Optimizing the mechanical performance of nacre-like hydroxyapatite/polymethylmethacrylate-polyacrylic acid composites with apatite-wollastonite

Biomimetic materials inspired by the hierarchical structure of nacre offer promising opportunities to enhance the mechanical strength and toughness of load-bearing bone implants. This study investigates the incorporation of apatite-wollastonite (AW) into nacre-like hydroxyapatite (HA)/polymethylmethacrylate-polyacrylic acid (PMMA-PAA) composites to improve their mechanical properties. The composite mimics the natural nacre structure, featuring a brick-and-mortar architecture where ceramic components form the "bricks," and the polymer phase acts as the "mortar." The results showed that adding AW significantly increased the density of the ceramic scaffolds and improved the sinterability of HA. However, at higher AW concentrations (e.g., 20 wt%), excessive grain growth was observed, which could negatively affect the composite's mechanical properties. The best mechanical performance was achieved at 15 wt% AW, which enhanced the sintering process and reinforced the HA through the formation of a stronger wollastonite phase, as revealed by x-ray diffraction analysis. Mechanical testing showed that the composite with 15 wt% AW exhibited superior properties, with flexural strength increasing from 158.2 +/- 7.1 MPa (for HA/PMMA-PAA) to 188.7 +/- 6.2 MPa (for AW-HA/PMMA-PAA). Fracture toughness also improved significantly, from 5.2 to 10.2 MPa m1/2. In addition to improved flexural strength and fracture toughness, the composite's Young's modulus of 28.5 +/- 1.6 GPa closely matched that of cortical bone, reducing the risk of stress shielding, a common issue with commercial implants. Acellular bioactivity tests demonstrated the formation of a biologically relevant apatite layer on the composite surface, indicating its potential to promote osseointegration. Cytocompatibility assays confirmed favorable cell proliferation and viability, supporting its suitability for clinical applications. In summary, the addition of AW enhanced sinterability and introduced a stronger wollastonite phase within the HA ceramic layer, leading to improved strength and toughness of the nacre-like HA/PMMA-PAA composites. However, there was an optimal AW concentration that balanced superior mechanical properties with controlled grain growth. The bioactive and cytocompatible composites showed great promise as candidates for next-generation load-bearing bone implants.