| The aim of this thesis is to attain better understanding of the electrode-tissue interface of the epiretinal prosthesis and neural prosthetic devices in general. Three studies explore this interface from different aspects. The overall goal is to improve this interface to produce safe and efficient electrical stimulation and also provide the basis for computational models to better understand electrical stimulation targeted at the retina.;A novel method is described and analyzed to design current waveform input to reduce the edge effect---the primary reason for non-uniform current density distribution on electrodes. Finite element modeling and mathematical analysis showed that waveform design can reduce the edge effect on disk electrode without the need to alter the electrode's geometry. Current waveforms with a slower rise to steady-state level compared to the abrupt rectangular step can reduce the current density non-uniformity by allowing current density to redistribute over time. Numeric method optimized the design for the waveform, which can be approximated via a RC circuit. The approximation of the optimized waveforms was tested in a pulsing experiment. The results showed reduced corrosion on the edge of platinum disk electrodes, therefore demonstrating the effectiveness of the waveform shaping method.;The peak resistance frequency (PRF) method---a simple method to extract tissue resistance from impedance spectroscopy measurement of the electrode-tissue interface---was explored for its mechanisms and inherent properties. The PRF method uses a variable frequency point at which the impedance phase is most resistive to estimate the tissue resistance. The previous study that first proposed the method showed that it works very accurately, compared to the large deviation of estimations from fixed frequency points. In this study, theoretical analysis and computational simulation reveal that the PRF method is only a good approximation for the tissue resistance. The PRF method has an inherent deviation that varies depending on the idealness of the electrode-tissue interface but is nevertheless correctable. Further simulations tested the realistic limitation of measurement noise and frequency sampling, and showed that the PRF method works reasonably well and reliable under realistic conditions. This work provides a solid theoretical foundation for the PRF method and means to correct the results when the parameters of the interface are not ideal.;As a first step of electrical mapping within the retina tissue, the resistivity profiles of healthy and degenerate retina were measured, utilizing the aforementioned PRF method. The study provided data on both healthy and degenerate mice retinas for the first time, which are relevant models for neurophysiology study of retinal prostheses. The experimental results show that the peak resistivity decreases with degeneration. Also, the resistivity profiles were thinner in degenerate retina and the thinning agreed with histology data. Therefore, the changes in tissue properties need to be taken into account for modeling study of retinal prosthesis. |
