Comparative Analysis of Bulk and Surface Erosion Behaviors in Semi-Crystalline PLLA and Amorphous PDLLA under Accelerated Enzymatic Conditions

While Polylactic acid (PLA) is extensively employed in biomedical applications, the stereochemical divergence in degradation mechanisms between its optical isomers remains a critical factor for clinical safety and material stability. This study presents a comprehensive comparative analysis of bulk and surface erosion mechanisms by simultaneously evaluating the long-term physicochemical property changes of 2D PLA films and the morphological degradation behavior of 3D PLA microparticles under accelerated in vitro enzymatic conditions utilizing Proteinase K and Lipase. Mechanical, thermal, and molecular monitoring of the films revealed that semi-crystalline Poly-L-lactic acid (PLLA) experiences water penetration into sparse amorphous regions, triggering internal chain cleavage and bulk erosion. This disproportionate degradation drastically increased the overall crystallinity of PLLA (up to 60.3%), ultimately resulting in a sudden and catastrophic brittle fracture. In contrast, the random distribution of enantiomers in amorphous Poly-D,L-lactic acid (PDLLA) suppresses crystallization, promoting a steady, linear reduction in mechanical properties without sudden structural failure, which is indicative of a controlled surface erosion process. Concurrently, morphological analyses via Scanning Electron Microscopy (SEM) and 3D Micro-Computed Tomography (MicroCT) on polymeric microparticles demonstrated that highly crystalline PLLA particles rapidly shattered into sharp, irregular fragments due to the sudden collapse of their internal structure. Alternatively, amorphous PDLLA microparticles featuring a densely packed sub-micron nanoporous sponge network effectively buffered the mechanical stress and enzymatic attack. These nanoporous PDLLA particles eroded uniformly layer-by-layer from the exterior, preserving their spherical integrity, structural convexity, and low aspect ratio throughout the degradation period. Our concurrent evaluations of both films and microparticles definitively prove that the amorphous structural capability of PDLLA, combined with a nanoporous architecture, prevents the generation of sharp, rigid fragments associated with PLLA's bulk erosion. Consequently, PDLLA provides superior morphological retention and a highly stable, biocompatible degradation profile optimally suited for long-term tissue integration.