Restoring Sight to Blind Rats

Seventies television aficionados might remember Steve Austin, the Six Million Dollar Man, who was rebuilt with prosthetics, including "bionic" eyes, after a terrible accident. Yet more than 40 years later, eye prosthetics remain, at best, clunky, energy-intensive, and prone to immune system attack. 

An organic retinal prosthesis that uses flexible conductive polymers rather than hard silicon electronics could change that. The device successfully restored sight to blind rats, lasted six to 10 months, and functioned without external power sources or wireless receivers. 

Fabio Benfenati, director of the Italian Institute of Technology's Center for Synaptic Neuroscience and Technology, led the research.

While the Six Million Dollar Man made it look easy, eye prosthetics challenge engineers. As anyone who ever used hard contact lenses knows, their rigid structure is a poor match for the eye's soft, curved surface, even before the immune system gets involved. 

In addition to mechanical and immune system problems, eyes present chemical challenges. Natural discharges of salty tears and other biological fluids can corrode and break down a prosthetic's structures. 

To understand how Benfenati's prosthetic eye works, consider the healthy eye. The lens focuses light onto special photoreceptor cells called rods and cones in the retina. These convert light into electrical signals, which activate nearby cells that pass information to neural cells that connect to the brain.

Several neuroscience studies have shown that, when photoreceptor cells have been lost due to disease or injury, researchers can electrically stimulate the brain cells behind the retina directly to restore some lost vision. 

Several researchers and startups have built prosthetic eyes. These use charge coupled devices (CCDs, silicon-based sensors like those found in cameras) to capture light and convert it to electrical pulses. Yet these require lots of onerous hardware, and they face long-term biocompatibility issues.

Read more about cutting-edge enhancement prosthetics from AABME.org. 

To address these issues, Benfenati looked at conjugated polymers, which are more flexible and electrically conductive. The one he chose, poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate), or PEDOT:PSS, is photovoltaic and converts light into electricity. 

"It's a remarkable material," he says. "We found that when we illuminated these polymers, we could excite and inhibit activity in neurons in a dish. The next step was to create an interface, one we could surgically attach to the eye, so we could try to use these materials to help restore sight in a rat model of blindness."

This required some design ingenuity. The rat eye is tiny, only 6 millimeters or so. The researchers had to buffer the conjugated polymer to create an implant that was flexible enough to be tolerated by a rather finicky body part. 

Instead, the researches created a sandwich. It consisted of a bottom semiconductor, poly(3-hexylthiophene); a middle conductive layer of PEDOT: PSS, and a passive layer of crystallized silk fibroin take from Bombyx Mori cocoons. 

After cutting the device to size, the team surgically implanted it under the retina of blind rats. To their delight, within a month, the rats' pupils began to dilate and contract reflexively when exposed to light. The researchers also measured electrical activity in the retina and increased metabolic activity in the eye, all without any undue friction, inflammation, or infection.

"Our device was able to restore normal vision functioning, both in terms of physiology and behavior, in these animals," says Jose Fernando Maya-Vetencourt, Ph.D., a scientist who works with Benfenati. "We saw nearly 65 percent recovery. The loss of these photoreceptors is the leading cause of adult blindness and does not currently have a treatment. So, our hope is being able to use this kind of device on humans in the future."

Toward that end, Benfenati and Maya-Vetencourt are now testing the device on pigs, an animal model that better reflects human physiology. But before they could start that study, they hit another design challenge--the silk fibroin wasn't working in larger devices.

"Unfortunately, the silk is not so uniform when we try to make the device bigger and use it for larger areas," Maya-Vetencourt says. "Instead, we've found a new material, polyethylene terephthalate, or PET, a plastic used in arterial and peritoneal implants. It can be created in a very similar thickness and is extremely regular on the surface, so the polymer's adhesion is even higher than before."

The researchers hope they will move to human clinical trials within the next few years, starting with a handful of patients. Benfenati and Maya-Vetencourt are confident that they can renovate damaged eyes. They have the technology, they just have to do the testing. 

"Once we work with patients, it will give us even more information on the kind of rescue that is really happening," Benfenati says. "The pig cannot tell us much at all. We cannot get the same information as you can get from a conscious person about what they are experiencing with the prosthetic. Perhaps then we can do even more to make the device comfortable and successful for this kind of blindness that doesn't have any other available treatments."

Kayt Sukel is an independent technology writer based in Houston, TX.