Throughout the world people who suffer from two of the most common types of blindness, macular degeneration and retinitis pigmentosa, face the reality of gradually losing their sight forever. Researchers are racing to restore vision to these sufferers. Both macular degeneration and retinitis pigmentosa are diseases that cause the death of essential cells in the eye. A healthy eye converts light to electrochemical signals through photoreceptor cells, located in the retina, called rods and cones. Macular degeneration and retinitis pigmentosa cause the loss of rods and cones, and what begins as tunnel vision in many cases ends in complete blindness. One way to correct the loss of sight is to replace the retina’s function with an electronic implant.
Vision researchers have learned an immense amount from the first successful bioelectronic interface – the cochlear implant. The cochlear implant uses only a few electrodes to communicate with tens of thousands of neural connections. At first, the recipient of the implant hears noise, but the brain quickly learns to interpret the electric signal as sound. When vision researchers apply this understanding to restoring sight they face an even greater challenge: instead of tens of thousands of connections, there are millions of connections conveying information to the optic nerve. The question arises: what is the minimum number of connections necessary before the brain can interpret the electric signal as an image?
Challenges of Operation
Operating on an eye provides additional complications. Seemingly simple engineering issues like power and life span of the implant are technically challenging when performed on so small a scale. Because the eye is a small and fragile organ, inserting a device is extremely difficult. The eye is only 2-centimeters in diameter and the retina no more than 300 microns in thickness, making the operation akin to attaching a computer chip to a wet piece of tissue paper within a jelly-filled orb. According to Dr. Wentai Liu, one of the most recognized researchers in the field, another significant problem is that an implant that generates any significant amount of heat, risks “cooking” the eye. Moreover, the device must fit the unique curvature of each retina and must move with the retina so as not to be displaced by the constant motion of the eye.
Research teams must also confront the issue of biocompatibility – whether or not the eye will reject the implant over time, and whether the electrical charges delivered to the eye will gradually overstimulate and eventually cause cell death in the optic nerve. In the face of all of this, research teams have stepped up to the challenge. What follows is an introduction to the state of this optical art.
Dr. Wentai Liu and doctoral student Elliot McGucken at North Carolina State University, have teamed with Dr. Mark Humayun, a researcher at Johns Hopkins University, to create an epiretinal chip that is implanted on top of the retina. The chip is powered by a laser mounted on a pair of glasses. The chip is 2mm square and works by generating electrical pulses at its electrodes which function as pixels. The pulses from the chip stimulate the ganglion cells and convey the information to the brain to be interpreted. Current resolution of the chip is 5 x 5 pixels, enough to distinguish a single letter. The team is already working on prototypes with 10 x 10 resolution, which should be enough to read large print. In the next couple of years they plan to support 20 x 20 resolution. Again, there is a fragile balance between how many pixels the brain requires to understand the image, and how many electrodes they can safely place on a microchip without threatening the health of the eye. Dr Liu admits that the laser-powered approach may not be the safest powering option, and says that they have begun to look closely at a radio frequency (RF) powered option.
Optobionics, a Wheaton, Illinois-based company, has a substantially different approach. Brothers Alan and Vincent Chow have developed a microchip no larger than 3 mm in diameter and 1/1000 of an inch in thickness that replaces the function of degenerated cells. The chip implanted subretinally, or behind the retina, is filled with microphotodiodes, tiny solar cells that “see” the light that comes into the eye and converts the light to thousands of impulses that are recognized by the surviving retina. In an interview, Dr. Alan Chow, CEO of Optobionics stated that “… each approach has its strengths, weaknesses, and difficulties. Our main strength is that potentially higher resolution and image quality may result from a relatively simple device.” This ingenious team has also solved the problem of power by designing the microchip to power itself using the very light it is interpreting. The team will not know until the first device is implanted, but Dr. Chow believes that recipients may see a pixelated image similar to that seen on a computer screen. According to Dr. Chow, “The artificial silicon retina (ASR) that we are developing responds to both visible and infrared light, patients with such an implant may also see into the infrared spectrum.”
Although the results of animal studies are promising, some issues, primarily long-term biocompatibility and function, can only be solved by testing in humans. Drs. Liu and Humayun have already been conducting some initial human research and consider it to be a major strength. Optobionics plans to begin conducting human research in the next two years. Interestingly, Germany has entered the race with over 13 million dollars invested in two teams mirroring the American efforts; one is working on an epiretinal approach and the other on a subretinal approach. Dr. Humayun recalled why this research began: “Despite being at the top hospitals (Duke and Johns Hopkins), nothing could be offered to a number of patients who were blind from retinal degeneration.” Now there is cause for optimism. With these researchers on the job, patients may soon be offered a chance to see again.