Engineering a living electrode: Growth of neural networks within a 3D hydrogel

Abstract

State-of-the-art neuroprosthetic devices rely on stiff metallic electrodes to communicate with neural tissues. It was envisioned that a soft, organic electrode coating embedded with functional neural cells will enhance long term integration of neuroprosthetic devices with target cells through enabling development of synaptic connections between device and cell. To this end, this thesis explored the development of a degradable hydrogel scaffold to support neural network growth as a critical element for next-generation living electrodes. A significant challenge in developing this technology is to engineer a hydrogel carrier that can support growth and function of complex neuronal networks. In this thesis, the hydrogel poly(vinyl alcohol) functionalized with tyramine (PVA-Tyr) was used to explore the mechanical and biological cues necessary to support growth of neural tissues. To provide an appropriate neural cell niche, it is desirable to have not only neural cells, but supporting glia such as Schwann cells (SC) within the hydrogel carrier. It was hypothesized that specific mechanical and biochemical milieu for SC development within a 3D cellular scaffold will support neuronal cell growth and differentiation into functional neural networks. PVA hydrogels (5 wt% and 10 wt%) were fabricated with mechanical moduli comparable to nerve tissue while incorporating the biomolecules sericin and gelatin (PVA-SG) to support cell survival and attachment. The neuron-like PC12 cell was studied in co-culture with SCs to understand appropriate conditions for supporting growth and development of both cell types. This study also demonstrated techniques for assessing neurite outgrowth, neuronal electrical excitability and expression of matrix proteins used to understand network development in a 3D hydrogel. While both 5 wt% and 10wt% PVA-SG hydrogels were able to support SC survival, development of physiologically relevant morphologies was more prolific in 10 wt% PVA-SG. SCs and PC12s were co-encapsulated within PVA-SG hydrogels extending neurite networks that were physically supported by SCs to generate excitable neural networks. Microchannels were shown to guide the direction of neural network growth. It was demonstrated that PVA-SG hydrogels can support growth and differentiation of complex neural networks. Future work will focus on evaluating the development of neural stem cells within PVA-SG hydrogel scaffolds.