Injectable hydrogels of optimized acellular nerve for injection in the injured spinal cord

Objective. Spinal cord injury (SCI) affects a quarter million individuals in the United States, and there is currently no clinical treatment. Both fresh and acellular peripheral nerve grafts can induce spinal axon regeneration and support functional recovery in experimental injury models. Nonetheless, a scaffold that can be injected into a spinal contusion would be far less invasive to apply. We aimed to develop the first injectable acellular nerve graft for promoting repair after contusion SCI. Approach. We report a method to enzymatically solubilize optimized acellular (OA) nerve-a decellularized peripheral nerve graft developed in our laboratory and currently used clinically-to obtain an injectable solution that undergoes thermal gelation under physiological conditions. We quantified multiple physical and compositional properties of this novel material as well as tested its efficacy at acute and chronic time points following cervical contusion SCI. Main Results. This injectable optimized acellular (iOA) nerve graft retains native chemical cues such as collagens and glycosaminoglycans. By varying hydrogel concentration, the rheological properties and compressive modulus of iOA were similar to that previous reported for rat central nervous tissue. iOA solution was compatible with rat Schwann cells in culture, and hydrogel injection into a rat cervical contusion model significantly reduced the ratio of M1:M2 macrophages after one week, favoring regenerative phenotypes (p < 0.05). Furthermore, while iOA treatment did not affect locomotor or respiratory recovery over an eight week period, the percentage of axonal coverage increased at the distal tissue interface (p < 0.05), suggesting enhanced axonal extension within this region. Significance. Our data indicate that this novel injectable form of acellular nerve grafts is amenable for use after contusion SCI and may bolster a simultaneous therapy by acutely modulating the inflammatory milieu and supporting axonal growth.