Millions of rods and cones are in the back of every healthy human eye. They are biological solar cells in the retina that convert light to electrical impulses -- impulses that travel along the optic nerve to the brain where images are formed. Many people are blind -- or going blind -- because of malfunctioning rods and cones. Retinitis pigmentosa and macular degeneration are examples of two such disorders. Retinitis pigmentosa tends to be hereditary and may strike at an early age, while macular degeneration mostly affects the elderly. Together, these diseases afflict millions of Americans; both occur gradually and can result in total blindness.
"If we could only replace those damaged rods and cones with artificial ones," says Dr. Alex Ignatiev, a professor at the University of Houston, "then a person who is retinally-blind might be able to regain some of their sight." Years ago such thoughts were merely wishful. But no longer. Scientists at the Space Vacuum Epitaxy Center (SVEC) in Houston are experimenting with thin, photosensitive ceramic films that respond to light much as rods and cones do. Arrays of such films, they believe, could be implanted in human eyes to restore lost vision.
"There are some diseases where the sensors in the eye, the rods and cones, have deteriorated but all the wiring is still in place," says Ignatiev, who directs the SVEC. In such cases, thin-film ceramic sensors could serve as substitutes for bad rods and cones. The result would be a "bionic eye."
The Space Vacuum Epitaxy Center is a NASA-sponsored Commercial Space Center (CSC) at the University of Houston. NASA's Space Product Development (SPD) program, located at the Marshall Space Flight Center, encourages the commercialization of space by industry through 17 such CSCs. At the SVEC, researchers apply knowledge gained from experiments done in space to develop better lasers, photocells, and thin films -- technologies with both commercial and human promise.
Scientists at Johns Hopkins University, MIT, and elsewhere have tried to build artificial rods and cones before, notes Ignatiev. Most of those earlier efforts involved silicon-based photodetectors. But silicon is toxic to the human body and reacts unfavorably with fluids in the eye -- problems that SVEC's ceramic detectors do not share.
"We are conducting preliminary tests on the ceramic detectors for biocompatibility, and they appear to be totally stable" he says. "In other words, the detector does not deteriorate and [neither does] the eye."
"These detectors are thin films, grown atom-by-atom and layer-by-layer on a background substrate -- a technique called epitaxy," continues Ignatiev. "Well-ordered, 'epitaxally-grown' films have [the best] optical properties."
Crafting such films is a skill SVEC scientists learned from experiments conducted using the Wake Shield Facility (WSF) -- a 12-foot diameter disk-shaped platform launched from the space shuttle. The WSF was designed by SVEC engineers to study epitaxial film growth in the ultra-vacuum of space. "We grew thin oxide films using atomic oxygen in low-Earth orbit as a natural oxidizing agent," says Ignatiev. "Those experiments helped us develop the oxide (ceramic) detectors we're using now for the Bionic Eye project."
The ceramic detectors are much like ultra-thin films found in modern computer chips, "so we can use our semiconductor expertise and make them in arrays -- like chips in a computer factory," he added. The arrays are stacked in a hexagonal structure mimicking the arrangement of rods and cones they are designed to replace.
The natural layout of the detectors solves another problem that plagued earlier silicon research: blockage of nutrient flow to the eye. "All of the nutrients feeding the eye flow from the back to the front," says Ignatiev. "If you implant a large, impervious structure [like the silicon detectors] in the eye, nutrients can't flow" and the eye will atrophy. The ceramic detectors are individual, five-micron-size units (the exact size of cones) that allow nutrients to flow around them.
Artificial retinas constructed at SVEC consist of 100,000 tiny ceramic detectors, each 1/20 the size of a human hair. The assemblage is so small that surgeons can't safely handle it. So, the arrays are attached to a polymer film one millimeter by one millimeter in size. A couple of weeks after insertion into an eyeball, the polymer film will simply dissolve leaving only the array behind. The first human trials of such detectors will begin in 2002. Dr. Charles Garcia of the University of Texas Medical School in Houston will be the surgeon in charge.
"An incision is made in the white portion of the eye and the retina is elevated by injecting fluid underneath," explains Garcia, comparing the space to a blister forming on the skin after a burn. "Within that little blister, we place the artificial retina."
Scientists aren't yet certain how the brain will interpret unfamiliar voltages from the artificial rods and cones. They believe the brain will eventually adapt, although a slow learning process might be necessary -- something akin to the way an infant learns shapes and colors for the first time. "It's a long way from the lab to the clinic," notes Garcia. "Will they work? For how long? And at what level of resolution? We won't know until we implant the receptors in patients. The technology is in its infancy." Ignatiev has received over 200 requests from patients who learned of the studies from earlier press reports. "I'm extremely excited about this," he says. He cautions that much more research is needed, but "it's very promising."