The Artificial Retina

By: Jacqui Mclaughlin

The Retina

The retina is a delicate membrane composed of neural tissue (Bouvier et al, 1996) located in the back of the eye. It is this part of the eye that gathers visual information such as light detection, object recognition (Fogler and Hush, 1996), motion direction, and color (Anonymous, 1997f) and sends it to the brain. There are roughly 800,000 cells in the retina that act as the eye's photodetectors (Anonymous, 1997q), called rods and cones. In an individual with normal vision, messages from these cells are interpreted by ganglion cells. The ganglion cells then send these messages to the brain via the optic nerve. However, when the rods and cones become inoperative, visual capability is lost. Over 10 million people suffer from retinal diseases leading to the loss of sight (Anonymous, 1997g). These disease include retinitis pigmentosa, affecting over 1.2 million world-wide, and age-related macular degeneration, the leading cause of blindness in the west (Roush, 1995).

In the cases of both of these diseases, the rods and cones no longer function, but the ganglion and optic nerve remain operative. In 1988, Dr. Mark Humayun of Johns Hopkins University demonstrated that the nerves behind the retina function regardless of the degenerative state of the retina (Anonymous, 1997g). By electrically stimulating points on the retina, a visually impaired individual could recognize points of light corresponding to those points activated on the retina (Liu et al, 1997). Thus, Dr. Humayun's results suggest that replacing the retina with a device capable of translating images into electronic impulses may restore vision (Anonymous, 1997g). It is this research that lays the foundation for a collaborative effort between researchers from North Carolina State University, University of North Carolina at Chapel Hill, and Johns Hopkins University to develop an artificial retina that will assist in restoring sight.

The Artificial Retina Component Chip

The artificial retina component chip (ARCC) is being developed by electrical engineering professor Dr. Wentai Liu of North Carolina State University and doctoral student Elliot McGucken of UNC-Chapel Hill (Anonymous, 1997h). This device consists of two main components: a photosensing, processing, and stimulus-driving chip powered by solar cells and a simple electrode array (Liu et al , 1997). The chip is placed just in front of the damaged retina (Anonymous, 1997g) where it can receive visual information and stimulate the retina with the proper current specifications. The current pulses are then passed to the attached electrode array (Liu et al, 1997).

Permission to reproduce image granted by Dr. Wentai Liu

Previously designed devices were too intrusive, utilized an unstable power supply, or lacked biocompatibility. However, the current ARCC is designed to account for these problems. It is two millimeters square and is being polished to less than 0.02 millimeters thick. This thickness will enable the passage of light and images through the chip to the photosensors at the back of the chip. The silicon microchip can be implanted near the vision center of the retina and may receive light and images through the pupil. It is powered by a laser, possibly attached to a pair of regular eyeglasses, that is aimed at a photovoltaic cell in the chip. This power source is ideal since the laser beam can pass through the cornea without damaging the corneal tissue and the regular eyeglasses replace clumsy or obtrusive headgear. When powered, the photosensor cells in the microchip convert the light and images into electric impulses, similarly to Dr. Humayun's demonstration, and stimulate the nerve ganglia behind the retina (Anonymous, 1997g).

The current ARCC has an array of 5 by 5 pixels which is just enough to identify individual letters (Liu et al, 1997). However, it is estimated that within five years, the chip may grow to a 20 by 20 array and eventually into an 250 by 250 array which is enough to read a newspaper (Anonymous, 1997h). Although the current chip does not restore clear vision, it does generate the ability to recognize movement direction and external forms (Anonymous, 1997f).

Currently, Liu and McGucken are working with Dr. Mark Humayun and Dr. Eugene de Juan of Johns Hopkins University on the design of the microchip. After testing is finished in Liu's lab, Humayun and de Juan will receive the microchip to test for biocompatibility. Researchers at Stanford University (Anonymous, 1997h) and Massachusetts Institute of Technology are also doing research concerning biocompatibility of the artificial retina (Rizzo et al, 1996). Whether or not the silicon implant is compatible with the body must be determined before the artificial retina is tested in humans. Once the biocompatibility testing is completed, the researchers will begin testing the retinal prosthesis in animals. The researchers must then receive FDA approval for application of the ARCC in humans (Anonymous, 1997g).

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