UC San Diego

Functional rehabilitation for traumatic central nervous injuries, both in the spinal cord and the brain, with interventional electrode arrays, have been shown to restore some degree of function and improve the quality of life for affected subjects. Arguably, the introduction of flexible, conformal materials such as Parylene-C and Polyimide, together with the development of 3-dimensional, low impedance materials has the potential to improve performance and functional outcomes. This thesis explores three components of flexible electrode design.The first part discusses the challenges with the scaling-up of broadly used passive electrode arrays, i.e., arrays with individually wired recording and/or stimulation contacts, and the potential for the integration of active electronics for amplification and switching signal pathways to smaller number of wires using amorphous Indium Gallium Zinc Oxide (a-IGZO) thin-film transistors (TFTs). We investigated the electrical properties of a-IGZO based TFTs and developed a physics-based model that captures the fabrication condition and size-dependent properties of a-IGZO TFTs. The second part of the thesis focuses on a more complete formulation of the underlying electrochemistry and the electrochemical safety limits for electrical stimulation through benchtop and in vivo experiments and the investigation of these safety limits by experimental validation in vivo. While modalities of tissue damage during stimulation have been extensively investigated for specific electrode geometries and stimulation paradigms based on water electrolysis, a comprehensive model that can predict the electrochemical safety limits in vivo doesn’t yet exist. We hypothesized that the factors that influence water electrolysis including electrode geometry and stimulation parameters can be systematically investigated and modeled with a single equation for the possible safe limits. Using detailed electrochemistry analysis and parametric immunohistochemistry for different stimulation regimens, we develop and validate a model for predicting the damage induced by electrical stimulation as a function of the stimulation and material design parameters. The third part of this thesis involves the application of electrical stimulation for highly focal subsurface neural recruitment in the spinal cord with using surface electrodes to target deeper neuronal pools for rehabilitating somatosensory function in patients with spinal cord injury (SCI). We demonstrate selective subsurface stimulation within the spinal cord using pulse-width modulated high frequency stimulation. We invoke brain, spinal cord, and muscle electrodes to investigate the mechanisms and effects of this novel stimulation paradigm.