Highly toughening of PLLA-Based micropart via stretching induced stereocomplex crystal microstructure evolution

• There is tenfold toughness increase in PLLA-based micropart by blending with low-MW PDLA, while keeping biodegradability. • Transformation of PLA stereocomplex crystal into microfibril and shish-kebab during drawing boosts mechanical property. • 10 wt% PDLA optimized composition achieves an optimal balance of 67 MPa tensile strength and 87.2 % elongation at break. • Resulted high-strength, tough, and biocompatible PLA material boosts bone growth, broadening biomedical application. Polylactic acid (PLA) holds great potentials in biomedical applications, but its inherent brittleness restricts its versatility to a considerable degree. This study proposed a novel and heterogeneous modifier-free approach to enhance PLA’s toughness by leveraging the stretching induced evolution of stereocomplex crystal (SC) microstructures in situ formed during microinjection molding. By incorporating 10 wt% poly(D-lactic acid) (PDLA) into poly(L-lactic acid) (PLLA) through combining melt compounding and microinjection molding featuring extremely intense shear, we achieved a remarkable 10-fold increase in elongation at break (from 8.7 % to 87.2 %) while maintaining tensile strength (∼67 MPa). The structural analyses revealed a transition from phase-separated sea-island morphologies to deformable stereocomplex crystal PLA phase domains, which could serve as the physical crosslinking points facilitating stress transfer, and can be transformed into microfibril and shish-kebab structures upon drawing. Such the phase structure evolution could efficiently distribute stress and hence enhance toughness without sacrificing biodegradability or biocompatibility. This work develops a streamlined approach in simplifying conventional stereocomplex reinforcement strategies and thus offers a scalable method for developing fully biodegradable, and high-performance PLA-based materials suitable for diverse biomedical applications, such as bone tissue reconstruction.