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May 13, 2012

Eye Implants

Technology Review - Retinal implants powered by light could reverse some vision loss with simple surgery.

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.



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. Nature Photonics/Stanford



BIONIC EYE - A system being tested in rats may partially restore sight for some blind people. A handheld computer processes images from a video camera that sits on specialized goggles. Lasers inside the goggles send that information to photovoltaic chips implanted in the eye, stimulating nerve cells that send information to the brain. The person then perceives the images seen by the camera.James Loudin/Nature Photonics




Specialized goggles that send information to solar cell–like chips implanted in the eyes may one day help some blind people. The new implants, which have been tested in rat retinas in a dish, would require less invasive surgery than similar devices now being tested and offer a higher-resolution view of the world.

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.

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