Electrical and magnetic stimulation separately modulates the extent and direction of neurite outgrowth in an ionically conductive hydrogel

Abstract Objective: The use of conductive materials for aiding peripheral nerve regeneration is a promising method to recapitulate native conductance of nerve tissue and facilitate the delivery of exogeneous stimulation for enhanced recovery. This study systematically investigated the effects of applying electrical or magnetic stimulation to neurons within new ionically conductive hydrogels.
Approach: The material properties of ionically conductive Gel-Amin hydrogels (Gelatin methacryloyl [GelMA] + Choline acrylate) were compared to those of GelMA hydrogels. Neonatal rat dorsal root ganglia (DRG) were encapsulated in both hydrogel formulations, subjected to electrical or magnetic stimulation, and evaluated for differences in neuronal extension. Peripheral glia, Schwann cells (SCs), were subjected to the same stimuli and their secretion of various neurotrophic analytes were investigated. 
Main Results: Gel-Amin hydrogels are 4x more ionically conductive than GelMA hydrogels. The application of electrical stimulation to the encapsulated cells led to a significant decrease (76%) in DRG outgrowth when encapsulated in GelMA versus the Gel-Amin hydrogel. In contrast, magnetic stimulation led to directional neurite extension in a direction perpendicular to the magnetic field gradient. 
Significance: We present here the first report of a controlled, direct comparison of electric and magnetic stimulation on whole DRG in synthetic materials. The combination of electrical and magnetic stimulation decreased total neurite outgrowth but led to more directional growth. Aspects of the material and type of stimuli were noted to reduce several cytokine secretion levels from primary SC cultures. These results highlight the importance of understanding material and biophysical interactions to enhance peripheral nerve regeneration.