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  • Silicon Implant Aims to Restore Sight
May 2012
STANFORD, Calif., May 14, 2012 — A new type of retinal prosthesis, which uses technology similar to that found in solar cells, could hold the key to those who suffer from degenerative eye diseases like macular degeneration and retinitis pigmentosa.

Researchers at the Stanford University School of Medicine have developed a pair of goggles that interfaces with a tiny chip implanted in the retina and converts light into electrical signals, stimulating the patient's optical nerves and allowing them to see once more.

"It works like the solar panels on your roof, converting light into electric current," said Daniel Palanker, associate professor of ophthalmology and one of the paper's senior authors. "But instead of the current flowing to your refrigerator, it flows into your retina."

The goggles are equipped with a camera and a pocket computer that feed an image data to a liquid crystal microdisplay. The image is then beamed to the implant using laser pulses of near-infrared light, where it is received by a photodetector silicon chip, which converts the light into an electrical current. The current stimulates the optical nerves, sending the image data to the vision centers of the brain. The whole process is similar to how a digital camera takes a picture.

This pinpoint-sized photovoltaic chip [upper right corner] is implanted under the retina in a blind rat to restore sight. The center image shows how the chip is comprised of an array of photodiodes, which can be activated by pulsed near-infrared light to stimulate neural signals in the eye that propagate then to the brain. A higher magnification view [lower left corner] shows a single pixel of the implant, which has three diodes around the perimeter and an electrode in the center. The diodes turn light into an electric current, which flows from the chip into the inner layer of retinal cells. (Image: Daniel Palanker)

The chip itself is the size of a pencil point and contains hundreds of light sensitive diodes. Using light to transmit the data instead of wire, coils, or antennae like other current implants, keeps the chip from being bulky and makes them easier to implant.

"The current implants are very bulky, and the surgery to place the intraocular wiring for receiving, processing and power is difficult," Palanker said. "The surgeon needs only to create a small pocket beneath the retina and then slip the photovoltaic cells inside it."

The researchers studied the effectiveness of the implants by putting them into the retinas of blind and normal rats. The retinal ganglion cells of the normal rats were responsive to stimulation by normal visible light as well as the near-infrared, which showed that the implants were responding to the non-visible light. In the blind rats, normal light generated very little ganglion response, whereas the near-infrared caused spikes in the rats' neural activity similar to that of the normal rats. However, the blind rats needed much more infrared light to achieve the same activity levels as the normal rats.

The work was funded by National Institutes of Health, the Air Force Office of Scientific Research and Stanford's Bio-X program. Others involved in the research include co-first authors Keith Mathieson and James Loudin; graduate students Georges Goetz, David Boinagrov and Lele Wang; senior research associate Philip Huie; research associates Ludwig Galambos and Susanne Pangratz-Fuehrer; and postdoctoral scholars Yossi Mandel and Daniel Lavinsky. In addition, Theodore Kamins, a consulting professor in electrical engineering, and James Harris, professor of electrical engineering, are co-authors.

The research was published online May 13 in Nature Photonics, and the researchers are seeking a sponsor to support human trials.

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The inability to perceive visual images (visible radiant energy). In human beings, blindness is defined as a visual acuity of less than one-tenth normal vision.
solar cell
A device for converting sunlight into electrical energy, consisting of a sandwich of P-type and N-type semiconducting wafers. A photon with sufficient energy striking the cell can dislodge an electron from an atom near the interface of the two crystal types. Electrons released in this way, collected at an electrode, can constitute an electrical current.
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