Delayed nerve repair often has poor functional recovery that results from the inability of host axons to reach and reform meaningful connections with the injured muscle. After nerve injury, axons in the distal nerve undergo Wallerian degeneration and Schwann cells temporarily form a pro-regenerative environment that promotes axon regeneration and facilitates muscle reinnervation. Over time, a prolonged period without axonal contact in the distal nerve leads to the degradation of the bands of Büngner, diminishing the potential for muscle reinnervation and ultimately functional recovery. Previous studies have shown that early reinnervation using axons from otherwise healthy nerves can promote functional recovery. However, there are no commercially available technologies that provide exogenous axons in the otherwise distal nerve.

This dissertation describes the engineering of an implantable microtissue designed to maintain the regenerative capacity and ultimately improve functional recovery following severe peripheral nerve injury. Moreover, emphasis is given to understanding the physiology underlying nerve injury and its relationship to the clinical challenges in peripheral nerve repair, clinically available various repair strategies, and the conception of our novel constructs from a translational perspective. Finally, this dissertation concludes with a prospective look at the field of peripheral nerve repair through the lens of these potentially transformative constructs.

Pursuant to the first aim of this dissertation, stretch-grown tissue engineered nerve grafts (TENG) are evaluated as a novel approach for simultaneously facilitating axon regeneration and preserving the regenerative capacity in two clinically relevant porcine models of peripheral nerve injury. Pursuant to the second aim of this dissertation, development and characterization of a miniaturized tissue engineered nerve graft (micro-TENG) designed to be a more translatable alternative for preserving the regenerative capacity via minimally invasive injection into the nerve was completed. Additionally, advanced fabrication methodologies were employed, including the optogenetic stimulation and aligned Schwann cells embedded in the construct. For completion of the third aim of this dissertation, the efficacy of micro-TENGs to preserve the regenerative capacity and improve functional recovery was evaluated in a rodent model of delayed nerve repair and chronic axotomy.

Collectively, this work shows that micro-TENGs integrate with denervated distal structures and preserve the regenerative capacity during prolonged periods without host innervation. By repopulating the distal sheath with exogenous axons, micro-TENGs also enable delayed nerve fusion, which was previously not achievable due to axon degradation after Wallerian degeneration. Furthermore, greater electrophysiological recovery, axon maturation, and muscle reinnervation was observed at 1 month following delayed nerve repair. Based on these findings, micro-TENGs appear to represent a transformative approach for restorative peripheral nerve surgery and potentially offer the possibility for functional recovery where virtually no hope currently exists.