Abstract
Abstract: :
Purpose: We evaluate physical constraints on size and density of the stimulating electrodes in a retinal prosthesis. Limitations on design of the implant and potential resolution are imposed by cross–talk between neighboring electrodes, heating of the tissue, electrolysis, and intensity of the ambient light on retina. Methods: Using a model of extracellular stimulation of the nerve cell soma we derive the threshold values of current, voltage and power as a function of electrode size and of the distance between the electrodes and the target cells. Constraints on these values set by the cross–talk, by heat diffusion and by electrochemistry, determine the maximal pixel density of the electrode array. Results: Cross–talk between electrodes, electrolysis and tissue heating, which limit maximal spatial resolution of the array, depend critically on separation between electrodes and cells. For example, a pixel density which geometrically corresponds to 20/80 acuity (2500 pix/mm2 and a pixel size of 20 µm) cannot be achieved unless the target neurons are within 7 µm of the electrodes. At a separation between electrodes and cells by 50 µm, the maximal pixel density drops to 44 pix/mm2, and at 100 µm it is further reduced to 10 pix/mm2. Electrochemical processes limit the size of Pt electrodes to not smaller than 25µm even if the target cells are in proximity to the electrode surface. More durable materials such as IrOx or Ag/AgCl should be used in high resolution implants.Our calculations show that ambient light on retina is too dim, by a factor of at least 1000, to activate a practical prosthetic chip, so that other power sources will be needed. Conclusions: Electrode spacing and potential visual resolution are limited critically by the distance between the electrodes and the target cells. To achieve functional levels of acuity (20/200 or better) the cell–to–electrode distances will need to be very small, in the range of 20 µm or less, depending on the desired resolution.
Keywords: visual acuity • retina: proximal (bipolar, amacrine, and ganglion cells) • age–related macular degeneration