ISSN 1070-3632, Russian Journal of General Chemistry, 2011, Vol. 81, No. 9, pp. 1755–1760. © Pleiades Publishing, Ltd., 2011. Original Russian Text © N.I. Ponomareva, T.D. Poprygina, S.I. Karpov, Yu.V. Sokolov, 2011, published in Zhurnal Obshchei Khimii, 2011, Vol. 81, No. 9, pp. 1415–1420.
Composite Coatings on Titanium N. I. Ponomarevaa, T. D. Popryginaa, S. I. Karpovb, and Yu. V. Sokolovb a
Burdenko Voronezh State Medical Academy, ul. Studencheskaya 10, Voronezh, 394000 Russia phone: (4732)437688 e-mail:
[email protected] b
Voronezh State University, Voronezh, Russia Received July 20, 2010
Abstract—Composite coatings on the surface of metallic titanium containing calcium carbonate, calcium phosphate, chondroitin sulfate and/or gelatin were obtained in a new way. By the methods of XRD, IR spectroscopy, elemental analysis, and electron microscopy the presence in the samples of calcite and hydroxyapatite crystals as the main phase was demonstrated. Inorganic coatings were found to show better adhesion and resistance compared with the coatings containing the corresponding biopolymers.
DOI: 10.1134/S1070363211090027 An urgent problem of reparative medicine is the creation of titanium implants with a rough surface that provides good adhesion between the implanted material and tissue. The major efforts in this direction concern biologically active coatings based on hydroxyapatite, which is intended to serve as an additional source of calcium and phosphorus at the resorption, as well as a “matrix” for the germination of the newly formed tissue. Many methods of applying coatings have been offered, such us plasma spraying, laser ablation, ion-induced deposition, magnetron sputtering, etc. [1]. It should be noted that all these methods are labor- and energy-consuming, in most cases they require expensive equipment, and often lead to the formation of nitrides, oxides, and other additional phases on the metal surface. Besides, the obtained coatings have low adhesion to the metal surface and insufficient thickness, which makes the material unsuitable for application in the reconstructive surgery. A separate problem is applying the coating on the medical products of complicated shape, and changes in phase composition, crystallinity and the strength of the films at the sterilization of the implants [2]. In this paper we suggest a new method of producing composite coatings containing calcium carbonate, calcium phosphate, and sometimes chondroitin sulfate or gelatin. The samples were tested in distilled water and saline. We found that the inorganic coatings have better adhesion and resistance compared with coatings containing chondroitin sulfate and gelatin.
In the developed method, a good adhesion of coating to the metal surface is achieved by processing of titanium with carbon dioxide gas formed at the decomposition of calcium hydrogen carbonate in aqueous solution. No publications adequately describing the process occurring therewith has been found. We assume that carbonic acid can partially dissolve the oxide film, containing mainly TiO2, promoting the formation of active sites on the titanium surface. At high concentrations of starting materials (over 0.5 M), the reaction occurs spontaneously even at low temperature (5ºC) in accordance with the equation: Ca(NO3)2 + 2NaHCO3 → CaCO3 + CO2↑ + H2O + 2NaNO3.
