Solar cell-like implant stimulates optic nerve

Ashley N. Paddock, ashley.paddock@photonics.com

A new retinal prosthesis that uses technology similar to that found in solar cells could restore sight to those who suffer from degenerative eye diseases such as macular degeneration and retinitis pigmentosa.

Researchers at Stanford University School of Medicine have developed goggles that interface with a tiny chip in the retina and convert light into electrical signals, stimulating the optic nerve and allowing patients to see once more.

“While high-fidelity color vision is a long way off, some patients with retinal prostheses have so far been able to read large fonts (with visual acuities on the order of 20/1000) and complete daily tasks in ways that they could not before treatment,” postdoctoral scholar Dr. James Loudin told Photonics Spectra. The research team hopes to achieve visual acuity better than 20/200 and is focusing its efforts on assessing the visual acuity and contrast sensitivity in vivo, said Dr. Daniel Palanker, an associate professor of ophthalmology at Stanford.

The goggles work similarly to solar panels on the roof of a building, converting light into electric current. Instead of the current flowing to household appliances, however, it would flow into retinas, Palanker said.

This pinpoint-size photovoltaic chip (upper right corner) is implanted under the retina in a blind rat to restore sight. The center image shows how the chip comprises an array of photodiodes, which can be activated by pulsed near-infrared light to stimulate neural signals in the eye that propagate 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. Courtesy of Daniel Palanker.

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

The chip 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 such as other current implants keeps the chip from becoming bulky and makes it easier for implantation.

Several other retinal prostheses are in the works, and at least two are in clinical trials. Second Sight of Los Angeles developed a device that was approved in April for use in Europe, and German prosthesis maker Retina Implant AG recently announced the results from its clinical testing in Europe.

“Second Sight uses RF telemetry to power and transmit data to an array of 60 electrodes over several square millimeters in their Argus II device,” Loudin said. “We have proven the functionality of single pixels as small as 70 µm, with pixel densities of up to 178 pixels per square millimeter. The Retinal Implant AG device has similarly high resolution but requires additional implanted hardware to power it.”

The advantages of Stanford’s approach, Loudin said, are “its high pixel density, easy scalability and lack of separate, bulky power-receiving hardware, which makes surgical implantation easier and thus reduces the risk of complications.”

The researchers tested the effectiveness of the implants in the retinas of both blind and normal rats. The retinal ganglion cells of treated normal rats were responsive to stimulation by plain visible light as well as to the near-infrared, which showed that the implants were responsive to nonvisible light. In the blind rats, the scientists observed that visible wavelengths generated very little ganglion response, whereas the near-infrared caused spikes in the rats’ neural activity similar to those in normal rats. The blind rats, however, needed significantly more infrared light to achieve the same activity levels as in normal rats.

Although these technologies induce color perception in patients, this perception is difficult to predict and control, Loudin said. These electrically stimulated percepts enable patients to see a variety of colors, including yellow, blue, red and white.

“A device with precise, predictable spatial control of the color of these percepts across many patients is many years off.” The scientists next will evaluate the in vivo perceptual resolution of these devices, and are working to understand how high a resolution is possible with this approach. They are seeking a sponsor to support human clinical trials, which they say will depend on the availability of industrial partners and funding.

The research was published online in Nature Photonics (doi: 10.1038/nphoton.2012.104).