Studies on Change in Solubility over Time of the Bioactive Material Amorphous Calcium Phosphate and Precipitation of Hydroxyapatite

In this study, we immersed amorphous calcium phosphate (ACP) powder in biochemical buffer solutions and performed analysis of its solubility and phase transformation of the precipitate. After preparing ACP powder that contains no impurities, we used 4-(2-hydroxyethyl)-1-piperazinetethanesulfonic acid (HEPES) buffer, one of the good buffers, as a buffer solution and measured the amount of calcium ions eluted from ACP and other calcium phosphate crystals. ACP was immersed in the buffer solution at 5°C, 20°C, and 37°C, and the amount of eluted calcium ions was measured from 15 min to 24 h thereafter. The precipitated solid phase was analyzed using X-ray diffraction and its morphology was observed using transmission electron microscopy. The precipitation of hydroxyapatite (HAp) was observed after 15 min in HEPES buffer solution. Furthermore, in this experimental group, the precipitates of the sample incubated in HEPES buffer solution at 37°C for 24 h produced the largest HAp crystals. From these results we concluded that ACP immersed in HEPES buffer solution easily releases calcium ions and phosphate ions, and a rapid phase transformation to HAp occurs. Moreover, we assume that, in addition to the thermodynamic effect, the crystal growth of HAp is enhanced by the buffer solution.