Three-Dimensional Polymeric Topographies for Neural Interfaces

Optimal integration of artificial interfaces with neural tissue is critical to develop novel neural regeneration scaffolds and neuroprosthetic implants. The topography of the implantable device can promote nerve repair as it affects neuronal growth via contact guidance. Furthermore, topography may be utilized to establish a stable and close contact with neural tissue required to improve the electrical neuron-device coupling. The goal of this thesis is to investigate the effects of nano- and microtopographies on the development and adhesion of embryonic cortical neurons. Three different polymer surfaces with topographical structures of varying dimensions were used – namely, i)anisotropic poly(N-isopropylacrylamide) (PNIPAAm) nanogels, ii) isotropic OrmoComp nanopillars, and iii) isotropic poly(3-hexylthiophene-2,5-diyl) (P3HT) micropillars. Anisotropic PNIPAAm nanogel arrays induced perpendicular alignment of major neurites and accelerated axon development, resulting in an ~80% increase in axon length compared to either unstructured nanogel substrates or glass substrates. Despite being relatively soft compared to glass substrates, unstructured nanogels did not induce substantial changes in neuronal morphology, indicating that neurons “perceive” unstructured nanogels as equivalent to glass. Isotropic OrmoComp nanopillars aligned neurites along topographically dictated angles (0°, 90°) with higher pillars (400 nm) confining neurites to a greater extent compared to lower pillars (100 nm). Furthermore, higher nanopillars promoted growth cone elongation and axon development resulting in ~40% longer axons compared to flat substrates. A larger surface area of the nanopillars was correlated with higher density of point contact adhesions in the growth cone and a reduction in actin retrograde flow rates, indicating a stronger coupling between the growth cone and the substrate which enables accelerated and persistent neurite outgrowth. Furthermore, F-actin accumulations and paxillin-rich adhesions were observed in the neuronal soma at nanopillars, indicating that neurons form a close contact with the nanoscale topography. Isotropic P3HT micropillars represent a relatively soft interface that enables neurons to achieve a close and conformal contact mediated by membrane rearrangements. Optical stimulation of embryonic neurons growing on photosensitive P3HT substrates induced a significant increase in neurite outgrowth compared to control substrates without deleterious effects on neuronal viability. The effects of photostimulation were further enhanced by the microtopography, indicating that P3HT acts as an active interface with possible applications in $\textit{in vitro}$ neural regeneration scaffolds. Furthermore, MEAs functionalized with P3HT micropillars yielded a significant increase in the signal-to-noise ratio (SNR) compared to flat MEAs, indicating that micropillars improve the cell-electrode coupling. Optical stimulation of spontaneous network activity on P3HT-functionalized MEAs induced neuronal firing and increased the firing rate. Although the process was not fully reproducible, optical excitation of P3HT interfaces provides a promising strategy for modulating network activity in a non-invasive manner