by Charles Q. Choi

Artificial organic light sensors could one day help lead to eye implants that integrate more naturally with the body to restore vision, researchers say.

Blindness often involves damage to the retina, the light-sensitive inner lining of the eye. For instance, retinitis pigmentosa, a group of inherited diseases that afflicts 1 in 4,000 people across the world, involves degeneration of the eye’s light-sensing cells, or photoreceptors.

Research teams worldwide are developing retinal prosthetics that seek to restore vision using electronics that essentially replace lost photoreceptors. In just one vein of study recently receiving attention, neuro-ophthalmologists at the University of Tuebingen in Germany have been implanting light-sensitive microchips within eyes that are helping patients recognize facial expressions and see the faces of their loved ones.

However, current retinal prosthetics typically use electrodes made of inorganic semiconductors like silicon to interface with the body.

“These need to be powered to electrically stimulate cells, and you need wiring to power the electrodes,” says neuroscientist Fabio Benfenati at the Italian Institute of Technology in Genoa. “The bulk of this can be problematic. The fact they need to be powered generates heat, which is detrimental for the retina. Also, the implants are more or less rigid, and the implant of a rigid device over time causes an inflammatory reaction in the eye, thereby greatly reducing its efficiency and contact with the rest of the retina.”

A better option: carbon-based materials

As such, Benfenati and his colleagues instead explored using organic materials based on carbon, which make up all life on Earth. Organic conducting polymers are already used for cellular interfaces in several applications, such as cellular scaffolds, neural probes, biosensors and drug-release devices.

The scientists began with a glass layer they covered with the transparent conductor indium tin oxide, which is often used in touchscreens, smartphones, flat-panel displays, solar panels and more. They next coated this material with poly(3-hexylthiophene), or P3HT, a polymer semiconductor commonly found in organic solar cells.

The researchers then placed on this coated glass the retinas of blind rats whose photoreceptors had been damaged by exposure to too much light. These damaged cells included the type called rods, which let animals tell dark from light, and cones, which let them see color.

The investigators found that when the polymer layer was lit with 20-millisecond pulses of light, it generated electrical signals that caused the retinal neurons to fire. “It really mimics the light absorption and electrical effect that naturally occurs in our photoreceptors,” Benfenati says.

The scientists note the polymer they used was quite sensitive, registering a broad spectrum of light normally seen outdoors during the day. They believe they can optimize devices further to operate over the full range of daylight conditions.

As good as the real thing?

In principle, all-organic retinal prosthetics using these light-sensitive compounds not only offer the potential advantage of compatibility with and acceptance by the body,  but also freedom from the need for a battery and wiring to power light sensors. Benfenati estimates devices based on this system could achieve visual resolutions about as good or better than current implants.

Future research needs to test how well implants based on this system are tolerated by the body, what material to use instead of glass in these implants, and the quality of vision they provide in terms of resolution and contrast.

“We are now implanting devices in the eyes of rodents with genetic degeneration of photoreceptors similar to retinitis pigmentosa or macular degeneration, and our preliminary results are going in the right direction,” Benfenati says. “We then aim to move to higher animals before final tests in humans.”

Benfenati and his colleagues detailed their findings online March 17 in the journal Nature Photonics.

Top Image: Visualization of a primary hippocampal neuron grown onto a photovoltaic photoactive poly(3-hexylthiophene) (P3HT) layer. A green light beam helps stimulate the neuron. Credit: Graphics by Nicola Martino, on an scanning electron microscope image of a hippocampal primary neuron courtesy of T. Limongi, M.Orlando, R. Ruffilli, F. Cesca.

Charles Q. Choi has written for Scientific American, The New York Times, Wired, Science and Nature, among others. In his spare time, he has traveled to all seven continents, including scaling the side of an iceberg in Antarctica, investigating mummies from Siberia, snorkeling in the Galapagos, climbing Mt. Kilimanjaro, camping in the Outback, avoiding thieves near Shaolin Temple and hunting for mammoth DNA in Yukon.