gms | German Medical Science

Purpose: Electronic retinal prostheses aim at restoring sight in blind patients with retinal degeneration by patterned electrical stimulation of surviving inner retinal neurons. Most designs use inductive or optical serial telemetry to wirelessly deliver power and data to implanted circuitry. The incoming signals are then decoded and electrical stimuli distributed to each pixel via an intraocular cable. We have designed and fabricated a photodiode-based prosthesis in which all pixels receive power and data optically in parallel and without the need for external circuitry or wiring, simplifying both the implant design and surgical procedures.

Methods: Processed camera images are projected onto the retinal implant by video goggles using pulsed infrared (905 nm) light. Silicon photodiodes convert this pulsed light into biphasic photovoltaic current. Each pixel has a central stimulating electrode surrounded by a photosensitive zone (covering 50% of the total area), and a peripheral return electrode. Trenches in 30 µm-thick monocrystalline silicon were etched to separate the pixels, leaving thin (~0.5µm) “springs” holding the array together. The resulting flexible implant conforms to the curvature of the eye, while the trenches electrically isolate neighboring pixels. Electrophysiological response of the retina to optoelectronic stimulation is being studied using multielectrode arrays.

Results: Single diode and three series diode implants were fabricated, with pixel sizes of 230, 115 and 58 µm containing 80, 40, and 20 µm diameter stimulation microelectrodes with corresponding pixel densities of 16, 64, and 256 pixels/mm². Implant sizes were 1x1.2 and 2x2 mm, for implantation into rat and cat eyes, respectively. Initial in vitro tests in phosphate buffered saline indicate a maximum charge injection of 1.7 mC/cm² per phase for SIROF microelectrodes. Surgical methods were developed, and successful subretinal implant placement evaluated using optical coherence tomography. Threshold light intensity for eliciting retinal response with 1 ms pulses was 1 mW/mm².

Conclusions: A photovoltaic subretinal prosthesis has been fabricated and tested. Since each pixel operates independently, they do not need to be physically connected to each other. Thus, segments of the array may be separately placed into the subretinal space, allowing for enlargement of the stimulated field, and greatly simplifying surgery.