Micro-photodiode arrays as artificial retina implant: electrical properties

Micro-photodiode arrays have been developed and are intended to serve as a replacement for degenerated photoreceptor cells in the retina. The chips are implanted into the sub-retinal space. Electric current generated in tiny micro-photodiodes is delivered to the neuronal network in the retina via micro-electrodes. Since the coupling between electrode and tissue is capacitive of nature, only transient signals may be used for stimulation. Accordingly, a high capacitance of the interface between electrodes and tissue is an important prerequisite for efficient charge transfer. In addition, the electric properties of the micro-photodiodes as are reflected in current/voltage traces have a profound influence on the overall charge delivery. Therefore, the entire system made up of micro-photodiode array, electrode/electrolyte interface and tissue has to be taken into consideration. Current delivery has been both calculated and measured based on current/voltage traces and electrode capacitance data. The authors' results indicate charged balanced operation of the device and a non-zero electrode polarization. The latter may be avoided, if an active current sink is employed in order to draw backward current.