 2 h under atmospheric conditions. After firing, crystalline carbonate-containing CDHA deposits of ~0.44 μm thick were formed. The tensile bond strength measurement and scratch test revealed an excellent adhesion of the CaPO4 deposits after immersion into phosphate buffered solution (PBS) solution [313, 316]. Later, the in vivo experiments showed a significantly higher percentage of bone contact of the coated implants if compared with the uncoated ones [314,316,317]. Positive results were also obtained with cells [318].

4.3. Wet techniques As follows from the definition, all types of wet deposition techniques occur from either solutions or suspensions both aqueous and nonaqueous. Therefore, all of them occur at moderate temperatures [319]. Depending on the solution pH, various CaPO4 might be precipitated (Table 1) and, therefore, be deposited. In general, the deposition process is usually based on the heterogeneous nucleation phenomenon, the kinetics of which depends on many parameters such as the solution supersaturation, concentration of the reagents, temperature, hydrodynamics, presence or absence of admixtures, nucleators, inhibitors, etc. As to the precipitation mechanisms of CaPO4 from aqueous solutions, this process appears to be rather complicated and for the biologically relevant CaPO4 (OCP, CDHA and HA) it occurs via formation of one or several intermediate and/or precursor phases, such as ACP, DCPD and/or OCP. The detailed description of the precipitation mechanisms is beyond the scope of current review; the interested readers are referred to the special literature on the topic [320–322]. For some types of the wet techniques, specific surface preparation appears to be necessary. For example, if CaPO4 needs to be biomimetically deposited on titanium or its alloys, a surface layer of titanium oxides, hydroxides and/or titanates should be created prior the deposition [323]. This can be done by various oxidation techniques, such as heat [324] or alkali [49,128,325,326] treatments, oxidation in H2O2 [128,326], micro-arc oxidation [327], pre-calcification in boiling Ca(OH)2 solution [328,329] or under the hydrothermal conditions [330], as well as by water vapor treatment [331]. Similar is valid for other chemically inert metals: prior biomimetic deposition of CaPO4, a surface layer of hydrated zirconium hydroxides, niobium hydroxides, tantalum hydroxides or their Na- or K-containing salts should be created on the surface of Zr, Nb and Ta, respectively [332,333]. The detailed information on the surface preparation of Mg and its biodergabable alloys is available elsewhere [42]. In addition, a positive influence of the presence of hydrated silica [334], sodium [335] or both (i.e., sodium silicates) [336] on CaPO4 precipitation onto substrates are known as well. Further details on this subject are available in literature [30,337–339].

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4.3.1. Electrophoretic deposition (EPD) According to Wikipedia, the free encyclopedia: “Electrophoretic deposition is a term for a broad range of industrial processes, which includes electrocoating, e-coating, cathodic electrodeposition and electrophoretic coating or electrophoretic painting.” [340]. A characteristic feature of this process is that charged colloidal particles suspended in a liquid migrate under the influence of a direct current electric field (electrophoresis) and are deposited onto a conductive substrate of the opposite charge. The entire process includes mass transport, accumulation of particles near the electrode and their coagulation to form deposits [341]. To the best of my findings, the idea of using electrodeposition to produce CaPO4 coatings was first proposed in USA. Namely, on June 1, 1975, a patent (an application was filed on April 16, 1974) describing “a method of improving orthopedic implant materials by the simultaneous electrodeposition of bone and collagen onto a prosthesis” was issued [342]. Although that patent describe deposition of bone particles, needless to explain that they contain CaPO4 of a biological origin as the major constituent. Thus, electrodeposition appears to be the earliest known technique to deposit CaPO4. Since EPD is designed to apply materials to any electrically conductive surface, it is used to deposit CaPO4 on metallic substrates only. This approach is especially useful for porous metallic structures. To perform deposition, CaPO4 powders are suspended in water, alcohol or other suitable liquids to produce a coating bath, followed by deposition onto a metallic surface [343–357]. Water is not preferred as a medium of dispersion of CaPO4, since the stability of the suspensions is poor without the help of dispersants and results in immediate sedimentation. Therefore, alcohols are often used instead. Among them, butanol is preferred over ethanol in order to lower the evaporation rate, which subsequently reduces cracking during drying of CaPO4 deposits. In addition, the proper dimensions of the particles are very important because they must be fine enough to remain in suspension during the coating process. EPD normally involves submerging a metallic substrate into a container or vessel, which holds the coating bath, and applying electricity using electrodes, where the substrate is one of the electrodes (anode or cathode). An applied electric field is the driving force of the deposition [341]. Depending on the mode and sequence of voltage applied, EPD of CaPO4 can be carried out at either DC or alternating current (AC) fields [352], as well as at either constant [348] or dynamic [349, 357] voltage. Among them, AC deposition leads to denser and less cracked coatings as compared to DC deposition at similar thickness [352], while dynamic voltage is preferable to put down gradient deposits [349]. After deposition, a substrate is normally rinsed off to remove the undeposited bath, followed by sintering in a high vacuum (10−6 to 10−7 Torr) at 850 to 950 °C [341]. The resulting deposits consist of a number of CaPO4 phases plus various random admixtures. For example, in the case of electrophoretically deposited CDHA, the sintering results in its transformation to biphasic (HA + β-TCP) coatings [344]. Their thicknesses can be varied by changing both the electrical field strength and the deposition time. Further, at the coating/substrate interface various metal-phosphorus compounds might be formed due to mutual inter-diffusion of CaPO4 and atoms of the metallic substrates. Unfortunately, due to densification during sintering, shrinkage and cracking of the deposits can occur. In addition, thermal stresses induced by the differences in thermal expansion coefficients between the core and the deposits during sintering and cooling can lead to cracking [8]. The surface morphology of the deposited CaPO4 was found to depend on applied voltage [347], deposition time [347] and powder concentration [348]. Namely, at 200 V the deposited particles had dimensions within 0.20–0.35 μm, at 400 V the particle size range increased up to 0.35–0.80 μm and at 800 V, the particle size range increased up to 0.80–1.20 μm. Furthermore, increasing voltages resulted in increasing of the amount of deposited CaPO4. Besides, porous and roughened deposits were obtained at a higher electric field, while dense ones of finer particle size were obtained at a lower electric field.

Similar effect was noticed for the deposition time: the shorter the time, the smaller particles were deposited [347]. Concerning the powder concentration in suspensions, for a low HA concentration, the deposits were very rough and a great level of agglomeration was noticed. At higher HA concentrations, they became uniform, crack-free and less agglomerated. At very high concentrations of HA, many cracks were found [348]. Thus, the powder concentration, deposition time and applied potential appeared to have a significant effect on the coating morphology. Although this is slightly beyond the subject, one should mention that some specific types of CaPO4 bioceramics might be prepared by EPD [345,346,358,359]. For example, hollow HA fibers of various diameters were fabricated. Initially, submicron HA powders were deposited on individual carbon fibers, their bundles and felts. Afterwards, they were sintered to oxidize carbon substrates and leave behind the corresponding CaPO4 replicas [345]. Similarly, uniform HA tubes were prepared by EPD of HA powders on carbon rods by repeated depositions at room temperature, followed by sintering [346]. In addition, using the same approach, both porous CaPO4 scaffolds [358] and coatings [360] were fabricated. Further details on the EPD are available in the topical review [361]. In addition, various modifications and hybrid technologies, such as plasma-assisted EPD [362] and a combination of micro-arc oxidation with electrophoresis [363] have been developed as well. 4.3.2. Electrochemical (ECD) or cathodic deposition As seen from the Wikipedia definition (see the previous section), the ECD appears to be a sub-division of EPD [340]. In ECD, supersaturated or metastable aqueous electrolytes containing both calcium and orthophosphate ions are used. A typical setup includes a platinum electrode (anode) and a metallic implant (cathode) connected to a current generator. The process is based on the various electrochemical reactions occurring in electrolytes, resulting in solution pH increasing around the cathode. This causes transformation of acidic orthophosphate ions − 3− HPO2− 4 and H2PO4 into PO4 with simultaneous formation of less soluble CaPO4. The electrochemical reactions occurring with ions during CaPO4 deposition are available in literature [364,365]. After the solution supersaturation has reached the critical value, CaPO4 crystals are nucleated and grow on the cathode, forming deposits [366–368]. Deposition can be carried out under constant current, voltage or potential modes and the choice of parameters strongly affects the phase and morphology of the coating obtained. However, when using high current densities, a large amount of H2 bubbles is produced at the vicinity of the cathode leading to non-uniform and weakly adherent deposits [369]. In order to overcome these problems, both a pulsed deposition [370–374] and a cyclic voltammetry method [375] were proposed. Since ECD always occurs on the negatively charged electrodes, in literature it is sometimes referred to as cathodic deposition [79,376,377]. A unique advantage of this method is that once the ceramic film has coated a certain part of a metallic substrate, the local current density will be automatically reduced due to the high resistance of ceramics. The current density will then concentrate on the bare metal. This negative feedback in current density results in high uniformity in both coating morphology and thickness, even on complex shapes. To compare the ECD and EPD techniques, one can claim the following. Both techniques might be used to coat the conductive materials only, while a lack of the electrochemical reactions in the EPD is the major difference between them. In addition, due to the substantial dimensional differences between suspended particles and dissolved ions, the EPD is an important tool for preparation of thicker deposits, while the ECD enables formation of thinner ones (Fig. 15) [377]. Since ECD of CaPO4 occurs in aqueous solutions, it might be performed at both ambient conditions [370,378–382] and elevated temperatures of 50–200 °C [35,36,40,128,375,383–390]. At temperatures exceeding 100 °C, autoclaves are used [388–390]. The ECD might be performed in various electrolytes, starting from a simple system of calcium nitrate and ammonium orthophosphate dissolved in water [128,375,


Fig. 15. Schematics of the electrophoretic and electrochemical depositions showing the motion of positively charged ceramic particles (left picture) and ions M+ (right picture), followed by their deposition on the cathode. Reprinted from Ref. [377] with permission.

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376,379–384,386,387] and ending with SBF [391–393]. Such parameters as deposition temperature, time, electrolyte composition and solution pH were found to influence both the structure and properties of the deposits (Fig. 16) [383]. For example, at solution pH ~ 5, DCPD is deposited, which afterwards might be converted into HA by treatment in a 1 M NaOH solution for 1 h at 80 °C or CDHA by treatment in SBF for 7 days at ambient conditions [381]. A coating thickness of less than 1 μm can be achieved. Reduction of the thickness leads to an increased resistance to delamination, which is observed frequently for thicker coatings [393]. Deposition of nano-sized crystals is also possible [394–398]. Natural materials, such as shells, have been tested as the source of calcium to perform ECD [399]. Unfortunately, ECD requires a sufficient volume of electrolyte to surface ratio (~20 ml of electrolyte are needed to coat 1 cm2 of implant). Additionally, hydrogen gas production hampers deposition due to formation of bubbles on the metallic surface, which results in non-uniform coatings [147]. In order to overcome the latter problem, a modulated technique was proposed [370]. According to the literature, nucleation of CaPO4 crystals during the ECD can occur either as instantaneous nucleation or as progressive one [400]. Nucleation is said to be instantaneous whenever the formation rate of a nucleus at a given site is expected to be at least 60 times greater than the expected rate of coverage of the site by growth only. Nucleation is said to be progressive when the expected coverage of a

Fig. 16. Typical SEM images of the CaPO4 coatings electrochemically deposited under different conditions: (a) pH = 4.2, 60 °C, 2 h; (b) pH = 4.2, 85 °C, 4 h; (c) pH = 6.0, 80 °C, 3 h; (d) pH = 6.0, 90 °C, 3 h; (e) pH = 6.0, 90 °C, 3 h, addition of 0.01 M KCl; and (f) pH = 6.0, 90 °C, 3 h, addition of 1.00 M NaNO2. All scale bars are 10 μm. Reprinted from Ref. [383] with permission.

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site by growth is at least 20 times greater than the coverage of the same site by the act of nucleation. In any case, after being formed, CaPO4 nuclei can grow in one, two or three dimensions resulting in different shapes of the deposits like needles, disks or hemispheres depending on deposit/substrate binding energy and their crystallographic misfit. In the case of CDHA, during the first ~12 min, the nucleation was instantaneous and accompanied by a two-dimensional growth. Afterwards, it became progressive and accompanied by a three-dimensional growth [400]. Data are available that ECD of CDHA occurs via intermediate formation of an OCP precursor phase [401]. Presumably, this might be a reason, why the electrodeposited CDHA coatings were found to consist of two distinct layers [402]. In general, electrochemically deposited CaPO4 have a uniform structure since they are formed gradually through nucleation and growth processes at relatively low temperatures [8]. The deposits might be porous [367]. Furthermore, HA coatings could be produced by ECD deposition of non-apatitic CaPO4, followed by additional treatments [403–406]. Subsequently, the deposited CaPO4 might be heat treated in water steam at 125 °C [407] and/or then calcined at temperatures up to 800 °C to densify and improve its bonding to the substrates. Besides, ECD might be combined with other deposition techniques. Namely, ECD was used to deposit CaPO4 seeds, followed by the secondary growth of CaPO4 crystals under hydrothermal conditions [408,409]. Further details on the ECD are available in literature [301]. To conclude, electrochemically deposited CaPO4 coatings on implants are commercially available, for example, BONIT® (DOT GmbH, Rostock, Germany) and BoneMaster® (BIOMET Corp., Warsaw, IN, USA). In addition, MedicalGroup (Velin, France) is a contract manufacturer, who, among other types of CaPO4-based business, also uses ECD to put down DCPD (trade name SALTINA™) on customers' orthopedic implants. 4.3.3. Sol–gel deposition By definition, a sol is a two-phase suspension of colloidal particles in a liquid, while gels are regarded as composites because they consist of a solid skeleton or network that encloses a liquid phase or an excess of the solvent. Thus, as the name implies, a sol–gel process is a wet-chemical technique that involves transition from a liquid ‘sol’ into a solid ‘gel’ phase. Colloidal particles can be in the size range of 1–1000 nm; hence, gravitational forces on these particles usually are negligible. Thus, the interactions among them are dominated by both short-range forces and surface charges. A sol–gel process goes back to the genesis of the modern chemistry. It was first identified in 1846 as an application technology, when Ebelmen observed a hydrolysis and a polycondensation of tetraethylorthosilicate [410]. To prepare sols, usually, inorganic salts and/or organoelement compounds such as alkoxides are used as precursors. For example, to synthesize HA, the orthophosphate precursors comprise P2O5, P(OC2H5)3, H3PO4 and (NH4)3PO4 dissolved in ethanol, while the calcium precursors are Ca(NO3)2·4H2O and Ca(CH3COO)2· H2O dissolved in either ethanol or water [284]. In addition, Ca(OC2H5)2 precursor might be used as well but it must be dissolved in a non-aqueous solvent. After mixing of the Ca- and P-containing precursors, sols are formed due to hydrolysis and condensation reactions. Besides, a sol might be prepared by dispersion of colloidal particles in a liquid, followed by destabilization of the sol to produce a particulate gel. Afterwards, the sol condenses into a continuous liquid gel phase. With further drying and heat treatment, the gel is then converted into dense ceramic or glass materials [411]. The deposited gels create coatings, films and layers, which commonly are put down using dip coating [412–427]; however, other deposition techniques such as spraying or spin coating [415,427–429] are used as well (the details of these techniques are given below). Therefore, to perform a sol–gel deposition of CaPO4, a Ca- and Pcontaining sol is deposited onto the substrates at low reaction temperatures. Shortly afterwards, the sol is transformed into a gel. After drying,

