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A Chip That Mimics a Retina but Strains for
From the New
Aug 09, 2001
By ANNE EISENBERG
LAST week in Illinois, three patients with severe retinal disease had an unusual operation: tiny silicon chips were implanted in their eyes.
The microchips are artificial retinas designed to fill in for damaged cells and help restore lost vision. Three other patients with damaged retinas had silicon chips implanted last year in similar operations.
The surgeries are part of a study, approved by the Food and Drug Administration, that allows the devices to be placed in 10 volunteers. "The first three patients have tolerated the chips for a year with no problems," said Dr. Alan Chow, a pediatric ophthalmologist who, with his brother, Vincent, invented the chip and is directing work on its development. Dr. Chow is chief operating officer of Optobionics, a privately held company in Wheaton, Ill., that produced the silicon implant.
In the healthy eye, cells within the retina, the lining in the rear of the eyeball, convert light into electrical signals through a series of chemical reactions. The new electronic retina, one of several being developed by research groups around the world, mimics this process it is made up of thousands of minuscule photodiodes that convert photons of light into electrical signals. These signals trigger the remains of the retinal circuitry to transmit signals to the brain by way of the optic nerve.
Dr. Chow said that there had been no signs of rejection or infection in the patients that received the chip a year ago and that the implants continued to produce electrical activity. The new patients will give him a larger statistical base on which to evaluate results, he said.
Citing F.D.A. guidelines, Dr. Chow said that he could not disclose whether any patient's vision had improved. "We'll compare results of this second group with results from the first group and communicate with the F.D.A.," he said. The results may be published later.
The patients in the study suffer from retinitis pigmentosa, a disease that damages the photoreceptor cells in the eye, called rods and cones. The silicon chip, which is implanted just behind the retina, uses about 3,500 miniature light detectors attached to metal electrodes to imitate the function of rods and cones. The light detectors absorb incident light and produce a small amount of electricity that stimulates the retinal neurons.
The chip is designed to treat not only retinitis pigmentosa but also macular degeneration, another disease that affects the photoreceptors. The artificial retina is not meant to treat glaucoma or other cases in which the nerve fibers leading to the optic nerve are damaged.
The replacement retina has a diameter of two millimeters, or less than one-tenth of an inch, and is about half as thick as a piece of paper.
The operation lasts about two hours, with an incision made in the white of the eye and the chip manipulated into a pocket under the retina.
Other scientists working on retinal prostheses were critical of some aspects of Dr. Chow's work. "It's important to understand that this kind of study is about whether the devices are safe can these chips be implanted in blind human patients without damaging their eyes," said Dr. Joseph Rizzo, a professor of ophthalmology at Harvard and co-director of the Harvard-M.I.T. Retinal Implant Project. "That's entirely different from understanding whether the chips will be helpful."
The retinal prosthesis Dr. Rizzo and his group are developing also stimulates the millions of neurons in the retina. But unlike Dr. Chow's subretinal implant, which is close to the light-detecting cells, the Rizzo group's chip will be positioned near the retinal layers where nerve impulses are sent to the brain, the ganglion cells. Their prototype uses a signal processor and a camera mounted in a pair of glasses to capture and transmit an image wirelessly to the embedded chip, from where it would travel to the brain.
Dr. Florian Gekeler of the University Eye Hospital in Tübingen, Germany, is part of a German consortium working on subretinal implants. He said that work published by his group indicated that light entering the eye would not be strong enough to power photocells to stimulate retinal neurons.
"Current photocells make use of about 16 to 18 percent of the energy in light," he said, "so ambient room light cannot be enough to stimulate the retina."
The device being developed by the Tübingen group uses an infrared diode mounted in a lens frame to deliver enough light to the implant.
Dr. Douglas B. Shire, a visiting scientist at the Cornell Nanofabrication Facility, has been working for the past four years on the actual structures that go into the eye in the Harvard-M.I.T. retinal prosthetic project. "The Chow device would work only if you shined extremely bright light into the patient's eye, but that's not what people carry around in their pockets," he said.
Dr. Mark S. Humayun, director of the intraocular prosthetic laboratory at the Wilmer Ophthalmological Institute at Johns Hopkins Hospital in Baltimore, said that the idea of replacing rods and cones with simple photodetectors was appealing. "A simple photodetector attached to a metal electrode that passes current to stimulate the retina is in many ways an ideal device," he said, because it has no power requirements other than ambient light and involves little if any heat dissipation from the electronics.
Dr. Humayun said that his group had at one time considered this approach. He, too, realized that the photodetectors weren't efficient enough to generate the current needed to stimulate retinal neurons, especially retinal neurons of a diseased or degenerated retina. "The intensity required is that of multiple suns," he said.
Instead, his group is focusing on transmitting an image from a small external camera to the implanted chip. "We can keep a large amount of the electronics in a glass frame and therefore simplify the electronics that are implanted," he said. In this scheme, the wearable electronics can be replaced or upgraded easily without subjecting the patient to a second operation, he said. Like Dr. Rizzo's device, Dr. Humayun's is designed to be placed by the ganglion cells.
Dr. Chow was familiar with many of the objections of his critics, but said that there were different ways to do any given experiment, including the ways that he had chosen. "For instance, the way that the Tübingen group is pursuing their experiments indicates to me that they might not be on the right track," he said.
Date last modified August 13, 2001