Preparation of Porous Spherical Calcium

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ratios of a-TCP and TTCP being 1.50 and 2.00, ... form in the range of 1.50 ( =a-TCP) to 1.67. ( =HAp) ..... with diameters of 1 to 10 ,11m was prepared by double.

Paper

journal of the Society of Inorganic Materials, japan 21, 22-30 (2014)

Preparation of Porous Spherical Calcium Orthophosphate Agglomerates by Double Nozzle Spray Pyrolysis and Its Application to Paste-Like Artificial Bone Kiyoshi IT ATANI, Tomohiro UMEDA, Kimiya TANAKA, Ian ]. DAVIES* and Yoshiro MUSHA ** (Department of Materials and Life Sciences, Faculty of Science and Engineering, Sophia University, 1, 7-chome, Kioi-cho, Chiyoda-ku, Tokyo 102-8554; *Department of Mechanical Engineering, Curtin University, GPO Box U1987, Perth, WA 6845, Australia; **2nd Department of Orthopaedic Surgery, School of Medicine, Toho University, 17-6, 2-chome, Oohashi, Meguro-ku, Tokyo 153-8515)

Calcium phosphate composite powder containing porous spherical adricalcium orthophosphate (cx-Ca 3 (P0 4) 2 ; a-TCP) and tetracalcium orthophosphate (Ca4(P04) 2 0; TTCP) agglomerates with diameters of 1 to 10 .urn was prepared by double-nozzle spray pyrolysis; the droplets containing (i) 0.30mol·dm- 3 Ca (N0 3 )z, 0.20mol·dm- 3 (NH 4) 2 HP0 4 and 0.10mass% colloidal silica for the preparation of a-TCP and (ii) 0.20mol·dm- 3 Ca (N0 3 ) 2 and 0.10mol·dm- 3 (NH 4) 2HP0 4 for TTCP, which had been formed using two air-liquid nozzles, were spray-pyrolyzed at 600°C. The spray-pyrolyzed powder was further heat-treated at 1250°C for 10 min in N2 atmosphere in order to prepare porous spherical a-TCP and TTCP agglomerates . According to X-ray diffractometry, the spray-pyrolyzed and heat-treated powder contained hydroxyapatite (Ca 10 (P0 4) 6 (0H)z; HAp) , together with a-TCP and TTCP. The formation of HAp could be avoided through the addition of glutaric acid to the spraying solution for TTCP; the carbon formed by the pyrolysis of glutaric acid during spray pyrolysis inhibited the solid state reaction to form HAp, and contributed to preparing the a-TCP/TTCP composite powder at 1250oC for 10 min. The setting of a-TCP / TTCP powder was conducted by mixing with malaxation liquid (12.5 mass% citric acid solution). The hardened body with a porosity as high as 68.2% could be obtained by immersing the set specimen in simulated body fluid at 37.0oC for 7 days. (Received Jun. 17, 2013) (Accepted Jul. 16, 2013) Key words:

Double-nozzle spray pyrolysis, a-Tricalcium orthophosphate, Tetracalcium orthophosphate, Porous spherical agglomerates, Paste-like artificial bone

(Ca 4 (P04) 20; TTCP) ) , have been utilized for bone and tooth implant materials 2 l. Calcium phosphates have been utilized for the medical treatment of many bone-associated diseases and injuries, examples of which include: (i) bone substitute materials and space filler for bioactive fixation (HAp) 2l·3) , (ii) bio-resorbable materials for the permeation of cells/tissues into the pores and subsequent bio-resorption ([3-TCP ceramics) 3l, and (iii) components of calcium phosphate paste for the repair of defects in living bones (a-TCP powder) 4l . The calcium phosphate cements or paste-like artificial bones, which are related to the present research subject, pave been developed as restorative materials of bone defect parts. The cement paste, which is

Introduction

In recent years the population of people aged 65 and over has increased significantly in developed countries with this number being anticipated to reach 35.86 million (38.8%) in Japan by 2050 1l . Due to such increases in aged population, the number of patients suffering from bone-associated diseases and / or injuries has increased year by year. Based upon the fact that the solid components in human bones are composed of hydroxyapatite (Ca 10 (P0 4) 6 (0Hh HAp) and organic compounds (chiefly collagen), various types of calcium phosphates, e.g., HAp, calcium orthophosphates (tricalcium phosphates (a- and [J-Ca 3 (P0 4 )z: cr and [3-TCP ) and tetracalcium phosphate 22

Kiyoshi ITATANI et a!.

