Fabrication of carbonate apatite blocks from octacalcium phosphate blocks through different phase conversion mode depending on carbonate concentration

Carbonate apatite (CO₃Ap) is an inorganic component of mature bone and is used as bone substitute instead of autografts. Recently, 100% pure CO₃Ap granules formed from calcium carbonate via a dissolution-precipitation reaction have been used as a clinical precursor for bone. In this study, the feasibility of fabricating blocks of CO₃Ap from blocks of octacalcium phosphate (OCP), which is an analog of the bone composition and serves as a precursor for bone, was investigated by immersion in NaHCO₃. The macroscopic structure of the OCP block was retained upon conversion to CO₃Ap. In addition, as the NaHCO₃ concentration increased from 0.0 to 2.0 mol/L, the carbonate content increased in the resulting CO₃Ap, reaching ≈ 14 wt% at maximum NaHCO₃ concentration. Moreover, the morphology of the CO₃Ap crystals varied as the NaHCO₃ concentration increased up to 0.5 mol/L: they were pseudomorphic plate-like when the NaHCO₃ concentration was low but formed particles ranging ~ 10 nm in size at high NaHCO₃ concentrations. The different phase transformation modes depended on NaHCO₃ concentrations closely related to CO₃Ap crystal morphology and were associated with a decrease in the mechanical strength of the block.