Development of Zirconia/Calcium Phosphate/Pyrolytic Carbon Composites with Nanoscale Lamellar-Structured Grain Boundary Phases to Control Crack Propagation for Biomedical Applications

Ceramic-based artificial bones that remain in the body for extended periods must exhibit high mechanical stability. However, the inherent brittleness of ceramics makes it difficult to ensure their long-term stability in vivo. In our previous work, we enhanced the damage tolerance of bulk calcium phosphate (CaP) ceramics by controlling the direction of crack propagation through lamellar structures. Although this material was compositionally suitable for artificial bone applications, its insufficient strength limited its practical use. Therefore, in this study, we developed an artificial bone composite with enhanced crack propagation control by incorporating a nanoscale lamellar-structured CaP phase into zirconia, a ceramic known for its high strength and toughness. The resulting composite features a controlled structure where hydroxyapatite and pyrolytic carbon form nanoscale lamellar structures at the grain boundaries of spherical tetragonal zirconia. The bending strength of this composite was found to be 360 MPa, which is significantly higher than that of dense hydroxyapatite sintered bodies, which are typical nonbiodegradable artificial bone materials. When a crack developed in this composite by indentation test, the damaged part detached. No evident cracks were found after the test, and the material as a whole maintained its integrity. This unique property is likely attributed to the nanoscale lamellar structures at the grain boundaries. Further, in vitro tests conducted using MC3T3-E1 cells confirmed that the composite exhibited no apparent cytotoxicity. Our results indicate that the developed composite can be potentially used to prepare novel nonbiodegradable artificial bone material with excellent long-term mechanical stability in vivo.