journal of the Society of Inorganic Materials, japan

generally prepared by calcium phosphate powder and malaxation liquid, may be injected into irregularlyshaped bone defects. Currently available commercial paste injected into bone defects generally sets within 6 min and then further hardens due to its conversion into apatitic phases within the body5l . The advantage of this paste is not only the rapid setting/hardening but also the heat evolution being virtually negligible during the setting process, in contrast to the noted heat evolution of organic cement such as poly (methyl methacrylate) (PMMA) 5l . Nevertheless, several problems relating to inorganic paste-like artificial bones still remain, in particular the retardation of bioabsorbability and subsequent replacement by new bone following the conversion into apatitic phases within the body6l . One method to promote the bioabsorbability of such apatitic phases may be to increase the area in contact with the body fluid (i.e. , increase porosity) and reduce the particle size 7l . The present authors have investigated the preparation of porous calcium phosphate agglomerates by spray pyrolysis, i.e., the simultaneous spray-pyrolysis of solutions containing the desired types and amounts of metal ions into the "hot zone" of an electric furnace. Such agglomerates are porous and comprised of nano-sized particles and have been found to possess enhanced bioabsorbability when compared to the case of solid particles with a low surface area 8l . The authors have previously prepared various types of calcium phosphate powders by spray pyrolysis, such as TTCP (Ca/ P=2.00) 9 l , HAp (Ca/ P= 1.67) 10l , [3- and a-TCP (Ca / P=l.50 ) 6 l . u) ~ 13 l , calcium diphosphate (Ca2 P 2 0 7 ) (Ca/ P=l.00) 14 l , and calcium metaphosphate (Ca(P0 3 ) 2 ) (Ca/P=0.50) 14 l. Among these calcium phosphates, a-TCP /TTCP

2 2·1

Experimental procedure

Materials and sample preparation

The starting solution for the TCP was prepared by mixing 0.30 mol·dm - 3 Ca (N0 3 h 0.20 mol ·dm - 3 (NH 4 )zHP0 4 and 0.10 mass% colloidal silica, whereas that for the TTCP was prepared by mixing 0.20 mol·dm- 3 Ca (N0 3) 2 and 0.10 mol·dm - 3 (NH 4) 2 HP0 4 • The solutions were sprayed using one or two air-fluid nozzle (s), i.e., single and double nozzles, respectively, with the resulting droplets being simultaneously introduced into the hot zone of an electric furnace heated at 600°C. The spray-pyrolyzed powders were heat-treated at a temperature between 1200 and 1300oC for 10 min in N2 atmosphere. Some of the spray-pyrolyzed powders were frozen at - 80oC and then freeze-dried at - 50oC for 24 h, prior to the heat treatment. The resulting powder was mixed with malaxation liquid ( 12.5~50.0 mass% citric acid solution) at a powder to liquid (P / L) ratio between 1.5 and 4.0. The mixture was packed into a cylindrical plastic mold of 7 mm diameter and 14 mm length. After setting, the specimen was immersed into simulated body fluid (SBF) at 37 ± 0.2°C for 3 h to 14 d. Evaluation

Phase identification of the powders was conducted using an X-ray diffractometer (XRD; Model RINT2100V /P, Rigaku, Tokyo; 40 kV, 40 rnA) with monochromatic CuKa radiation. The phase changes during heating from room temperature to 1250oC were examined by thermogravimetry (TG; Model Thermo Plus TG8120, Rigaku, Tokyo). The agglomerate morphologies were observed using a field-emission scanning electron microscope (FE-SEM: Model S4500, Hitachi, Tokyo; accelerating voltage, 5 kV); the analysis of chemical composition was conducted using an energy-dispersive X-ray microanalysis (EDX) whose apparatus was attached to the FE-SEM.

A/B

~

Oo

3 3·1 Fig. 1

23

composite powder is a promising candidate for pastelike artificial bones and may have the possibility of being prepared, not only by conventional spray pyrolysis (i.e., single nozzle mode), but also by double nozzle spray pyrolysis that was previously examined for the preparation of HAp-Zr0 2 composite powder (see Fig. 1) 15) ,16) . On the basis of such information, the present paper describes not only the preparation conditions of a-TCP /TTCP powder by double nozzle spray pyrolysis but also the possibility of applying them to paste-like artificial bones.

2·2

(a)

21, (2014)

(b)

Schematic diagrams for the preparation of calcium phosphate agglomerates by (a) single nozzle spray pyrolysis and (b) double nozzle spray pyrolysis.

