Florian Gekeler of the University Eye Hospital in Tuebingen, 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 the Tuebingen group is developing uses an infrared diode mounted in a lens frame to deliver enough light to the implant.
Douglas 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-MIT 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.
Mark 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 appealed. "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.
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.
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 Tuebingen group is pursuing their experiments indicates to me that they might not be on the right track," he said.
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