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Preparation and Characterization of Macroporous Carbonate-Substituted Hydroxyapatite Scaffold Yoong Lee,*,† Yeong Min Hahm,‡ Dong Hyun Lee,† Shigeki Matsuya,§ Masaharu Nakagawa,| and Kunio Ishikawa| Department of Chemical Engineering, Sungkyunkwan UniVersity, Suwon-city, Gyeonggi-do, 440-746, Korea, Department of Chemical Engineering, College of Engineering, Dankook UniVersity, Yongin-city, Gyeonggi-do, 448-701, Korea, Section of Bioengineering, Department of Dental Engineering, Fukuoka Dental College, 2-15-1 Tamura, Sawara-ku, Fukuoka 814-0193, Japan, and Department of Biomaterials, Faculty of Dental Science, Kyushu UniVersity, 3-1-1 Maidashi, Higashi-ku, Fukuoka 812-8582, Japan
Human bone mineral is different in composition from stoichiometric hydroxyapatite (Ca10(PO4)6(OH)2) in that it contains additional ions, of which CO32- is the most abundant species. Two macroporous carbonatesubstituted hydroxyapatite (CHA) scaffolds were prepared by treating a macroporous CaCO3 column (porosity, 88%; pore and hole size, 160 and 71 µm), prepared in this study, hydrothermally at 120 °C in 1 M phosphate solutions of (NH4)2HPO4 and K2HPO4. Especially, it was found that the CaCO3 was transformed into CHA after hydrothermal treatment (HT) for 24 h irrespective of the type of phosphate solution. Under HT for 24 h, the crystallite size and crystal shape of CHA prepared in K2HPO4 solution (KCHA) and those of CHA prepared in (NH4)2HPO4 (NH4CHA) were quite different from each other. Moreover, NH4CHA contained 2.09 wt % A-type CO32- out of 6.05 wt % CO32-, but KCHA contained a small amount of A-type CO32(0.08 wt % out of 8.24 wt % CO32-). The chemical formulas were Ca9.23(NH4)0.12(PO4)5.36(CO3)0.64(OH)0.54(CO3)0.34 for NH4CHA and Ca8.18K0.5(PO4)4.72(CO3)1.28(OH)0.1(CO3)0.02 for KCHA. Therefore, the properties, such as carbonate type and content, and crystal shape, of CHA are significantly affected by the type of phosphate solution. 1. Introduction The mineral phases of bone, dentine, and enamel are more or less nonstoichiometric forms of hydroxyapatite (HA, Ca10(PO4)6(OH)2) containing a variety of impurity ions such as carbonate, sodium, and potassium.1-5 One of the most abundant ions is carbonate (up to 8 wt %).6-8 Synthetic carbonatesubstituted HA (CHA) has been proposed to improve the biological activity of HA because the incorporation of carbonate into HA causes an increase in solubility, a decrease in crystallinity, a change in crystal morphology, and an enhancement of chemical reactivity owing to the weak bonding.2,9,10 CHA is actually more soluble in vivo than HA and increases the local concentration of calcium and phosphate ions that are necessary for new bone formation.11-13 For these reasons, recent research has focused on CHA in comparison with HA. Carbonate ion in CHA can be present at two different sites in the apatite lattice.14-18 One is in the hydroxyl ion site (channel site), called A-type CHA (A-CHA), and the other is in the phosphate site, called B-type CHA (B-CHA). These two different carbonate types can be characterized by their infrared spectra: A-type carbonate has a doublet band at about 1545 and 1450 cm-1 (asymmetric stretching vibration) and a singlet band at 880 cm-1 (out-of-plane bending vibration), and B-type has these bands at about 1455 and 1410, and 875 cm-1. A-CHA and B-CHA are abundant in bone of the old and the young, respectively.19 A-CHA is generally prepared using a solid-phase method at high pressure or at high temperature (800-1000 °C) under dry carbon dioxide atmosphere as a source of carbonate.10,19,20 In * To whom correspondence should be addressed. † Sungkyunkwan University. ‡ Dankook University. § Fukuoka Dental College. | Kyushu University.