Results and discussion

Preparation of calcium phosphate composites by single nozzle spray pyrolysis

The authors firstly examined the possibility of preparing a-TCP / TTCP composites by single nozzle spray pyrolysis, due to the control of the Ca/ P ratios within the spraying solutions. Based upon the Ca/ P

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Preparation of Porous Spherical Calcium Orthophosphate Agglomerates by Double Nozzle Spray Pyrolysis and Its Application to Paste-like Artificial Bone

ratios of a-TCP and TTCP being 1.50 and 2.00, respectively, and with no TTCP being expected to form in the range of 1.50 ( =a-TCP) to 1.67 ( =HAp), the starting Ca/P ratios were determined to be fixed at 1.80 and 1.90. According to the XRD investigation, the spray-pyrolyzed powders contained poorly-crystalline HAp 17 l (XRD patterns have not been presented here). When the droplets are introduced into the hot zone, evaporation of the solvent occurs so as to form solid material on the surface of the droplets; thermal decomposition and subsequent crystal growth of solid material would then occur to form HAp as the solubility product of HAp is the lowest among the calcium phosphates 18 l. 10Ca2 + +6P0 43 - +20H Ca 10 (P04)6(0H)z

(1)

Since the Ca / P ratios of the spraying solutions were 1.80 and 1.90, respectively, the spray-pyrolyzed powders must include not only HAp (Ca / P= 1.67) (i.e., the crystalline compound detected by XRD) but also amorphous calcium phosphates with Ca/P ratios higher than that of the spraying solutions. Based upon our preliminary investigation regarding heat treatment conditions, the spray-pyrolyzed powders were heat-treated at 12oo·c for 10 or 30 min in order to check whether or not the objective compounds could be formed. Crystalline phases determined by X-ray diffractometry have been shown in Fig. 2, together

0

10

20

30

40

50

201 • CuKa.

Fig. 2

XRD patterns of powders obtained by the heat treatment of spray-pyrolyzed powders. Note that heat treatment of the spray pyrolyzed powders was conducted at 120o·c for 30 min (Ca/P ratios of calcium phosphate solutions: 1.80, 1.90 and 2.00) and 1200°C for 10 min (Ca/ P ratio of solution: 1.50) _ 0: a-TCP, e : HAp, 0: TTCP

with those of a-TCP 19 l (Ca / P ratio= 1.50) and TTCP 20 l (Ca/ P ratio=2.0 ) powders both obtained by single nozzle spray pyrolysis. The spray pyrolysis of solution with a Ca/P ratio of 1.90 and subsequent heat treatment at 12oo·c for 30 min resulted in the formation of TTCP. In contrast to this, spray pyrolysis of the solution with a Ca/ P ratio of 1.80 and subsequent heat treatment at 12oo·c for 30 min resulted in the formation of both TTCP and HAp. According to our previous research on the preparation of a-TCP by single nozzle spray pyrolysis, we found that the spray pyrolyzed powder contained [3TCP and poorly crystalline HAp6l _ Such poorly crystalline HAp may be regarded as the calcium-deficient HAp generally expressed as Ca 10 -x (HP04) x(P04) 6- x (OH) 2 - x (x~1) 2 1 l or typically Ca9 (HP0 4) (P0 4 ) 5 (OH) (x= 1) _ 9Ca2 + + 6P0 43 - + HzO Ca9 (HP0 4) (P0 4 )s(OH)

(2)

The thermal decomposition of calcium-deficient HAp leads to the formation of [3- TCP and subsequent phase transformation of [3- to a-TCP in the range of 1120 to 118o·czll. On the other hand, we have previously reported that TTCP may be formed by the solid state reaction of HAp with CaO, following single nozzle spray pyrolysis9 l. Ca 10 (P0 4) 6(OH) 2 + 2Ca0 3Ca4(P04) zO + HzO

(3)

No formation of a-TCP but the formation of TTCP/ HAp by single nozzle spray pyrolysis may be explained on the basis of Eq. (3) . The particle morphology of heat-treated powders was studied using a FE-SEM with typical micrographs being presented in Fig. 3. FE-SEM micrographs indicated the heat-treated powders to be composed of spherical agglomerates with diameters of 1 to 10 ,urn; the agglomerates were further composed of small primary particles, and the observation of fractured agglomerates showed these spherical agglomerates to be hollow. On the basis of previous work 6l, the hollow spherical agglomerates were considered to be formed during heat treatment via the routes of: (i) the formation of agglomerates with closely-packed primary particles due to the evaporation of solvent during spray pyrolysis and (ii) the formation of shells due to the rearrangement of primary particles and coalescence of pores. Furthermore, careful morphological observation of a-TCP agglomerates indicated the presence of numerous pores on the surfaces of spherical agglomerates. Since the phase transformation of [3- to a-TCP is known to occur in the range of 1120 to 1180.CZ 1l' volume changes due to the phase transformation 11 l may have the possibility of creating pores during the heat treatment. Such creation of

LBO

1.50

Fig . 3

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Journal of the Society of Inorganic Materials, Japan 21, (2014)

Kiyoshi ITATANI eta!.