Fig. 17. A schematic sketch of biphasic deposits prepared by a sol–gel technique, in which nano-sized particles of β-TCP were embedded into a continuous coating of fluoridated HA (FHA). Reprinted from Ref. [420] with permission.

solid deposits are left on the surface. To prepare thicker deposits consisting of several layers, the deposition-and-drying cycle is repeated several times. As formed, sol–gel CaPO4 deposits are porous, less dense and have a poor adhesion to the substrate. To improve their properties, the coated substrates are annealed at temperatures of 400–1000 °C [380,412–429]. Depending upon both the Ca/P ratio and the calcining temperature, different CaPO4 compounds are obtained. The resulting deposits can be extremely dense and adhere strongly to the underlying substrate [8]. Biphasic deposits, in which particles of one type of CaPO4 were embedded into a continuous coating of another type of CaPO4, could be prepared as well (Fig. 17) [420]. In order to improve the bond strength between the deposits and substrates, an intervening layer of another compound might be applied prior the sol–gel deposition of CaPO4 [430]. 4.3.4. Wet-chemical and biomimetic deposition Since biomimetics (synonyms: bionics, biomimicry) seeks to apply biological methods and systems found in nature, biomimetic deposition appears to be a method whereby a biologically active bone-like apatite deposits are formed on substrates by immersion in various simulating solutions, such as Hank's balanced salt solution (HBSS), PBS or SBF [37,39,51,56,68–71,324,327,431–443]. A wet-chemical deposition appears to be very similar to the biomimetic one. The differences between them lay in both the solution compositions (solutions for wet-chemical deposition may contain any possible ions and additives, while those for biomimetic deposition must contain only the biologically relevant ones) and temperatures (wet-chemical deposition may be performed at

Fig. 18. A scanning electron microscopy of a typical biomimetically deposited carbonated apatite coating. Inset: an EDX spectrum of the coating. Bar is 200 μm. Reprinted from Ref. [147] with permission.


temperatures from 0 to 100 °C, while biomimetic deposition must be performed at temperatures from ~20 to ~40 °C). Historically, it was in 1990, when CaPO4 was first biomimetically deposited onto a substrate [431]. This method involves a heterogeneous nucleation and growth of bone-like crystals of ion-substituted CDHA on the surface of implants at physiological conditions (temperatures 25 or 37 °C and solution pH within 6–8) for several days or even weeks. In addition, OCP might be biomimetically deposited [14,391, 442–445]. The thickness of such CaPO4 deposits varies within several microns (Table 3), while, according to the XRD measurements, the majority of the biomimetic precipitates appear to be either amorphous or poorly crystalline [8]. A typical example of a biomimetically deposited CaPO4 is shown in Fig. 18 [147]. The mechanism of bone-like apatite formation on an oxidized surface of titanium was investigated in details [446,447]. Briefly, it looks as follows. First, a layer of amorphous sodium titanate is formed on the Ti surface after alkali pre-treatment. Then, immediately after immersion into SBF, the sodium titanate exchanged Na+ ions for H3O+ ions in the fluid to form Ti–OH groups on its surface. Later, the Ti–OH groups incorporated calcium ions from the SBF to form a layer of amorphous calcium titanates. After longer soaking times, the amorphous calcium titanates incorporated orthophosphate ions from the SBF to form ACP coatings with a Ca/P atomic ratio of ~1.4. Thereafter ACP converted into bone-like ion-substituted CDHA with a Ca/P ratio of ~ 1.65 [446]. In the next study, the authors specified that after exchanging Na+ ions for H3O+ ions various types of titania gels might be formed but only those with the anatase or rutile structure induced apatite formation [447]. Interestingly although Ti and Zr belong to the same group of the Periodic Table of Elements, the surface reactions of biomimetic deposition of CaPO4 were found to be different for the Ti and Zr substrates [448]. Further specific details on this topic are available in literature [449]. Since biomimetic deposition of CaPO4 is a slow process, ways were sought to make it faster. Using condensed versions of the simulating solutions is the most popular option. For example, time for apatite induction in the 1.5-fold SBF was significantly shortened compared to that in the standard SBF [39,56,439]. Therefore, the concentration of SBF was increased further. Namely, 2-fold [439,450–452], 5-fold [453–456], 7-fold [436] and even 10-fold [66,436,457–459] SBF solutions were used to accelerate precipitation and increase the amount of precipitates. However, the precipitation might become too fast. In order to slow down the fast precipitation from the condensed SBF solutions, they might be acidified by CO2 bubbling to pH ~ 5.8. As CO2 gas evolved out of solution, the pH value slowly increased inducing the deposition of CaPO4 on the substrates [391]. A similar effect might be achieved by addition of a urea–urease combination, in which urease causes an enzymatic decomposition of urea into CO2 and NH3, resulting in pH increasing [460]. A stable solution containing high concentrations of calcium and orthophosphate ions was prepared in another study. This solution became supersaturated after NaHCO3 addition. Uniform deposits of ~ 40 μm thick and adjustable composition from ionsubstituted CDHA to DCPD were obtained after immersion for 24 h [461]. However, one should keep in mind that application of the condensed solutions of SBF was found to result in changes in the chemical composition of the precipitates; namely, the concentration of carbonates increased, while the concentration of orthophosphates decreased [462]. In addition, deposits produced from lower concentration SBF solutions was found to have a high initial Mg2+ incorporation producing relatively smooth surface topography, while higher concentration SBF solutions resulted in a lower Mg2 + incorporation producing a highly rough surface topography consisting of micrometer-sized plates [436]. The nucleation and growth of CaPO4 deposits on Ti–6Al–4V alloy substrates from 5-fold SBF was investigated in details by both atomic force and environmental scanning electron microscopes [455]. Namely, scattered deposits of ~ 15 nm in diameter were found to appear after only 10 min of immersion in 5-fold SBF. Then they grew up to 60–

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100 nm after ~ 4 h. With increasing immersion time, packing of the CaPO4 deposits with size of tens of nanometers in diameter formed larger globules and then continuous coatings on the substrates. The coatings were composed of nano-sized crystals. A direct contact between CaPO4 and the Ti–6Al–4V surface was observed [455]. Simplification of the ionic composition of the standard simulating solutions is another option to increase the deposition kinetics [444, 445,460,463–467], while heating is one more option [468]. For example, a fast (a few hours instead of 14 days with SBF) biomimetic deposition of CDHA on Ti–6Al–4V alloy substrates was obtained by using a slightly supersaturated Ca/P solution with an ionic composition simpler than that of SBF. Thin film XRD indicated that the deposits obtained after ~ 3 h consisted of poorly crystalline ion-substituted CDHA and their content increased on increasing the soaking time up to 3 days [464]. Although this is slightly beyond the subject, one should mention that some specific types of CaPO4 bioceramics might be prepared by wet-chemical technique. For example, hollow CaPO4 tubes were synthesized. Initially, CaPO4 was precipitated on the surface of anodic aluminum oxide membranes by mixing diluted Ca(OH)2 and H3PO4 aqueous solutions. In order to remove the membranes and recover isolated tubes, the coated membranes were dipped in an etching agent (1:1:3 by volume, NH3:H2O2:DI water) for 15 days (pH = 11). A clear colloid was obtained which was centrifuged at 3500 rpm for 20 min, followed by washing, drying and calcining [469]. To conclude this section, one should mention that biomimetically and wet-chemically deposited CaPO4 coatings, films and layers on implants are commercially available. The examples comprise PeriApatite™ HA (Stryker Corp., NJ, USA) and HAnano Surface (Promimic AB, Sweden). According to the manufacturer, the HAnano Surface of 20 nm thick can be applied to substrates in three different ways through spraying, dripping or dipping, while an excess coating solution is removed by spinning and pressurized gas followed by a short heat treatment. Therefore, one can claim that such techniques as dip and spin coating, as well as cold spraying (the description of these techniques is given below) are also commercially used for CaPO4 deposition. 4.3.5. Dip coating Dip coating is a simple and, therefore, a popular deposition technique suitable for various substrates. It consists of several successive steps. A substrate is immersed into a suspension of the material (in our case, CaPO4) at a constant speed. Wet precipitates are selfdeposited on the substrate while it is pulled up. Usually, to avoid any jitters, withdrawing is carried out at a constant speed. The speed determines the thickness (the faster–the thinner). An excess of the suspension is drained from the surface. Simultaneously, a solvent is evaporated densifying the deposits. For volatile solvents, such as alcohols, evaporation starts already during the deposition and drainage steps. After being dried, solid deposits are achieved [470]. To increase the thickness, the dip-and-dry treatment might be repeated several times [52,471,472]. There are two mechanisms which govern the deposition during dip coating. The first mechanism is known as liquid entrainment. It occurs when a specimen is withdrawn from slurry faster than it can drain from the surface, leaving thin deposits [473]. The second mechanism is slip casting, which is a filtration process, in which porous substrates are dipped into a suspension for a certain time and then removed from it. Owing to the capillary forces, the liquid phase is sucked from the suspension (slip), while the solid particles are concentrated at the substrate/suspension boundaries and a wet membrane (a cake) is formed on the surface [474]. Thus, both the withdrawal velocity and the suspension properties (volume fraction of solids, viscosity) have an influence on the liquid entrainment mechanism, while the surface microstructure of the substrate (porosity and pore diameter) together with the suspension properties have an influence on the slip casting mechanism. By modifying these parameters, deposits as thin as 2 μm and as thick as 0.5 mm might be prepared [473,474].

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a complicated mixture of various compounds with the predominant quantity of β-TCP. In addition, the adherence of the deposits to the substrates was low and they could be removed by a light scraping [471].

Fig. 19. A photograph showing the application of the dip-and-dry deposition technique on metals. A stainless steel square and five porous (pore size ~ 500 μm) tantalum specimens in the upper half were treated 8 times in a supersaturated CaPO4 solution, followed by drying. The samples show a uniformly white color of the deposited coating. As a contrast, the as-received specimen in the lower half show the gloss of metals. Reprinted from Ref. [52] with permission.

By means of dipping, CaPO4 were deposited onto various substrates [52,92,93,471–483]. An example of dip-coated substrates is shown in Fig. 19 [52]. If a post-deposition sintering is performed [471], such technique might be called “frit enameling” [8]. Furthermore, a dipping stage is commonly used in other deposition techniques (e.g., the aforementioned sol–gel deposition and several below mentioned techniques); therefore, the entire amount of studies on dip coating exceeds the cited examples. To finalize this section, one should briefly mention on a possibility to dip substrates into melted CaPO4. Since the melting points of the thermally stable CaPO4 (α-TCP, HA, FA, TTCP) exceeds 1500 °C, substrates having even higher melting points (Ti, Ta, alumina, zirconia, etc.) might be coated by this technique only. Such experiments were performed and this technique was called “immersion coating” to differentiate it from the standard dip coating because there is nothing “wet” in it (“wet” refers to “moisture”, while high temperature melts cannot contain any moisture). Similar to the aforementioned thermal spraying techniques, the deposits from melted HA powders appear to represent

4.3.6. Spin coating Spin coating is a procedure used to apply uniform thin deposits to flat substrates, such as disks and plates. It is rather similar to dip coating. The process consists of four stages: deposition, spin up, spin off and evaporation. A substrate is dipped into a solution, a suspension or a sol and then withdrawn at a constant speed, usually with the help of a motor. Since a substrate initially is dipped and then spinned, such technique is occasionally called a dip–spin method [484]. As an alternative, a solution, a suspension or a sol might be either poured or dropped onto the flat surface of a rotating substrate [485]. Rotational draining and solvent evaporation result in deposition (Fig. 20). After drying, the procedure can be repeated one or several times to prepare thicker deposits. After several consecutive depositions have been performed, a final heat treatment is usually done to improve the physical properties of the deposits. A machine used for spin coating is called a spin coater, or simply a spinner [486]. Just a few publications on spin coating of CaPO4 were published [427–429,484,485]. 4.3.7. Hydrothermal deposition Hydrothermal deposition is a simple process and one of the most cost-effective techniques. It is rather similar to the aforementioned biomimetic deposition and wet-chemical precipitation; however, since the hydrothermal treatment is performed at elevated (N 80 °C) temperatures during a relatively prolonged period of time (N1.5 h), the CaPO4 deposits are usually crystalline. Hydrothermal process was used to deposit CaPO4 on both metallic [408,409,487–498] and polymeric [499,500] substrates. Once this technique was called as “a chemical bath method” [501]. The process is based on the formation of EDTA–Ca2 + chelate compounds due to cooperative dissolution of a Ca-containing salt and EDTA at ambient conditions. As the temperature increases, thermal dissociation of the EDTA–Ca2 + chelate compounds occurs, which supplies sufficiently high concentration of Ca2+ ions to perform precipitation in the presence ions. Namely, by hydrothermal treatment for 2 h at 90 °C, the of PO3− 4 researchers succeed to form well crystallized HA and OCP deposits on both pure Mg and Mg–Al–Zn alloys from a 0.25 M EDTA–Ca2 + and KH2PO4 treatment solution in a pH range from 5.9 to 11.9. Both HA and OCP deposits were found to consist of an outer porous layer and

Fig. 20. A schematic drawing of a spin-coating technique.


an inner continuous layer, while both the crystal phases and the microstructures of the deposits were found to vary with the pH of the treatment solutions. Namely, in weak acidic (pH = 5.9) solutions, dual-layer deposits were formed: an outer coarse layer consisted of plate-like OCP crystals and an inner dense layer consisted primarily of HA crystals. In weak alkaline (pH = 8.9) solutions, the dual-layer structure was also formed: an outer coarse layer consisted of rod-like HA crystals and an inner dense layer consisted of well packed HA crystals. Both layers were found to grow with an increase of the treatment periods. In strong alkaline (pH = 11.9) solutions, needle-like HA crystals were formed. A thin Mg(OH)2 layer was also formed at the boundary between the deposited CaPO4 and Mg substrates. The HA and OCP deposits were found to improve the corrosion resistance of both pure Mg and Mg–Al–Zn alloys in both HBSS and a 3.5 wt.% NaCl solutions; among them, the corrosion resistance of HA deposits was always higher than that of OCP ones. The authors revealed that the protection level of CaPO4 deposits could be varied by their crystal phase, microstructure and thickness. In addition, the deposits showed good adhesive properties with slight plastic deformations under cyclic stresses below the fatigue limit [491–497]. To conclude this section, one should note that hydrothermal deposition of CaPO4 might be also performed from both SBF [498,499] and aqueous suspensions [496], as well as onto the previously deposited CaPO4 seeds [408,409]. 4.3.8. Thermal substrate deposition Thermal substrate deposition is based on the solubility differences at low and high temperatures. Namely, by heating a substrate in suitable saturated aqueous solutions, solid precipitates can be directly deposited onto the substrate. Various heating techniques have been proposed. Namely, conductive substrates, such as metallic foils or wires, can be heated by electric current through them. Non-contact techniques, such as induction heating, can be used to heat materials with complex shapes. In such cases, the technique is called induction heating deposition [60,135,502–507]. In addition, there is an ultrahigh frequency induction heating deposition, which is called “chemical liquid vaporization deposition” [508]. In any case, the immersed samples can be heated up to 160 °C in solutions, giving local supersaturations to perform crystallization. By thermal substrate deposition CaPO4 were put down on various substrates [60,135,502–516]. For example, in the case of Ti, an alternating current was passed through the substrates immersed in aqueous solutions containing calcium and orthophosphate ions. The deposition was performed for 10–30 min at solution pH 4–8 and temperatures up to 160 °C. The type of precipitates was varied depending on the solution pH, temperature and ion concentrations. Namely, high quality deposits, whose predominant component was CDHA (at pH N 6) or DCPA (at pH = 4), were obtained on Ti [509–516]. Similarly, DCPA was deposited on carbon at solution pH ~ 4.5 [507], followed by hydrothermal treatment in alkaline solutions to convert DCPA to HA [60,135, 503–508]. In all studies, the content of apatites in the deposits was found to increase with increasing temperature and heating time. 4.3.9. Alternate soaking An alternate soaking deposition was developed in 1998 [517]. The process consists of several successively performed deposition cycles. In one cycle, a substrate is soaked in a Ca-containing solution, rinsed with water, afterwards soaked in a PO4-containing solution and then rinsed again [518]. When repeatedly applied, CaPO4 are deposited [519–527]. Simple inorganic salts, such as calcium chloride or nitrate are used as the Ca-source and sodium or ammonium hydroorthophosphate are used as the PO4-source. The duration of each soaking stage can vary from 1 min [522] to 2 h [525] depending on the substrate. At even shorter soaking times (b1 min), the deposition technique is called “alternate dipping” [528]. Regarding examples, an alternate soaking was used to deposit CaPO4 on Ti substrates [522]. The

