Effect of calcium and phosphate on compositional conversion from dicalcium hydrogen phosphate dihydrate blocks to octacalcium phosphate blocks

Octacalcium phosphate (OCP) has attracted much attention as an artificial bone substitute because of its excellent osteoconductive and bone replacement properties. Although numerous studies have investigated OCP powder fabrication, there are only a few studies on OCP block fabrication. Therefore, in this study, the feasibility of optimizing dicalcium hydrogen phosphate dihydrate (DCPD) blocks, as a precursor for OCP block fabrication, under a pH 6 adjusted acetate buffer solution at 70^◦ C for 2 days was investigated. When a DCPD block was immersed in acetate buffer, the block was partially converted to OCP, with a large amount of dicalcium hydrogen phosphate anhydrate (DCPA), and its macroscopic structure was maintained. When the DCPD block was immersed in a Ca-containing solution, it was converted to mainly hydroxyapatite (HAp) with DCPA. On the other hand, when the DCPD block was immersed in a PO₄-containing solution, the block was converted to OCP, and its macroscopic structure was maintained. In other words, the PO₄-induced calcium phosphate with a Ca/P molar ratio lower than 1.0 may represent an intermediate phase during the compositional transformation from a DCPD block to an OCP block through the dissolution–precipitation reaction.