A new retinal implant sits mostly outside the eye.
For many blind or partially sighted people, implants that stimulate healthy nerve cells connected to their retinas could help restore some normal vision. Researchers have been working on such implants since the 1980s but with only limited success. A major hurdle is making an implant that can stay in the eye for years without declining in performance or causing inflammation.
Now researchers with the Boston Retinal Implant Project, which was spun out of MIT, Harvard Medical School, and the Massachusetts Eye and Ear Infirmary in 1988, have developed hardware they say overcomes such issues. The implants have been tested in animals, and the group plans to start human trials by 2010.
In retinal diseases such as acute macular degeneration and retinitis pigmentosa, the light-sensing cells of the retina may no longer work, even though the neurons that carry signals from these cells to the brain are still healthy. The Boston project uses an array of electrodes to stimulate these cells and reproduce a simplified visual image in the subject’s brain. A camera mounted on a pair of eyeglasses captures an image, which is rapidly processed by a microcontroller to produce a simplified picture. This is then wirelessly beamed to the implant, which activates 15 electrodes inside the eye. The implant also receives power wirelessly from the microcontroller.
In its current form, the implant can reproduce only a 15-pixel image, but the group is working on a version with around 100 pixels and hopes to get up to 1,000 eventually.
The latest implant has been successfully tested in pigs, whose eyes are comparable in size to our own. It hasn’t yet been tested in people, but the research group is confident it will restore enough vision to let people walk around unaided. The electrode array has previously been tested for short periods in patients who reported seeing clouds, red spots, and other images when the electrodes were activated one by one. “We know the concept works; now we need to get the device prepared,” says Shawn Kelly, a visiting scientist at MIT who works for the Boston project.
Previously, the device was housed in a flexible plastic case wrapped around the outside of the eye. But over long time periods, the plastic absorbed water. The electronics inside the new device are housed within a waterproof titanium case similar to those used for heart pacemakers. The new retinal implant case is also the smallest ever made, and it has a large number of feed-through holes for the wires that connect to the electrode array, the wireless power component, and the data coil. Machine-milling such a small, intricate case posed a major challenge, says Kelly. But the new case makes for easier, safer surgery because it sits on the side of the eye, away from the entry point for the electrode array. It is also mechanically more stable, Kelly notes.
Other retinal implants sit completely inside the eye, which can cause biocompatibility issues. Kelly says the goal is to develop implants that last for years, so that patients can learn to process the images produced by the implant.
Another arm of the Boston research group led by John Wyatt, a professor of electrical engineering at MIT, is working on algorithms for converting the camera signal into an image the brain can more easily interpret. An initial goal is enabling people to see enough to walk around a room unaided, so the system’s software focuses on edge detection. The Boston group is also working on algorithms that help people recognize faces, which is a far more challenging problem.