Basic Optical and Electrical Properties of the Mini AERP:

The most crucial purpose of any retinal prosthesis is to transform incoming light into electrical signals of appropriate physiological strength able to stimulate the retinal ganglion cells (RGCs). As outlined by Zrenner et al. [9, 10] the output of microphotodiodes is insufficient to produce enough current to stimulate RGCs . Therefore the basic light transforming elements we have designed for the present mini AeRP are photodiodes of 1mm2, which produce electrical currents in the picoamps to microamp range (for details and testing on such photodiode cells see Appendix, which details a First Generation Prototype). Because the photodiodes are purely photovoltaic there will be no significant accumulation of heat. Color information is an important aspect of normal vision. The photodiodes will receive light of all wavelengths and depending on their specifications will peak at a particular wavelength. To mimic color perception we have placed thin color filters or colored fiber optic strands, in front of the photodiodes. The efficiency, so-called color currents, of the filter covered photodiodes with respect to the basic colors blue, green and red light have been tested. Finally the photodiodes are arranged in a circular array with a diameter of 8 mm, which thus contains 50 photodiodes.

A second important aspect of the mini AeRP is that light focused on the retina, by cornea and eye lens, has to be efficiently channeled onto the photodiodes. To this end we have placed in front of each photodiode a fiber optic bundle, which together form a fiber optic plate (FOP). In front of the FOP an array of microlenses is placed which will amplify the light input transferred through the fiber optic bundle to each individual photodiode. In this way the incoming light will be channeled on the individual color coded photodiodes and the photodiode array will roughly represent the light intensity of the image projected on the retina. The electric output of the photodiodes is send to the retina by biphasic leads consisting of electrically conducting polymer strands. The leads are held in place by a biocompatible, non-electrically conducting "umbrella"-shaped polymer sheet, which directly adheres to the inner limiting membrane (ILM) of the retina. The leads end in the "umbrella" at a distance of 10-20 ?m from its surface. When placed on the ILM, which is about 50 ?m in thickness and not very resistive, the distance between the photodiode leads and the RGC's is about 60-70 ?m. This should give descent resolution.

The basic design of the MINI AeRP and the photodiodes are schematically given in Figures below.

There are two major aspects of the design, which need theoretical and if possible experimental verification viz. the photometry of the system and the matrix algebra of the photodiode array. Experimentation is planned and in some aspects is underway.