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J. Loudin, D. Palanker; Photovoltaic Retinal Prosthesis. Invest. Ophthalmol. Vis. Sci. 2008;49(13):3014.
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© ARVO (1962-2015); The Authors (2016-present)
Electronic retinal prostheses aim at restoring sight in patients with retinal degeneration by stimulating the retinal neurons with electrical pulses applied via an array of microelectrodes. Most implants involve inductive or optical transmission of data and power to an intraocular receiver, with a subsequent distribution of the decoded signals to retinal electrodes through an intraocular cable. Surgical complexity could be minimized by an "integrated" prosthesis, in which both power and data are delivered directly to the stimulating array without any other discrete components and cables. We present a design of a photovoltaic retinal prosthesis, in which photodiodes integrated into the stimulating array receive both power and data.
Processed video camera images are displayed on a goggles-mounted LCD screen and projected onto the retinal implant with a pulsed infrared beam. We explored the photovoltaic performance of a series of photodiodes connected to platinum disk electrodes, bathed in phosphate buffered saline. Current and voltage waveforms, as well as light-to-current conversion efficiency were measured for 0.5 ms pulses of 635 nm light, for both anodic and cathodic polarities.
Efficiency of light-to-current conversion of the two-photodiode pixels exceeded that of a single diode at light intensities exceeding 75 uW/pixel. Efficiency of the 3-diode pixels was slightly lower than that of the 2-diode ones due to an additional voltage step over the third diode. 2-diode pixels could generate up to 40 µA of current with linear light-to-current conversion, corresponding to maximum charge injection of 1.0 mC/cm2, as opposed to 0.25 mC/cm2 with a single diode. The number of pixels that can be chronically powered in this approach is limited by the maximum permissible ocular exposure (MPE) to IR radiation. For an MPE of 1 mW/mm2, pulse duration of 0.5 ms, and repetition rate of 25 Hz, the minimum pixel size for generating currents up to 20 µA is 30 um.
Photovoltaic pixels composed of a series of 2 or 3 photodiodes provide significantly higher charge injection when compared to a single photodiode, and can generate sufficient current for cellular stimulation without an external bias. Such a design eliminates the need for the intraocular power supply which is necessary for single diode pixels operating photoconductively. Since visual information is projected to all pixels simultaneously, the retinal prosthesis is compact and simple, and maintains the natural link between image perception and eye movements. The pixel size in such a photovoltaic implant can be as small as 30 µm, and multiple arrays can be placed independently to cover a larger field of view.
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