An in vitro model of a retinal prosthesis

The electrode-retina interface of an epi-retinal prosthesis implant has been studied using finite-element modeling, electrochemical methods, and retinal array electrophysiology. The studies presented in this thesis have relied on the flexibility of custom designed and microfabricated multielectrode arrays to investigate the current distribution at the electrode-electrolyte interface and its interaction with retinal tissue.; A flexible photolithographic microfabrication process flow has been optimized for the fabrication of conformal MEAs for hippocampal and retinal slice electrophysiological experiments. (Seven different electrode pad layouts were used in the fabrication of various arrays.) Electrostatic finite-element modeling was used to model the inhomogeneous current distribution at the electrode surface.; Using cyclic voltammetry (CV), biphasic pulsing, and electrochemical impedance spectroscopy (EIS) it has been shown that (1) microelectrodes with diameters ≤100 mum exhibit increased charge density, and (2) that the impedance of microelectrodes transitions from a perimeter to an area dependent regime (e.g., 1r→1r2 ) as the frequency is decreased from 100 KHz to 10 Hz. The later, proves that the current distribution at the electrode surface is dynamic, and evolves during the time course of a stimulus pulse.; MEAs with 200 mum diameter stimulating electrodes and 10 mum diameter recording electrodes were employed in isolated tiger salamander retina electrophysiology studies. Pharmacological agents were used to isolate direct excitation of ganglion cells from excitation of other inner retinal cells. Strength-duration data suggests that if amplitude will be used for the coding of brightness in retinal prostheses, shorter pulses (200 musec) will allow for a smaller region in the area of the electrode to be excited over a larger dynamic range compared with longer pulses (1 ms). Both electrophysiological results and electrostatic finite-element modeling show that electrode-electrode interactions can lead to increased thresholds for sites half way between simultaneously stimulated electrodes (29.4 +/- 6.6 nC) compared with monopolar stimulation (13.3 +/- 1.7 nC). Presynaptic stimulation of RGCs with both 200 mum and 10 mum diameter electrodes yielded threshold charge densities of 12 +/- 6 nC/cm2 and 7.66 +/- 1.30 mC/cm2, respectively, while the required charge was 12.5 +/- 6.2 nC and 19 +/- 3.3 nC.