the case of B-CHA, it is synthesized using a liquid-phase method, such as precipitation and hydrothermal treatment, employing various starting materials. It has been reported that human trabecular osteoclastic cells have low affinity toward A-CHA surface compared to HA,10,21 whereas B-type substitution can enhance the solubility without changing the surface energy of CHA that affects the initial cell attachment and the collagenous matrix deposition.22,23 Even though much research has been carried out concerning the preparation of CHA, it was mostly related to CHA in the form of powder.7,9,16,19,20 We reported that calcium hydroxide compact was carbonated in a wet CO2 atmosphere to form calcite (CaCO3) monolith in our previous study.24 This monolith could be then transformed into carbonate apatite by treatment in several phosphate solutions at 60 °C. In the present study, in order to prepare macroporous CHA scaffold, macroporous CaCO3 prepared by the carbonation of calcium hydroxide (Ca(OH)2)/sodium chloride (NaCl) columnar composite at ambient temperature was treated hydrothermally in two different phosphate solutions. Both CHA scaffolds were characterized and compared with each other in the aspect of chemical and physical properties. 2. Materials and Methods 2.1. Preparation of Macroporous CaCO3 Column. Calcium hydroxide (Ca(OH)2; Wako Chemicals, Japan) and sodium chloride (NaCl; Wako Chemicals, Japan) were used for the preparation of a macroporous CaCO3 column. NaCl was pulverized mechanically and sieved to obtain 106-300 µm particulates before use. A mixture of Ca(OH)2 and 85.7 wt % NaCl was arranged manually and subjected to pressure uniaxially in a stainless steel die using an oil pressure press machine (Riken Power, Riken Seiki, Japan) at 10 MPa molding pressure to prepare the Ca(OH)2/NaCl composite (10 mm in diameter
10.1021/ie071474a CCC: $40.75 © 2008 American Chemical Society Published on Web 03/21/2008
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and 9 mm in height). The composite was carbonated in a carbon dioxide reaction vessel for 90 h at room temperature. The reaction vessel, approximately 5 L, was saturated with water vapor, and carbon dioxide gas was supplied at a rate of 0.150.2 L/min. Subsequently, it was washed with distilled water at 90 °C over 3 days to drive off soluble components including NaCl and dried at 60 °C for 24 h. 2.2. Preparation of CHA Scaffold. The macroporous CaCO3 column was treated hydrothermally at 120 °C in 1 M diammonium hydrogenphosphate ((NH4)2HPO4; Wako Chemicals, Japan) and 1 M dipotassium hydrogenphosphate (K2HPO4; Wako Chemicals, Japan) solutions, respectively, as phosphate sources. The overall Ca/P molar ratio was 10/18 in the reaction system, meaning that P was 3 times in excess with respect to a stoichiometric Ca/P ratio (1.67) of pure hydroxyapatite. Subsequently, each scaffold was immersed in distilled water for 6 h to eliminate soluble ions. 2.3. Characterization. 2.3.1. X-ray Diffraction (XRD) Analysis. Phase purity and crystallite size were determined by XRD (RINT 2500V, Rigaku, Japan) analysis. Their XRD patterns were taken at 40 kV and 100 mA in the range of 2θ from 10 to 60° in a continuous scanning mode at a rate of 2°/ min. 2.3.2. Scanning Electron Microscopy (SEM) Analysis. The morphology of the fracture surface and crystal shape were observed by SEM (JSM 5400LV, JEOL, Japan). The average NaCl-print size that can be attributed to the pore size and hole size formed in pores was analyzed by means of image analysis software (USB Digital Scale V1.0, Scalar Corporation). 2.3.3. Fourier Transform Infrared (FTIR) Analysis. Infrared spectra between 400 and 4000 cm-1 were recorded on an FTIR spectrophotometer (Spectrum 2000, Perkin-Elmer, USA) to examine structural changes during the hydrothermal treatment (HT), in particular, with respect to CO32- ions substituting for PO43- and/or OH- ions. 2.3.4. Mechanical Strength Analysis. Compressive strength was evaluated by use of a universal testing machine (SV-301, IMADA, Japan) at a constant crosshead speed of 1 mm/min. Five samples were tested for each evaluation. 2.3.5. Chemical Analysis. Carbon content and nitrogen content were determined using a CHN elemental analyzer (CHN coder-MT-6, Yanaco, Japan). Calcium and potassium contents were determined by an atomic absorption spectrometer (Analyst 300, Perkin-Elmer, Japan), and phosphorus content was determined by a spectrophotometer (Ubest-50, Jasco, Japan). 2.3.6. Porosity Measurement. The relative density was calculated by the ratio of the apparent density to a true density. True density was measured using a picnometer. The total porosity is defined as follows:
total porosity (%) ) 100 (%) - relative density (%) The total porosity was obtained as the average value of five samples. 3. Results and Discussion 3.1. Preparation and Characterization of CaCO3 Column. Figure 1 shows XRD patterns of Ca(OH)2/NaCl composite with and without washing after carbonation. The composite was converted into CaCO3/NaCl after carbonation for 90 h, but 19% of Ca(OH)2 still remained. Pure CaCO3 column was obtained by eliminating the unreacted Ca(OH)2 and NaCl through washing with distilled water.