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Fig. 21. A schematic drawing of an alternate soaking deposition technique. Reprinted from Ref. [522] with permission.

pretreated substrates were subsequently soaked in 0.5 M CaCl2 and after washing with distilled water soaked in 0.1 M Na2HPO4 solution (Fig. 21). The deposited amount of CaPO4 increased with the number of reaction cycles, but it was independent on both the solution temperature and soaking time, indicating that the deposition process depended on ion exchange and adsorption on the pretreated surface [522]. Similar findings were obtained in other studies [519–528]. In 2011, the process was automated allowing deposition of substantial quantities of CaPO4 with a minimum of labor and energy [529]. To finalize this section, one should mention that alternate soaking might be performed at the initial stages only just to deposit CaPO4 precursors, such as ACP, or seeds followed by the standard biomimetic deposition from SBF [34,74,76]. Furthermore, alternate soaking might be performed under the hydrothermal conditions [41]. 4.3.10. Micro-arc oxidation (MAO) MAO (synonyms: plasma electrolytic oxidation, anodic spark deposition, anodic plasma-chemical treatment, micro-arc discharge oxidation, spark anodizing) is a combination of plasma-chemical and electrochemical processes that appeared to be applicable to deposit ceramic coatings on various metals and alloys. The MAO process combines an electrochemical oxidation of the metallic surface by a high-voltage (up to 500 V, frequently, of an alternating current) spark treatment performed in aqueous electrolytic baths. During the process, sparks appear and move rapidly across the treated surface, while temperature and pressure inside a discharge channel can reach ~104 K and ~103 MPa, respectively, which are sufficiently high for inducing thermochemical interactions between the substrate and electrolyte [530]. Usually the electrolytic baths also contain modifying elements in the form of dissolved salts (e.g., silicates, borates) to be incorporated into the resulting deposits. Thus, the MAO deposits consist mainly of metal oxides and/or hydroxides combined with silicates, borates, etc. depending on the chemical composition of the electrolytic baths. Furthermore, the nature of the substrates was found to influence both the morphology and the composition of the MAO deposits [531]. In the presence of Ca- and P-containing salts, such deposits also contain various Ca- and Pcontaining compounds (but rarely CaPO4) [327,532–537]. For example, MAO coatings on Ti were found to consist of a complicated mixture of titanium oxides (both anatase and rutile phases), β-Ca2P2O7, CaTiO3, α-TCP and Ca2Ti5O12 [327]. Similar results were obtained in other studies [532,533,537]. Besides, for the same purposes, nano-sized CaPO4 powders were dispersed in the electrolytic baths and, thus, the MAO process was performed from aqueous suspensions [538]. Since either formation of CaPO4 was not detected in the MAO coatings or they were presented as admixture phases only, all the aforementioned cases should be considered as a still another pre-deposition technique

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(see Section 3 above). A schematic setup of a MAO system is available in literature [30,146]. However, further deposition of CaPO4 over the aforementioned MAO coatings might be performed through either sequential or hybrid processing routes. Namely, a MAO treatment of metals is followed by direct deposition of CaPO4 using other techniques, such as EPD [363,539,540] and ECD [541,542], wet chemical or biomimetic precipitation [327, 543], electron beam evaporation [544], sol–gel coating [545]. In the case of EPD, CaPO4 could be co-deposited and incorporated into growing MAO coatings in situ from electrolytes containing suspended nanoparticles [539]. In addition, the MAO process could be performed in aqueous electrolytes, containing dissolved Ca- and PO4-containing compounds only [542–553]. In this case, CaPO4 was deposited in the single stage. For example, MAO was performed on Ti in an electrolyte containing calcium glycerophosphate and calcium acetate using a direct current power supply. Porous and rough deposits consisting of a homogeneous mixture of TiO2 with ACP were formed as the result. Then, the substrates were hydrothermally treated in aqueous solutions with pH within 7.0–11.0 (adjusted by NaOH) at 190 °C for 10 h in an autoclave. This procedure converted the ACP deposits into HA ones, while the amount of precipitated HA increased with solution pH increasing. The authors suggested that the hydrothermal treatment forced ACP diffusion from the inner layers into the surface, where it was hydrolyzed and precipitated as HA [547]. Similar results were obtained for Ti alloys [550–552]. However, when aqueous electrolytes containing dissolved CaCl2 and KH2PO4 were used, CaPO4 was deposited without a necessity to perform additional treatments [548]. Furthermore, the MAO process can be performed in electrolytes, representing suspensions of HA powders in aqueous solutions of KOH or KOH + K3PO4; in both cases deposition of HA was observed [554]. The list of the available electrolytes is available in literature [555]. The elemental composition of the MAO-deposited CaPO4 coatings on Ti was investigated (Fig. 22). The deposits were found to consist of CDHA-based outer layer and TiO2-based inner layer, in which the outer layer contained small amounts of α-TCP and CaCO3, while the inner layer contained CaTiO3 [549]. Similar results were obtained in another study [550].

4.4. Other deposition techniques: miscellaneous Prior describing the below mentioned CaPO4 deposition techniques, one should note that they are rare and the majority of them were

Fig. 22. A cross-sectional morphology and an element distribution of a MAO deposited CaPO4 coating on Ti. Reprinted from Ref. [549] with permission.

mentioned in just a few research papers. Therefore, the detailed description is not always possible. 4.4.1. Hot isostatic pressing (HIP) HIP is a process used to reduce porosity and increase the density of many types of materials. The HIP process subjects a component to both elevated temperature and isostatic gas pressure in a highpressure containment vessel. To perform deposition, initially, solid cores are covered by a CaPO4 (usually, HA) powder. Both organic binders and some other additives are usually used to improve fixation. To remove these additives, a furnace is constructed within the highpressure vessel and the coated substrates are placed inside. Then, the specimens are heated at temperatures within 700–1200 °C and simultaneously pressed at 20–100 MPa. The obtained deposits are usually thick (0.2–2.0 mm) and always dense [556]. However, the HIP can be used as a post-deposition technique to reduce porosity of the CaPO4 deposits put down by other methods [557]. The HIP technique was used to deposit CaPO4 on various materials [471,558–561]. For example, HA granules (32–38 μm in diameter) were implanted into a substrate of superplastic titanium alloy. First, the granules were spread over the surface and, then, hot pressed at 750 °C and 17 MPa for 1 h with a plunger to implant them into the substrate. After 10 min of the treatment, the implantation ratio was ~20% and some granules were not on the substrate; however, after 60 min of the treatment, the implantation ratio was 100%, but the upper areas of granules were exposed [561]. However, the majority of the CaPO4 deposits produced by HIP technique are contaminated by metals and SiO2 particles, due to the use of glass encapsulating tubes [8]. Furthermore, it is difficult to coat complex substrates by this method. Moreover, since high temperatures and pressures are required, there is a thermal expansion mismatch between the substrates and deposits. 4.4.2. A double layered capsule hydrothermal hot pressing Just a few publications were found on the topic and they were devoted to HA deposition on AZ31 Mg alloy [562] and Ti [9,563,564]. In order to create suitable hydrothermal conditions, double-layered capsules were used: the inner capsule encapsulated the deposited materials and a substrate, while the outer one was subjected to isostatic pressing under the hydrothermal conditions. In the Mg alloy paper, an alloy rod and a powder mixture of DCPD and Ca(OH)2 were placed into a polyethylene tube. The powder mixture was loaded into the tube such that the Mg alloy rod was concentrically positioned with respect to the tube axis. Both the ends of the tube were fastened with paper staples. Secondly, the entire tube was further encapsulated using a poly-vinylidenechloride film. Then, alumina powder was placed between the tube and film. The entire construction was put into a batch-type high temperature and pressure vessel for hydrothermal treatment. Then, the vessel was heated up to 150 °C for 3 h, while the pressure was kept at 40 MPa using a pressure regulator. After the treatment, the vessel was cooled down to a room temperature and the HA-coated AZ31 samples were removed. Afterwards, pullout tests were conducted in order to measure the adhesive properties of HA coating to AZ31 substrates. The average value of the maximum shear stress was determined to be 6.1 ± 1.0 MPa. In addition, it was revealed that HA remained on the surface of the Mg alloy after the pullout fracture tests. Thus, by means of this technique, HA deposits could be bonded to Mg and its alloys with good adhesive properties [562]. In the Ti papers, the authors demonstrated that HA deposits of ~50 μm thick could be put down onto the surface of Ti cylindrical rods at 135 °C under the confining pressure of 40 MPa. The deposited HA had a porous microstructure with the relative density of ~60%. According to the results of pullout tests, the shear strength was in the range of 4.0–5.5 MPa. These results also revealed that a crack propagation occurred within the HA deposits but not along the HA/Ti interface.


Therefore, the fracture property of the HA/Ti interface was higher than that of the HA bioceramics only [9,563,564]. 4.4.3. Detonation gun spraying Detonation gun spraying is both a high temperature and a high velocity technique, which introduced a higher degree of melting to starting powder. Just 3 publications are available on the subject [102, 152,153] and, among them, the most recent ones were published in 2001. As the name implies, the spraying equipment was like a gun, which consisted of a 1.4-m long water-cooled barrel. In this process, a mixture of oxygen and acetylene was fed into the barrel together with a CaPO4 powder. The hot gasses generated in the detonation chamber travel down the barrel at a high velocity, heated the CaPO4 particles to a plasticizing stage, and accelerated them to velocities up to ~800 m/s. The high kinetic energy of the hot CaPO4 particles on impact with a substrate resulted in a buildup of very dense and strong deposits, which had a higher proportion of amorphous phases (ACP) with some evidence for the appearance of β-TCP. The thickness developed on the substrate per shot depended on the ratio of combustion gasses, CaPO4 particle size, carrier gas flow rate, frequency and distance between the barrel end and the substrate. Depending on the required thickness, the detonation spraying cycle could be repeated at the rate of 1–10 shots/s. A lower crystallinity and higher residual stress found in the detonation gun sprayed CaPO4 deposits resulted in a faster dissolution rate both in vitro and in vivo [102,152,153]. 4.4.4. Aerosol–gel deposition An aerosol is a colloid suspension of fine solid particles or liquid droplets in a gas. The liquid droplets or solid particles have diameter mostly smaller than ~ 1 μm; therefore, they are commonly called as microdroplets and microparticles, respectively. Examples of aerosols are dust, clouds and air pollution, such as smog and smoke. While gels are regarded as composites because they consist of a solid skeleton or network that encloses a liquid phase or an excess of the solvent. Therefore, as the name implies, the aerosol–gel process is a gas–chemical technique that involves transition from a gaseous ‘aerosol’ into a solid ‘gel’ phase. The aerosol–gel technique was applied to put down highly porous CaPO4 deposits onto the surface of various materials [565–568]. Calcium nitrate and either H3PO4 [565] or triethylphosphate [566] dissolved in ethanol were used as precursor solutions. The aerosol can be produced by an ultrasonic pulverization either of both solutions independently but simultaneously [566] or after their previous mixing which resulted in pulverization of CaPO4 suspensions [565]. After production of a steady state aerosol, the microdroplets were forwarded into a deposition chamber by an air flux. After being deposited, the coated substrates were sintered at temperatures within 500–1000 °C. In order to produce thicker deposits, a multilayer deposition could be performed [565]. The composition, structure and morphology of the final deposits were found to fit highly porous polycrystalline HA. An adhesive strength, measured by means of indentation techniques, was found to be about 100 MPa [565–568]. 4.4.5. Aerosol deposition (AD) According to this technique, an aerosol flow is produced by mixing fine (from ~0.1 μm to ~1 μm) CaPO4 powders with a stream of a carrier gas of a high velocity (from ~ 100 m/s to ~ 400 m/s) and the obtained aerosol is then accumulated on substrate surfaces via impact adhesion. Dense CaPO4 deposits are formed due to the high kinetic energy impacts of the particles, which enables the deposits to exhibit high adhesion strengths. Since deposition is carried out at ambient conditions, both the starting powder and the resultant deposits have the same chemical composition. Therefore, the composition of deposits can be precisely controlled by changing the powder composition, making it possible to obtain multi-component coatings.

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An AD technique was used to deposit HA onto the surface of Ti [15, 569,570], Ti alloys [571], Mg previously covered by either MgF2 [80] or poly(ε-caprolactone) (PCL) [572], as well as onto various polymers [573,574]. Namely, to perform AD, HA powder was sprayed onto Mg samples in a deposition chamber using oxygen carrier gas at a flow rate of 5 × 10−4 m3/s under a pressure of 9.2 Torr. According to SEM observations, when HA was deposited onto Mg with the PCL interlayer, it was partially embedded into this interlayer, forming composite-like structures [572]; however, when HA was deposited onto Mg with the MgF2 interlayer, no composite-like structures were observed [80]. In addition, the HA deposits on Mg with the PCL interlayer were found to have better stability during deformation if compared to those on Mg without the interlayer [572]. Due to the ambient processing conditions, various types of CaPO4based biocomposites might be deposited by AD [575–578]. For example, to deposit HA/chitosan biocomposites on AZ31 Mg alloy substrates, researchers employed a slit-type nozzle with a 10 × 0.5 mm2 rectangular opening and air as a carrier gas with a flow rate of 30 L/min. The 5 μmthick HA/chitosan deposits were put down over the entire surface of the substrates by scanning them by a motorized X–Y stage for 1 min at a scanning speed of 1 mm/s. The biocomposite deposits were found to exhibit high adhesion strengths ranging from 24.6 to 27.7 MPa and possess good corrosion resistances [578]. 4.4.6. Cold spraying (CS) A CS technique appears to be rather similar to the previously mentioned AD. The principle differences between them comprise coarser (1–50 μm in size) CaPO4 particles (particles of such dimensions cannot form aerosols), higher velocities (from ~300 m/s to ~1200 m/s) of a carrier gas and elevated (300–800 °C) deposition temperatures [579]. Nevertheless, one should stress that, in contrast to the aforementioned thermal spraying (Section 4.1), CS deposition occurs solely by deformation of solid particles occurring at temperatures much lower than the melting point. This is realized due to high kinetic energies of the particles upon impact on substrates. Similar to the previously mentioned AD, in CS process, spraying CaPO4 particles experiencing both a little change in microstructure and a little oxidation and/or decomposition are accelerated to very high velocities by a supersonic jet of a compressed gas stream passing through a Laval type nozzle. The stream of the particles in a gas is forwarded into a deposition chamber, in which a substrate is located. The deposition system consists of a gas pressure regulator, a gas heater, a powder feeder and a spray gun. The particle temperature upon impact depends on various factors such as gas temperature, nozzle design and heat capacity of the particles. Therefore, all phenomena occurring at high temperatures, such as thermal decomposition and phase transformations are avoided. Deposition of CaPO4 particles takes place through intensive plastic deformations [580–585]. However, for successful bonding, the deposited particles have to exceed the critical velocity, which is dependent on the properties of the particular sprayed material [579]. Deposition of CaPO4containing composites is also possible by CS [585–587]. 4.4.7. Blast coating Abrasive blasting is an operation of forcibly propelling a stream of abrasive material against a surface under high pressure to smooth a rough surface, roughen a smooth surface, shape a surface, or remove surface contaminants. A pressurized fluid, typically air, or a centrifugal wheel is used to propel the blasting material. By studying this process in details, researchers discovered that abradants could stick to the surface. This happens because impacts of particles onto the substrate surface result in the transfer of both their momentum and kinetic energy. The energy is partly absorbed by the substrate surface causing its melting within a microscopic range, which is enough to fix the particles. In addition, a larger area of lattice defects is observed. The dimensions of both melting and lattice defect zones depend on both the

