Elastic Properties and Bulk Microstructure of Poly(L-Lactide)–Hydroxyapatite Composites Under Long-Term In Vitro Hydrolytic Degradation

Predicting the long-term degradation of bioresorbable poly(L-lactide) (PLLA)–hydroxyapatite (HA) composites remains a critical challenge in orthopedic implant design. An artificial implant must support bone tissue and maintain its mechanical and elastic properties for a certain period of time, corresponding to the rate of regeneration of damaged tissue; the time can reach several months. This study investigates the evolution of elastic properties and bulk microstructure in highly filled PLLA–HA composites (5–20 wt.% HA) during 76 weeks of in vitro hydrolytic degradation at 37 °C. Using high-frequency pulsed scanning acoustic microscopy (100 MHz), microstructural transformations and local elastic moduli were monitored non-destructively, complemented by mechanical testing and density measurements. Results indicate a concentration-dependent degradation mechanism: while initial stiffness increased with HA content, filler concentrations exceeding 10 wt.% accelerated degradation via early interfacial debonding and cavity formation around filler agglomerates. Conversely, the 5 wt.% HA composite exhibited superior stability, maintaining an elastic modulus of 6.7 GPa over 64 weeks with minimal microstructural damage. High-frequency ultrasound effectively quantified internal void formation and degradation kinetics in a non-invasive manner. These findings identify 5 wt.% HA as the optimal concentration for balancing mechanical reinforcement with controlled resorption rates. This work provides fundamental insights into the structure–property–degradation relationships in biocomposites and validates ultrasonic diagnostics as a vital tool for predicting the service life of resorbable implantable devices.