April 1, 2005, Stanford, CA--Researchers from the Departments of Ophthalmology and Neurobiology and the Hansen Experimental Physics Laboratory at Stanford University have designed an optoelectronic retinal prosthesis system that can stimulate the retina with resolution corresponding to a visual acuity of 20/80--sharp enough to recognize faces, read large fonts, and, of most importance, allow users to lead an independent life.
The researchers hope their device, first described in the Feb. 22 issue of the Journal of Neural Engineering, may someday bring artificial vision to those blind due to retinal degeneration. They are testing their system in rats, but human trials are at least three years away. The project is funded in part by the U.S. Air Force and VISX Corp., which licensed the technology through Stanford's Office of Technology Licensing.
The researchers plan to directly stimulate the layer underneath dead photoreceptors using a system that looks like a cousin of the high-tech visor blind engineer Geordi La Forge wore in Star Trek: The Next Generation. It consists of a tiny video camera mounted on transparent "virtual reality" style goggles. There's also a wallet-sized computer processor, a solar-powered battery implanted in the iris and a light-sensing chip implanted in the retina.
The chip is the size of half a rice grain--3 millimeters--and allows users to perceive 10 degrees of visual field at a time. It's a flat rectangle of plastic (eventually a silicon version will be developed) with one corner snipped off to create asymmetry so surgeons can orient it properly during implantation. How does the system work when viewing, say, a flower? First, light from the flower enters the video camera. The video camera then sends the image of the flower to the wallet-sized computer for complex processing. The processor then wirelessly sends its image of the flower to an infrared LED-LCD screen mounted on the goggles. The transparent goggles reflect an infrared image into the eye and onto the retinal chip. Just as a person with normal vision cannot see the infrared signal coming out of a TV remote control, this infrared flower image is also invisible to normal photoreceptors. But for those sporting retinal implants, the infrared flower electrically stimulates the implant's array of photodiodes. The result? They may not have to settle for merely smelling the roses.
In the Stanford system, image amplification and other processing occur in the hardware, outside the eye. If amplification occurred inside the implant's pixels, as it does in one German design, there'd be no way short of surgery to make adjustments.
Current retinal implants provide very low resolution--just a few pixels. But several thousand pixels would be required for the restoration of functional sight. The Stanford design employs a pixel density of up to 2,500 pixels per millimeter, corresponding to a visual acuity of 20/80, which could provide functional vision for reading books and using the computer. Physical limitations regarding electrical stimulation most likely make it impossible for implants to impart a visual acuity of 20/10 (the sharpness required to see the bottom line on an eye chart), 20/20 (the so-called standard of good vision) or even 20/40 (the level to which vision must be correctable to be eligible for a California driver's license).
Graduate students Ke Wang in applied physics and Neville Mehenti in chemical engineering are currently working with Fishman of the Stanford Ophthalmic Tissue Engineering Laboratory on carbon nanotube electrodes and on chemical stimulation of the retinal cells. Medical student Ian Chan continues to develop lithographic fabrication technology for the implants. Alex Butterwick, a graduate student in applied physics, is studying the mechanisms of cellular damage and the safe limits of electrical stimulation.