Microchip has potential to restore lost vision

MIT scientists and colleagues report significant progress on the development of a microchip that might one day restore vision to people who suffer from certain diseases of the retina.

The microchip, which would be surgically implanted in the eye, would work in conjunction with a miniature camera and laser that fit on a pair of eyeglasses. Team members recently developed a prototype of the microchip and they hope to begin preliminary work with blind human volunteers in about six years.

The Retinal Implant Project is led by Professor John L. Wyatt of the Department of Electrical Engineering and Computer Science and the Research Laboratory of Electronics, and by Dr. Joseph F. Rizzo of the Massachusetts Eye and Ear Infirmary and Harvard Medical School.

If successful, the work could aid people suffering from two retinal diseases, retinitis pigmentosa and macular degeneration. Retinitis pigmentosa is the leading inherited form of blindness, affecting about 1.2 million people worldwide. The condition causes a slowly progressive vision loss that first affects peripheral vision but eventually consumes all vision. Macular degeneration impairs central vision and removes one's ability to read, though peripheral vision is maintained. It affects roughly 10 million Americans, about one million of whom are legally blind with the disease.

In both conditions the rods and cones-the cells that receive light-are destroyed. As a result, the healthy retinal ganglion cells that would have passed on the visual signals from the rods and cones cannot transmit that information to the brain. This renders a person blind.

To address this problem the researchers are working to create an ultra-thin microchip that can be surgically implanted on the retina. The microchip serves to bypass the defective rods and cones by stimulating healthy ganglion cells directly with tiny electrical currents. The researchers hope that this device will restore some vision to blind patients.

To date the team has designed and successfully bench-tested a prototype of the microchip powered by an external laser. The laser beam powers the chip via an invisible infrared beam that will also convey the visual information sensed by a tiny electronic camera (the researchers have not yet tested the laser with the camera). The camera and laser will both fit on a pair of eyeglasses.

The researchers have also developed novel techniques for implantation through dozens of surgical experiments with animals and have completed a number of tests to determine the electrical stimulation thresholds of ganglion cells.

In addition, they have verified the biocompatibility of several implant materials in preliminary studies. They have since begun a comprehensive, year-long study of all materials that will be used in the implant.

While significant progress has been made since 1989, when the project started, many challenges still lie ahead. According to Professor Wyatt, the greatest of these is the potential for damage to delicate retinal tissue that can occur at the interface between the retina and the implant.

The immediate objective of the research team is to refine the method for applying the silicone coating now used on the implant. Tests have revealed tiny leaks in the coating, so a more reliable encapsulation method must be developed, possibly employing new materials. Even the smallest leak of salt from the eye into the implant would destroy the function of the chip.

The research team has already successfully recorded signals from the visual part of the brain of experimental animals following electrical stimulation to an area of the retina roughly as large as the implant will stimulate. The next major goal will be to surgically implant the completed prosthesis and verify the brain's response to the implant.

From there, the team will tackle other challenges. First, they plan to develop strategies to make the electrical stimulation as selective as possible for the desired cell types, with the hope of improving the quality of visual perception. Second, they will design and build a second-generation implant capable of driving each of the stimulating electrodes separately rather than simultaneously as in the prototype version. Third, they will attempt to restore vision to animals that have been blinded by retinitis pigmentosa.

Fourth, assuming approval is obtained from the internal research approval boards, the team anticipates beginning work with blind human volunteers in about six years.

A dozen researchers at MIT and elsewhere are involved in the Retinal Implant Project. At Lincoln Laboratory, David Edell is an expert in neural prosthetic devices, Jack Raffel and Jim Mann contribute to microelectronics, and Terry Herndon contributes to microfabrication and materials. MIT research scientist David Brock is a polymer expert. He and Mike Socha of the Draper Laboratory are in charge of miniature mechanical design.

Ralph Jensen, a retinal neurophysiologist at Southern College of Optometry in Memphis, studies the electrical stimulation thresholds of retinal ganglion cells. Sumiko Miller, a research affiliate at MIT's RLE who is also affiliated with the Massachusetts Eye and Ear Infirmary, measures cortical responses of experimental animals to retinal stimulation. Dianna D'Souza is surgical and administrative assistant at Mass. Eye and Ear. MIT graduate students Andy Grumet and Alan Gale, both of EECS, are doing research on electrical stimulation of nerves and signal processing, respectively.

The work is currently supported by private grants from the Seaver Institute, the Lions Club and the Joseph Drown Foundation. Additional funds are being sought to carry the work forward to the stage of human testing.

A version of this article appeared in MIT Tech Talk on February 1, 1995.

Topics: Computer science and technology, Health sciences and technology

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