Novel Micro-Photodiodes for Retina Stimulation
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Novel Micro-Photodiodes for Retina Stimulation M. Rojahn and M.B. Schubert Univ. of Stuttgart, Institute of Physical Electronics, Pfaffenwaldring 47, D-70569 Stuttgart, Germany Email: [email protected] ABSTRACT We present a new design of micro-photodiodes for in-vitro tests to electrically stimulate the ganglion cells of chicken and rat retinae upon light exposure of the photodiodes. Based on amorphous silicon, our laterally series connected double-stacked micro-photodiodes provide an open circuit voltage of 2.3 volts. Photolithographic steps as well as etching procedures for patterning the back contact, the amorphous silicon layers and the front contact are described. We analyse current- voltage-measurements performed with direct contact of the metal needles of a micro-positioning system to the device’s electrodes. In order to test the performance of an individual micro-photodiode in an electrolyte environment, the stimulation electrode of the device is also contacted with a micro-droplet of buffer solution. Further improvement is needed, mainly addressing the problem of long-term stability of the device in electrolyte environments. INTRODUCTION Over the last few years a number of research groups have started working on technical solutions to repair certain malfunctions of the light sensitive retina part of the eye. Diseases such as retina pigmentosa cause the photoreceptor layer of the retina to gradually degrade, eventually leading to complete blindness. The subretinal approach [1-2] aims at the direct functional replacement of the photoreceptor layer by a device that delivers electrical stimulation upon daylight illumination. Such a device will be implanted in a way that the bipolar and ganglion cell layers remain intact and come in close contact with the device’s electrodes. Our German consortium has made some progress [3,4], especially in better defining the requirements for successful retina cell stimulation. Figure 1 depicts the general idea of the subretinal prosthesis. Experiments show that ganglion cell activity can indeed be triggered by applying an electric field in transverse direction over the still functional retina layers [9-10]. However, the ganglion cell activity strongly depends on the applied voltage. Due to series resistance and electrode polarization, generally more than 1.5 volts of photodiode output voltage will be needed to trigger cell responses that clearly correlate with the applied field. The main objective of our current work is to provide micro-photodiode arrays (MPDA) which deliver up to 2.3 volts open circuit voltage (Voc). As a result of the above mentioned in-vitro stimulation tests, we know that the electrical coupling of any device’s electrodes and the cell layers in the natural electrolyteenvironment is governed by capacitive charge transfer. Consequently, the distance between the electrodes and the cells as well as the geometric size of the electrodes are of great importance. With our new device we can test a variety of electrode sizes in in-vitro experiments.
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