Tiny "solar panels" implanted into the eye could one day restore vision to the blind without the need for any ugly wires.

Around 15 million people worldwide have some form of blindness. In people with these conditions the retina's photoreceptors, which transform light hitting the eye into electrical impulses, are often damaged, preventing visual information from being sent to the brain.

Several companies, such as Second Sight in Sylmar, California, have developed prosthetic retinas, some of which are currently in clinical trials. Such technologies generally use a camera to detect visual information that is then relayed through a wire to an implant inside the eye.

The implant effectively replaces the damaged photoreceptors. However, these prostheses tend to require numerous wires to connect the implant to an external power source and to transmit information from the camera to the implant, says James Loudin of Stanford University in California.

Wireless vision

So along with Daniel Palanker and colleagues, Loudin set out to build a tiny wireless implant out of photovoltaic pixels – similar to those found in rooftop solar panels.

The idea is that a video camera set on a pair of glasses would pick up visual information and relay it to the photovoltaic implant using a beam of low-intensity infrared light. The implant would then convert the light into electrical activity to stimulate neurons, sending the visual information to the brain without the need for any wires into the implant.

Sensors on previous prostheses cannot be stimulated individually, and are instead activated in bunches, limiting the resolution of the resulting image. Because Loudin's pixels are only 70 micrometres wide, one-third of the width of a human hair, they are sensitive enough to respond to a single photon of infrared light. This enables each pixel to be activated individually, similar to the way that biological photoreceptors work. Loudin hopes that this will create better resolution than in previous prostheses.

Second sight

To test the implant, the team removed retinas from dead rats whose photoreceptors had been destroyed. They implanted the photovoltaic chips into the retinas and flashed infrared light onto them. The pixels converted the light into electrical energy which activated neurons within the retina. In a live animal, these neurons would then relay that information to the brain.

It is difficult to know just how similar the visual resolution would be compared with real vision, Loudin says. Current prostheses show the basic geometry of a shape, but since large numbers of neurons are activated at once, colours appear at random, similar to when you rub your eyes. Implants that profide real colour vision, Loudin says, are still a long way off.

He says they are now testing their prosthesis in live rats with some success, and ultimately aim to move on to clinical trials in humans. First they want to see if they can make the pixels any smaller, enabling them to fit more onto the implant, increasing potential resolution still further.

Journal reference: Nature Photonics, DOI: 10.1038/nphoton.2012.104


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