Key Engineering Materials Vols. 309-311 (2006) pp 515-518 online at http://www.scientific.net © (2006) Trans Tech Publications, Switzerland Online available since 2006/May/15
Properties of Calcium Phosphate Powder Prepared from Phosphoryl Oligosaccharides of Calcium Tomohiro Umeda1a, Kiyoshi Itatani2b, Hiroko Mochizuki2b, Ian J. Davies3c, Yoshiro Musha4d and Seiichiro Koda2b 1
Olympus Biomaterial Corporation, 2-3 Kuboyama-cho, Hachioji-shi, Tokyo 192-8512, Japan 2 Faculty of Science and Engineering, Sophia University, 7-1 Kioi-cho, Chiyoda-ku, Tokyo 102-8554, Japan 3 Department of Mechanical Engineering, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, Australia 4
Faculty of Medicine, Toho University, 5-21-16 Omorinishi, Ota-ku, Tokyo 143-8540, Japan a
[email protected], b
[email protected],
[email protected], d
[email protected]
Keywords: Hydroxyapatite, carbonate apatite, hydrothermal treatment, nano-sized powder, phosphoryl oligosaccharides of calcium
Abstract. The phosphoryl oligosaccharides of calcium (POC), extracted from potato starch, are composed of phosphorus oligosaccharides and calcium ions. Ultrafine calcium phosphate particles, whose main phase was hydroxyapatite (Ca10(PO4)6(OH)2: HAp), could be prepared through the hydrothermal treatment of POC solution at a temperature between 110 and 130˚C; X-ray diffraction indicated the crystallinity of HAp in the resulting powder to be poor and similar to that of living bone. The present HAp powder was regarded to be calcium deficient carbonate apatite with the OHgroup being partly substituted by a carbonate (CO32-) group. The solubility of the resulting powder in dilute hydrochloric acid was higher compared to that of commercially available HAp, suggesting excellent bioabsorbability for the present powder. Introduction Hydroxyapatite (Ca10(PO4)6(OH)2: HAp) is a main inorganic component in bone and, due to its excellent biocompatibility, has found use as a biomaterial in applications such as artificial bone and dental roots. Synthetic HAp powder has frequently been produced using reaction between calcium (Ca2+) and phosphate (PO43-) ions in solution. However, the presence of various cations and anions within the solution, in addition to Ca2+ and PO43-, is known to influence the crystal orientation of HAp. In addition, the presence of organic compounds may give rise to preferred-orientation of the HAp particles and alter the shape of particles, e.g., formation of needle-like particles when a monosaccharide such as D-glucose is added to the starting calcium phosphate solution [1]. Thus, it is important to examine the influence of organic compounds in order to control the crystal size, preferred orientation and surface area of the resulting HAp powder. Unfortunately, no systematic information regarding this area has so far been available until now. POC extracted from potato starch is composed of phosphorus oligosaccharides and calcium ions [2]. It is expected from this chemical structure that highly functional calcium phosphate powders modified with saccharides may be directly prepared through the hydrolysis of such POC. In addition, this technique has the advantage of being prepared using a simple technique, i.e., without the requirement for a precipitant. On the basis of the above information, this paper describes the conditions necessary for the preparation of nano-sized calcium phosphate powders from POC.
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Materials and Method Commercially available POC was used as the (a) starting powder; the Ca and P contents were 5.0 and 3.7 mass%, respectively, and the Ca/P ratio was calculated as 1.0. The chemical structure of POC is shown in Fig. 1, together with a scanning electron micrograph (SEM). Calcium phosphate particles were prepared using the hydrothermal treatment of an aqueous solution containing 10 mass% of POC at a temperature between 80 and 140˚C for 2 to 7 h; the heating rate from room temperature up to the desired (b) -1 temperature was approximately 2˚C·min . Phase changes during the hydrothermal treatment were examined using an X-ray diffractometer (XRD; RINT2100C/P, Rigaku, Tokyo, 40 kV and 40 mA) and Fourier transform infrared spectrometer (FT-IR; Model 8600PC, Shimazu, Kyoto). Morphologies of the resulting particles were observed using a SEM (Model S-4500, Hitachi, Tokyo, accelerating voltage 15.0 kV) Fig. 1 (a) Chemical structure and (b) SEM and transmission electron microscope (TEM; micrograph of POC powder. Model JEM-2011, JEOL, accelerating voltage 200 kV) whilst the Ca/P ratio and specific surface area (SSA) was measured using EDX (Model EMAX-5770, Horiba, Kyoto) and the BET method, respectively. The solubility of the resulting powder (150 mg) in diluted hydrochloric acid (HCl; 100 cm3) at 37.0 ± 0.2˚C and pH 5.5 was examined according to the Nancollas method: [3] any rise in pH, due to the partial dissolution of paste in the HCl solution, was adjusted back to pH 5.5 using 0.1 mol·dm-3 of HCl with the amount of HCl required to maintain a pH of 5.5 being defined as the solubility to the acid. Results and discussion Although concentrated ammonia was added to the solution containing 10 mass% of POC, no precipitates were formed. Nevertheless, precipitation occurred without the addition of ammonia following hydrothermal treatment of the solution at a temperature between 110 and 140˚C. Changes in pH of the solution containing 10 mass% POC during hydrothermal treatment are shown in Fig. 2, together with a schematic representation of the precipitate color. The solution pH was 5.65 at 80˚C but reduced with increasing temperature to reach 3.64 at 140˚C. Precipitates were formed at temperatures ≥110˚C with a treatment time of 5 h being required to form precipitates at 110˚C. The precipitates exhibited the lightest brownish color for a hydrothermal treatment of 2 h at 110˚C. It is
Fig. 2 Changes in pH of solution containing 10 mass% of POC during the hydrothermal treatment, together with the color of powders.