(1)
Processing titanium with the liberated carbon dioxide leads to a better adsorption of calcium carbonate. The surface of the metal becomes of a dull whitish hue. The coating thickness increases at the longer keeping the plates in the reaction mixture. We found that additional treatment of titanium surface (cleaning with sandpaper, boiling in alkali solutions, etching with acids or iodine) does not lead to better results. According to XRD phase analysis, the calcium carbonate formed on the titanium surface crystallizes as calcite. The diffraction pattern allows the identification of the titanium interplanar distances (d = 2.556, 2.341, 2.241, 1.728, 1.477, 1.336 Å) and those of the formed calcite (d = 3.035, 2.495, 2.285,
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2.095, 1.913, 1.875 Å) [3]. Under the used experimental conditions, titanium does not form additional phases. According to elemental analysis, the coatings contain up to 32 wt % of Ca, 12 wt % of C, and 57 wt % of O. Keeping the samples in distilled water and saline for 14 days did not lead to significant changes in the phase and elemental composition, as well as in the coating strength and uniformity. At heating at the temperature above 400°C partial peeling of coating occurs, but on the titanium a thin film remains containing calcium carbonate. Since the solubility product of calcium carbonate Ks(CaCO3) = 3.8×10–9 is much higher than the values for phosphate and hydroxophosphate {Ks[Ca3(PO4)2] = 2.0×10–29, Ks[Ca10(PO4)6(OH)2] = 2.6×10–116}, it is also possible to obtain calcium phosphate coatings on titanium by heterogeneous substitution along reaction (2) and (3): 3CaCO3 + 2PO43– → Ca3(PO4)2 + 3CO32–, 10CaCO3 + 6 PO43– + 2OH– → Ca10(PO4)6(OH)2 + 10CO32–.
(2) (3)
It was found experimentally that calcium phosphate is formed faster than hydroxyapatite, which is related to the peculiarities of the restructuring of the crystal lattice, and to slowing of reaction in viscous solutions: at adding ammonia or alkali the reaction mixture for the aqueous synthesis of hydroxyapatite gets a gel-like structure, which hinders the diffusion of ions to the titanium surface. Besides, at the immersion of samples in the hydroxyapatite suspension the deposition of new layers does not occur: The hydroxyapatite particles in the solution absorb rapidly the calcium ions from the titanium surface to complete construction of its own calcium-deficient lattice. The surface of the metal takes its original luster in a few minutes. This method can be used rather for cleaning metal surface to remove the sediment, but not for coating. Therefore, to obtain just the hydroxyapatite coating on titanium we used the method of sequential immersion in solutions of ammonium hydrogen phosphate and calcium nitrate (see Experimental). Successive immersion of plates in solutions of calcium nitrate and ammonium hydrogen phospate should also lead to “healing” the defects of the crystal lattice. The analysis showed that the product was a carbonate-hydroxyapatite with varied composition Ca10(PO4)6(CO3)x(OH)2–2x, where x < 1. In
addition, after 7 days the coverage still contains the hydroxyapatite precursor phases, which in time should transform into the thermodynamically more stable hydroxyapatite. IR spectra of the coatings do not differ from the published spectra of the carbonate-hydroxyapatite [4–6]: They include pronounced bands of stretching and bending vibrations of phosphate groups, ν(PO43–) 1047, 604, 568 cm–1 and the carbonate group ν(CO32–) (1420, 1387 cm–1). Vibration bands of free hydroxy group is observed at ν(OH) = 3620 cm–1, of the group associated through intermolecular hydrogen bonds, at 3426 cm–1 [7, 8]. On micrographs of the hydroxyapatite coatings the spherical aggregates of particles are seen with the size about 1 μm. The typical diffraction pattern allowed us to determine the typical interplanar spacings in the hydroxyapatite (d = 3.44, 2.82, 2.78, 2.72, 2.63, 2.26, 1.94, 1.84 Å). The diffractogram does not contain the peaks of titanium, rutile, or other possible phases. The coating is not peeling at heating to 400°C and at keeping in distilled water or saline for 14 days. Composite coatings containing biopolymers were obtained by processing the samples coated with calcium carbonate in the solutions of chondroitin sulfhydroxyapatitete and/or gelatin, with further sequential exposure to the hydroxyapatitete and calcium solutions (see Experimental). After 7 days the coating contains calcite and forming hydroxyapatite and after 14 days, mainly the hydroxyapatite. We found that the composite coatings on titanium with thickness more than 1 μ, obtained in several stages, are easily peeled, which indicates that adhesion of the hydroxyapatite to the biopolymers is better than to the titanium surface. As the crystals grow, the surface becomes unstable and nonuniform by thickness and composition. According to elemental analysis, the coatings based on hydroxyapatite with gelatin contain 27–33 wt % of Ca, 13–16 wt % of F, 7–10 wt % of C, and 47–51 wt % of O, less than 1 wt % of impurity metals (Cu, Fe). The mean molar (atomic) ratio of Ca:P is 1.65, that is, the phosphate formed in the composites is calciumdeficient carbonate-substituted hydroxyapatite. According to XRD, the samples do not contain additional phases (Fig. 1). Since the film was easily scraped from the surface, it became possible to conduct IR spectroscopic determination.