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material properties of the substrate and on the energy of the blasting grains [588]. Therefore, an idea appeared to use this property to deposit CaPO4 on metallic (Ti and its alloys) substrates [589–597]. The process was performed at room temperature using a sandblaster and either a HA powder only [589,590,592], or a mixture of a HA powder with a blast medium (a CoBlast™ process developed by ENBIO, Dublin, Ireland) [592–597]. In addition, blast coating can be performed by a composite ceramic consisting of an alumina core (a carrier material) covered with a porous HA shell [591]. As the result, the surface of the metallic substrates was coated by HA homogeneously and completely. The authors noticed that the deposited HA particles stuck together at room temperature as if they were sintered. The deposits were stable against ultrasonication in water for at least 5 min and it was difficult to remove by nail scratching. All coated surfaces exhibited higher adhesive strengths if compared with plasma-sprayed HA, which could be attributed to a combination of chemical and mechanical bond formation during deposition. Furthermore, the composition of the HA deposits closely matched that of the feedstock HA. In addition, the coated substrates showed promising results in vivo [589,590,592]. Similar results were obtained for the CoBlast™ process [592–597]. In addition, there is a shot peening deposition, which differs from the CoBlast™ process just by the nature of the blast medium (silica is used in shot peening, while alumina is used in CoBlast™) [598]. In addition, one should mention that BrainBase Corporation (Tokyo, Japan) developed an ABS process (ABS = Apatite Blasted Surface) for blasting of Ti dental implants by biphasic HA + β-TCP bioceramics, followed by prolonged ultrasonic cleaning in pure water. Although almost no traces of CaPO4 were found on the Ti surface (the chemical analysis of blasted surface revealed that the remaining CaPO4 content was less than 1% toward all over the surface), this value appeared to be sufficient to improve biocompatibility. To conclude, blast coating appears to be rather similar to the previously mentioned CS technique and, perhaps, both techniques could be combined. 4.4.8. Direct laser melting According to the direct laser melting technique, a starting CaPO4 powder is mixed thoroughly with a solvent to create a suspension. Then the suspension is deposited onto the substrate surfaces, using any suitable technique, such as spraying, spin or dip coating. Afterwards, the coated substrates are air-dried to remove moisture, followed by direct laser melting of CaPO4 deposits using either continuous wave or pulsed laser beams to produce strong bonds between the deposits and the substrate. The surface textures of the obtained deposits might be controlled by varying the laser spot overlap with a change in laser traverse speed. However, due to the high processing temperatures, necessary to melt CaPO4, thermal decomposition of the latter occurs. Therefore, the deposits always represent a complicated mixture of various compounds and phases [146,258,599–605]. A schematic setup of a direct laser melting system is available in literature [146]. 4.4.9. Transmission laser coating Transmission laser coating process is rather similar to the previously mentioned direct laser melting process. The differences between the two techniques lay in the melting material. Namely, CaPO4 deposits are melted in the direct laser melting, while the surface of the substrates is melted in the transmission laser coating. The latter process is based upon the great differences in light interactions between the ceramics and metals. Namely, most types of ceramics (including CaPO4) appear to be opaque to ultraviolet light but transparent to infrared light, while the absorption of metals to laser is the highest in infrared range. Therefore, infrared laser beam easily penetrates through deposited CaPO4 almost without heating them and melts the surface of a metallic substrate. After solidifying, the CaPO4 particles appear to be tightened by the metal. This process was found to provide both a strong coating/

substrate interfacial strength and a low processing temperature for CaPO4 to reduce their decomposition [606]. 4.4.10. Laser cladding CaPO4 powders might be deposited on various substrates by laser cladding. The method has noticeable similarities with the aforementioned direct laser melting and transmission laser coating techniques. According to the powder supply mode, laser cladding can be categorized as pre-placement and synchronous feeding process. The former one is similar to the aforementioned direct laser melting, while in the latter technique particles of either CaPO4 or their precursors are injected into a stream of a carrier gas. This powder stream is then injected into the area irradiated by a laser beam. The beam heats the deposited material and creates a molten pool on the metallic substrate where the particles are impinged. To avoid oxidation in the interaction zone, a shielding inert gas is usually applied. Rapid quenching takes place when the molten pool leaves the laser-irradiated area and deposits onto substrates [607–609]. Except of CaPO4 themselves [610], their precursors, such as mixed powders of CaCO3 and DCPA/DCPD, can be used for laser cladding [611–617]. Depending on the Ca/P ratio, the reactions between the precursors can produce both well crystallized HA (if Ca/P ~ 1.67), α-TCP, βTCP or their mixture (if Ca/P ~ 1.50), as well as complicated mixtures of TTCP, α-TCP, β-TCP, ACP, HA, Ca2P2O7 and CaO, if the Ca/P ratio is deviated from both numerical values. Furthermore, when deposition was performed on metals (e.g., Ti), other admixtures, such as CaTiO3, might be formed due to oxidation. Since the reactions between the aforementioned precursors produce gaseous by-products (CO2 and water vapor), porous deposits are prepared. As a laser power increased, the amount of TTCP, HA and CaO in the deposits decreased gradually and, finally, only α-TCP and CaTiO3 remained. Nevertheless, the amount of HA could be increased greatly by a post-deposition calcining at 800 °C for 5 h followed by furnace cooling, due to the total transformation of TTCP and α-TCP to HA [611–617]. A computer-controlled version of laser cladding technique was called “laser rapid forming”; it was able to perform deposition on complex shapes [618]. In addition, some CaPO4-containing composites might be deposited by laser cladding [619]. 4.4.11. Laser-engineered net shaping (LENS™) LENS™ (Optomec, Albuquerque, NM, USA) is a commercial rapid prototyping process, introduced in 1996 [620], which was also used to deposit CaPO4 on metallic substrates [621–623]. It also has noticeable similarities with the aforementioned direct laser melting, transmission laser coating and laser cladding techniques; however, the full details about this process remain undisclosed. According to LENS™, a laser power is focused onto a substrate to create a molten layer on its surface. A CaPO4 powder is then injected by means of a carrier gas (Ar) through a deposition head onto the molten metallic layer, where it is also melted. As the laser head moves on, the molten metal and CaPO4 solidify rapidly. Thus, solidified CaPO4 particles appear to be embedded into the surface of metallic substrates with formation of biocomposites. The entire substrate is scanned back and forth to create a pattern of deposited CaPO4 with a finite thickness. This procedure is then repeated many times until the desired thickness is prepared. The deposition process could be regulated by variations of a laser power, a scan speed and powder feed rates. By means of this technique, CaPO4/metal composite deposits were fabricated [621–623]. Due to a partial oxidation of metals, namely Ti, formation of calcium titanates was detected [621]. To reduce oxidation, the LENS™ process could be performed in a controlled oxygenfree atmosphere. In the case of stainless steel substrates, a small amount of Fe3P phase was discovered on their surface [623]. A schematic setup of the LENS™ deposition system is available in literature [621]. Although a good biocompatibility of the deposited CaPO4–Ti biocomposites was detected [621,623], the LENS™ deposition system was combined with RF induction plasma spraying process [624]. The advantage of such a combination was obvious: the laser processed


compositionally graded Ti–HA interlayer acted as a diffuse and strong interface between the HA coating and the Ti substrate, which eliminated problems, associated with sharp interfaces and enhanced the crystallinity of the HA deposits due to lower cooling rates. In vitro biocompatibility tests, using human fetal osteoblast cells, revealed a clear improvement in cellular activity if compared with LENS™-TCP coated Ti [624]. 4.4.12. Matrix assisted pulsed laser evaporation (MAPLE) A MAPLE process was developed in the late 1990s at the U.S. Naval Research Laboratory as an alternative to PLD in order to provide a gentler pulsed laser evaporation process for functionalized polymers. In the classical MAPLE process, a laser target comprise of a frozen matrix consisting of a solution of a polymeric compound dissolved in a relatively volatile solvent. The solvent and solute concentration are selected so that the material of interest can fully dissolve to form a dilute, homogeneous solution, with the goal of having most of the laser energy initially absorbed by the solvent rather than the solute. As in conventional PLD, both solute and solvent molecules are ejected from the target region illuminated by the laser pulse and deposited over the substrate surfaces. Thus, the MAPLE technique provides a gentle mechanism for transferring different compounds, including large molecular weight species, and it is expected to ensure an improved stoichiometric transfer, a more accurate thickness control and a higher uniformity of the deposits. A good schematic setup of this process is available in literature [625, 626]. Regarding the subject of this review, the MAPLE technique was used for a delicate and an accurate deposition of both CaPO 4 alone [626,627] and CaPO4 combined with organic and/or biologic materials (biocomposites) [628–630]. The examples comprise deposition of hydrated forms of CDHA [626] and ion-substituted OCP [627], as well as CaPO4-based biocomposites with sodium maleate [628], alendronate [629] and silk fibroin [630]. 4.4.13. Liquid phase laser deposition The technique represents employing of a laser irradiation to nucleate CaPO4 precursors on the surface of various substrates, followed by wetchemical or biomimetic deposition. It was applied to deposit CaPO4 onto various inorganic [631,632], polymeric [633,634] and even HA [635] substrates. The experimental set-up comprised an open system, in

Fig. 23. A schematic set-up used in the liquid phase laser deposition and a design of the laser irradiation (upper left corner). Reprinted from Ref. [632] with permission.

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which a supersaturated aqueous precursor solution, containing ions of Ca2 + and PO34 − (such as SBF), was poured into a container (Fig. 23). Then, substrates were immersed into the solution at room temperature and simultaneously irradiated by a laser. The laser beam was focused on the substrate surface through the solution. By means of a scanning system, a surface pattern of seven concentric squares, each separated by a distance of 200 μm was formed at the edges of inorganic substrates as seen in the upper left corner of Fig. 23, while the middle of the substrates was not irradiated and used as the control. After the irradiation, the substrates were subsequently immersed into SBF for 24 h under the physiological conditions. The authors revealed that ion-substituted CDHA was rapidly deposited from SBF onto the irradiated area of inorganic substrates, while no deposition occurred onto the surface of non-irradiated area of the same substrates. This occurred due to formation of small CaPO4 crystals during irradiation, which further facilitated the growth of thicker deposits on the irradiated area of the substrates [632]. Similar results were also obtained for both polymeric [633,634] and HA [635] substrates, where deposition of CaPO4 was attributed to laser absorption by the substrate surface, which both caused its modifications and resulted in temperature increasing of the surrounding solution. The CaPO4-forming ability was found to increase with an increase in both the laser power and irradiation period. The entire deposition process appeared to be simple, mild and area specific [631–635]. 4.4.14. Electrostatic spray deposition (ESD) Electrostatic spray deposition (ESD, synonyms: electrospraying, electrostatic atomization, electrohydrodynamic atomization spraying) was developed at the Laboratory for Inorganic Chemistry, Delft University of Technology (Netherlands) during the early 1990s to fabricate porous, thin ceramic deposits with a controlled morphology for solid electrolytes and lithium battery [636]. The technique is based on generation of an aerosol composed of organic solvents containing inorganic precursors under the influence of high voltages. This is accomplished by pumping the liquid through a nozzle. Usually spherical droplets are then formed at the tip of the nozzle, but if a high voltage is applied between the nozzle and substrate, each droplet is transformed into a conical shape and fans out to form a spray of highly charged droplets. A high voltage is applied between the nozzle and substrate. Consequently, droplets coming out the nozzle are dispersed into a spray and this spray is deposited upon grounded and heated substrates due to the applied potential difference. Therefore, the droplets appear to be impinged onto the substrates, where they lose their charge. After complete solvent evaporation, thin deposits are left on the surface. Parameters such as concentration, flow rate, nozzle-to-substrate distance, voltage and relative humidity could all be varied to control droplet size and thereby the final properties of deposits. A schematic setup of the ESD technique is available in literature [270,637–639].

Fig. 24. A scanning electron microscopy of an electrostatic spray deposited CaPO4 coating, characterized by a porous surface morphology. Reprinted from Ref. [641] with permission.

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To perform ESD, a soluble calcium salt (nitrate or chloride) and phosphoric acid were dissolved in an alcohol. The obtained solutions were pumped, quickly mixed prior the nozzle and electrostatically sprayed onto a substrate, while the substrate itself might be heated to 300–450 °C [637–642]. Except solutions, CaPO4 powders might be suspended in alcohols and the obtained suspensions are electrosprayed [643–649]. Among them, the solution approach allows putting down of the thinner deposits than the suspension approach. The chemical and morphological characteristics of the CaPO4 deposits were found to be strongly dependent on both the composition of the precursor solutions (pH, absolute and relative precursor concentrations) and the deposition parameters, such as temperature, the nozzle-to-substrate distance, the liquid flow rate, as well as the geometry of the spraying nozzle. By varying these parameters, several phases and phase mixtures were deposited by ESD technique: carbonate apatite, α-TCP, β-TCP, DCPA, β- and γ-calcium pyrophosphates, calcium metaphosphate, CaCO3, CaO [640–642]. Since ESD might be performed at ambient temperatures, thermally unstable compounds could be deposited as well. As seen in Fig. 24, the electrostatically sprayed CaPO4 deposits might be porous [641–643,646,649,650]. Nevertheless, after the deposition, the coated substrates might be annealed at high temperatures. The annealing stage is necessary to aggregate and/or melt the deposited CaPO4 particles and form highly dense and homogeneous coatings. To conclude the ESD description, combined techniques, such as sol– gel assisted ESD [651] and ESD vapor deposition [652], have been developed as well. Further details are available in the literature [270]. 4.4.15. Spray pyrolysis (pyrosol) Spray pyrolysis (pyrosol) deposition is a CVD technique, resembling the aforementioned MOCVD. The difference between them lies in the evaporation technique of the precursor solutions: it is thermal in MOCVD and ultrasonic in spray pyrolysis. In the latter case, an aerosol is formed [653–660]. That is why, spray pyrolysis might be called “aerosol CVD” [656] or “ultrasonic spray pyrolysis” [658,659]. Similarly to MOCVD, complicated Ca- and P-containing organic precursors might be used. For example, solutions of calcium bis(2,2,6,6-tetramethyl 3,5heptenedionate) dihydrate and triethylphosphate dissolved in di(ethylene glycol) dimethyl ether were evaporated ultrasonically and, similarly to MOCVD, the obtained aerosol was forwarded into a reactor with a heated substrate inside [656]. In another study, a solution of Ca acetylacetonate hydrate in N,N-dimethylformamide and an aqueous solution of H3PO4 were used [660]. Deposition could be performed under both continuous and pulsed (intermittent) modes; the latter one was found to reduce cracking and increase bonding strength of the CaPO4 deposits [659]. Dense, flat and homogeneous CaPO4 deposits were formed, which could be both porous [653,654] and continuous [659,660]. 4.4.16. Drop-on-demand (DOD) micro-dispensing A DOD technology was first introduced for ink jet printers. According to this technology, ink jet print heads project digitally micro drops of ink (a few picoliters) through nozzles producing directly images on the substrate [661]. Recently, the DOD technology was also applied for deposition of both pure and doped CaPO4 powders on various substrates [662–664]. A schematic setup of the DOD technique is available in literature [664]. Briefly, the deposition system consists of a precision XYZ stage controller, a micro-valve driver, a pneumatic system, a dispensing unit, a vision system, and an external heating patch. A micro-valve print head with an inner diameter of 300 μm was used to produce droplets at an applied constant pressure of 2 bar and an operational on-time of 500 μs. The movement of the droplets was achieved by the XYZ stage. The results revealed that DOD technique did not alter the properties of the CaPO4 powders during the deposition. After deposition, no observable changes in powders' morphology were found either. The deposits appeared to be phase-pure, retained all functional groups and displayed rod-like particles with dimensions ~ 60 nm in length and ~ 15 nm in