Figure 1. Powder XRD patterns of the Ca(OH)2/85.7 wt % NaCl composite prepared at 10 MPa molding pressure after 90 h of carbonation.
Figure 2. Scanning electron microscopic observation of the CaCO3 scaffold obtained after 90 h of carbonation: (a) fracture surface; (b) crystal shape.
Fracture surface and crystal morphology of pure CaCO3 column are illustrated in Figure 2. NaCl prints, which can be ascribed to pore size, were seen on the surface in Figure 2a, and most of the pores were interconnected through the holes, judging from black parts in the pores. The average pore and hole sizes were 160 and 71 µm, respectively. The pore size is in the range 100-400 µm, which allows bone tissue ingrowth, making direct integration with bone to improve the mechanical fixation of the implant at the implantation site.25-27 Porosity, compressive strength, and crystallite size were 88%, 53 kPa, and 600 Å, respectively. The crystal shape, as shown in Figure 2b, was stick-like. 3.2. Comparison of Physical and Chemical Properties of CHA Scaffolds Prepared in Two Different Phosphate Solutions. Figure 3 shows XRD patterns of pure CaCO3 column after HT in each phosphate solution for different times. Hydroxyapatite (HA) phase was detected regardless of HT time and type of phosphate solution. In the case of CaCO3 column treated in (NH4)2HPO4 solution, CaCO3 was remarkably transformed into HA after HT for 6 h and the transformation was completed after HT for 24 h, as shown in Figure 3a. On the
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Figure 4. FTIR spectra of CHA scaffolds prepared in two phosphate solutions under HT for 24 and 48 h: (a) (NH4)2HPO4 solution; (b) K2HPO4 solution.
Figure 3. Powder XRD patterns of CHA scaffolds prepared in two phosphate solutions according to HT time: (a) (NH4)2HPO4 solution; (b) K2HPO4 solution.
other hand, a strong diffraction peak (indicated with an arrow) corresponding to CaCO3 still remained after HT for 6 h in K2HPO4 solution (Figure 3b). Although CaCO3 disappeared after HT for 24 h, a small amount of CaCO3 was still found in HA scaffold after HT for 48 h in K2HPO4 solution. It is thought to be one that was generated during HT as mentioned by Erna et al.28 HA scaffold obtained after HT for 12 h in (NH4)2HPO4 solution contained a small amount of Ca(OH)2, and also a relatively strong diffraction peak of Ca(OH)2 was found in Figure 3b when CaCO3 column was treated in K2HPO4 solution for 12 h of HT. This Ca(OH)2 can be attributed to one that generated during HT because pure CaCO3 column was used as the starting material. However, Ca(OH)2 present in both HA scaffolds almost disappeared after HT for 24 h. Crystallinity of HA scaffold prepared in (NH4)2HPO4 solution was lower than that of HA scaffold prepared in K2HPO4 solution, judging from the half-width of peaks. Under HT for 24 h, the crystallite size of HA scaffold prepared in (NH4)2HPO4 solution was 312 Å and that of HA scaffold prepared in K2HPO4 solution was 601 Å. Figure 4 shows FTIR spectra of HA scaffolds prepared after HT of CaCO3 column for 24 and 48 h in the phosphate solutions. Typical peaks of B-type CO32- and PO43- bands (980-1100, 960, and 560-600 cm-1) were observed in Figure 4. Therefore, HA scaffolds were CO32--substituted HA (CHA). In addition, two peaks at 1455 and 1410 cm-1 were asymmetric and the asymmetric extent in Figure 4a was relatively large compared
Figure 5. Scanning electron microscopic observation of CHA scaffolds prepared by treating CaCO3 scaffold hydrothermally for 24 h in two phosphate solutions: (a) fracture surface of A/B-NH4CHA; (b) crystal shape of A/B-NH4CHA; (c) fracture surface of mB-KCHA; (d) crystal shape of mB-KCHA.