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CaCO3
* – Ti
2θ, deg
Transmission
Fig. 1. Diffractogram of the composite hydroxyapatite–gelatin on titanium.
ν, cm–1 Fig. 2. IR spectra: (1) hydroxyapatite, (2) gelatin, and (3) hydroxyapatite composite with gelatine.
The IR spectra of the coatings (Fig. 2) revealed the bands of the stretching and bending vibrations of phospate groups, ν(PO43–) 1047, 604, 568 cm–1, and carbonate group, ν(CO32–) (1420, 1387 cm–1). Vibration bands of the free hydroxy group are observed at ν(OH)
= 3620 cm–1, of those associated by intermolecular hyd-rogen bonds, at 3426 cm–1 [7, 8]. Figure 3 shows the micrograph of the surface of the obtained composite coatings. The coating peels off
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A typical diffraction pattern (Fig. 4) includes individual peaks of hydroxyapatite (d = 3.43, 2.80, 2.61, 1.86, 1.73 Å), calcite (d = 3.028 Å), and titanium (d = 2.56, 2.34, 2.24, 1.73, 1.47 Å). Figure 5 shows the surface micrograph of the obtained composite coating: needle crystals fuse together to form aggregates resembling snowflakes. IR spectra of the composites are shown in Fig. 6. These composite coatings also are unstable at storage in a dry place and peel within 1–2 weeks. Fig. 3. Photomicrograph of composite coatings containing hydroxyapatite with gelatine. Tag 10 μm.
easily during the week after drying, but after keeping in distilled water or saline it remains unchanged for indefinitely long time. The coatings based on hydroxyapatite with chondroitin sulfate and gelatin contain 19–25 wt % of Ca, 12–13 wt % of P, 10–14 wt % of C, and 48–52 wt % of O, less than 1 wt % of S and admixed metals (Cu, Fe). The mean molar (atomic) Ca:P ratio is 1.66, indicating the formation of calcium deficient carbonate-substituted hydroxyapatite.
Thus, inorganic coatings exhibit better adhesion and resistance compared with coatings containing the corresponding biopolymers. While producing the composite coatings based on hydroxyapatite we found also that gradual addition of ammonium hydrogen phosphate leads to better results than immersion in a solution of this salt at pH = 10. A similar result was previously observed in the synthesis of hydroxyapatite and composites based on it in aqueous solution [9, 10]. EXPERIMENTAL We used plates made of titanium (GOST 16071– 72) with a surface area 1 and 3 cm2, as well as dental
I/I0 ○ – Hydroxyapatite ▼–
CaCO3
* – Ti
2θ, deg Fig. 4. Diffractogram of the composite hydroxyapatite–chondroitin sulfate–gelatin on titanium. RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 81 No. 9 2011
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Fig. 5. Photomicrograph of the composite coatings containing hydroxyapatite with chondroitin sulfate and gelatin. Tag 10 μm.