width [663,664]. Uniform CaPO4 deposits were produced with a thickness of 34.5 ± 1.0 μm and the critical load before failure of 69 mN [664] and a thickness of ~ 3.5 μm and the critical load before failure of 160 mN [663]. 4.4.17. Mechanochemical synthesis or ball impact technique A rather unusual mechanochemical deposition of CaPO4 was proposed [665,666]. In both papers, the authors used a high-energy ball milling to perform CaPO4 deposition on metallic substrates. Namely, HA powder, Ti powder (250 μm) and 10 × 8 × 2 mm Ti alloy substrates were used in that work. The powders and the substrate were placed into a vibration steel vial (50 Hz) and treated for the different times with optimum ball to powder ratio of 40:1. The milling process was carried out without any process control agent. To prevent contamination from the atmosphere, the chamber was sealed. Deposits of ~ 50 μm thick were put down as the result [665]. Similar results were obtained in another study [666]. According to the authors, the impacts of the milling balls both significantly decreased the dimensions of the HA particles and simultaneously activated the metal surface, which led to the robust cold-welding of HA particles to the metal surface [665,666]. No information on the properties of deposits was disclosed. 4.4.18. Polymeric deposition route A polymeric deposition route is a CVD technique, which appears to be rather similar to the aforementioned molecular precursor and thermal decomposition methods. It is also based on deposition of a mixture of the Ca- and P-containing organic precursors with the desired Ca/P ratio onto substrates, followed by drying, calcining and/or sintering. The difference lays in an intermediate formation of viscous polymeric solutions, into which substrates were dipped. Namely, to perform polymeric deposition, initially, phenyldichlorophosphine (C6H5PCl2) was mixed with acetone and hydrolyzed with water. Afterwards, a solution of an exact stoichiometric quantity of calcium nitrate in acetone was added. Then an obtained mixture was oxidized by air bubbling. After 1 h, a considerable increase in viscosity was observed, which indicated to polymerization. Then, various thermally stable substrates were dip coated by this viscous solution, followed by drying and sintering. As a result, thin and nearly fully dense deposits, whose XRD patterns indicated the presence of HA and β-TCP, were fabricated [667]. 4.4.19. Autocatalytic deposition This deposition technique appears to be a variation of the wetchemical deposition from supersaturated aqueous solutions (Section 4.3.4). The presence of a catalyst (the authors used PdCl2) and reductive P-containing compounds (the authors used NaH2PO2· H2O) were the characteristic features of the autocatalytic deposition. To put down CaPO4 coatings by autocatalytic deposition, two types of Ca-containing solutions were studied: one was slightly alkaline (pH ~ 9.2) and another was slightly acidic (pH ~ 5.3). The entire process was based on a catalytic oxidation of hypophosphite ions into those of orthophosphate. After being formed, the latter ions reacted with dissolved calcium ions to precipitate CDHA on polymeric substrates [47, 668,669]. A bit later, this process was applied for CaPO4 deposition on metallic substrates [49,669]. To achieve this, the former authors used H3PO4 as the P source and either PdCl2 or AgCl as the catalysts [49], while the later authors used the slightly acidic bath only [669]. A proposed mechanism of the autocatalytic deposition is schematically shown in Fig. 25. 4.4.20. Cyclic electrodeposition Cyclic electrodeposition (synonym: cyclic electrochemical deposition) represents a combination of an alternate soaking deposition with an ECD techniques. It is based on independent calcium and orthophosphate electrodeposition cycles. In this way, a substrate is sequentially immersed into a Ca- and a PO4-containing solution, while the electrical current is reversed in accordance, allowing both species to be deposited


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Fig. 25. A hypothetical mechanism of CaPO4 deposition on a Ti surface by using PdCl2 as the catalyst. Please, note that an intermediate layer of sodium titanates was created on Ti prior deposition of CaPO4. Reprinted from Ref. [49] with permission.

during a given number of cycles. The main advantage of cyclic electrodeposition is that the individual variables of each step can be controlled and studied independently. That means that the main ECD parameters such as pH, current density and reaction time can be adjusted independently for Ca- and PO4-containing baths [670,671]. Namely, to perform cyclic electrodeposition of CaPO4 on porous silicon, ethanol-containing 0.1 M aqueous solutions of CaCl2 (pH was adjusted to 5.5) and Na2HPO4 (pH was adjusted to 7.2) were used. The silicon substrates were alternately immersed into the Ca- and PO4-containing solutions under the following standard conditions: 1 mA/cm2 of cathodic current density in Ca-solution and 1 mA/cm2 of anodic current density in PO4-solution, during 30 s controlled by a potentiostat. The substrates were rinsed in ethanol between each step and 20 Ca- and PO4-deposition cycles were made in total. All reactions were carried out at room temperature but a thermal post-annealing at 800 °C for an hour was applied to enhance crystallinity of the deposits [670,671]. 4.4.21. Cyclic spin coating Similar to the cyclic electrodeposition, cyclic spin coating also represents a combination of an alternate soaking deposition but currently with a spin coating. The method also based on independent calcium and orthophosphate coating cycles, in which a substrate is sequentially spin coated by a Ca- and a PO4-containing solutions. The deposition of CaPO4 was performed at 2500 rpm during 60 s using the solutions,

described in cyclic electrodeposition. This deposition process was performed at room temperature and repeated for ten cycles in a row [671]. 4.4.22. Biomediated deposition (biosynthesis) The biomediated deposition of CaPO4 is based on using calcified bacteria and just one publication on the subject is available [672]. According to the authors of this technique, fresh saliva samples were collected and calcified bacteria were isolated. The total viable bacterial counts were enumerated by standard pour plate method using modified Mueller Hinton agar medium. The composition of the modified Mueller Hinton agar medium was as follows: casein acid hydrolysate 17.5 g/l, beef infusion 2.0 g/l, starch soluble 1.5 g/l, CaCl2 1 g/l and agar 20 g/l. To perform CaPO4 deposition, 300 ml of the broth were poured into a 500 ml clean conical flask and the broth was inoculated by 10 ml of calcified bacteria cultures. Afterwards, metallic substrates were immersed into the broth under sterile condition and kept an incubator for 48 h at 37 °C. After incubation, the substrates were removed from the medium, washed with distilled water to remove unattached bacteria, followed by gentle heating to form CDHA deposits. The CDHA deposits revealed an improved corrosion resistance behavior and possessed a lower passivation current density [672]. 4.4.23. Emulsion route To perform an emulsion route, stable oil-in-water emulsions containing CaPO4 particles should be prepared. To do this, aqueous

Fig. 26. Representative optical micrographs (background) and schematic representations (inserts) of nano-sized HA-stabilized dichloromethane emulsion of PLLA in water (left) and fabrication of nano-sized HA-coated PLLA microspheres after dichloromethane evaporation (right). Reprinted from Ref. [673] with permission.

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dispersions of nano-sized HA particles with a solid content of 0.04 wt.% were prepared by serial dilution. Then, aliquots of these dispersions (25 g) were hand-shaken with poly(L-lactic acid) (PLLA) solution in dichloromethane (total 2.5 g, 1.0–9.1 wt.% solid contents) at 25 °C for 30 s. HA-particle stabilized emulsion was prepared as a result (Fig. 26, left). Since dichloromethane was a volatile organic solvent, the HA-coated PLLA microspheres were prepared via its evaporation in situ from the emulsion at 25 °C (Fig. 26, right). Experiments with cells revealed that the HA-coated PLLA microspheres promoted the cell adhesion and spreading if compared with non-coated PLLA microspheres [673]. HAcoated PCL microspheres were prepared by the same technique [674]. However, for polymers having a low melting point, such as PCL (melting point 60 °C), a solvent-free emulsion route was developed via preparation of stable melt-in-water emulsions containing CaPO4 particles. To perform deposition, the same aliquots of the same dispersions of nano-sized HA particles were mixed with 0.125 g of PCL pellets in glass tubes. Then the tubes were heated in a water bath at 80 °C for 1 h to melt PCL. To prepare emulsions, the hot mixtures were homogenized using a homogenizer at 20,500 rpm for 1 min. Afterwards, the emulsions were cooled in air to room temperature and HA-coated PCL microspheres were prepared as the result [675]. Similar can be done with polymers having a melting point within 100–160 °C; however, in such cases, stainless-steel high-pressure reactors and oil baths should be used [675]. Furthermore, to gain additional properties, such as magnetism, some fillers, such as nano-sized Fe3O4, could be incorporated into the polymeric microspheres prior deposition of CaPO4 particles using the emulsion route [676]. 4.4.24. Slurry processing A slurry processing was performed to deposit CaPO4 on Ti. As follows from the name, initially a CaPO4 slurry should be prepared. To do this, a fine CaPO4 powder was mixed with distilled water in almost equal proportions. Afterwards, Ti substrates were completely buried into the slurry in a ceramic crucible and then heated in air for 2 h at temperatures within 450–750 °C. The heating stage oxidized the Ti surface and promoted a mutual solid-to-solid interdiffusion between titanium oxides and CaPO4 with simultaneous fixation of CaPO4 deposits on the surface. After heating, the substrates were removed from the slurry, washed ultrasonically in distilled water for 10 min and dried at 60 °C in air. Composite gradient deposits comprising HA and TiO2 of 200– 1000 nm thick were fabricated on Ti substrates. At higher temperatures, oxygen diffused more readily into the substrate, which increased both the thickness and roughness of the TiO2 layer [677,678]. 4.4.25. Slip coating A slip coating deposition has substantial similarities to the aforementioned dip- and spin coating techniques, as well as with slurry processing. Briefly, to perform deposition, an aqueous CaPO4 slip (i.e., a suspension, a slurry, etc.) should be prepared and put down onto the surface of pre-sintered ceramic substrates by a soft brush, followed by drying and sintering. Since the substrates were pre-sintered, they had a good water-absorption ability, which facilitated formation of thicker CaPO4 deposits in one stage. For a better adhesion, various additives, such as dispersants, binders, and surfactants might be added to the CaPO4 slips. Similar to dip- and spin coating techniques, the procedure can be repeated several times to prepare even thicker deposits. Microporous CaPO4 deposits with good adhesion strength were fabricated as a result [679–681]. 4.4.26. Deposition by solvent evaporation This simple but elegant technique is based on HA dissolution in an “apatite-dissolved solution” followed by the solvent evaporation. Initially, the “apatite-dissolved solution” was prepared by dispersion of 1 g commercially available high-purity HA powder in 1000 ml of de-ionized water, followed by bubbling of CO2 gas for 1 h. CO2 bubbling resulted in pH drop, which cased HA dissolution. Afterwards, a substrate

(the authors used graphite) was immersed into the “apatitedissolved solution” and the entire system was heated by either an external heater (at 90 °C for 1 h) or a microwave oven (at 100 °C for 5 min) to both evaporate water and eliminate CO2 (when an external heater was used) or just eliminate CO2 in microwave oven. The combination of external and microwave heating was found to be effective in deposition of large amounts of CaPO4, because the seed crystals formed by the evaporation of solvent from the “apatite-dissolved solution” (i.e., external heating) contributed to enhancing the deposition of CaPO4 through the microwave heating. The deposits were formed by stacking the plate-like crystals of ~ 4.9 μm thick and were identified as carbonate-containing CDHA with Ca/P ratio of 1.72. Subsequent in vivo implantations into femur and tibia of Japanese white rabbits for 4 months revealed formation of calcified bone at the interfaces, which was a good indication to an excellent biocompatibility [682]. 4.4.27. Powder mixed electrical discharge machining (PMEDM) Electric discharge machining (EDM, synonyms: spark machining, spark eroding, burning, die sinking, wire burning or wire erosion) is a material removal process, in which a desired shape is obtained using electrical discharges (sparks). Since there is no direct contact between a tool and a workpiece, the EDM process enables machining of any electrically conductive material, irrespective of its hardness, shape and strength. A material is removed from workpieces by a series of rapidly recurring current discharges between two electrodes, separated by a dielectric liquid. Electrical sparks locally increases the surface temperatures above the materials boiling points leading to a formation of crater-covered surfaces. An addition of fine powder into the dielectric liquid is a relatively new advancement in the EDM process, known as PMEDM [683]. A study is available, in which a fine HA powder was added into water (a dielectric liquid) and a PMEDM treatment of the surface of Ti–6Al–4V alloy substrates was performed. The addition of HA was found to drastically alter both the surface topography and chemical composition of the treated substrates, indicating that HA powder had a significant influence on the entire process. In addition, the results of EDS analysis of the substrate surface clearly demonstrated the presence of strong peaks of Ca and P due to HA deposition. Thus, the authors concluded that PMEDM technique could be used for CaPO4 deposition [684]. 4.4.28. Investment casting Since metallic prostheses are usually manufactured by investment casting of molten metals into molds made from high temperature stable and chemically inert inorganic compounds (e.g., graphite, zirconia, alumina, silica), this process might also be used for CaPO4 deposition [685–688]. The technology is rather simple: prior casting of a molten metal, a cavity of the mold must be covered by CaPO4. Therefore, an aqueous suspension of HA powder was prepared and pasted on the cavity using a paintbrush. When water was completely dried, the HA particles adhered to the cavity walls [685–687]. However, a more complicated HA deposition technique on the cavity walls by using wax patterns, followed by de-waxing might be applied as well [688]. Afterwards, a molten metal was poured into the HA-covered mold, followed by cooling, mold disassembling and metallic specimen retrieving. This technique was found to be feasible and the cast specimens were easily separated from the mold without difficulties such as chemical reaction or sticking. An uptake of CaPO4 by the surface of the metallic castings was detected and an analysis of those deposits revealed that they consist of the mixtures of various compounds such as HA, α-TCP, β-TCP and CaO [685]. Similar results were obtained in other studies [686–688]. 4.4.29. Adsorption Finally, deposition of CaPO4 on some substrates could be performed by adsorption. For example, to perform adsorption of nano-sized HA


crystals on the surface of PLLA microspheres, initially, the microspheres were treated with alkali (pH = 11.0, adjusted with 25% ammonia solution) for 1 h at room temperature in order to introduce carboxyl groups on their surfaces, washed with water and then dried under reduced pressure. Then, the alkali-treated and dried PLLA microspheres were washed with ethanol and immersed into a 1.0% ethanol dispersion of nano-sized HA for 1 h at room temperature under stirring. Afterwards, the HA-coated microspheres were washed five times with ethanol under sonication for 3 min and dried under a reduced pressure. The surface of the PLLA microspheres was found to be uniformly coated by HA through the ionic interaction between the calcium ions of HA and the surface carboxyl groups of the alkalitreated PLLA [53].

