Carbonate apatite‐coated calcium carbonate (CO₃Ap/CaCO₃) was fabricated through a dissolution–precipitation reaction using CaCO₃ granules as a precursor to accelerate bone replacement based on superior osteoconductivity of the CO₃Ap shell, along with Ca²⁺ release from the CaCO₃ core and quicker resorption of the CaCO₃ core. In the present study, CaCO₃, 10% CO₃Ap/CaCO₃, 30% CO₃Ap/CaCO₃, and CO₃Ap granules were fabricated and examined histologically to evaluate their potential as bone substitutes. Larger contents of CaCO₃ in the granules resulted in higher Ca²⁺ release and promoted cell proliferation of murine preosteoblasts at 6 days compared with CO₃Ap. Interestingly, in a rabbit femur defect model, 10% CO₃Ap/CaCO₃ induced significantly higher new bone formation and higher material resorption compared with CO₃Ap at 8 weeks. Nevertheless, CO₃Ap showed a superior osteoconductive potential compared with 10% CO₃Ap/CaCO₃ at 8 weeks. All tested granules were most likely resorbed by cell mediation including multinucleated giant cell functions. Therefore, we conclude that CO₃Ap/CaCO₃ has a positive potential for bone tissue engineering based on well‐controlled calcium release, bone formation, and material resorption.

Carbonate apatite (CO₃Ap) foam has gained much attention in recent years because of its ability to rapidly replace bone. However, its mechanical strength is extremely low for clinical use. In this study, to understand the potential of gelatin-reinforced CO₃Ap foam for bone replacement, CO₃Ap foam was reinforced with gelatin and the resulting physical characteristics were evaluated. The mechanical strength increased significantly with the gelatin reinforcement. The compressive strength of gelatin-free CO₃Ap foam was 74 kPa whereas that of the gelatin-reinforced CO₃Ap foam, fabricated using 30 mass % gelatin solution, was approximately 3 MPa. Heat treatment for crosslinking gelatin had little effect on the mechanical strength of the foam. The gelatin-reinforced foam did not maintain its shape when immersed in a saline solution as this promoted swelling of the gelatin; however, in the same conditions, the heat-treated gelatin-reinforced foam proved to be stable. It is concluded, therefore, that heat treatment is the key to the fabrication of stable gelatin-reinforced CO₃Ap foam.