Abstract

Hydroxyapatite is one of main inorganic constituents of human bones and teeth. It is widely used as a biomaterial due to its great bioactivity and biocompatibility. The hydroxyapatite crystals have two kinds of crystal axes: an a(b)-axis and a c-axis. There are some advantages in the hydroxyapatite crystals with an a(b)-axis orientation. In recent years, a synthesis method has been reported for plate-shaped hydroxyapatite powders with an a(b)-axis orientation. However, the Ca/P molar ratio of the synthesized powders was lower than that of hydroxyapatite with stoichiometric composition. For this reason, they were thermally decomposed into β-tricalcium phosphate over 700°C. In order to solve the problem, we utilized dissolution-deposition reactions caused by hydrothermal treatment. In this study, the hydrothermal treatment was performed with variation of two hydrothermal conditions: Ca²⁺ ion concentration and hydrothermal temperature. Furthermore, we examined a thermal stability by heating the hydrothermaltreated powders at 1000°C for 1 h. When the synthesized powders were hydrothermally treated at 180°C for 2.5 h in a calcium chloride solution (1.0 mol•dm^-3), the Ca/P molar ratio of them increased from 1.38 to 1.64. In addition, the optimized hydroxyapatite powders were the thermally stablest among examined samples still after heating. From the above, the hydrothermal process is considered to be useful for the preparation of the plate-shaped hydroxyapatite powders with closer stoichiometric Ca/P molar ratio and the development of their thermal stability.