Retinitis pigmentosa, a hereditary eye disease that affects the retina, can cause progressive vision loss and blindness and has no known cure. However, an implantable microchip might one day improve the vision of humans affected by the disease.In normal vision, photoreceptor cells, which are neurons in the retina, capture light and convert it to electrical signals. If these cells degenerate and die, vision loss occurs. Scientists around the world have been working on various types of implants to stimulate the damaged photoreceptor cells. However, the implants are often invasive, and most require external devices such as computers, lasers or radio-frequency transmitters.Researchers implanted a microchip (orange disk on the right) into a cat’s retina to see whether it could stimulate retinal cells and improve vision. Courtesy of Kristina Narfstrom.Professors Kristina Narfstrom from the University of Missouri-Columbia and Machelle Pardue from Emory University in Atlanta have been working with an artificial silicon retina microchip designed by Optobionics Corp. in Naperville, Ill., that doesn’t require any external devices to function. The 23-μm-thin, 2-mm-diameter disk consists of several thousand microphotodiodes that catch light and covert it to photoelectric signals in the same way that healthy photoreceptor cells would. The artificial signals induce electrical signals in any remaining functional retinal cells, which then send the visual information to the brain via the optic nerve.Narfstrom has been testing implantation of the chip in the retinas of severely visually impaired or blind Abyssinian cats. She said that the cats suffer from visual problems very similar to human retinitis pigmentosa. A cat’s eye also is very similar physiologically to the human’s. When performing the surgery, Narfstrom makes an incision in the lateral eyelid and opens up all the structures down to the sclera (the outer layer surrounding the eyeball). She makes two small cuts into the sclera, one for a light pipe so she can see where to operate and the other for the actual procedure. After removing the vitreous liquid that fills the back part of the eye, she makes a small blister in the retina, which she opens up just wide enough to place the microchip inside.So far, the researchers are pleased with their preliminary results, which indicate the microchip does stimulate damaged retinal cells. Narfstrom said that the microchip is minimally invasive for the cats if the surgery is done correctly.The researchers plan to implant the microchip into cats with early stages of the disease to see if the chip also can prevent retinal cells from dying. They hope that the device, if implemented early enough, will ultimately have a protective effect on the retina, as well.