5. Deposition of ion-substituted CaPO4 and CaPO4-containing biocomposites Since chemically pure CaPO4 bioceramics possess some serious mechanic limitations [3–6], to improve their properties, both ionsubstituted CaPO4 [287,689–692] and CaPO4-containing biocomposites [59,693–699] were deposited on various substrates. In general, such types of deposits might be prepared by two approaches: (i) direct deposition or co-deposition of either such compounds or their precursors and (ii) incorporation of the desired dopants and/or compounds into the previously deposited pure CaPO4. According to the available literature, all the afore-described deposition techniques of pure CaPO4 might also be used for deposition of ion-substituted CaPO4 and CaPO4-containing biocomposites as well; however, an applicability of each specific technique strongly depends on the nature and properties of the desired dopants and/or compounds. For example, in the case of inorganic dopants, chlorapatite is thermally stable, while carbonates are thermally unstable. Therefore, the former might be easily deposited by plasma spraying [700], while any type of a thermal spraying technique appears to be inapplicable to deposit carbonate apatite; however, it might be easily deposited by wet techniques. In addition, wet deposition techniques also allow incorporation of the thermally stable dopants as well. For example, let us consider deposition of CaPO4 doped by various metals. Ag-doped CaPO4 were put down using sol–gel [701,702] and electrophoretic [703] deposition, magnetron sputtering [704,705], thermal [706], plasma [707] and cold [708] spraying, IBAD [709], PLD [710] and thermal substrate [711] techniques, while CaPO4 doped with Sr [437,712], Zn [713] and Si [437,714] were deposited using a biomimetic technique. However, in no case this means that CaPO4 doped by Sr, Zn, Si and other dopants can be deposited by the biomimetic technique only [707,715–717]. In addition, CaPO4-containing biocomposites could be put down by electrodeposition [718], dip coating [719], biomimetic [720 and references therein] and hydrothermal–electrochemical co-deposition [721] techniques. Furthermore, MAO, MAPLE and AD techniques provide the opportunities to deposit various composites. For example, CaPO4/hydrated titania biocomposites were deposited on Ti by MAO coupled with EPD [722]. Nevertheless, since the amount of possible dopants and/or additives is very big, while the room is limited, the topic on deposition of both the ion-substituted CaPO4 and CaPO4based biocomposites is not detailed further; the interested readers are referred to the original publications. Concerning incorporation of the desired dopants into the deposited CaPO4 coatings, films and layers, sorption and ion exchange methods appear to be the most common techniques. Electrodeposition might be used as well [622]. To perform doping, the CaPO4-coated substrates are immersed into solutions for a while, followed by various postdeposition treatments. However, this approach has some limitations. Namely, the incorporated dopants are reside mostly on the outer surface of the CaPO4 deposits and are quickly depleted both in vitro and in vivo without a long-term effect [622,716,723–725].

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6. Conversion-formed CaPO4 deposits Not many publications are available on this topic. Due to a specific formation mechanism of the conversion-formed deposits, they might be fabricated only on the surface of solid substrates, containing either ions of calcium or those of orthophosphate in their initial chemical composition. Therefore, to prepare CaPO4 on their surface, such substrates should be treated by a soluble chemical, containing the missing ion. Therefore, a Ca-containing solid is treated by an aqueous solution, containing dissolved PO4 ions. Namely, a surface of marble (i.e., CaCO3) samples after a gentle and prolonged (72 h) treatment by a diluted (1 g/l) aqueous solution of MCPM, was found to be covered into CDHA (presumably, carbonate-containing CDHA) [726]. In addition, for the same purposes, aqueous solutions of (NH4)2HPO4 [727–732], (NH4)3PO4 [732], NH4H2PO4 [732], K2HPO4 [733,734], KH2PO4 [735], etc. might be used instead of the MCPM ones. In some cases, a mixture of CDHA with OCP might be deposited on marble [732]. Interestingly the well known solid-state process of hydrothermal transformation of CaCO3 skeletons of marine corals into CaPO4 (preparation of coralline HA) [736–738] appears to be the same chemical process and, if stopped at the intermediate stages, CDHA deposits of various thickness could be fabricated on corals. In addition, a combined deposition–conversion technique is possible. Namely, a surface of substrates might be covered by CaCO3 using any possible deposition technique, followed by a conversion of CaCO3 deposits into CaPO4 ones using any PO4-containing conversion solution. For example, chitosan/carbonate apatite composites were deposited onto a surface of Ti–6Al–4V alloy substrates according to the following successive steps: (i) deposition of CaCO3 coatings on the substrates by EPD; (ii) their transformation into carbonate apatite ones in PBS; and (iii) formation of the composite coatings by modification with chitosan [739]. To conclude this topic, it is worth mentioning that a spontaneous formation of CDHA on the surface of ancient marble items was discovered [740,741]; therefore, the conversion-formed CaPO4 deposits were proposed for both consolidation of carbonate stones [727–729] and conservation of outdoor marble artworks and relics [726,728,730]. 7. Properties Biomedical applications of CaPO4 deposits involve a series of important conditions. Namely, non- or slowly-resorbable deposits of high crystallinity have been recommended to retain the bonding strength with implants. However, this contradicts the statement that the ideal interface between the implants and surrounding tissues should match the tissues being replaced. For example, in the case of HA, its crystallinity degree is in the inverse proportion to its bioactivity [742]. Therefore, from the bioactivity point of view, CaPO4 deposits should be of a low crystallinity and, ideally, should contain various bone-mimicking ionic substitutions, such as Na, Mg and carbonates. Thus, since one of the first steps in bonding involves dissolution of the coating surface, it might be suggested that deposits prepared from less crystalline and/or better resorbable CaPO4 would be more beneficial for early bone ingrowth than those prepared from high crystalline HA [30]. However, soluble deposits will weaken the bonding strength between them and substrates. In particular, a rapid dissolution of CaPO4 deposits may loosen the bonding strength between the implant surface and the host bone. For example, a comparative study on the biological stability and osteoconductivity of HA coatings on Ti produced by PLD and plasma spraying was conducted. After 24 weeks of implantation, the plasma sprayed HA deposits showed considerable instability and reduction in thickness but no statistical difference to the uncoated Ti (the control), while the PLD ones remained almost intact but showed a significantly higher amount of bone apposition [743]. Thus, the authors revealed that the coating stability prevailed over its solubility. Furthermore, the excessive amount of the dissolved ions from the soluble deposits may

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cause local inflammatory reactions. Therefore, both the composition and the structure of the ideal deposits should be complex. Namely, a rapidly resorbing ACP phase should be located mainly at the surface where it can support the development of bone-like apatite, while the crystalline and slower dissolving HA should be regionally located toward the coating/implant interface to both ensure long-term survival and provide adequate bond strength. Nevertheless, a certain amount of the crystalline phases needs to be incorporated into the ACP phase, so that bone-like apatite can appropriate a foundation when other phase components dissolve. In addition, apart from inducing faster bone regeneration and bonding between the implant surface and the newly formed bone, CaPO4 deposits should be well adherent and they should not release particles that could damage other components of the implants. Moreover, to reduce the intrinsic residual stresses in the implantation area, ideally, their elastic modulus should have an intermediate value between that of the substrate and that of the bone. Except of the preparation techniques and processing conditions employed, which directly influence the structure, crystallinity and chemical composition, a number of other factors appear to influence the physical, chemical and mechanical properties of CaPO4 deposits. They include thickness (this will influence adhesion and fixation — the agreed optimum now seems to be within 50– 100 μm), phase and chemical purity, fatigue resistance, porosity and adhesion [8,182]. An abrasion resistance might be important as well [163]. 7.1. Elastic modulus and hardness Hardness is a property related to yield strength and it is useful to predict strength of the deposits, a stress state within the deposits, as well as an abrasion resistance on their surface. Ideally, to minimize a stress shielding effect (which occurs due to a mismatch between stiffness of bones and that of implants), the mechanical properties of CaPO4 deposits should be in between of those of bones and those of the substrates. That is why, the mechanical performance of the CaPO4 deposits was studied a lot [31,32,55,141,169–172,189,206,210,247, 391,429,744–751]; however, the obtained numerical values appeared to vary in a great extent. This is due to several important reasons: an anisotropic behavior of CaPO4 deposits, variations in nature and type of substrates, various techniques used for deposition, diverse deposition conditions, using or non-using of additives (binders, surfactants, porogens, etc.), as well as application or non-application of both preand post-deposition treatments. Due to all these variables, the mechanical properties of the CaPO4 deposits prepared by different researchers who used dissimilar techniques (Table 3) under divergent conditions appear to be almost incomparable. In addition, the measurement technique itself might influence. Namely, the elastic modulus and hardness of magnetron spattered CaPO4 were determined by dynamic nanoindentation with a Vickers tester. The immersion speed of the Vickers diamond pyramid with an angle between opposite faces of 136° was 5 mN/min and the time of a loading/unloading cycle was 2 min. The authors concluded that the nanohardness and the Young's modulus of the CaPO4 deposits themselves were ~10 GPa and ~110 GPa, respectively. They also found that the nanohardness value decreased when the indenter penetration depth increased, a phenomenon known as indentation size effect [237]. The numeric values of nanohardness of 3.4– 4 GPa and the Young's modulus of 122–150 GPa were measured in other studies [229,747]. In addition, almost similar values were obtained for the microplasma sprayed HA [169–173]. Similarly, the numerical values for the Young's modulus and hardness of both the crystalline and amorphous CaPO4 coatings deposited by PLD on both titanium and silicon substrates appeared to be ~ 93 GPa (on Ti), ~ 127 GPa (on Si) and ~1.6 GPa (on Ti), ~2.3 GPa (on Si) for the crystalline coatings and 74– 107 GPa (on Ti), ~ 68.5 GPa (on Si) and 0.55–1.06 GPa (on Ti), ~0.40 GPa (on Si) for the amorphous coatings, respectively [744]. Nanoindentation tests indicated that the Young's moduli of the IBAD coatings were higher than 91.7 ± 3.6 GPa while microhardness values were

higher than 5.27 ± 0.32 GPa [225]. Furthermore, for thick plasma sprayed HA deposits, the top surface microhardness values were found to be higher if compared to those at the center of crosssection [750]. However, for plasma sprayed HA coatings on tungsten, the numerical values of Young's moduli in both tension and compression measurements evaluated by the cantilever beam bend test were found to be below 6 GPa [31]. Interestingly hardness of the CaPO4 deposits were found to increase with the substrate temperature increasing, being as low as 5 GPa at 30 °C and reaching a high value of 28 GPa at 700 °C [263]. The numerical values of hardness and Young's modulus for CaPO4 deposited by wet techniques appear to be lower. For example, for HA deposited by sol–gel technique the numerical values appeared to be ~ 0.25 and ~ 28 GPa, respectively [429], while the numerical values of Young's modulus for biomimetically deposited CaPO4 was found to be ~ 4.5 GPa [436]. Further details on the mechanical testing methods of CaPO4 deposits are available in literature [752]. 7.2. Fatigue properties Several studies have demonstrated that cyclic loading of the CaPO4coated samples leads to fatigue failure of the CaPO4 deposits [89,90, 496]. Further, it has been shown that a combination of an aqueous environment with a stress can result in delamination or accelerated dissolution of deposited CaPO4, which can influence the long-term stability of the implants [153,753–758]. For example, various types of CaPO4coated substrates were mechanically tested in either dry or wet (SBF [755–757], HBSS [758] and Ringer's [153] solutions) conditions. The results demonstrated that the fatigue properties of amorphous (ACP) and crystalline CaPO4 deposits appeared to be different. In addition, the fatigue behavior revealed substantial differences when tested in either dry or wet conditions [755–758]. Nevertheless, studies are available in which no fatigue effect was detected [759,760]. For example, cyclic fatigue testing of HA coated samples in a lactated Ringer's solution to 5 million cycles showed no changes in response to fatigue loading [759]. In another study, after up to 10 million cycles of bending in air and SBF, no significant microcracking or coating spalling on the surface of plasma-sprayed HA deposits, nor significant changes in thickness, weight, crystallinity or residual stress were found [760]. As already mentioned in the previous section, these differences in the fatigue properties are due to numerous processing variables, such as nature of substrates, deposition technique used, application or non-application of pre- and post-deposition treatments. To improve fatigue stability of CaPO4 deposits, surface modifications of substrates are commonly used. For example, application of a HA/Ti bond coating onto the surface of Ti substrates prior deposition of CaPO4 was found to prolong the fatigue life [89,90]. 7.3. Thickness Depending on the production technique, the thickness of the CaPO4 deposits varies from nanometric dimensions to several millimeters (Table 3) and this parameter appears to be very important. Namely, if CaPO4 deposits are too thick, they are easy to break. In addition, the outer layers might tend to detach from the inner ones, which in time can result in implant mobility. On the contrary, if they are too thin, the deposits are easy to dissolve, because resorbability of HA, which is the second least soluble CaPO4 (Table 1), is about 15–30 μm per year under the physiological conditions [761]. Needless to explain, except of FA, dissolution of other CaPO4 would be faster. Thus, the optimal thickness of the CaPO4 deposits appears to be a compromise between the dissolution and the mechanical properties. In addition, the choice of thickness with allowance for resorption depends also on its atomic structure (amorphous or crystalline), the degree of dispersion of the grain structure, the phase and chemical compositions. To complicate the situation, the failure mechanisms for thinner and thicker deposits


appear to be different. Namely, the failure mode of thinner (~50 μm) HA coatings on a Ti alloy was found to be conclusively at or near the coating/bone interface, while that of thicker (~ 200 μm) HA coatings was found to be at the coating/bone interface, inside the HA lamellar splat layer, as well as at the coating/Ti alloy substrate interface [762,763]. A similar conclusion was made in another study, in which the mechanical behavior of thin (0.1, 1 and 4 μm) CaPO4 deposits was compared [764]. Furthermore, thickness of CaPO4 deposits was found to influence on both the mechanical fatigue behavior [765] and implant fixation in vivo [766]. However, no statistically significant difference was found in the degree of bone formation as it related to the thickness of HA deposits [767]. In addition, the corrosion resistance of metallic substrates was found to depend on both thickness and grain size of the protecting CaPO4 deposits [422,483]. Considering all these points, the commercial plasma sprayed HA deposits have thickness between 50 and 200 μm [182], though cells and tissues interact with only top surface and, thus, thickness of ~10 nm would be sufficient for the cell activities. To illustrate a complex correlation between the thickness of CaPO4 deposits and their structure + chemical composition, the following examples can be mentioned. Depending upon the deposition times, CaPO4 coatings of various thicknesses ranging from 170 nm up to 1.5 μm [249] and from 40 nm up to 5 μm [250] were put down by PLD. Their morphology was found to be grain-like particles and droplets. During growth, the grain-like particles grew in size, partially masking the droplets and a columnar structure was developed. The thinnest (~170 nm) deposits were found to consist mainly of ACP, those of ~ 350 nm thick also contained HA, whereas even thicker ones contained some α-TCP in addition to HA. All types of deposits failed under the scratch test by spalling from the diamond tip, however, the failure load increased as thickness decreased until only plastic deformation and cohesive failure for the thinnest coating was observed [249]. Similar results were obtained in another study [250]. Therefore, both the structure and the phase composition of CaPO4 deposits might depend on their thickness. 7.4. Adhesion and cohesion In surgical practice, both failure of implants and undesirable tissue responses take place when detachment of coatings occurs. This leads to micromotion of the implants and increased fretting and production of debris particles [768]. A typical morphology of the rupture surfaces after detachment of CaPO4 is shown in Fig. 27 [691]. It can be seen that both the adhesive and cohesive failures were present. Therefore, all types of CaPO4 deposits must be both strong enough to sustain cohesion and adhere satisfactorily to the underlying substrate irrespective of their intended functions. Since neither substrates nor CaPO4 deposits are perfectly flat, the bottom surfaces of the deposits are never in the full contact with the substrates. The areas that are in contact are called

