Palanker Lab: BioMedical Physics

Restoration of Sight to the Blind:

Optoelectronic Retinal Prosthesis

Blindness is one of the most devastating consequences of disease. We develop electronic retinal prosthesis for 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 a prolonged period of time.

Electrical stimulation of the remaining retinal neurons can produce phosphenes - perception of light, and the first retinal implants involving a small number of electrodes (16 to 60) yielded encouraging results in patients with retinal degeneration. However, thousands of pixels are likely to be 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 that compensates for the partial loss of the retinal neural network, and many others.

Due to highly interdisciplinary nature of this project our group includes specialists from four departments at Stanford: Ophthalmology, Hansen Experimental Physics Lab, Electrical Engineering, and Neurobiology.

System Design Data stream from a video camera is processed by a pocket PC, and the resulting images are displayed on a liquid crystal microdisplay (LCD), similar to video goggles. The LCD corresponding to approximately 30 degrees of visual field is illuminated with a pulsed (0.5 ms) near-infrared (~900 nm) light, projecting the images through the eye optics onto the retina. The IR image is then received by the photovoltaic pixels in a subretinally implanted chip. Each pixel converts the pulsed light into a proportional pulsed bi-phasic electric current that introduces visual information into diseased retinal tissue. Retinal chip is approximately 3 mm in diameter, corresponding to 10 degrees of visual field. The 30 degree visual field is accessible by eye scanning.

Optical approach to information delivery 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, segments of the array may be separately placed into the subretinal space, greatly simplifying surgery.

Proximity of Electrodes to Target Cells Addressing the problem of proximity, we have found that certain 3-dimensional microstructures prompt the retina to migrate into very close proximity to the implant with its neural circuitry largely intact. One strategy involves pillar microelectrodes that, upon retinal migration, reach the required layer of cells.	Scanning Electron Micrograph of a lithographically manufactured 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.
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