November 20, 1996

Engineer Works on Artificial Retina Design

Dr. Wentai Liu, associate professor of electrical engineering, holds the artificial retina chip embedded in a test strip. UNC-CH graduate student Elliot McGucken developed the photoreceptors used in the chip.

Restoring sight to the blind may sound like a miracle to some, but not to a team of researchers from North Carolina State University, UNC-Chapel Hill and Johns Hopkins University. Dr. Wentai Liu, professor of electrical engineering at NC State, Elliot McGucken, a doctoral student at UNC-CH, and Drs. Mark Humayun and Eugene de Juan of Johns Hopkins University are working on the design of a microchip that could restore sight to people with retinal pigmentosa.

The idea for the project began in 1988 when Humayun demonstrated that a blind person could be made to see points of light by stimulating the nerve ganglia behind the retina with electrical current. The demonstration proved that the nerves behind the retina still functioned even though the retina had degenerated. Scientists reasoned that if the retina could be replaced with a device that could translate images to electrical impulses, then vision could conceivably be restored.

An implantable artificial retina device could enable many of the 10 million people afflicted with retinal diseases such as retinal pigmentosa and age-related macular degeneration to regain a sense of sight. In these diseases, the rods and cones that work much like pixels on a video screen become inoperative, but the ganglion cells lining the retina remain intact. Unfortunately, many of the devices developed thus far have been impractical for implantation.

"The problem was that the devices being developed were too intrusive or had unstable power supply," said McGucken. "We had to design a device that wouldn't damage the tissues, was small enough to fit in the eye and still have a means for providing power."

"There are very many complex engineering problems in this project," explained Liu. "We had to consider biocompatibility of the device and how to provide a reliable power supply. We also had to design an electrical circuit that conforms to the biological specifications."

With research funding from the National Science Foundation and the Fight for Sight Foundation, Liu and McGucken have developed an artificial retina component chip (ARCC) that seems to fit those specifications. Just two millimeters square, the wafer-thin silicon microchip is imbedded with photosensor cells and electrodes. At that size, the ARCC can be implanted in the blind person's eye near the vision center of the retina. Powered by an exterior laser aimed at a photovoltaic cell, the photosensor cells in the microchip receive light and images through the pupil. The photosensor cells convert the light and images into electrical impulses that stimulate the nerve ganglia behind the retina. By stimulating the retina with a pattern of electrodes, the device partially recreates the visual information.

Using specifications sent by Humayun and de Juan, Liu and McGucken have created a prototype chip that is being polished to less than .02 millimeter thickness that will enable light and images to pass through the chip to the photosensors located at the back of the chip.

"While the current design of the ARCC will not restore clear vision, it can produce vision compatible with limited mobility such as the ability to see forms or direction of movement," said McGucken.

Liu and McGucken have designed the chip to be as noninvasive to the eye tissues as possible. By using an external laser to power the chip, they have eliminated the need for future surgeries to replace the power source and eliminated the problem of how to keep a battery viable in the wet, salty environment of the eye. The laser and photovoltaic cell is the power source of choice because the laser beam can pass through the cornea without damaging the corneal tissue. Also, the laser, powered by a small battery pack, can be fitted to a pair of regular eyeglasses and aimed at the ARCC's photovoltaic cell without clumsy or obtrusive headgear.

To further protect the eye tissues, the ARCC is designed so that the electrodes do not pass current to stimulate the ganglia. Instead the electrodes charge a plate that stimulates the ganglia. By charging a plate instead of passing current, the ARCC protects the retinal tissues from damage from the electrical current.

Once tested in Liu's lab at NC State, the chips will be sent to Johns Hopkins for biocompatibility testing by Humayun and de Juan. Then, FDA approval will be necessary for testing in humans.

"It's too early to tell if the chip could actually work in a human eye," said Liu. "The question of biocompatibility of the silicon being implanted in the body has to be answered first. But we are very optimistic."

The prototype chips are expected to be ready for shipment to Johns Hopkins for testing in the spring of 1997.

This illustration by Dr. Mark Humayan shows how the chip, placed just in front of the damaged retina, converts the image of the large "E" into electrical impulses that stimulate the ganglia. The brain processes the message from the ganglia and interprets the signal as a large "E."