Photovoltaic Retinal Prosthesis for Restoring Sight to the Blind

We develop electronic restoration of sight to patients blinded by degenerative retinal diseases, such as Retinitis Pigmentosa and Age-Related Macular Degeneration. In these conditions the photoreceptor cells slowly degenerate, leading to loss of sight. However, many of the inner retinal neurons that transmit signals from photoreceptors to the brain are preserved to a large extent. Electrical stimulation of the remaining retinal neurons can reintroduce information into the visual system.

Clinical trials of retinal prostheses demonstrated feasibility of eliciting perception of patterns of light in patients blinded by retinal degeneration. However, these implants include extraocular power supplies, which require complex surgery involving trans-scleral cables, making them prone to various complications. They also provided very low visual acuity - typically below 20/1200, while much higher acuity is required for functional restoration of sight, such as reading and face recognition.

Development of a high resolution retinal prosthesis involves multiple engineering and biological challenges, such as delivery of information to thousands of pixels at video rate, placement of the tiny electrodes in close proximity to the target neurons, 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

In our photovoltaic system, 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. Images are then projected onto retina using pulsed (1-10 ms) near-infrared (~880 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 stimulate the nearby inner retinal neurons, and thereby introduce 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 visual perception. Since each photovoltaic pixel operates independently, they do not need to be physically connected to each other. Thus, small (1-2 mm) modules can 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 of the array with 70um pixels placed on retinal pigment epithelium 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 circumferential return electrode. Pixels are separated by 5 um trenches to improve diffusion of nutrients through the implant.

We study characteristics of prosthetic vision in-vivo and ex-vivio, and develop this system for upcoming clinical trials.

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 degenerate 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.