to that in Figure 4b, which was not changed significantly by HT time. This asymmetry is presumably due to the overlap of B-type with A-type CO32- and seems to depend on the amount of A-type CO32- present in CHA scaffold. Landi et al.19 reported that a weight ratio of A- and B-type CO32- could be estimated from the intensity ratio of two peaks at 880 and 873 cm-1 in the IR spectrum. These two peaks were also employed in this study to calculate the weight ratio using CHA scaffolds prepared after HT for 24 h in both phosphate solutions. Since two peaks were overlapped, they were separated graphically using the computer software Peak Fit (SPSS Inc., Chicago, IL). This analysis pointed out an A-/B-type weight ratio of 0.53 for CHA scaffold prepared in (NH4)2HPO4 solution (A/B-NH4CHA) and 0.01 for CHA scaffold prepared in K2HPO4 solution (mB-
Ind. Eng. Chem. Res., Vol. 47, No. 8, 2008 2621 Table 1. Chemical Composition of CHA Scaffolds Prepared under 24 h Hydrothermal Treatment salt
HT time [h]
Ca [wt %]
P [wt %]
Ca/P molar ratio
NH4 [wt %]
(NH4)2HPO4 K2HPO4
24 24
38.22 34.74
17.16 15.5
1.72 1.73
0.22
KCHA). Therefore, the type of CO32- was mostly B-type in the case of mB-KCHA, causing two almost symmetric peaks at 1455 and 1410 cm-1. It has been reported that the increase in A-type CO32- content caused the decrease in crystallinity, which was attributed to the fact that the hydroxyl site, differing from the phosphate site, can accept more vacancies and the carbonate ion has consequently more degree of freedom.19 In this study, the crystallinity of A/B-NH4CHA, as shown in Figure 3, was also lower than that of mB-KCHA. Besides, adsorbed water bands were located at 3000-3700 and 1640 cm-1 and there were no peaks of hydrogenphosphate (HPO42-; 906, 852, and 530 cm-1) irrespective of types of phosphate solutions and HT time.9,29,30 The following analyses were carried out to characterize and compare the chemical and physical properties of A/B-NH4CHA with those of mB-KCHA. The results of the chemical analysis of both CHA scaffolds are summarized in Table 1. CO32- and K+ ion contents in mBKCHA, as shown in Table 1, were larger than CO32- and NH4+ ion content in A/B-NH4CHA. The main reason for this result can be attributed to the difference in ionic radius between K+ (1.38 Å) and NH4+ (1.43 Å), meaning that K+ ions can be more easily replaced with Ca2+ (1.00 Å) ions in order to compensate for extra negative charge caused by the substitution of CO32for PO43- ions. A-type CO32- content on the basis of FTIR analysis was 0.08 wt % out of 8.24 wt % for mB-KCHA and 2.09 wt % out of 6.05 wt % for A/B-NH4CHA. From the above results, it is found that the crystallinity of CHA scaffold seems to be more affected by the type of CO32- than by the total content of CO32-. Chemical formulas were expressed as eq 1 for A/B-NH4CHA and eq 2 for mB-KCHA based on Table 1 and FTIR analysis.