1.go

.----------''

L

+

'

t

Ca/P ratio of

2 _00

spraying solution

FE-SEM micrographs of powders obtained by the heat treatment of spray-pyrolyzed powders. Note that heat treatment of the spray pyrolyzed powders was conducted at 1200'C for 30 min (Ca/ P ratios of calcium phosphate solutions: 1.80, 1.90 and 2.00) and 1200'C for 10 min (Ca/ P ratio of solution: 1.50) .

pores, together with the preservation of hollow spherical agglomerates, may be aided by the colloidal silica which was added to the solution for TCP, due to the restriction of sintering among TCP particles and agglomerates. 3·2

Preparation of calcium phosphate composites by double nozzle spray pyrolysis

As mentioned in Section 3.1, one of the objective compounds (ex-TCP) could not be prepared by single nozzle spray pyrolysis, with HAp still remaining in the resulting powder. As a result of this, the preparation conditions of porous spherical ex-TCP and TTCP agglomerates were examined by double nozzle spray pyrolysis, i.e., two nozzles available for the preparation of TCP and TTCP, respectively. A typical XRD pattern of the resulting powder has been shown in Fig. 4, together with a FE-SEM micrograph. According to the XRD analysis, the spray-pyrolyzed powder contained poorly-crystalline HAp 17l . On the other hand, the FE-SEM micrograph indicated the presence of spherical agglomerates with diameters of 1 to 10 ,urn. As the spray-pyrolyzed powder contained poorlycrystalline HAp, there appeared to be no difference in reaction product between the single and double nozzle spray pyrolyses. As mentioned previously, the formation of HAp from the droplets must be occurred at the initial stage of spray pyrolysis due to its lower solubility product when compared to TCP and TTCP 18l . The diameters of spherical agglomerates prepared by double nozzle spray pyrolysis were almost the same as those prepared by single nozzle spray pyrolysis. This fact indicates that most of the sprayed droplets for preparing TCP and TTCP separately pyrolyzed, prior to their coalescence to form agglomerates.

10

30

20 28 I

Fig . 4

o

40

50

CuKa

XRD pattern of the powder prepared by double nozzle spray pyrolysis at 600'C, i.e., 0.10 mol· dm - 3 calcium phosphate solution for preparing TTCP and 0.15 mol· dm - 3 calcium phosphate solution for preparing TCP, together with a typical FE-SEM micrograph. e:HAp

The present powder was further heat-treated at a temperature between 1200 and 1300oC for 10 min in order to check whether the objective products (i.e., exTCP and TTCP) could be formed . Typical XRD patterns have been presented in Fig . 5, as a function of heat treatment temperature. XRD pattern of the powder heat-treated at 1200oC for 10 min indicated the presence of HAp, TTCP and CaO. When the temperature increased to 1250°C, ex-TCP appeared, together with HAp and TTCP. Even at the temperature of

26

Preparation of Porous Spherical Calcium Orthophosphate Agglomerates by Double Nozzle Spray Pyrolysis and Its Application to Paste-like Artificial Bone Ca4 (P0.,) 20 Ca 10(PO.,),(OH) 2

p-Ca,(P0.,) 2

;;;;;,(!i;;c;s: "CID~il~ l k. (1)0.30 mol· dm - 3 Ca (N0 3) 2 , 0.20 mol· dm - 3 (NH 4) 2 HP0 4 to J:: U 0.10 mass% ::1 P 1 -7· JvV IJ 1J ( a-TCP IDiil~ffll.KliH~ ) t, (2) 0.20 mol· dm - 3 Ca 1)

(N03)z:tci J:: U 0.10 mol· dm- 3(NH4)zHP04(TTCP ID~il~ffll.K~~) O)~imill.KRiHit~ 2 M!O)=DfEi* / :A;v(-7.7 Jv / :AJv) ~~ J::--::> -c~~~~ l-c 600°C -c~:5.tM l tc.. l!lB~:5tM~1*1i ~ G ~~ 1250°C -c 10 min, N2 ~lm~-c~~~ l k... X *[email protected]:fJTf~ J:: ~ t, ~~~~{;$:~~~± a-TCP--? TTCP O)ft!! ~~. 7.KM7 ;\ -71 r (Ca 10 (P04)6(0Hh HAp ) O) :i_S