The effects of gradients of nerve growth factor immobilized PCLA scaffolds on neurite outgrowth in vitro and peripheral nerve regeneration in rats

Introducing concentration gradients of nerve growth factor (NGF) into conduits for repairing of peripheral nerve injury is crucial for nerve regeneration and guidance. Herein, combining differential adsorption of NGF/silk fibroin (SF) coating, the gradient of NGF-immobilized membranes (G-Ms) and nanofibrous nerve conduits (G-nNCs) were successfully fabricated. The efficacy of NGF gradients was confirmed by a quantitative comparison of dorsal root ganglia (DRG) neurite outgrowth on the G-Ms or uniform NGF-immobilized membranes (U-Ms). Significantly, the neurite turning ratio was 0.48 ± 0.11 for G-M group, but it was close to zero for U-M group. The neurite length of DRGs in the middle of the G-Ms was significantly longer than that of U-M group, even though the average NGF concentration was approximated. Furthermore, 12 weeks after implantation in rats with a 14 mm gap of sciatic nerve injury, G-nNCs achieved satisfying outcomes of nerve regeneration associated with morphological and functional improvements, which was superior to that of the uniform NGF-immobilized nNCs (U-nNCs). Sciatic function index (SFI), compound muscle action potentials (CMAPs), total number of myelinated nerve fibers, thickness of myelin sheath were similar for the G-nNCs and autografts, with the G-nNCs having a higher density of axons than the autografts. Our results demonstrated the significant role of introducing NGF gradients into scaffolds in promoting nerve regeneration.