Retinal implants aim to provide artificial sight
Over the past decade it has become commonplace to use prosthetic devices such as artificial hips and pacemakers to replace or enhance parts of the human anatomy. Twenty years ago, the development of the cochlear implant--tiny electrodes that stimulate the acoustic nerve--gave some previously deaf patients the gift of sound. Now, researchers, such as those at North Carolina State University (NCSU; Rayleigh, NC), are developing neuroprosthetic devices to replace lost visual functions (see Vision Systems Design, June 1999, p. 31).
To develop a prosthesis capable of functioning as photoreceptors when implanted into the retina, Eberhart Zrenner at the University Eye Hospital (Tübingen, Germany) and others are also developing a prosthetic retinal device. It is hoped that such devices, designed to produce electrical impulses to the visual system, will give patients with retinal lesions but otherwise intact visual systems a limited capacity to see.
In the development of the prosthetic retina, Zrenner and his colleagues used prototypes of photodiode arrays built at the Institute of Microelectronics (Stuttgart, Germany). These devices contain 7000 microelectrodes that are 20 x 20 μm in size, allowing a photodiode density comparable to that of photoreceptors in the retinal periphery. For stimulating the retinal cells, the microphotodiodes (MPDs) are attached to 10-μm gold, iridium, or titanium electrodes at the Natural Sciences and Medical Institute (Reutlingen, Germany). According to Zrenner, these materials were selected because they are corrosion-proof, establish good contact with the nerve cells, and are biologically compatible with the nervous system.
Fabricated in silicon, MPDs can be charged either negatively or positively. When light illuminates the photodiodes, electric current flows from the front to the back of the devices allowing the nerve cells adjacent to the cell membranes to briefly become positively polarized.
If the electric impulse is strong enough, a neurotransmitter substance of the nerve cells is released and signal conduction can be achieved. Depending on the adjacent cell type, either a negative or a positive stimulating current can lead to release of neurotransmitter substance. In this process, it is essential that the electric charge generated by the MPD exceeds at least 1 nC (nanoCoulomb) of electrical charge.
The transmission of the low electrical charge generated by the MPDs must be optimized to the nervous tissue. Therefore, the surface of the MPDs must be coated so that the nerve cells can be close to the implant. To improve the contact, protein molecules will be bound to the surface of the MPD. This will result in micropattern so that nerve-cell adhesion can be precisely controlled. Researchers are continuing to devise a coating that will impel the nerve cells to grow onto the stimulating electrode.