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“welding points” or “active zones”. Voids of various shapes and dimensions are located among them. In general, the greater the contact area is, the better adhesion is [769]. Since the chemical bonds between the deposited CaPO4 and substrates are rare, a mechanical anchorage is the main mechanism involved in adhesion. Therefore, shape and sizes of the grains with intimate contact with the substrate were found to affect the adhesion of HA deposits and the hexagonal grains with enlarged sizes (e.g., up to 250 nm in side length) deteriorated the adhesion, while the granular nano-sized grains enhanced the bonding of the deposits through improving the interlocking by the substrate [207]. Furthermore, the adhesion strength was found to be a linear function of the average surface roughness [770]; thus, highly roughened substrate surfaces exhibited higher bond strength as compared to smooth substrate surfaces [330,771]. Therefore, substrate preparation techniques, such as grit blasting, are used to increase roughness prior to spraying and hence increase the adhesion strength (see Section 3. Brief knowledge on the important pre- and post-deposition procedures). On the other hand, the amount of mechanical anchorage is reduced if a large amount of shrinkage occurs during solidification of the particles [150]. Since the strength of human bones is ~18 MPa, all types of the deposits on the implant surface should have either higher or, at least, comparable bond strength. Thus, according to the ISO requirements, the adhesion strength of CaPO4 deposits should not be less than 15 MPa [19–21]. Specifically, the adhesion forces of CaPO4 deposits should be high enough to maintain their bioactivity after a surgical implantation. Generally, tensile adhesion testing according to standards ASTM C633 [772] and ASTM F-1147-05 [773] is the most common procedure to determine the quantitative adhesion values to the underlying substrates. A graphical sketch of the procedure is shown in Fig. 28. Furthermore, fatigue [148,774], scratch [744,749,775–779] and pullout [775] testing, as well as wear resistance [777] are among the most valuable techniques to provide additional information on the mechanical behavior of CaPO4 deposits. The scratch test is performed with reference to ISO 20502:2005 [780]. Changes in the surface topography can give an indication on the wear resistance. For example, deposits with good adherence to the substrate have shown less alteration of its surface roughness, while the study on the different parameters revealed that deposition time was the most influential factor in the wear behavior [777]. The latter was attributed to its correlation with coating thickness. The load at which complete removal of the coating occurs is usually taken as an indication of the adhesion strength. Similarly to the aforementioned properties, the adhesion strength of CaPO4 also depends on very many parameters. In the first instance, it strongly depends on the deposition technique (Fig. 29) [781]. For example, HA deposited by PLD, showed greater adherence to a titanium alloy, when compared with plasma sprayed HA [782]. Besides, the adhesion strength might depend on the deposit thickness and its chemical

Fig. 27. Typical morphologies of rupture surfaces of CaPO4 deposits. A left image shows a typical adhesive failure where a piece of the coating is detached from the substrate. A cohesive failure can be observed in a right image. It happens within the coating and is spot flaking around the surface of the remaining coating. Reprinted from Ref. [691] with permission.

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Fig. 28. A schematic drawing of the standardized tensile adhesion measurement.

composition. Namely, deposits of 50 μm thick gave higher values of the adhesion strength than those of 240 μm thick [783], while scratch tests revealed that the sol–gel fluorinated HA (FHA) adhered to Ti-alloy substrate up to 35% better as the fluorine concentration increased [418]. In another study on FHA deposits with variable amounts of F, the adhesion strength was found to increase up to ~40% and the fracture toughness increased for ~ 200 to 300% with the degree of fluoridation increasing [421]. Furthermore, the nature, structure and chemical composition of the substrate surface also play an important role [36]. Besides, the adhesion strength of plasma-spayed deposits was found to decrease when either the plate power was reduced (from 28 to 22 kW) or the working distance was increased (from 90 to 130 mm) [186]. Moreover, increasing the intensity of a magnetic field was found to improve the adhesive strength of HA deposits [133]. Additionally, the bond strength of CaPO4 deposited on Ti plates pre-treated in an alkali solution followed by heattreating (600 °C for 1 h) in air had a higher value (~35 MPa) if compared to those followed by heat-treating a vacuum (~21 MPa). This was attributed to the structural and compositional differences in the interfacial layer of sodium titanates [784]. Furthermore, a correlation between adhesion and residual stress was found [188]. For plasma-assisted deposition techniques of CaPO4, a good overview on the adhesion strength values is presented in Table 3 of Ref. [148].

However, applying of various inter-layers (synonym: buffer layers) seems to be the most important way to influence the adhesion strength of CaPO4 deposits. A big variety of the available deposition techniques (see Section 4 above), which should be multiplied to a big selection of various substrates, results in a great number of potentially appropriate chemicals to be used as inter-layers between the substrates and deposited CaPO4. For example, for plasma-assisted deposition, such chemicals as TiO2 [785], TiN [248,786–788], ZrO2 [248] or Al2O3 [248] were used as buffer layers. TiO2 [789,790] and TiN [229] were also used as underlayers for RF magnetron sputtering. Similarly, formation of intermediate layers of hydrated titanium oxides on Ti [323,330,429] or polar hydrophilic groups on polymers [51,53,60,69–71] is required to improve the adhesion of wet deposited CaPO4. Namely, the adhesive strength of the deposited apatite was increased from 3.5 to 8.6 MPa, from 1.1 to 3.4 MPa and from 0.6 to 5.3 MPa by NaOH pre-treatment of the polymeric surfaces for polyethyleneterephthalate, polymethylmethacrylate and polyamide 6, respectively [51]. Positive effects were discovered for both the glow-discharge [69,70] and ultraviolet [47,70,71] pretreatments of the same polymers. Other types of pre-treatments of various surfaces were found to influence as well [36]. To complicate things even further, one should mention, that mutual inter-diffusion of atoms, ions and molecules of the deposited CaPO4 from one side and those of substrates from another side might occur. Especially this is valid for high temperature techniques; however, the mutual inter-diffusion might also happen at the post-deposition annealing (see Section 3. Brief knowledge on the important pre- and post-deposition procedures). Various types of non-stoichiometric inter-layers are formed as the result. For example, the width of such inter-layer between a HA coating and magnesium substrate formed by IBAD technique was found to be ~ 3 μm [791]. Such inter-layers can reduce the mismatch of thermal expansion coefficients between the deposits and substrates, influence the surface area, wettability or heat conductivity; thus, increasing the bonding strength without affecting biocompatibility. Since the subject of inter-layers appears to be very broad, additional details are not specified further. Let me mention some more examples. For dense CaPO4 deposits under tensile loading, failures were found to occur at the deposit/ substrate interface because the cohesive strength was higher than the bond strength. For porous CaPO4 deposits, the cohesive strength was low and the fracture occurred inside them [403]. The amorphous (ACP) deposits were found to have a more brittle nature and less adhesion if compared to the crystalline ones [247]. The bond strength of apatite layer to Ti metal substrate was reported to range from 10 to 30 MPa [792,793]. Similar values were obtained in another study, where CaPO4 were deposited on Ti substrates by a biomimetic method from two types of SBF. The results indicated that both the ionic concentrations of the SBF and the surface roughness of the substrates had a significant influence on formation, morphology and bond strength of CaPO4 precipitates. The highest bond strength of the precipitated coatings was ~15.5 MPa [794]. To finalize this section, one should mention that except of adhesive failures of CaPO4 deposits, cohesive ones also could happen [45].

7.5. Biodegradation

Fig. 29. The adhesion strength values of HA coatings deposited by various techniques on Ti–6Al–4V alloy samples. Reprinted from Ref. [781] with permission.

Biodegradation (synonyms: biotic degradation or biotic decomposition) is a chemical dissolution of materials by bacteria, cells and/or other biological means. Since the chemical composition of the body fluids might be considered as constancy, biodegradation of the all types of deposits is controlled solely by the properties of CaPO4 themselves, which include their chemical and phase composition, Ca/P ratio, crystal structure, crystallinity, porosity, lattice defects, particle sizes and purity [8]. Therefore, until CaPO4 deposits become so thin that body fluids will get an access to the surface of the substrates; their biodegradation appears to be similar to that of CaPO4 bulk bioceramics having the same


structure, composition and properties (porosity, roughness, topography, etc.). In general, the biodegradability requirement of CaPO4 deposits appears to be a matter of debates. Namely, which quality is the optimum? Should they stay on the implant surface permanently or should their functions be temporary? In this regard, two directions are possible. One way comprises creation of stable deposits to enhance the bonding strength between them and implants and another way goes to creation of resorbable ones to increase their bioactivity. Therefore, studies on behavior of CaPO4 deposits in various solutions keep going. For example, the dissolution kinetics of deposited HA was studied using the dual constant composition method and dissolution rates decreased with HA crystallinity increasing [795]. Similar results were obtained in vivo: after implantation, HA coatings with crystallinity of ~ 55% were found to degrade faster and possess better osteoinductivity than those with crystallinity of ~ 98% [796]. Although biodegradation supposed to be an in vivo process, various in vitro simulations are widely investigated. However, to be closer to the in vivo conditions, the biodegradable assessments of CaPO4 deposits are performed in various simulating solutions, such as SBF [158,159,289,794,797–804], HBSS [236,788,805], aqueous saline solution [806,807], Ringer's solution [808,809], PBS [236,810,811], and Eagle's minimum essential medium (EMEM) [812]. In addition, media containing osteoclast-like cells are used [813]. Since the simulating solutions usually contain dissolved ions of calcium and orthophosphates, both partial dissolution of CaPO4 deposits and their re-crystallization are occurred [159,798,801,802, 812]. For example, as written in the abstract of Ref. [159]: “The soaking in SBF homogenizes the morphology of coatings. The sintered zone disappears and the pores get filled by the reprecipitated calcium phosphates.” One should stress that, due to the presence of other ions in the chemical composition of the simulating solutions, in the vast majority of the cases the ion-substituted CaPO4 are crystallized. Usually, the biodegradation kinetics of deposited CaPO4 appears to be proportional to the solubility values of the individual compounds, listed in Table 1. For example, both bone bonding and bone formation of plasma sprayed HA, α-TCP and TTCP were evaluated by mechanical push-out tests and histological observations after 3, 5, 15 and 28 months of implantation. Among them, α-TCP (which was the most soluble phase) showed the most significant degradation after ~3 months of implantation, while HA and TTCP showed significant signs of degradation only after ~ 5 months of implantation [814]. Similar results were obtained in another study [804]. This resulted in lesser values of the mechanical push-out tests for α-TCP-coated implants, if compared with those coated by HA and TTCP [815]. Plasma sprayed HA was

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found to dissolve faster than the stoichiometric HA did because a high temperature melted HA powder and partly decomposed it into more soluble compounds, such as high-temperature ACP and OA [816]. Similarly, as-deposited magnetron spattered CaPO4 were almost amorphous (i.e., consisted of ACP) and, therefore, they completely dissolved after exposure to PBS for only 24 h, while the dissolution rate of the same deposits after annealing (they became crystalline) was found to be more restrained [810]. Additionally, HA deposits were found to be less stable than those of FA [817–820] and of a similar stability with magnesium-whitlockite (i.e., Mg-substituted β-TCP) ones [818,819]. On the other hand, there are cases [761,821–823], in which the biodegradation kinetics of CaPO4 deposits appeared to be correlated imperfectly with their solubility values, mentioned in Table 1. For example, three types of CaPO4 (HA, ACP and β-TCP) deposited by the laser ablation technique were immersed in SBF in order to determine their behavior. Neither HA nor ACP coatings were found to be dissolved in SBF, while the β-TCP coating slightly dissolved. Precipitation of an apatitic phase was favored onto both HA and β-TCP; however, no precipitation occurred onto ACP [822]. Additionally, degradation rates of dental implants with 50- and 100-micron thick coatings of HA, FA and fluorhydroxylapatite (FHA) were studied [761]. The implants were inserted in dog jaws and retrieved for histological analysis after 3, 6, and 12 months. Both HA and FA coatings (even of 100-micron thick) were almost totally degraded within the implantation period. In contrast, the FHA coatings did not show significant degradation during the same period [761]. Similar results on FHA were obtained in another study, in which FHA deposits with 25% of fluorine substitution were found to be less degradable in SBF, if compared with both HA deposits and FHA deposits with 60% of fluorine substitution [823]. A mechanistic model was presented for the development of the CaPO4 splats immersed in SBF and influence of their structure on the formation of bone-like apatite deposits (Fig. 30) [797]. Since the melted sections of HA splats represented a complicated mixture of various phases (high temperature ACP, α-TCP, β-TCP, OA, TTCP, calcium pyrophosphates, calcium metaphosphates, CaO), they exhibited higher dissolution rates if compared to that of HA, whereas the un-melted splat centers consisted of the original feedstock powder, i.e. crystalline HA that had a slower dissolution rate. Therefore, the melted sections of HA splats disappeared rapidly without development of bone-like apatite, while, in contrast, the remaining cores offered their surface for biomimetic precipitation of ion-substituted CDHA. The authors concluded that, to offer a foundation for bone-like apatite precipitation, the ideal CaPO4 deposits should consist of a mixture of fast-dissolving phases and crystalline structures, for instance, evolving from un-

Fig. 30. A schematic illustration of a dissolution–precipitation behavior of individual HVOF sprayed HA splats showing that the surrounding melted and afterwards solidified part were nearly totally dissolved and only the un-melted HA core is remained: (−1) is the original splat; and (−2) is the corresponding aged morphology. The decrease of HA particle size, (b) compared to (a), causes further phase dissolution. Reprinted from Ref. [797] with permission.

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Fig. 31. A schematic representation of the processes, occurring with the plasma sprayed CaPO4 deposits immersed into aqueous solutions: a) disintegration (grain boundary dissolution), b) dissolution of more soluble phases (e.g., ACP) and precipitation of less soluble ones (e.g., CDHA), c) hydrolysis of metastable phases (e.g., OA), d) partial dissolution, e) precipitation from the supersaturated solutions (e.g., SBF), f) hydrolysis of CaO, followed by dissolution of Ca(OH)2. Adapted from Ref. [187] with permission.

melted cores [797]. A comprehensive scheme, describing the complete variety of possible events, occurring in aqueous solutions with plasma sprayed CaPO4 deposits is shown in Fig. 31 [187].

7.6. Interaction with cells and tissue responses

Fig. 32. Water contact angle measurements of (a) non-coated and (b) HA-coated Ti samples. Reprinted from Ref. [357] with permission.

