The most promising treatment for blindness caused by the loss of light-sensitive cells in the retina is to implant retinal prostheses that provide artificial vision through the electrical stimulation of the remaining functional neurons in the retina.
“But current implants produce very poor results, and their wearers are still considered legally blind,” says Diego Ghezzi, PhD, the Medtronic Chair in Neuroengineering at the School of Engineering at the École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland. “In order to lead what is considered a ‘normal’ life, the implantee must recover a visual field of at least 40 degrees. Current implants achieve only 20 degrees.”
Since 2015, Ghezzi and his team have been developing a retinal implant that works with camera-equipped glasses and a microcomputer. “Our system is designed to give blind people a form of artificial vision by using electrodes to stimulate their retinal cells,” says Ghezzi.
The way the technology works is that a camera embedded in the glasses captures images in the wearer’s field of vision and sends the data to a microcomputer placed in one of the eyeglasses’ end-pieces. The microcomputer turns the data into light signals which are transmitted to electrodes in the retinal implant. The electrodes then stimulate the retina in such a way that the wearer sees a simplified, black-and-white version of the image.
This simplified view of the world is made up of dots of light that appear when the retinal cells are stimulated. However, wearers must learn to interpret the many dots of light to figure out shapes and objects. “It’s like when you look at stars in the night sky—you can learn to recognize specific constellations. Blind patients would see something similar with our system,” says Ghezzi.
The system has not been tested on humans, yet. “We aren’t yet authorized to implant our device in human patients, since obtaining the medical approval takes a long time. But we came up with a process for testing it virtually—a type of work-around,” says Ghezzi. More specifically, the engineers developed a virtual reality program that can simulate what patients would see with the implants.
Their findings have been published in the journal Communication Materials in an article entitled “Photovoltaic retinal prosthesis restores high resolution responses to single-pixel stimulation in blind retinas.”
The health of a person’s vision is measured by the field of vision and the resolution. The team of engineers also used these parameters to evaluate their system. The retinal implants they developed contain 10,500 electrodes. Each one generates a dot of light.
“We weren’t sure if this would be too many electrodes or not enough. We had to find just the right number so that the reproduced image doesn’t become too hard to make out. The dots have to be far enough apart that patients can distinguish two of them close to each other, but there has to be enough of them to provide sufficient image resolution,” says Ghezzi.
The engineers also had to make sure that each electrode could reliably produce a dot of light in the retina. “We wanted to make sure that two electrodes don’t stimulate the same part of the retina. So we carried out electrophysiological tests that involved recording the activity of retinal ganglion cells (a type of neuron at the inner surface of the retina). And the results confirmed that each electrode does indeed activate a different part of the retina.”
Next, the team used the virtual reality program to test whether the 10,500 dots of light provide sufficient resolution. “Our simulations showed that the chosen number of dots, and therefore of electrodes, works well. Using any more wouldn’t deliver any real benefits to patients in terms of definition,” says Ghezzi.
Finally, the researchers attempted to expand the cone of the field of vision. So far, implanting the tiny photovoltaic cells that convert light into electricity in a tiled fashion had restored the visual angle to a maximum of 11 degrees in rabbit studies.
“We started at five degrees and opened up the field all the way to 45 degrees. We found that the saturation point is 35 degrees—the object remains stable beyond that point,” says Ghezzi.
These experiments demonstrated that the system’s capacity does not need to be improved any further, and that it is ready for clinical trials.