Ca9.23(NH4)0.12(PO4)5.36(CO3)0.64(OH)0.54(CO3)0.34 (1) Ca8.18K0.5(PO4)4.72(CO3)1.28(OH)0.1(CO3)0.02
(2)
These formulas were obtained by setting the total number of the subscripts of PO43- and CO32- ions to 6 and the weight percent OH was calculated from eq 3 on the basis of the electroneutrality condition.4
wt % Ca wt % K [or NH4] wt % OH )2 + MOH MCa MK [or NH4] 3
wt % CO3 wt % PO4 -2 (3) MPO4 MCO3
where MX in eq 3 is the atomic or ionic mass of X. Six types of basic substitution mechanisms have been proposed in the literature.4,15 Among those, Ca2+ + PO43- + OH- S VCa + CO32- + VOH [mechanism I] and Ca2+ + PO43- S M+ + CO32- [mechanism III] for B-type, and 2OH- S VOH + CO32[mechanism V] for A-type carbonate substitutions were reported as main mechanisms.4,19 M+, in the present study, stands for potassium ion or ammonium ion, and Vi represents a vacancy on a regular apatite lattice site occupied by “i” (Ca2+ or OH-). If CO32- is substituted for PO43- or OH- according to mechanism I or mechanism V, charge compensation should be necessary for keeping electroneutrality due to the charge
K [wt %]
CO3 [wt %]
OH [wt %]
2.06
6.05 8.24
0.95 0.18
difference between CO32- and PO43- or OH-, thus resulting in a vacancy in each site. The chemical formula based on these mechanisms can be described as follows for A/B-CHA.
Ca10-x-yMy(PO4)6-x-y(CO3)x+y(OH)2-x-2z(CO3)z
(4)
, where x, y, and z represent the contribution of mechanisms I, II and V, respectively, and M stands for alkali or ammonium ion. In the present study, these three mechanisms are responsible in the case of A/B-NH4CHA, whereas another mechanism including these three should be also taken into account in the case of mB-KCHA because the values of x (0.78), y (0.5), and z (0.02) calculated from the subscripts for K and CO3 in eq 2 gave the subscript of 8.72 for Ca and 1.18 for OH in eq 4, which are quite different from those in eq 2. Therefore, it can be inferred that several mechanisms are simultaneously exerting and that the preferential substitution mechanism depends on the experimental condition. Figure 5 shows the fracture surface and crystal morphology of both CHA scaffolds. Porosity, average pore size, and average hole size were 89%, 153 µm, and 67 µm, respectively, for A/BNH4CHA and 88%, 154 µm, and 69 µm, respectively, for mBKCHA, which are not quite different from those of CaCO3 as the starting material shown in Figure 2a. On the other hand, crystal shapes of both CHA scaffolds, shown in Figure 5b,d, were quite different from that of the CaCO3 shown in Figure 2b. Flake-like crystals were formed in the case of A/B-NH4CHA (Figure 5b), and globule-like crystals were formed in the case of mB-KCHA (Figure 5d), meaning that crystal shape largely depends on the type of phosphate solution. The compressive strength of A/B-NH4CHA was 123 kPa, which is over twice as hard as that (53 kPa) of the CaCO3 column, and that of mB-KCHA was 64 kPa. This remarkable difference in compressive strength of both CHA scaffolds is presumably related to the difference in crystallite size and crystal shape because other properties, such as porosity and average hole size, are similar to each other. 4. Conclusions Two CHA scaffolds were prepared by the hydrothermal treatment of pure CaCO3 column at 120 °C in two different phosphate solutions, 1 M (NH4)2HPO4 and 1 M K2HPO4. By characterizing the physical and chemical properties of the scaffolds, the following conclusions can be drawn. CaCO3 is transformed into CHA when it is treated hydrothermally for 24 h irrespective of the type of phosphate solution. The porosity and average pore size of mB-KCHA and A/BNH4CHA are not quite different from those of CaCO3, but the crystal shapes of both CHAs are different from that of CaCO3. In addition, the type and content of CO32- in CHA scaffold are dependent on the type of phosphate solution. It is desirable to use K2HPO4 solution to prepare B-CHA scaffold. That is, B-type CO32- (8.16 wt % out of 8.24 wt %) was dominant in mBKCHA holding globule-like crystals compared to that (3.96 wt % out 6.05 wt %) in A/B-NH4CHA holding flake-like crystals. Literature Cited (1) Rau, J. V.; Cesaro, S. N.; Ferro, D.; Barinov, S. M.; Fadeeva, I. V. FTIR study of carbonate loss from carbonated apatites in the wide temperature range. J. Biomed. Mater. Res., Part B: Appl. Biomater. 2004, 71B (2), 441-447.
