Synthesis of functionally graded CO3 apatite as surface biodegradable crystals

Apatite, which will have a degradable surface and a relatively stable core, was synthesized at 80±1°C and pH 7.4±0.2 using a specially generated gradient carbonate supply system. X-ray diffraction analysis showed that the (3 0 0) reflection peak shifted to the higher angle direction due to the substitution of CO2-3 ions into the PO3-4 positions and was broader than that of homogeneous hydroxyapatite. Scanning electron microscopy indicated that the apatite was composed of small coagulated ellipsoidal crystals differing from needle-like crystals with hexagonal section of hydroxyapatite. Electron spectroscopy for chemical analysis showed a negative gradient of carbonate concentration with depth in the crystals. The apparent solubility in 0.5 mol l-1 acetate buffer solution (37°C, pH 4.0) was much higher than that of homogeneous hydroxyapatite. The residual sample showed slim needle-like crystals. These results suggested that graded carbonate-containing apatite, namely CO3 apatite with solubility gradient in its crystal structure, was formed by this process and may be useful from the view point of surface biodegradable materials as bone representatives.