Multifaceted characterization of hydroxyapatite-doped polylactide as bone-graft substitute

Abstract Purpose This study investigates whether polylactide (PLA)/hydroxyapatite (HA) composites fabricated by industrially scalable melt-processing can achieve a favorable balance between mechanical performance and in vitro bioactivity for bone-graft substitute applications, and identifies the HA concentration thresholds that govern key material transitions. Methods PLA composites containing 2.5, 5, and 10 wt% HA were produced by twin-screw extrusion and injection molding. Composites were characterized by capillary rheometry, thermogravimetric analysis, differential scanning calorimetry, dynamic mechanical analysis, tensile testing, and Charpy impact testing. In vitro bioactivity was assessed by immersion in simulated body fluid at 36.5 °C for up to 42days, with surface evolution monitored by optical microscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. Results HA addition progressively reduced melt viscosity (up to about 21-fold at 300 s⁻¹ for 10 wt% HA) and lowered the maximum degradation temperature by about 70 °C, consistent with HA-catalyzed chain scission during melt processing. Young’s modulus increased modestly from 3.6 to 4.3 GPa, but tensile strength, elongation at break, and impact resistance decreased markedly above 2.5 wt% HA, indicating a ductile-to-brittle transition. In vitro SBF immersion confirmed the formation of a bone-like apatite layer on composites containing ≥ 5wt% HA within 21 days, a response absent in neat PLA and minimal at 2.5 wt% HA. Conclusions A 5 wt% HA loading represents an optimal trade-off, conferring confirmed bioactivity while retaining acceptable mechanical properties for melt-processed PLA-based bone substitutes. The HA content can be adjusted to match the mechanical demands of different anatomical sites, highlighting the versatility of this composite system for bone-regeneration applications.