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Oct. 18, 2000 Wired

The 411 on Retinal Implants

by Jessie Seyfer

Retinal implants could restore sight to millions of people around the world who suffer from degenerative eye diseases. The technology is still in its infancy, but has finally progressed to human trials. Here's the basics on the technology and where it's headed:

What are retinal implants?

Retinal implant technology has the same basic premise as cochlear implants, which allow tens of thousands of deaf people around the globe to hear sounds. Both implants bridge the "missing link" between the damaged organ and the brain. Cochlear implants detect sounds in the ear and transmit them to the brain's auditory nerve, and retinal implants detect light signals and transmit them to the optic nerve.

There are two kinds of retinal implants, subretinal and epiretinal.

Subretinal devices, like the Optobionics chips now in human trials, are only 2 millimeters in diameter and are composed of tiny electrodes that are powered by 3,500 microscopic solar cells. In the Optobionics chips, light coming into the eye both powers the device and is transmitted to the brain as an image by the device.

There is some speculation as to whether solar cells can generate an electrical pulse strong enough to be picked up by the brain as an image. Optobionics says 10 years of study and animal trials lead them to believe the solar cells are sufficient.

But a group of German researchers at the University of Tuebingen said their research shows solar cells aren't enough, so they have developed a subretinal implant with an outside power supply.

The other type of implants, epiretinal devices, go on top of the damaged retina, and are far more complicated than subretinal chips. At Johns Hopkins University, researchers have developed an implant system where a video camera captures images, a chip processes the images, and an electrode array transmits the images to the brain.

The Johns Hopkins chips, which drew the attention of blind musician Stevie Wonder, garnered a lot of media attention last year when they were implanted into patients for about 45 minutes. During that time, patients said they could see some colors and the outlines of letters.

Who are retinal implants good for?

As it turns out, not Stevie Wonder. That's because his blindness is not caused by a gradual breakdown of the retina, but from complications at the time of his birth.

Experts say retinal implants are best suited for people who suffer from diseases like retinitis pigmentosa and macular degeneration. Retinitis pigmentosa is a hereditary disease in which the retinal cells gradually die over time, robbing sufferers of their vision. An estimated 100,000 people in the United States have the disease. Macular degeneration is the leading cause of blindness for people 55 and older, and is marked by a gradual loss of central vision.

What are some of the challenges researchers face?

"In a lot of ways the technology is progressing faster than the biology," said Fanta Tumminia, a researcher at the Foundation Fighting Blindness, one of the largest private donors to eye research in the United States.

Microchips may be getting smaller and more efficient every day, but the eye's wet conditions will always pose a challenge, Tumminia said.

"The eye is very salty, and electronics and salt don't go well together," she said. "There is also the question of whether the body will reject the implant as something foreign, and whether it will stay in place or float around where it's not supposed to go."

How far are we from developing a technological cure for blindness?

Tumminia thinks it'll be 10 to 15 years before scientists develop a chip to help people see more than just letters or light-and-dark outlines. Other experts agree.

"As with the development of the cochlear implant, we must be satisfied to move ahead incrementally step by step," Dr. Thomas Weingeist, of the University of Iowa, recently told the American Academy of Ophthalmology. "There is no expectation that a solution will occur soon. In the meantime, physicians and patients should remain hopeful, but not be unrealistic or deceived by the hype."

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Date last modified November 24, 2000