A tiny microchip in a lab at Johns Hopkins University receives an electronic impulse, inducing an even tinier grid of electrodes to display a capital letter E. The feat may seem like news from the past in this high-rez age - until you consider that the two-millimeter microchip will one day be implanted in the filmy tissue of the human retina. For patients blinded by degenerative diseases such as retinitus pigmentosa, the resolution might look pretty good.
The implantable artificial retinal device is intended to bridge the gap left in the network of impulses that runs from the eye, along the optic nerve and to the brain to generate vision.
"If the optic nerve is intact, then you can use the blind eye to stimulate vision," said Mark Humayun, assistant professor of opthamology at the Wilmer Eye Institute at Johns Hopkins University.
Humayun, Eugene de Juan, and researchers at the University of North Carolina say the chip, which went into fabrication this week, will be tested on retinal tissue extracted from laboratory animals to see how well it withstands the stresses of an implant.
"Retinas have the consistency of wet tissue paper; they can't take something heavy. The next step is to develop an implantable device," said Humayun.
Once the researchers complete their stress-testing of retinal tissue, they must tackle the problem of power. Last fall, the chip was laid on top of - but not actually implanted in - the retinas of 11 test subjects, and doctors ran very fine insulated wires from an external power source.
Once implanted, though, the chip will need to generate its own juice.
The answer may be in the light. Elliot McGucken, graduate researcher at University of North Carolina and Dr. Wentai Liu, professor of electrical engineering at NC State University, are developing solar cells tiny enough to rest on the chip at the back of the eye.
"The cells will harvest the energy from surrounding light," said McGucken. But sunlight and lamplight will not be enough to power the chip. Humayun believes patients will wear special glasses that project a laser light onto the chip inside the eye.
For people who suffer from degenerative diseases such as retinitus pigmentosa - which Humayun said affects approximately 5 to 10 percent of the 400,000 sightless people in the US - the photo receptors at the back of the eye are no longer functional.
But Humayun, de Juan, and others had to overcome the skepticism among researchers that the retinas of blind people are non-functional. Humayun believed differently. "We looked at the retinas of dead people who had had retinitus pigmentosa and found that about 70 percent of the tissue [of the retina] was functional," he said.
Earlier efforts to restore the sight of the blind focused on the brain's visual cortex. Researchers at the National Institutes of Health tested volunteers by stimulating the brain with a similar microelectrode array with similar functional vision results. Unfortunately, this technique requires brain surgery.
"The risks to operating on a blind eye are considerably less than operating on a functional brain," said Humayun.
Even if researchers overcome the technological hurdles of a building a functional retina chip, they are quick to point out its limitations. Rather than a miracle sight restoration, they anticipate it will offer only ambulatory vision - a depth of field of about 5 feet.