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PO43PO43-
PO43PO43-
H 2O CO32-
(a)
4000
3000
2000
1000
PO43-
PO43-
Fig. 3 XRD patterns of the powders prepared by the hydrothermal treatment at 110˚C for 5 h, 120˚C for 2 h, 130˚C for 2 h, and 140˚C for 2 h. ○: Ca10(PO4)6(OH)2 △: CaHPO4
(b)
H 2O
Transmi Transmitttance
H 2O CO32-
(c)
known that the phosphate-ester linkage is easily Wavenumber Wavenumber / cm-1 severed due to hydrothermal treatment, releasing phosphate ions into the solution [2]. The change Fig. 4 FT-IR spectra of powders prepared by the hydrothermal treatment at (a) 110˚C for 5 h, in pH from 5.65 to 3.64 is therefore attributed to (b) 120˚C for 2 h, and (c) 130˚C for 2 h. the release of phosphate ions into the solution. Phases present in the resulting powders were examined using XRD and FT-IR. Firstly, XRD patterns for these powders are in Fig. 3. Whereas the powders hydrothermally-treated between 110 and 130˚C contained poorly crystalline HAp, together with a small amount of calcium hydrogenphosphate (CaHPO4), the powder prepared at 140˚C for 2 h contained only CaHPO4. Since HAp was present in the temperature range of 110 to 130˚C, FT-IR analysis of these powders was carried out with the results being presented in Fig. 4. The FT-IR pattern of the powder hydrothermally-treated at 110˚C contained absorption peaks at 420, 570, 600, 960, 1050, 1090 and 1650 cm-1, together with an absorption band at 3200-3700 cm-1. These absorption peaks were assigned to PO43-, except for the case of the absorption peak at 1650 cm-1 and the band at 3200 to 3700 cm-1 (physically adsorbed water). An additional absorption peak appeared at 1542 cm-1 for hydrothermal treatment temperatures of 120 and 130˚C with this peak being assigned to carbonate (CO32-) ions that were replaced by hydroxyl groups (A site) in the HAp. The following reactions proceeded during the hydrothermal treatment of POC solution: 10Ca2+ + 6PO43- + 2OH- Ca10(PO4)6(OH)2 (1) 2+ 3+ (2) Ca + PO4 + H CaHPO4 EDX results indicated the Ca/P molar ratio of the resulting powder to be approximately 1.50. On the basis of the XRD, FT-IR and EDX results, the HAp present within the powder was regarded to be calcium-deficient and carbonate-containing apatite, similar to that found in living bone. Specific surface areas of the precipitates are shown in Fig. 5, as a function of the hydrothermal temperature. Whilst the SSA of the powder was 97.9 m2·g-1 following hydrothermal treatment at 110˚C for 5 h, this decreased with increasing hydrothermal temperature to reach 38.1 m2·g-1 at 140˚C. The high SSA values indicate that the powder was composed of nanometer-scaled particles with sizes below 100 nm. The particle shapes of these powders were observed using SEM and TEM with results being presented in Fig. 6, together with electron diffraction patterns. Powders hydrothermally treated in the temperature range of 110 to 130˚C were composed of rod-like particles with long-axis sizes of approximately 50 nm or less; the electron diffraction patterns exhibited broad
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Fig. 5 Changes in SSA of the powder as a function of hydrothermal treatment. ●:110˚C for 5 h, ○:120, 130 and 140˚C for 2 h
Fig. 6 SEM and TEM micrographs of the powders prepared by the hydrothermal treatment at rings that indicated a poorly crystalline struc- (a) 110˚C for 5 h (TEM), (b) 120˚C for 2 h ture. In contrast to this, the powder hydro- (TEM), (c) 130˚C for 2 h (TEM), and (d) 140˚C thermally treated at 140˚C was comprised of for 2 h (SEM). rectangular solid particles with long-axis sizes of approximately 30 µm. Finally, the solubility of these powders in dilute HCl was examined in order to evaluate their bioabsorbability with the results being shown in Fig. 7, together with those of the commercially available powder (SSA: 69.4 m2·g-1). The solubility of the present powder was higher compared to that of the commercially available HAp powder. These results suggest the present powders to be novel and promising with the potential for use as bone absorption/substitute materials. Fig. 7 Solubilities of (a) commercially available HAp and (b) powder hydroUltrafine calcium phosphate particles whose main phase thermally treated at 110˚C for 2 h. was HAp could be prepared through the hydrothermaltreatment of POC solution at a temperature between 110 and 130˚C; crystallinity of the resulting powder was poor and similar to that of living bone. The present HAp powder was regarded to be calcium-deficient and carbonate apatite with the OH- being partially substituted by a carbonate group. The solubility of the resulting powder in dilute hydrochloric acid was higher when compared to that of commercially available HAp, suggesting the potential for excellent bioabsorbability. Conclusion
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