High fracture toughness: Nacre-mimetic HA-CaCO₃/SA layered composites without sintering

The fabrication of composites with nacre-like structures through the ordered integration of ceramics and organic components represents an effective approach to developing high strength and toughness bone implant materials. Traditional preparation methods typically involve high-temperature sintering followed by the infiltration of organic substances. While these methods can achieve high strength, the uniformity and depth of organic substance penetration into the ceramic matrix were constrained by pore size, leading to an uneven distribution. Additionally, the interface bonding between the organic and ceramics was limited to physical bonding, which restricts the enhancement of fracture toughness. In this study, nacre-mimetic hydroxyapatite (HA)-Calcium carbonate (CaCO₃)/sodium alginate (SA) layered composites were fabricated utilizing a combination of roll rolling and the sinterless characteristics of calcium phosphate bone cement (CPC). The SA contains oxygen groups that form bonds with Ca²⁺ in the HA layer. The bending strength of nacre-mimetic HA-CaCO₃/SA layered composites can achieve 9.04 MPa, which was comparable to that of human trabecular bone. Repeated roll rolling lead to a lamellar dispersion of SA in the composite, with an average thickness of 0.19 μm. The fracture toughness of the layered composite was 13.89 MPa m^1/2, which was higher than that of cortical bone due to deflection extension path of the crack under bending stress, crack passivation, crack bifurcation and crack bridging. The layered composites exhibit excellent biocompatibility, promoting the proliferation and adhesion of osteoblasts. This characteristic was crucial for developing bone implant materials that match both the mechanical and biological properties of human bones.