Carbonate apatite (CO₃Ap) block, which is a bone replacement used to repair defects, was fabricated through a dissolution–precipitation reaction using a calcium hydrogen phosphate dihydrate (DCPD) block as a precursor. When the DCPD block was immersed in NaHCO₃ or Na₂CO₃ solution at 80 °C, DCPD converted to CO₃Ap within 3 days. β-Tricalcium phosphate was formed as an intermediate phase, and it was completely converted to CO₃Ap within 2 weeks when the DCPD block was immersed in Na₂CO₃ solution. Although the crystal structures of the DCPD and CO₃Ap blocks were different, the macroscopic structure was maintained during the compositional transformation through the dissolution–precipitation reaction. CO₃Ap block fabricated in NaHCO₃ or Na₂CO₃ solution contained 12.9 and 15.8 wt % carbonate, respectively. The diametral tensile strength of the CO₃Ap block was 2 MPa, and the porosity was approximately 57% regardless of the carbonate solution. DCPD is a useful precursor for the fabrication of CO₃Ap block.

Interconnected pore forming calcium phosphate cement is useful for the reconstruction of bone defects as well as scaffold fabrication in tissue engineering. In this study, interconnected pore forming calcium phosphate cement was fabricated using α-tricalcium phosphate (α-TCP) foam granules. When α-TCP foam granules were mixed with acidic calcium phosphate solution prepared from monocalcium phosphate monohydrate (MCPM) and phosphoric acid solution, brushite crystals were precipitated. These crystals bridged the α-TCP foam granules immediately upon mixing. As a result of the brushite bridge between the α-TCP foam granules, fully interconnected macroporous α-TCP was obtained. The amount of brushite precipitate and the mechanical strength of the set cement increased with acidic calcium phosphate concentration.

We recently reported that when an acidic calcium phosphate solution is mixed with β -tricalcium phosphate ( β -TCP) granules, the resulting dicalcium phosphate dihydrate (DCPD) crystals form bridges between the β -TCP granules, creating a set interconnected porous structure in approximately 1 min. Although this self-setting β -TCP granular cement ( β -TCPGC) is useful for clinical applications, the short setting time is a key drawback for handling. In this study, the setting time of β -TCPGC was adjusted with the addition of citric acid, which is a known inhibiter of DCPD crystal growth. As the concentration of citric acid in the acidic calcium phosphate solution increased, the amount of DCPD formation in the set β -TCPGC decreased, and the crystal morphology of DCPD became elongated. β -TCPGC prepared with various citric acid concentrations were used as grafting material in rat calvarial bone defects to evaluate bone regeneration in vivo . Four weeks after implantation, no inflammatory reaction and approximately 20% new bone formation were observed, regardless of the presence or absence of citric acid in the liquid phase of β -TCPGC. We concluded, therefore, that citric acid might be a useful retarder of β -TCPGC setting times.