Electrochemical models for electrode behavior in retinal prostheses

Abstract

The focus of this thesis is the modeling, characterization, and improvement of microfabricated electrodes for the Retinal Implant Project. The ultimate goal of the Project is to build a retinal prosthesis able to restore a limited degree of visual function in people suffering from certain types of blindness. An important step in this process is the design and fabrication of a safe, efficient, and effective electrode array. Designing such an array will require a detailed understanding of electrode properties and accurate models for their electrical and chemical behavior. This thesis represents a few initial steps towards that goal. Besides providing useful data on the current arrays, it is hoped that this thesis will also provide a good general introduction to electrode modeling and help others in the research group better understand the devices they are using. The thesis followed four main steps. The first step was to find an appropriate circuit model for the behavior of microfabricated electrodes in an electrolyte. After some preliminary observations, the Randles model was chosen as a convenient starting point. Several aspects of this model were discussed, including its impact on electrode design, its expected behavior using different measurement techniques, and its major limitations. The second step was to calculate experimental values for the individual elements in the model. This was done for a number of different electrode designs under various physical conditions. The data was collected using several different electrochemical measurement techniques, each of which was explained in reasonable detail. The third step was to understand the physical basis of each model parameter and find chemical or physical theories to explain and predict the observed values. This modeling work focused on the series resistance and double layer capacitance. The resistance was well fit by a recessed disk model with an additional term for the oxide film. Several important aspects of the capacitance scaling were explained by a simple model involving nonuniform current density at the electrode surface, but a great deal of work remains to be done in this area. The final step of the thesis was to suggest possible improvements on the current electrode design and point out several directions for future work.