Composite coatings containing biopolymers were applied in several stages. The above method was used to obtain calcium carbonate coating on titanium, then the plate with the calcium carbonate was washed with distilled water, dried, wiped with filter paper. Some samples were immersed in 0.5% gelatine solution, prepared with 0.1 m solution of calcium nitrate, others in the same solution containing chondroitin sulfate, the remaining plates were used to compare the results. All plates were pre-dried in air, then placed in distilled water and then 0.06 M solution of ammonium hydrogen phosphate brought to pH = 10 with ammonia. After 2 days, the films were washed and left
Transmission
titanium pins I-POST TITANIUM from Itena Clinical Products, France (Titanium 5 in line with the ISO 5832–3 and ASTM F136 biocompatibility standards for the use as surgical implants for the fixation of hip and vertebral bone tissue) and titanium foil. The plates were degreased with ethanol. Since preliminary tests showed that polishing, drying, cleaning with sandpaper do not lead to better results, the metal surface was not processed additionally. The coating was performed at temperatures of 12, 20, and 100°C. A solution of sodium hydrogen carbonate (analytical grade) was poured to a solution of calcium nitrate or chloride (reagent grade) in the stoichiometric ratio 2:1. Titanium plates or pins were placed into the reaction mixture at the beginning of the carbon dioxide liberation. To avoid the concentration flows during the formation of crystals, the mixture was stirred occasionally, therewith the carbon dioxide bubbles began to release more intensively. Some samples were washed and dried for 15 min, others were left in the solution till the reaction completion. The washed films were left for 2 days in contact with a solution of 0.6 M (NH4)2HPO4, followed by 2 days in a solution of 1 M Ca(NO3)2, and then again in a solution of 0.6 M (NH4)2HPO4 for 2 days. The samples then were washed with distilled water and dried in air at a temperature of 20°C. Some plates were left in distilled water or saline (0.9 % NaCl) for 14 days.
ν, cm–1 Fig. 6. IR spectra: (1) composite hydroxyapatite with chondroitin sulfate, (2) hydroxyapatite, and (3) chondroitin sulfate. RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 81 No. 9 2011
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in a solution of calcium nitrate, and then after 2 days, again they were left in water, and ammonium hydrogen phosphate was added dropwise to the solution. The coverings thus obtained were washed with distilled water and dried in air at a temperature of 20°C. Some plates were left in distilled water or saline for 14 days. Electron microscopy was carried out on an instrument JSM-6380 LV (Japan), elemental analysis was performed using the device Energy-250. IR spectra in the range 4000–400 cm–1 were recorded on Infralyum FT-02 and Specord-IR-75 spectrometers from the samples pressed into pellets with KBr. The diffraction patterns were obtained on DRON-3M and DRON-7 instruments, radiation CuKα, scanning rate 2° min–1. REFERENCES 1. Barinov, S.M. and Komlev, V.S., Biokeramika na osnove fosfatov kal’tsiya (Bioceramics Based on Calcuium Phosphates), Moscow: Nauka, 2005.
2. Hans, E., Sheel, J., and Fukuda, T., Crystal Growth Technology, New York: Wiley, 2003. 3. Mirkin, L.I., Spravochnik po rentgenostrukturnomu analizu polikristallov (Handbook on XRD Analysis of Polycrystals), Moscow: Fizmatgiz, 1961. 4. Rodicheva, G.V., Orlovskii, V.P., Privalov, V.I., and Barinov, S.M., Zh. Neorg. Khim., 2001, vol. 46, no. 11, p. 1798. 5. Ezhova, Zh.A., Orlovskii, V.P., and Koval’, E.M., Zh. Neorg. Khim., 2002, vol. 47, no. 8, p. 1246. 6. Zakhydroxyapatiterov, N.A., Orlovskii, V.P., Klyuev, V.A., and Toporov, Yu.P., Zh. Fiz. Khim., 2001, vol. 75, no. 5, p. 948. 7. Nakanisi, K., Infrared Spectra and Structure of Organic Compounds, Moscow: Mir, 1965. 8. Bellami, L., Infrared Spectra of Complex Molecules, Moscow: Inostrannaya Literatura, 1957. 9. Ponomareva, N.I., Poprygina, T.D., Lesovoi, M.V., and Karpov, S.I., Zh. Obshch. Khim., 2008, vol. 78, no. 4, p. 538. 10. Ponomareva, N.I., Poprygina, T.D., Karpov, S.I., Lesovoi, M.V., and Agapov, B.L., Kondensirovannye Sredy i Mezhfaznye Granitsy, 2009, vol. 11, no. 3, p. 239.
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