The interactions of CaPO4 deposits with either cells in vitro or surrounding tissues in vivo have been studied a lot [36,120,130,236,244, 269,314,316,317,391,478,559,566,574,824–842]. Similar to the aforementioned, until the CaPO4 deposits become so thin that cells and tissues will get the direct access to the surface of the substrates, their interactions with cells appear to be similar to that of CaPO4 bulk bioceramics having the same structure, composition and properties. Namely, the in vitro trials using different cell lines revealed that in the vast majority of the cases, CaPO4 deposits enhanced cellular adhesion, proliferation and differentiation, while the results of the in vivo studies revealed that they promoted bone regeneration. For example, a combination of surface geometry and CaPO4 deposits was found to benefit the implant-bone response during the healing phase [843]. Sodium bisphosphonate-immobilized CaPO4 deposits appeared to be effective in the promotion of osteogenesis on the surfaces of dental implants [130]. A greater percent of bone contact lengths was detected for the CaPO4-coated Ti implants compared to the uncoated controls 3 and 12 weeks after the implant placement [844]. Similar results were obtained in other studies [845–847]. Obviously, a better wettability of CaPO4-coated substrates, if compared to the uncoated controls (Fig. 32) [357], promotes adhesion and proliferation of various types of cells. Additional electrical polarization of CaPO4 deposits was found


to improve biomedical properties of the coated substrates even more [10,123,165]. Concerning the experiments with cells, human gingival fibroblasts attachment, spreading, extracellular matrix production and focal adhesion plaque formation were investigated on commercially pure Ti, HA-coated Ti and porous TCP/HA-coated Ti. TCP/HA and HA deposits exhibited that both the attached cell number and cell spreading area were higher than that on pure Ti and focal adhesion plaque formed earlier than that of uncoated substrate. The attached cell number and type I collagen formation on TCP/HA deposits were more than that on HA ones [848]. Osteoblasts and osteoblast-like cells were successfully grown on the surface of OCP [255], CDHA [813,849,850] and HA [44,46,255,851] deposits and all types of the deposits were found to favor cell proliferation, activation of their metabolism and differentiation. Furthermore, the in vitro cell culture studies using MG63 osteoblast-like cells were performed on CaPO4 deposited by plasma spray, sol–gel and sputtering techniques. The study demonstrated the ability of cells to proliferate on the materials tested. Among them, the sol–gel deposits were found to promote higher cell growth, greater alkaline phosphatase activity and greater osteocalcin production compared to the sputtered and plasma sprayed coatings [269]. In another study, deposited CaPO4 were found to induce significantly higher cell differentiation levels than the uncoated control [852]. In addition, the nature and scale of the response appeared to be influenced by the surface topography of the coated substrates. Namely, CaPO4 were deposited onto substrates with varying topography. Then, a layer of fibronectin was deposited from solution onto each surface and the response of MG63 osteoblast-like cells was studied. The results revealed that cells on the fibronectin-coated CaPO4 with regular topographical features in the nanometer range showed statistically significant differences in focal adhesion assembly, osteocalcin expression and alkaline phosphase activity compared to the CaPO4 deposits without those topographical features [839]. Thus, both an adsorbed bioorganics and a surface topography of the substrates appear to influence cell adhesion and differentiation. In vivo results correlate well with the in vitro ones. Osseointegration rates of porous-surfaced Ti–6Al–4V implants with control (unmodified sintered coatings) were compared to porous-surfaced implants modified through deposition of either an inorganic or organic route sol–gel-formed CaPO4. Implants were placed in distal femoral rabbit condyle sites and, following a 9-day healing period, implant fixation strength was evaluated using a pullout test. Both types of CaPO4 deposits significantly enhanced the early rate of bone in-growth and fixation as evidenced by higher pullout force and interface stiffness compared with controls [853]. To conclude this section, one should note that the positive clinical benefits of CaPO4 deposits were not always detected. For example, studies were undertaken to evaluate the processes involved in biological responses of the Ti–6Al–7Nb alloy with and without HA deposits using both in vitro and in vivo tests. The results with coated samples appeared to be similar to those obtained with the uncoated ones [854–856]. Similarly, neither positive, nor negative influence of the presence of HA deposits on the surface of implants was detected during the 8-year [857], 8- to 12-year [858], 10-year [859], 13-year [860], 15year [861], 15- to 16-year [862], 15- to 18-year [863] and undisclosed [864,865] follow-ups. However, there are studies, in which short-term (4 weeks) advantages of the CaPO4-coated implants were found, whereas no significant differences to the uncoated samples were found after 6 months [866]. In addition, BoneMaster® (BIOMET Corp., Warsaw, IN, USA) surfaces showed significantly greater alkaline phosphatase activity and osteocalcin production compared with the uncoated controls; however, no difference was found between the gold-coated and uncoated BoneMaster® samples, indicating that the BoneMaster® topography was the main contributing factor but not the CaPO4 nature [867]. Furthermore, inflammatory tissue reaction cases were detected [868,869]. More to the point, there are reports of adverse events

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associated with these deposits, which may fragment, migrate and even cause increased polyethylene wear secondary to third body abrasive wear [861,870–872]. Interestingly the short-term inflammatory response against HA deposits on Ti was lower in comparison to the DCPD ones. The observed differences between the Ti–HA implants and the Ti–DCPD ones were attributed to their dissolution characteristics: the HA deposits showed an increased stability and hence a reduced the inflammatory response [869]. Furthermore, HA deposits were found to be a risk factor for cup revision due to aseptic loosening [873]. Thus, precautions to prevent contamination (asepsis) and/or infection (perioperative antibiotics) appear to be more important for the CaPO4-coated implants, if compared with the uncoated controls [874]. 8. Biomedical applications In 1979, the first patent was issued on the development of thermal sprayed HA coatings on metallic implants [139]. In 1987, the results of the first clinical study were published [875]. Shortly afterwards, Furlong and Osborn, two leading surgeons in the orthopedics field, began implanting plasma spray deposited HA stems in patients [876]. Other clinicians followed them [877,878]. Since then plentiful reports have been published about the advantages of such coated implants. To summarize the available information on the biomedical and biomechanical properties of implants coated by CaPO4, one can claim the following. If compared to uncoated controls, deposited CaPO4 were found to induce bone-to-implant contacts [574,818,819,879–890], improve initial stability [891], implant fixation [763,892–896] and nanomechanical properties of adjacent bones [897], show higher torque values [881, 886,896,898] and push-out strength [899], seal the interface from wear particles [900], facilitate bridging of small gaps [901,902], reduce ion release from the metallic substrates [806,903–905], slow down metal degradation and/or corrosion [38,42,72,73,414,791, 906–908], accelerate bone growth [909–911], remodeling [912,913] and osteointegration [35,437,480,767,914–917], improve biocompatibility [918], induce osteoconductivity [846,919–922], osteoinductivity [923] and osteogenesis [130,889,896,924,925], improve the early bone [437,895,925–927] and healing [928] responses, prevent from formation of fibrous tissues (Fig. 33) [147,929], increase ectopic bone formation [459], osteoblast density [930] and their proliferation [669], as well as the clinical performance of orthopedic hip systems (see below). Furthermore, antibacterial properties of deposited CaPO4 were detected in some studies [38,930]. Interestingly even CaPO4 bioceramics (namely, a biphasic formulation consisted of HA and β-TCP) was also coated by another type of CaPO4 (namely, nanodimentional HA) to improve its osteoinductivity [931]. One must stress that all these cases represent a collection of the positive effects of CaPO4 deposits put down by various techniques, while comparison studies revealed that these effects strongly depended on the deposition techniques. Namely, if compared with uncoated controls, the electrochemically deposited CaPO4 were found to contribute to the fixation between bone and implant, whereas the biomimetically deposited ones had a little effect on the fixation [932]. As written in Section 5 above, CaPO4 can be deposited both with various dopants and as diverse biocomposites. In addition, amino acids, drugs and other important biologically active compounds, such as peptides, hormones, growth factors, genes and DNA can be incorporated into them [58,718,720,933–944]. The antibiotic-containing CaPO4 deposits were found to exhibit significant in vivo improvement in preventing infection compared with the standard ones [934,943, 944]. Similar effect was found for Ag-doped deposits [701–708]. These delivery methods of bioactive molecules extend the functions of CaPO4 deposits to enhance new bone formation on orthopedic implants. However, there are still many unresolved issues regarding the methodology of antibiotic incorporation and the optimal release kinetics. In the case of porous implants, deposited CaPO4 enhance bone ingrowth into pores [945]. Furthermore, studies concluded that there

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Fig. 33. Comparison of bone-integrative properties of non-coated (left) and biomimetically coated by CaPO4 metal implants (right) after implantation in the femur of goats for 6 weeks. Reprinted from Ref. [147] with permission.

was significantly less pin loosening in CaPO4-coated groups [946]. Thus, the majority of the clinical studies are optimistic on the in vivo performance of CaPO4-deposited implants. However, to be objective, one must mention the studies, in which no positive effects have been detected [947,948]. Besides, the presence or absence of the positive effects might also depend on the deposition technique [932,949,950], as well as on the coating vendor [845]. Furthermore, an application or nonapplication of post-deposition treatments also influences the biological response of CaPO4 deposits [951]. These uncertainties might be due to

several reasons, such as variability in chemical and phase composition, porosity, and admixtures, as well as due to various surgeon and patient factors that often confound clinical trials. In biomedical applications, bone grafts are usually much thicker than CaPO4 deposits applied to them. Nevertheless, the coated implants combine the surface biocompatibility and bioactivity of CaPO4 with the core strength of strong substrates (Fig. 34) [952]. The clinical results for the coated implants reveal that they have much longer life times after implantation than uncoated devices and, therefore, they are particularly

Fig. 34. Shows how a plasma sprayed HA coating on a porous titanium (dark bars) dependent on the implantation time will improve the interfacial bond strength compared to uncoated porous titanium (light bars). Reprinted from Ref. [952] with permission.


beneficial for younger patients [953]. Their biomedical properties are approaching those of bioactive glass-coated implants [954,955]. Since, among the available types of CaPO4, HA appears to be the most popular material to be deposited, the vast majority of the clinical investigations was performed with HA. For example, HA coating as a system of fixation of hip implants in vivo was found to work well in the short to medium terms (2 years [956], 5 years [957], 6 years [958], 8 years [959, 960], 9 to 12 years [961], 10 years [962,963], 10 to 13 years [964], 10 to 15.5 years [965], 10 to 17 years [966], 13 to 15 years [967], 15 years [968], 15 to 21 years [969], 16 years [970], 17 years [971], 18 years [972] and 19 years [973]). In 2004, a special book summarizing the studies on HA-coated implants and the “state of the art” of HA coatings in orthopedics at the close of 2002 was published [974]. Similar data for HA-coated dental implants are also available [975–977]. Nevertheless, even longer-term clinical results are still awaited with a great interest. Additional details on the subject might be found in other reviews [30, 146,270,301]. To finalize this section, one must stress that although the majority of the experiments on the in vivo studies of CaPO4 deposits have indicated a stronger and faster fixation, more bone ingrowth at the interface, etc., one should always keep in mind the negative results as well, the reasons of which must be carefully investigated and understood. Therefore, the clinical performance of CaPO4 deposits is still far from perfection. Some of the major concerns can be listed as follows [824–826]: • The degradation and resorption of CaPO4 deposits in a biological environment, could lead to their disintegration, resulting in the loss of both coating–substrate bond strength and the implant fixation. • Coating delamination and disintegration with the formation of particulate debris. • CaPO4 deposited on polymers may also lead to increased polymeric wear from the acetabular cup and thereby alleviate the problem of osteolysis.

In addition, still an insufficient amount of the in vivo studies is available in the literature. The limitations to such experiments may be attributed to any of the following reasons: • Difficulty in selection of a suitable animal model to simulate the actual mechanical loading and unloading conditions the implant might undergo in a human body environment. • The need to sacrifice a large number of animals, since most of these experiments demands a statistical analysis to validate the results. • A high cost and a long period of clinical testing these experiments demand. • Lack of coordination among material scientists and biologists and thereby an insufficient understanding of this interdisciplinary subject. • Serious ethical concerns on the use of animals for experimental studies, as they are subjected to painful procedures or toxic exposures during the course of test.

9. Future directions A potential drawback of the majority of the deposition techniques is their relatively high cost for a large-scale production. Therefore, to decrease processing time and make their manufacturing commercially viable, it is desirable to process the thinnest CaPO4 deposits able to improve the biological response substantially [978]. Questions related to the necessity and efficacy of CaPO4 deposits in different anatomic sites, their robustness to withstand physiological loads without fragmentation and problems related to third body wear limit their more widespread use. Further research to answer these questions will improve the mechanical and biologic aspects of CaPO4 deposits and optimize their safety and efficacy. More attention should be paid to functionally graded deposits with an amorphous top layer and a crystalline

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layer underneath. This allows adjusting the resorption rates to the values at which new bone grows at early stages, when it is of the most importance for the bone mineralization process [979]. Furthermore, therapeutic capabilities of the CaPO4 deposits as templates for the in situ delivery of drugs and osteoinductive agents (peptides, hormones and growth factors) at the required times should be elucidated much better. These biomedical properties may be augmented further by adding growth factors and other molecules to produce a truly osteoinductive platform [980]. Future directions are aimed at creating therapeutic deposits possessing a dual beneficial effect: osteoconductive properties combined with the ability to deliver therapeutic agents, proteins and growth factors directly into the deposits. These new types of CaPO4 deposits may offer the ability to stimulate bone growth, combat infection, and, ultimately, increase implant lifetime. In addition, such types of deposits might be useful for a non-viral transfection of stem cells [981]. Furthermore, combinations of CaPO4 deposits with cells seem to be the most promising direction [982].

10. Conclusions Solid implants prepared from various materials often possess a poor biocompatibility with a simultaneous lack of the osteogenic properties to promote bone healing. In addition, direct bone-to-implant contacts are desired for a biomechanical anchoring of the implants rather than fibrous tissue encapsulation. Therefore, the aim is to provide the implants with a friendly surface for adsorption of proteins, adhesion of cells and bone apposition. All these problems might be solved by applying CaPO4 deposits simply due to their chemical similarity to the calcified tissues of mammals. Therefore, the available knowledge on CaPO4 deposits (coatings, films and layers) on various substrates has been summarized in this review. As seen from above, there are about 50 different approaches to perform deposition (and new methods are continuously introduced) but each technique requires both the equipment and conditions of its own. In addition, each deposition technique has both advantages and shortcomings of its own (Table 3). Since none of them is able to provide the perfect covering, any type of the CaPO4 deposits always have some imperfections, such as cracks, pores, second phases, as well as residual stresses and/or poor adhesion. All these imperfections reduce durability of the CaPO4 deposits, which might lead to a partial or complete disintegration in body fluids. Thus, still there are no standard guidelines for putting down CaPO4 deposits onto the implant surfaces. In addition, the solubility requirements for CaPO4 deposits remain to be controversial. Namely, a partial dissolution of the deposited CaPO4 improves implant osseointegration and is the basic requirement for bioactivity. However, this dissolution diminishes the stability and increases the potential for loosening of the implants. CaPO4 deposits of lower solubility and higher stability are desirable for the long-term performance of implants because they promote faster initial bone fixation, bridge larger gaps in the misfit and degrade at a controlled rate. Although numerous animal and in vitro studies reported on the substantial benefits of using CaPO4-coated implants, the majority of them did not consider the chemical and structural characterization of the deposits. Under these conditions, making meaningful comparisons among various reports and studies are difficult. Therefore, future investigations will have to include clinical trials to get better understanding of bone responses to coated-implant surfaces, as well as extensive studies on coupling of CaPO4 deposits with drugs, growth factors and cells. Although it has been generally accepted that deposited CaPO4 improve bone strength and initial osteointegration rate, the optimal properties required to achieve maximal bone response are yet to be reported. As such, for the well-characterized CaPO4 deposits cell culture experiments, animal studies and clinical investigations should be well documented to avoid controversial results.

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