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(2) Baig, A. A.; Fox, J. L.; Su, J.; Wang, Z.; Otsuka, M.; Higuchi, W. I.; Legeros, R. Z. Effect of carbonate content and crystallinity on the metastable equilibrium solubility behavior of carbonated apatites. J. Colloid Interface Sci. 1996, 179, 608-617. (3) Tang, R.; Henneman, Z. J.; Nancollas, G. H. Constant composition kinetics study of carbonated apatite dissolution. J. Cryst. Growth 2003, 249, 614-624. (4) Pieters, I. Y.; Maeyer, E. A. P. D.; Verbeeck, R. M. H. Stoichiometry of K+- and CO32--containing apatites prepared by the hydrolysis of octacalcium phosphate. Inorg. Chem. 1996, 35, 5791-5797. (5) Wenk, H. R.; Heidelbach, F. Crystal alignment of carbonated apatite in bone and calcified tendon: results from quantitative texture analysis. Bone 1999, 24 (4), 361-369. (6) Landi, E.; Tampieri, A.; Celotti, G.; Langenati, R.; Sandri, M.; Sprio, S. Nucleation of biomimetic apatite in synthetic scaffold fluids: dense and porous scaffold development. Biomaterials 2005, 26, 2835-2845. (7) Barralet, J.; Best, S.; Bonfield, W. Carbonate substitution in precipitated hydroxyapatite: an investigation into the effects of reaction temperature and bicarbonate ion concentration. J. Biomed. Mater. Res. 1998, 41 (1), 79-86. (8) Barralet, J.; Akao, M.; Aoki, H. Dissolution of dense carbonate apatite subcutaneously implanted in wistar rats. J. Biomed. Mater. Res. 2000, 49 (2), 176-182. (9) Suchanek, W. L.; Shuk, P.; Byrappa, K.; Riman, R. E.; Tenhuisen, K. S.; Janas, V. F. Mechanochemical-hydrothermal synthesis of carbonated apatite powders at room temperature. Biomaterials 2002, 23, 699-710. (10) Redey, S. A.; Nardin, M.; Assolant, D. B.; Rey, C.; Delannoy, P.; Sedel, L.; Marie, P. J. Behavior of human osteoblastic cells on stoichiometric hydroxyapatite and type A carbonate apatite: role of surface energy. J. Biomed. Mater. Res. 2000, 50 (3), 353-364. (11) Barralet, J. E.; Aldred, S.; Wright, A. J.; Coombes, A. G. A. In vitro behavior of albumin-loaded carbonate hydroxyapatite gel. J. Biomed. Mater. Res. 2002, 60, 360-367. (12) Matsumoto, T.; Okazaki, M.; Inoue, M.; Ode, S.; Chien, C. C.; Nakao, H.; Hamada, Y.; Takahashi, J. Biodegradation of carbonate apatite/ collagen composite membrane and its controlled release of carbonate apatite. J. Biomed Mater. Res. 2002, 60, 651-656. (13) Barralet, J. E.; Best, S. M.; Bonfield, W. Effect of sintering parameters on the density and microstructure of carbonate hydroxyapatite. J. Mater. Sci.: Mater. Med. 2000, 11, 719-724. (14) Tonsuaadu, K.; Peld, M.; Leskela, T.; Mannonen, R.; Niinisto, L.; Veiderma, M. A Thermoanalytical study of synthetic carbonate-containing apatites. Thermochim. Acta 1995, 256, 55-65. (15) Feki, H. E.; Savariault, J. M.; Salah, A. B.; Jemal, M. Sodium and carbonate distribution in substituted calcium hydroxyapatite. Solid State Sci. 2000, 2, 577-586. (16) Oliverira, L. M.; Rossi, A. M.; Lopes, R. T. Gamma dose response of synthetic A-type carbonated apatite in comparison with the response of tooth enamel. Appl. Radiat. Isot. 2000, 52, 1093-1097. (17) Fleet, M. E.; Liu, X. Location of type B carbonate ion in type A-B carbonate apatite synthesized at high pressure. J. Solid State Chem. 2004, 177, 3174-3182.
