Just as the cochlear implant can restore hearing in deaf people, an artificial retina may some day offer sight to the blind.

More than 10 million people worldwide suffer blindness due to a damaged retina, the thin layer of cells lining the back of the eye that convert light into nerve impulses. These impulses travel the optic nerve to the visual cortex of the brain, where they are interpreted as the images we see.

But when the photoreceptors in the retina -- more commonly known as rods and cones -- are damaged by diseases such as retinitis pigmentosa or macular degeneration, the retina cannot process the light. The brain never receives the impulses and cannot form a complete image. The result is blindness.

Bypassing the damaged retina with an artificial retina is the goal of many researchers around the world, including a team of biomedical engineers led by Mark Humayun, M.D., Ph.D., at The Johns Hopkins University's Wilmer Eye Institute in Baltimore. Their approach resembles that of the cochlear implant, which converts sound into patterned electrical impulses to stimulate the auditory nerve in the inner ear.

At the heart of Humayun's vision system is an artificial retinal chip, a small, tissue-thin silicon wafer with an array of tiny platinum electrodes on one side. A camera mounted on a pair of eyeglasses captures video images. The images are converted into electrical signals by an external device, then transmitted with high-frequency radio waves to the retinal chip via an antenna implanted in the front part of the eye. A separate, low-frequency antenna implant transmits power to the chip.

The chip, mounted directly on the retina, converts the data signals from the implanted antenna into electrical impulses, then sends them along the electrodes to stimulate healthy neurons just below the damaged photoreceptors. Below these neurons lie ganglion cells, which connect to the optic nerve.

"The light-sensing part of the eye may be damaged, but the rest of the cells are still intact," says Humayun. "What we do is jump-start these cells, and the rest of the normal human circuitry carries the information to the brain."

Early on, Humayun and others worried that the brain might interpret simultaneous stimulation on different spots of the retina as a single, incoherent blur. But experiments showed otherwise. Several tiny electrodes were temporarily implanted in an eye of 15 blind subjects. The electrodes were connected to a computer chip outside the eye, which emitted impulses representing various patterns.

The patients were able to make out some simple shapes and crude letters -- sometimes in color -- demonstrating that remaining neurons in the eye can be artificially activated.

"The problems now lie in bringing this to the point of functional sight," says Humayun. Functional sight doesn't necessarily mean crystal-clear vision, researchers point out. A completely blind person most likely would benefit from what normal-sighted people would consider low-quality vision.

The quality of vision from the artificial retina depends on how many stimulating electrodes are used. Up to now, research involved 25 electrodes arranged in a 5-by-5 array on the macula, the area of highest acuity vision roughly the size of a match head in the retina's central region. This gives an image of up to 25 dots, or pixels, enough, say researchers, to allow patients to distinguish high-contrast shapes or tell night from day.

Work is being done now to begin testing a chip with a 10-by-10 array, or 100 electrodes. This would give patients a higher-resolution (100-pixel) image. Researchers say that should be enough resolution to read signs, recognize faces, and serve as a navigation aid. Humayun's team hopes someday to add even more electrodes.More electrodes mean smaller electrodes.

"Obviously, if you have a fixed space to work with, the smaller you can make an electrode the more electrodes you can put in the space," says James Weiland, Ph.D., a member of Humayun's team who is studying the interface between the electronics and the retina. "If you can make the electrode smaller and more efficient, you essentially increase the number of pixels in the limited macula area."

Each electrode is a flat disc a fraction of a millimeter across, less than the width of the period at the end of this sentence. Supported by a Whitaker research grant, Weiland will attempt to optimize the electrodes by making them with different metals, including iridium oxide and titanium nitride. Weiland says he hopes these materials will help cut the electrode's diameter by more than one fourth, while transferring equal or greater amounts of power than the current platinum electrodes.

Other concerns lie in how human tissue, especially the retina, will react to the electrodes and the long-term stress of the system's implanted components. While Weiland's research will touch on some of these biocompatibility problems, Gianluca Lazzi, Ph.D., at North Carolina State University is investigating the heat generated by the system's electromagnetic fields.

Lazzi, who is also supported by the foundation, is creating high-resolution computer models of the entire head and its more than 30 different tissue types. He hopes to minimize heat buildup while safely and efficiently transferring data and power to the retinal chip.

Other research thrusts will include finding the best method of affixing the chip to the retina and protecting the chip. The harsh environment inside the eye could cause the chip to disintegrate or malfunction, say researchers, so the entire device will need to be encased in a strong but flexible coating. The chip's total weight also must be kept to a minimum.

Despite the hurdles, Humayun says he is convinced that achieving some level of functional vision for many blind patients is "a realizable goal" in the near future. "None of the various problems are insurmountable."

Second Sight, LLC, a company in Valencia, Calif., agrees. Its president, Robert Greenberg, M.D., Ph.D., is a former graduate student under Humayun and was supported by Humayun's Whitaker grant.

Greenberg was recently awarded a Bioengineering Research Grant by the National Institutes of Health for $12.5 million over five years to develop a prototype retinal prosthesis. Humayun's team is also part of that grant.