Morphologically bioinspired hierarchical Nylon 6,6 electrospun assembly recreating the structure and performance of tendons and ligaments

Reconstructions of ruptured tendons and ligaments currently have dissatisfactory failure rate. Failures are mainly due to the mechanical mismatch of commercial implants with respect to the host tissue. In fact, it is crucial to replicate the morphology (hierarchical in nature) and mechanical response (highly-nonlinear) of natural tendons and ligaments. The aim of this study was to develop morphologically bioinspired hierarchical Nylon 6,6 electrospun assemblies recreating the structure and performance of tendons and ligaments. First, we built different electrospun bundles to find the optimal orientation of the nanofibers. A 2nd-level hierarchical assembly was fabricated with a dedicated process that allowed tightly joining the bundles one next to the other with an electrospun sheath, so as to improve the mechanical performance. Finally, a further hierarchical 3rd-level assembly was constructed by grouping several 2nd-level assemblies. The morphology of the different structures was assessed with scanning electron microscopy and high-resolution X-ray tomography, which allowed measuring the directionality of the nanofibers in the bundles and in the sheaths. The mechanical properties of the single bundles and of the 2nd-level assemblies were measured with tensile tests. The single bundles and the hierarchical assemblies showed morphology and directionality of the nanofibers similar to the tendons and ligaments. The strength and stiffness were comparable to that of tendons and ligaments. In conclusion, this work showed an innovative electrospinning production process to build nanofibrous Nylon 6,6 hierarchical assemblies which are suitable as future implantable devices and able to mimic the multiscale morphology and the biomechanical properties of tendons and ligaments.