(18) Fleet, M. E.; Liu, X.; King, P. L. Accommodation of the carbonate ion in apatite: an FTIR and X-ray structure study of crystals synthesized at 2-4GPa. Am. Mineral. 2004, 89, 1422-1432. (19) Landi, E.; Tampieri, A.; Celotti, G.; Vichi, L.; Sandri, M. Influence of synthesis and sintering parameters on the characteristics of carbonate apatite. Biomaterials 2004, 25, 1763-1770. (20) Fleet, M. E.; Liu, X. Carbonate apatite type A synthesized at high pressure: new space group (P3) and orientation of channel carbonate ion. J. Solid State Chem. 2003, 174, 412-417. (21) Redey, S. A.; Razzouk, S.; Rey, C.; Assollant, D. B.; Leroy, G.; Nardin, M.; Cournot, G. Osteoclast adhesion and activity on synthetic hydroxyapatite, carbonated hydroxyapatite and natural calcium carbonate: relationship to surface energies. J. Biomed. Mater. Res. 1999, 45, 140147. (22) Ellies, L. G.; Lelson, D. G. A.; Featherstone, J. D. B. Crystallographic structure and surface morphology of sintered carbonate apatites. J. Biomed. Mater. Res. 1988, 22, 541-553. (23) Doi, Y.; Koda, T.; Wakamatsu, N.; Goto, T.; Kamemizu, H.; Moriwaki, Y.; Adachi, M.; Suwa, Y. Influence of carbonate on sintering of apatites. J. Dent. Res. 1993, 72, 1279-1284. (24) Matsuya, S.; Lin, X.; Udoh, K.; Nakagawa, M.; Shimogoryo, R.; Terada, Y.; Ishikawa, K. Fabrication of porous low crystalline calcite block by carbonation of calcium hydroxide compact. J. Mater. Sci.: Mater. Med. 2007, 18 (7), 1361-1367. (25) Navarro, M.; Valle, S. D.; Martinez, S.; Zeppetelli, S.; Ambrosio, L.; Planell, J. A.; Ginebra, M. P. New macroporous calcium phosphate glass ceramic for guided bone regeneration. Biomaterials 2004, 25, 4233-4241. (26) Almirall, A.; Larrecq, G.; Delgado, J. A.; Martinez, S.; Planell, J. A.; Ginebra, M. P. Fabrication of low temperature macroporous hydroxyapatite scaffolds by foaming and hydrolysis of an R-TCP paste. Biomaterials 2004, 25, 3671-3680. (27) Tadic, D.; Beckmann, F.; Schwarz, K.; Epple, M. A novel method to produce hydroxyapatite objects with interconnecting porosity that avoids sintering. Biomaterials 2004, 25, 3335-3340. (28) Erna, A. P.; Maeyer, D.; Ronald, M. H. V.; Didier, E. N. Optimalization of the preparation of Na+- and CO32--containing hydroxyapatite by the hydrolysis of monetite. J. Cryst. Growth 1994, 135, 539547. (29) Bouhaouss, A.; Bensaoud, A.; Laghzizil, A.; Ferhat, M. Effect of chemical treatments on the ionic conductivity of carbonate apatite. Int. J. Inorg. Mater. 2001, 3, 437-441. (30) Dekker, R. J.; Bruijn, J. D. D.; Stigter, M.; Barrere, F.; Layrolle, P.; Blitterswijk, C. A. V. Bone tissue engineering on amorphous carbonated apatite and crystalline octacalcium phosphate-coated titanium discs. Biomaterials 2005, 26, 5231-5239.
ReceiVed for reView October 31, 2007 ReVised manuscript receiVed February 9, 2008 Accepted February 9, 2008 IE071474A
