Retinal implants powered by light could reverse some vision loss with simple surgery.
Light-powered eyes: This photovoltaic retinal prosthesis is a flexible sheet of silicon pixels that convert light into electrical signals that can be picked up by neurons in the eye. A scanning-electron micrograph shows the implant in a pig’s eye.
The new implant, which works like a combination digital imaging chip and photovoltaic array, requires much less bulky hardware than previous designs. The devices have yet to be tested in live animals or human patients, but the implants are creating excitement among researchers because they have greater pixel densities and may restore more vision than other retinal prosthetics being worked on.
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People suffering from macular degeneration (the most common cause of blindness among older people) and some other forms of blindness have lost the light-sensing cells in the retina but still have the underlying nerve cells that convey visual information to the brain. Retinal implants use electrodes to stimulate those nerves. Typically, the prosthetics require bulky electronics that sit on the eye to supply power, image data, or both to a chip inside the retina. The more hardware that’s installed in the body, the greater the risk to the patient. And the complexities of the electronics have typically limited the pixel counts of these systems.
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The new design, described today in the journal Nature Photonics, gets around these problems by using light as both image and power source. The device, designed by researchers at Stanford University in Palo Alto, California, combines infrared video-projection goggles with a small, wire-free chip implanted inside the retina.
A camera on the goggles transmits video to an image processor, which sends a signal back to infrared projection screens inside the goggles. Other researchers have tried to develop photovoltaic retinal implants in the past, but it didn’t work. “The light that you get into the back of the retina at the equator on a sunny day is not enough to power a retinal implant,” says James Loudin, a researcher at Stanford. So the Stanford system doesn’t rely on the light that comes into the eye; it uses a projection system to make much more intense signals. The researchers selected infrared light because it won’t damage or heat up any of the eye tissues and will not be picked up by any remaining light-sensitive cells and confuse the image, says Loudin.
The infrared image is picked up by a compact array of photovoltaic pixels implanted right where the light-sensing cells would be in a healthy eye. Each pixel contains three infrared-sensitive diodes facing the inside of the eye. The diodes convert light into electricity that’s pulsed out to the nerve cells by electrodes facing the back of the eye.
The Stanford scientists have mapped the resulting nerve activity in mice. Now they’re experimenting with various designs, including a flexible silicon array that can bend to the curvature of the eye. The most pixel-dense so far has 178 pixels per square millimeter. By comparison, the first retinal prosthesis to go to market (in Europe last March), made by Second Sight of Sylmar, California, has 60 pixels in total and requires bulkier hardware.
The next step for the Stanford device is a few more years of safety testing before clinical trials.
