Stretch-Induced Microstructural Evolution of Electrospun Polycaprolactone Microfibers for Biomedical Applications

The performance and degradation of polymeric medical yarns are strongly dependent on their microstructure, which can evolve significantly during fabrication. This work investigates and models how the microstructure of microfibrous electrospun (ES) filaments changes during the critical postprocessing step of uniaxial stretching. Specifically, we studied filaments designed for use in a knee ligament regeneration implant made from biodegradable, semicrystalline polycaprolactone (PCL). Structural changes were characterized at both the fiber and the molecular scales. Stretching led to fiber alignment, thinning, and coalescence, as revealed by microcomputed tomography (μCT) and scanning electron microscopy (SEM). At the molecular scale, the crystalline microarchitecture transformed profoundly, as shown by differential scanning calorimetry (DSC), one-dimensional (1D) and two-dimensional (2D) X-ray diffraction (XRD), and dynamic mechanical thermal analysis (DMTA). Based on these findings, we propose a conceptual model for stretch-induced microstructural evolution: at lower strains, chain-folded crystals (CFCs) fragment while amorphous chains extend; at higher strains, CFCs unfold and recrystallize with extended chains into more thermodynamically stable chain-extended crystals (CECs) aligned with the stretch axis. This mechanism clarifies how uniaxial strain reorganizes semicrystalline domains in PCL, with important implications for thermomechanical and degradative properties relevant to implant performance. Understanding how microstructure responds to stretching enables the future development of more accurate simulations of complex fibrous materials under physiological conditions and informs the optimization of fabrication and design parameters for next-generation medical yarns.