Technique Aims to Restore Vision

A pinpoint-sized photovoltaic chip implanted under the retina of a blind rat (upper right). The background image shows the chip’s array of photodiodes. A single pixel of the implant (lower left) shows 3 diodes around the perimeter and an electrode in the center. The diodes turn light into an electric current that flows from the chip into the inner layer of retinal cells.The Palanker lab

In a proof-of-principle study, researchers developed retinal implants that can potentially deliver images to surviving neurons in the eye and restore vision.

Retinal degenerative diseases such as age-related macular degeneration are the leading cause of vision loss and blindness in older Americans. These diseases lead to destruction of photoreceptors, the cells in the retina that detect light. Inner retinal neurons, which deliver signals to the brain for processing, largely survive these diseases. Researchers have thus been working to develop artificial retinal implants to substitute for the damaged photoreceptor cells. They've made impressive progress, with some now in clinical trials. But current implant designs have drawbacks, including bulky power-receiving hardware and the inability to use natural eye movements to scan a scene.

Drs. Daniel Palanker and James Loudin of Stanford University wanted to design a better approach. The system they conceived involves a thin retinal implant and a pair of goggles with a miniature camera. One advantage to their strategy is that the bulky components are in the goggles, not the eye itself. The camera is connected to a portable computer to process the video. A miniature projection system similar to conventional video goggles then projects images into the eye using near-infrared laser light.

The core of their approach is a thin photovoltaic silicon chip implanted beneath the retina. Near-infrared light isn't visible to the naked eye, but photodiodes on these chips can sense it and convert it into electrical pulses. Surviving neurons in the inner nuclear layer of the retina would then pass the electric signals to the brain, potentially restoring vision. The team fabricated the photodiode arrays and collaborated with Dr. Alexander Sher's laboratory at the University of California, Santa Cruz, to test whether their approach could work. Their study was funded in part by NIH's National Eye Institute (NEI).

The scientists placed isolated rat retinas between a photodiode array and an array of 512 electrodes capable of detecting the activity of about 100 retinal neurons. They projected pulses of patterned near-infrared light onto the photodiode arrays and measured the responses of both normal and damaged retinas. Their results appeared in the early online edition of Nature Photonics on May 13, 2012.

Rat retinas normally wouldn't react to near-infrared light. But with the photodiode array in place, retinas from normal rats responded to both normal visible light and near-infrared light. The degenerated retinas, no longer responsive to normal light, were able to generate strong electrical spikes in response to the near-infrared light. The degenerated retinas required greater amounts of near-infrared light to achieve the same level of activity as the normal rat retinas. However, the irradiation was still well within established safety limits.

“It works like the solar panels on your roof, converting light into electric current,” Palanker says. “But instead of the current flowing to your refrigerator, it flows into your retina.” Every pixel in the array is like a little solar cell, generating current and stimulating neurons in the inner nuclear layer of the retina, which then pass on the visual information to the brain.

The scientists have implanted the photodiodes in rats' eyes. While the study is still under way, preliminary data suggests the visual signals are reaching the rats’ brains.

—by Harrison Wein, Ph.D.