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

Abstract

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.

Effect of an Amorphous Calcium Phosphate Agent on Artificial Dentin Caries Lesions

“…The synthetic ACP and HEPES buffer (pH 7.4) were mixed in the proportion of 1:2,000 and incubated at 37ºC for 30 minutes, after which the reaction was halted by treating the centrifuged precipitate with 100% ethanol, and the ethanol was evaporated at room temperature to obtain HAP powder. The precipitate obtained by this method is HAP that has undergone a phase transition from ACP over a short time period, and it has been confirmed to be HAP by X-ray diffraction 2) .…”

“…We have previously reported that HAP is deposited within 15 minutes at an ACP:HEPES ratio of 1:2,000 (we./wt. ratio) 2) . However, a mixture of ACP and HEPES with a P/S ratio of 1:2,000 is practically liquid, and it would therefore be highly likely to run out of the cavity immediately after filling.…”

“…In recent years, we have synthesized amorphous calcium phosphate [ACP: Ca 3 (PO 4 ) 2 ] and identified a method of using HEPES (2-[4-(2hydroxyethyl)-1-piperazinyl] ethanesulfonic acid) buffer to induce hydroxyapatite [HAP: Ca 10 (PO 4 ) 6 (OH) 2 ] deposition from ACP in vitro 2) . These in vitro experimental results showed that HAP was deposited from ACP in 15 minutes by a buffer solution at physiological pH, presaging the possibility of tooth restoration treatment in a matter of minutes by a material equivalent to the biological substance.…”