Could This Bionic Vision System Help Restore Sight?
Time is not your body’s friend. Years will wear away the color of your hair, dull the bounciness of your joints, erase the elasticity of your skin. Among these many indignities of age, however, one of the worst is the potential loss of sight.
The leading cause of age-related vision loss is macular degeneration—a disease that slowly eats away at the central vision, leaving a blurry or dark hole in the middle of your field of view. The National Institutes of Health estimates that by 2020 nearly three million Americans over the age of 40 will suffer from some stage of the disease. But vision loss isn’t restricted to the elderly. Retinitis pigmentosa, a genetically inherited disease, also strikes around 1 in 4,000 people in the United States—both young and old.
The diseases target the photoreceptors, which are the rod- and cone-shaped cells at the back of the eye. These cells convert light into an electrical signal that travels to the brain via the optic nerve. Macular degeneration and retinitis pigmentosa break down these photoreceptors. In the most advanced forms of the disease, many tasks become nearly impossible without assistance: reading text, watching TV, driving a car, even identifying faces.
Though the impacts are severe, not all hope is lost. The remainder of the retina’s neurons and cells that transmit the electrical signals are often left intact. That means that if scientists can rig a device that can essentially imitate the function of the rods and cones, the body can still process the resulting signals.
Researchers and developers around the world are attempting to do just that. A team at Stanford is using a small and sleek solution: tiny photodiode implants, a fraction of the width of a hair across, that are inserted underneath the damaged part of the retina.
“It works like the solar panels on your roof, converting light into electric current,” Daniel Palanker, professor of ophthalmology at Stanford University, says in a press release about the work. “But instead of the current flowing to your refrigerator, it flows into your retina.”
Dubbed PRIMA (Photovoltaic Retinal IMplAnt), the minute panels are paired with a set of glasses that have a video camera embedded in the center. The camera takes pictures of the surroundings and wirelessly transfers the images to a pocket computer for processing. Then the glasses beam the processed images to the eyes in the form of pulses of near infrared light.
The tiny array of silicon “solar panel” implants—each roughly 40 and 55 microns across in PRIMA’s latest iteration—picks up the IR light and converts it into an electrical signal, which is sent through the body’s natural network of neurons and converted into an image in the brain.
To test out the device, the team implanted the tiny PRIMA panels in rats, then exposed them to flashes of light, measuring their response by electrodes implanted over the visual cortex—the part of the brain that processes imagery. Using the 70 micron implants they had developed at the time, the researchers found that the rats had around 20/250 vision—slightly above legal blindness in the U.S., which is 20/200 vision. This means that a person can see at 20 feet what a person with perfect vision can see at 250 feet, rendering most of their surroundings blurry.
“These measurements with 70 micron pixels confirmed our hopes that prosthetic visual acuity is limited by the pixel pitch [or the distance from the center of one pixel to the center of the next pixel]. This means that we can improve it by making pixels smaller,” Palanker writes via email. They’ve already developed pixels three quarters the size. “We are now working on even smaller pixels,” he writes.
PRIMA is, of course, not the only team chasing this goal. A device called Argus II from Second Sight, a California-based company, has already made it to market in the U.S. Approved in February 2013 by the Food and Drug Administration for patients with severe retinitis pigmentosa, the basic setup is similar to PRIMA. But instead of a solar panel, the implant is a grid of electrodes, which is attached to a pea-sized electronics case and internal antennae. A glasses camera takes an image that is processed by a small computer and then wirelessly transmitted to the implant, which fires electrical signals to create the image.
But there are several drawbacks to this system. The implant’s electronics are bulky and the antennae can experience interference from home appliances or other antennae-reliant gadgets, such as cell phones. The device also has limited resolution, restoring vision to around 20/1,260 without additional image processing. Because of this limited resolution, the FDA only has approved its use in patients who are almost completely blind.
“The FDA doesn’t want to run the risk of damaging the vision in an eye that already has some, because the amount of visual restoration is minimal,” says William Freeman, director of the Jacobs Retina Center at the University of California San Diego. “You can get a little bit, but it’s not a lot.”
Many more technologies are also in the works. A German company Retinal Implant AG uses a digital chip, similar to what is found in a camera. But preliminary tests for the technology in humans have been mixed. Freeman is part of another company, Nanovision, which employs nanowire implants that are barely larger than a wavelength of light. Though they work similarly to PRIMA’s photodiodes, Freeman says they have potential to be more sensitive to light and could help future patients see on a grayscale—not just black and white. The technology is still in animal trials to evaluate its effectiveness.
″[For] all these technologies, there are limitations that are intrinsic,” says Grace L. Shen, director of the retinal diseases program at the National Eye Institute. Though not directly involved in prosthesis research, Shen serves as the program officer for one of the grants that supports Palanker’s work.