Photovoltaic Retinal Prosthesis for Restoring Sight to the Blind

Blindness is one of the most devastating consequences of disease. We work on electronic restoration of sight to patients suffering from degenerative retinal diseases such as Retinitis Pigmentosa and Age-Related Macular Degeneration. In these conditions the photoreceptor cells slowly degenerate, leading to blindness. However, many of the inner retinal neurons that transmit signals from the photoreceptors to the brain are preserved to a large extent for extended periods of time.

Electrical stimulation of the remaining retinal neurons can percepts of light (phosphenes), and the first retinal implants involving a small number of electrodes (60) yielded encouraging results in patients with retinal degeneration. However, thousands of pixels are required for functional restoration of sight, such as reading and face recognition.

Development of a high resolution retinal prosthesis faces multiple engineering and biological challenges, such as delivery of information to thousands of pixels at video rate, placement of the electrodes in close proximity to the target cells, avoidance of fibrotic encapsulation of the implant, signal processing to compensate for the partial loss of the retinal neural network, and many others.

 

System Design

Data stream from a video camera is processed by a pocket PC, and the resulting images are displayed on a head-mounted microdisplay, similar to video goggles. From the microdisplay the images are projected onto retina using pulsed (1-10 ms) near-infrared (~900 nm) light. These light pulses are photovoltaically converted into bi-phasic pulses of electric current flowing between the active and return electrode in each pixel, which stimulates the nearby inner retinal neurons, and thereby introduces visual information into the retinal neural network.

Optical delivery of information and power allows for simultaneous activation of thousands of pixels in the implant, and retains a natural link between the eye movements and the visual perception. Since each photovoltaic pixel operates independently, they do not need to be physically connected to each other. Thus, small (1-2 mm in size) segments of the array may be separately placed into the subretinal space to tile a large visual field, greatly simplifying surgery.

Image on the right shows a 1-mm wide array implanted subretinally in a rat eye. SEM demonstrates a higher magnification view of the array with 70 um pixels placed on retinal pigment epithelium (RPE) in a porcine eye.

Color insert on the left shows a single pixel in the hexagonal array. Each pixel includes 3 photodiodes connected in series between the central active electrode (brown disk) and the circumpherential return electrode. Pixels are separated by 5 um trenches to improve diffusion of the nutrients through the implant.

 

Short animation about Photovoltaic Retinal Prosthesis

Proximity of Electrodes to Target Cells

Addressing the problem of proximity between the electrodes and neurons, we have found that certain 3-dimensional microstructures prompt the retina to migrate into the voids in the implant, with its neural circuitry largely intact. One strategy involves pillar microelectrodes that, upon retinal migration, reach the target layer of neurons.

 

Scanning Electron Micrograph of an array with pillars of 10 µm in diameter and 65 µm in height.

Histology of the RCS rat retina 6 weeks after implantation of a pillar array into a subretinal space. Tops of the pillars achieve an intimate proximity with the cells in the inner nuclear layer.

 

Conceptual diagram of the photovoltaic pixels with pillar electrodes (1) penetrating into the inner nuclear layer. The return electrodes (2) are located in the plane of the photodiodes.