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

Abstract Carbonate apatite (CO 3 Ap) is an inorganic component of mature bone and is used as bone substitute instead of autografts. Recently, 100% pure CO 3 Ap 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 CO 3 Ap 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 NaHCO 3. The macroscopic structure of the OCP block was retained upon conversion to CO 3 Ap. In addition, as the NaHCO 3 concentration increased from 0.0 to 2.0 mol/L, the carbonate content increased in the resulting CO 3 Ap, reaching ≈ 14 wt% at maximum NaHCO 3 concentration. Moreover, the morphology of the CO 3 Ap crystals varied as the NaHCO 3 concentration increased up to 0.5 mol/L: they were pseudomorphic plate-like when the NaHCO 3 concentration was low but formed particles ranging ~ 10 nm in size at high NaHCO 3 concentrations. The different phase transformation modes depended on NaHCO 3 concentrations closely related to CO 3 Ap crystal morphology and were associated with a decrease in the mechanical strength of the block. Graphical abstract Different phase conversion modes from OCP to CO 3 Ap depended on CO 3 concentration fx1 [ABSTRACT FROM AUTHOR]