Photovoltaic Retinal Prosthesis for Restoration of Sight in Retinal Degeneration

Retinal degenerative diseases lead to blindness due to loss of the “image capturing” photoreceptors, while neurons in the “image-processing” inner retinal layers are relatively well preserved. Information can be reintroduced into the visual system using electrical stimulation of the secondary retinal neurons, the bipolar cells, which then transfer their responses to the rest of the retinal neural network. This approach enables preservation of many features of the retinal signal processing, and thereby allows restoration of sight. We developed a photovoltaic subretinal prosthesis, which converts incident light into pulsed electric current, stimulating the nearby inner retinal neurons. Results of the clinical trial with our implants (PRIMA, Pixium Vision) having 100um pixels, as well as preclinical measurements in rodents with 75 and 55um pixels, confirm that spatial resolution of prosthetic vision can reach the sampling density limit.

For a broad acceptance of this technology by patients who lost central vision due to age-related macular degeneration, visual acuity should exceed 20/100, which requires pixels smaller than 25um. Radial expansion of electric field in front of the flat arrays precludes scaling the pixels to such small dimensions. We are working on 3-dimensional electro-neural interfaces which should enable such a high resolution, and may even reach single-cell selectivity.

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System Design

Animation illustrating the conceptPRIMA system design

In our system, data stream from a video camera is processed by a pocket PC, and the resulting images are displayed on the augmented-reality glasses, shown on the right. Images are then projected from a microdisplay onto the subretinal implant (lower image on the right) using pulsed (1-10 ms) near-infrared (~880 nm) light. These light pulses are photovoltaicly 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, thereby introducing visual information into the retinal neural network. Optical delivery of the information and power allows for simultaneous activation of thousands of pixels in the implant, and retains the natural link between the eye movements and visual perception.

subretinal photovoltaic arrayIn preclinical studies, we found that prosthetic vision with subretinal implants preserves many features of natural visual processing, including flicker fusion at high frequencies (>20 Hz), adaptation to static images, antagonistic center-surround organization and non-linear summation of subunits in receptive fields, providing high spatial resolution. Results of the clinical trial with our implants (PRIMA, Pixium Vision) having 100um pixels confirm that spatial resolution of prosthetic vision can reach the pixel pitch (20/420 visual acuity with 100um pixels). Patients also demonstrated simultaneous perception of the peripheral natural and the central prosthetic vision.

We study the mechanisms of neural stimulation and characteristics of prosthetic vision ex-vivio and in-vivo, and optimize the system to enable high resolution prosthetic vision. These studies include modeling of electric field in tissue, ion channel dynamics, electrode-electrolyte interface, circuit performance, fabrication of the implants, and electrophysiological assessement of the retinal, cortical and behavioral responses to visual stimuli. We also participate in the design and data analys of the clinical trials of our PRIMA system manufactured by Pixium Vision.

3-dimensional electro-neural interface

For a broad acceptance of this technology by patients with geographic atrophy, visual acuity should exceed 20/100, which requires pixels smaller than 25um. However, limited penetration depth of the electric field formed by a planar electrode array constrains such miniaturization.

We developed a novel 3-D honeycomb confguration of an electrode array with vertically separated active (red) and return (blue) electrodes. This geometry is designed to leverage migration of the inner retinal cells into voids in the subretinal space. Insulating walls surrounding each pixel align the electric field vertically, thereby decoupling the field penetration depth from the pixel width. Alignment of electric field along the bipolar cells reduces the stimulation threshold and enables scaling the pixel size down to cellular dimensions.