A survey is given on the status of developments, concerning a subretinal electronic microphotodiode array that aims at replacing degenerated photoreceptors. Various prototypes have been developed, tested, and implanted in various experimental animals up to 18 months. The fact that electrical responses were recorded from the Visual cortex of pigs after electrical stimulation by subretinal electrodes and the fact that responses are also recorded in-vitro in degenerated rat retinae, shows the feasibility of this approach. However, there are a number of open questions concerning the biocompatibility, the long-time stability,and the type of transmitted image to be solved before application in patients can be considered.

Micro-photodiode arrays have been developed which are intended to eventually replace the function of degenerated photoreceptor cells in the retina. Electric current generated by 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. Therefore, high capacitance of the interface between electrodes and tissue is an important prerequisite for efficient charge transfer. In addition, the electrical properties of the micro-photodiodes as are reflected in current/voltage traces have a profound influence on the charge delivery. For the first time the electrical properties of the entire system consisting of micro-photodiode array, electrode/electrolyte interface and tissue were studied. Our results indicate charged balanced operation of the device and the presence of a non-zero electrode polarization. The latter may be avoided, if an active current sink is employed in order to draw current to discharge the electrode capacitance.

The German 'SubRet' consortium has fabricated various types of microphotodiode arrays, the photovoltaic response of which will induce a retina response after implantation in the subretinal space. Ultrathin and flexible devices have been designed as well as CMOS-based chips with different pixel sizes and electrode configurations. From in-vitro electrostimulation studies, it becomes clear that a purely photovoltaic mode of operation will not be feasible, but an additional energy input by near-infrared radiation or radio frequency power transmission needs to be incorporated. Implantation experiments on pigs and rabbits prove the biocompatibility, general function and local stability of the implants. The inner retina architecture is well preserved, and literally no glia proliferation is found from long-term monitoring and histological examination. In conclusion, the subretinal approach is very promising and our results are encouraging, although major obstacles cannot be excluded concerning the application in human.

M.B. Schubert, A. Hierzenberger, H.N. Wanka, M. Graf*, H.G. Graf*,W. Nisch°Institute of Physical Electronics, University of Stuttgart,Pfaffenwaldring 47, D-70569 Stuttgart, Germany; *Institute forMicroelectronics Stuttgart, Allmandring 30a, D-70569 Stuttgart, Germany;°Natural and Medical Science Institute at Univ. of Tubingen,Eberhardstr. 29, D-72762 Reutlingen, Germany The development of an ultrathin, flexible Micro-Photodiode Array aimsto replace degenerated photoreceptors in the human eye to re-establisha certain amount of vision. For realizing devices according tobiocompatible requirements, a broad technological approach has beenchosen which in addition to crystalline silicon microelectronics alsoincludes amorphous silicon photodiodes for subretinal implantation.1. IntroductionSeveral types of diseases destroy theouter retinal layers and, in the finalstage, people become blind. Anambitious neurotechnology programwas started in Germany in 1995,pursuing the goal of developing aretinal implant. Two research teamshave chosen fundamentally differentapproaches to solve this problem,namely the subretinal [1] and theepiretinal [2] one. We report on thesubretinal approach here, which aimsat directly replacing degeneratedphotoreceptors with a Micro-Photodiode Array (MPDA) in thesubretinal space of the eye. TheseMPDAs have to be flexible, in order