Retinitis pigmentosa and age-related macular degeneration are two of the leading causes of blindness for which there is no curative treatment. But scientists at the Massachusetts Institute of Technology (MIT; Boston) are developing a retinal prosthesis that would take over the function of lost retinal cells by electrically stimulating the nerve cells that normally carry visual input from the retina to the brain.

A small company, Second Sight Medical Products (Sylmar, California), earlier this year was granted FDA approval for a 20-patient clinical trial of a similar implant in development, but the MIT team is betting their version has a key advantage that will move its prosthesis to the head of the pack in the race to commercialization.

"Second Sight has a huge amount of capital and team three times the size of ours, but we think our approach is a much better idea," John Wyatt, MIT professor of electrical engineering and project leader, told Medical Device Daily. "We used to put them inside and fix them to the retina. The tissue beneath the retina is the most vascular in the body. We decided to come in through a little incision in the white and come through to the retina from the back."

The implant under development, inspired by the functionality of cochlear implants, would not restore normal vision, but could help blind people to more easily navigate their environments.

The MIT group, as well as Second Sight, initially developed implants that were implanted inside the ocular cavity, called epiretinal implants.

"Our group initially focused on an epiretinal approach, but after nearly a decade of sobering experience we decided it best to pursue a subretinal approach instead," Wyatt said, adding that epiretinal approach introduces a greater danger of bleeding in the eye when mounting an array or an entire electronic system epiretinally using either adhesives or retinal tacks. There's also a postsurgical risk of eventual detachment and bleeding, among other potential complications.

"As we discovered these impediments, we eventually decided to switch to a subretinal approach to the implant design," Wyatt said.

A new design emerged – one in which the bulk of the implant is attached to the outside of the sclera except for a thin microelectrode array penetrating the sclera to stimulate the retina electrically from a subretinal location.

"A little bit of bleeding into the eye is extremely destructive," Wyatt said. "We though it was safer our way."

Once the implant is in place, the patient would wear glasses mounted with a tiny camera that sends images to a microchip attached to the eye. The glasses also contain a coil that wirelessly transmits power to receiving coils surrounding the eyeball. When the microchip receives visual information, it activates electrodes that stimulate nerve cells in the areas of the retina corresponding to the features of the visual scene. The electrodes directly activate optical nerves that carry signals to the brain, bypassing the damaged layers of retina.

Aside from testing the system in animals, the MIT group implanted six patients with a prototype for a single day to do experiments, which confirmed that retinal stimulation can produce some kind of organized vision in blind patients. But they won't know exactly how well the implant works until they test it fully in humans with chronic blindness, "because cortical learning [like what happens with cochlear implants] happens over weeks to months, not hours," Wyatt said.

With backing from the VA Center for Innovative Visual Rehabilitation, the National Institutes of Health, the National Science Foundation, the Catalyst Foundation (Lancaster, California) and the MOSIS (Marina del Rey, California) microchip fabrication service, the group intends to start testing in blind patients within the next two years pending FDA approval. In the midst of this, Wyatt said, they are working to either spin out a company or outlicense the technology.

A special surgical technique has been developed to facilitate subretinal implants and the MIT group has reported a success rate of about 90% in pigs. Additionally, they have switched from using a silicone exterior to hermetically sealed titanium, which can stand up to saline exposure for decades unlike silicone.

In May, Second Sight reported that FDA granted approval for up to 20 people who are blind or who have severely impaired vision due to the genetic eye disease, retinitis pigmentosa, to participate in the Argus II Retinal Implant feasibility study in the U.S.

Lynn Yoffee, 770-361-4789;

lynn.yoffee@ahcmedia.com