Bi-material nanofibrous electrospun junctions: A versatile tool to mimic the muscle–tendon interface

Soft robotics aims to replicate the structure and mechanics of skeletal muscles. The challenge lies in seamlessly integrating these muscle-inspired soft actuators with the joints they intend to actuate, resembling the natural connection between muscles and tendons (i.e., myotendinous junction). This study addresses this issue by producing electrospun bundles of aligned nanofibers using a thermoplastic polyurethane, mimicking the muscle fascicle, and nylon 6.6 for the tendon one. A novel method was developed to create electrospun bi-material bundles with two different types of myotendinous-inspired junctions, called flat and conical. Scanning electron microscopy and microtomography analyses confirmed that conical junctions mimicked natural myotendinous structures better than flat ones. Tensile mechanical tests demonstrated that bi-material junctions reached stress at failure comparable to polyurethane bundles (11 ± 2 MPa), with the conical junction showing stiffness (0.13 ± 0.02 N/mm) and net elastic modulus (153 ± 10 MPa) values closer to the natural myotendinous ones. Cyclic tests verified the mechanical stability of junctions and their ability to dampen nylon 6.6 hardening over time. Moreover, all bundles withstood cyclic loading without breaking. These findings suggest the potential of biomimetic electrospun junctions for applications in soft robotics, marking a significant step toward advancing this field.