Development of Amorphous Silicon Carbide Ultramicroelectrode Arrays for Neural Stimulation and Recording

Interest in restoring lost function using neuro-prosthetic devices and treating neurological disorders or neurodegenerative diseases through electrical stimulation of neural activity has increased in recent years. For example, implantable cortical neural interfaces allow investigation of sensorimotor learning, and control of both natural and prosthetic limbs through recording of volitional intent and stimulation of neural activity. However, these interfaces decline rapidly in performance over chronic timescales. Foreign body response is believed to limit their recording and stimulation reliability. The resulting glial scar isolates the indwelling microelectrodes from healthy neuronal cells. The consequence is recording from large populations of weak neural signals and the requirement for high current amplitudes to deliver the necessary charge for neural activation. Recently, carbon fiber microelectrodes with small cross-sectional dimensions (below 10 µm) have been shown to reduce insertion damage to neurons and microvasculature, minimize adverse tissue reaction, and provide stable neural recording over chronic timescales. Despite these achievements, the development of carbon fiber MEAs faces the issue of micro-assembly, micromanipulation, and the general lack of control of the geometric surface area (GSA) of the active sites. This dissertation addresses these issues by developing ultrathin cellular or sub-cellular scale microelectrode arrays (MEAs) based on amorphous silicon carbide. Amorphous silicon carbide (a-SiC) deposited by plasma enhanced chemical vapor deposition has similar mechanical properties to carbon fiber but is amenable to thin-film microfabrication methods, thus permitting a wide variety of designs, control of GSA, and batch fabrication of microelectrode arrays. Challenges associated with residual stress control in the a-SiC and those associated with metal patterning needs to be addressed to use the a-SiC in ultrathin MEA designs. Also, implantation strategies for ultrathin MEA shanks and the burden of using small contact sites for electrochemical measurement, electrical stimulation and electrophysiology need to be addressed. In this dissertation, microelectrode arrays based on a-SiC were fabricated, characterized for their electrochemical properties in a saline model of the interstitial fluid, and evaluated functionally in songbird and rat brain. We describe stress engineering in the multilayered structure to regulate the curvature of the a-SiC MEAs. Engineering challenges associated with process controls to produce penetrating probes of reduced cross-sectional shank dimensions are discussed. We have developed implantation strategies to insert ultrathin a-SiC MEAs into rat motor cortex. We show that a minimum a-SiC thickness of 6 µm is required to insert 2 mm long a-SiC MEAs shanks into rat cortex without the need for insertion guides or temporary support structures. Below this thickness, we demonstrate that a-SiC MEAs will require temporary support structures such as polyethylene glycol or in situ designs that increases the critical buckling load of the implanted shanks. With the reduced shank dimensions, the electrode sites on the a-SiC MEA are small with high electrode impedance and low charge injection properties. We investigated low impedance coatings such as titanium nitride, sputtered iridium oxide and electrodeposited iridium oxide films as a means of improving the electrochemical performance for neural stimulation and recording. We show that cathodal charge injection capacities greater than 17 mC/cm2 can be achieved with the coated ultramicroelectrode site with appropriate biasing.