CONTROLLED DELIVERY SYSTEMS FOR NEURONAL TISSUE ENGINEERING

Complete transection of peripheral nerves can result from trauma, tumor removal, infection, or as adverse consequences of various surgeries. Current commercially available nerve guides cannot repair large nerve defects because these guides are engineered to provide mechanical support for the developing axon and do not actively promote axonal growth. For large nerve gaps, targeting axonal growth is particularly important because the length of the nerve that must be regrown is the distance from the lesion to the innervated muscle. Therefore, there is enormous clinical potential for a nerve guide capable of improving axonal outgrowth across large nerve defects. Our underlying hypothesis is that delivery of Glial Cell Line-Derived Neurotrophic Factor (GDNF) from a nerve guide will improve peripheral nerve regeneration across large defects. To test this hypothesis, biodegradable poly(caprolactone) (PCL) nerve guides were prepared with manufacturing parameters optimized for protein delivery and retention at the injury site. Quantitative changes in the diffusion of small molecular weight proteins and glucose through PCL conduit walls were measured to determine the independent and combinatorial effects of three fabrication variables: wall thickness, pore size and porosity percentage. Double-walled microspheres were then fabricated as a method of sustained protein delivery, and were incorporated within the luminal wall of PCL nerve guides using a novel solvent specific embedding technique. The overall efficacy of our nerve guide design was confirmed by encapsulating and delivering GDNF in the rat sciatic nerve injury model. Evaluation of sensory reinnervation following a long gap, 1.5cm nerve injury at 16 weeks showed a significant increase in animal response time to stimuli from animals treated with GDNF as opposed to negative control PCL guides. Furthermore, the measured gastrocnemius contraction force in animals treated with GDNF was significantly higher than negative controls and was not significantly different from the isograft positive control group. Histological assessment of explanted conduits after 16 weeks showed improved tissue integration within GDNF releasing nerve guides compared to negative controls. Nerve fibers were present across the entire length of GDNF releasing guides, while nerve fibers were not detectable beyond the middle region of negative control guides. Therefore, the results reported within this dissertation support our original hypothesis that; the long-term delivery of a neurotrophic factor from nerve guides results in improved functional recovery above negative controls following large axonal defects in the peripheral nervous system