A curvable silicon retinal implant [electronic resource]
Keywords: Electrical Engineering

In age-related macular degeneration (AMD) and retinitis pigmentosa (RP), two leading causes of blindness, the photoreceptor layer of the retina is degenerated while the rest of the retina is well preserved. The function of the photoreceptors is very similar to that of solar cells. Upon receiving light, they activate the inner layers of the retina by producing electrical and chemical signals. These signals are then processed and compressed by a complex circuit of retinal neurons (horizontal cells, bipolar cells, amacrine cells, and ganglion cells) and sent to the brain. The brain perceives these data as sight. Electronic retinal implant systems seek to restore sight in AMD and RP by electrical stimulation of the surviving retinal neurons. Currently the more dominant systems consist of a microelectrode array, which is placed directly on the ganglion cells (epiretinal). In this approach, a camera mounted on goggles captures video which is then processed by a pocket computer. The power and data are then transmitted wirelessly to an extraocular receiver unit. Through an intraocular cable, the receiver unit sends appropriate electrical stimuli to the array of microelectrodes. The electrodes then stimulate the ganglion cells by passing current pulses through the tissue. These stimulations are perceived by the brain as spots of light. The epiretinal approach has some disadvantages. Because the electrodes directly stimulate the ganglion cells, the image processing and data compression capabilities of the retina are not utilized. Placing the extraocular receiver unit and connecting it via a cable to the microelectrode array significantly complicates the surgical procedure and increases the chance of post surgical complications. Additionally, the perceived images are independent of the eye movements. We have developed an integrated photovoltaic monolithic silicon retinal implant that requires no electrical power or data connection. In our design, a miniature camera captures video that is processed by a pocket computer before being projected into the eye at a near-infrared wavelength ([Lambda] = 905 nm) onto the silicon implant located in the subretinal space (area in the back of the retina). The implant consists of a two-dimensional array of photovoltaic pixels. The projected image is provided in pulsed fashion and each pixel element consists of up to three series-connected photovoltaic cells such that the pixels deliver current pulses that are sufficiently strong to stimulate the remaining functional neural cells. The current pulses are interpreted as visual images by the brain. Placing the implant in the subretinal space allows for utilization of the existing image processing and data compression functions of the retina. Each pixel receives both power and data directly through laser radiation. This eliminates the need for a separate wireless receiver unit and simplifies the surgical procedures and reduces the post-surgical risks. Additionally, in this approach, eye movements change the perceived images. The novelty of the work reported here is the integration of photovoltaic devices in a MEMS process that allows the implant to deform to the natural curvature of the eye, while also providing isolation between the bodies of the three series-connected subpixels that make up each pixel. This was achieved by patterning the implants into an array of pixels connected together by deformable silicon flexures. In addition, the trenches also provide electrical isolation between the three series-connected photovoltaic subpixels. Curving the implant is advantageous since the complete implant is in focus, resulting in optimum quality of vision perceived. Curved implants can also be substantially larger than planar implants and can hence cover a larger part of the field of view. A curvable implant also causes no mechanical strain or abnormality in the eye. Usage of three series-connected subpixels per pixel improves the impedance matching to the surrounding tissue and enhances the injected current per pixel allowing for higher resolution implants. Fabricated implant with a resolution of 64 pixels/mm2 can inject sufficient current for neural activation at safe optical intensities. Additionally, the exchange of nutrients and waste, which is necessary for the survival of the retinal cells, is provided by diffusion through the trenches that define the silicon devices.