The Retinal Implant
An eye is a precious part of a human which plays an integral sensory role for the visual capability of the vision. Because of many factors this part may come to loose its visionary sensing power in some human beings. For example, Age-Related Macular Degeneration (ARMD) is a progressive disease of the retina wherein the light-sensing cells in the central area of vision (the macula) stop working and eventually die. By replacing the dead part of retina with the retinal implants, theoretically and practically, the diseases related with retina can hopefully be controlled.
The principle and working of an Eye
The construction of the Eye is similar to that of a camera. In principle, the visible light is focused by a lens on a screen called the retina and the image is realized. Fig.1 (please find at http://webvision.med.utah.edu/sretina.html) shows a cross-section of human eye ball with its different parts. The Retina, being the most significant in the functioning of the eye, is composed of layers with different cells as shown in Fig. 2 (please find at [http://www.farlops.com/images/photos/retina.jpg] )
The light first enters the Nerve Fiber Layer and the Ganglion Cell Layer, under which most of the nourishing and aiding blood vessels of the retina are located. This is where the nerve begins, picking up the impulses of the acquired image from the retina and transmitting them to the brain. The light is received by the Photoreceptor cells called Rods and the Cones. These cells convert light into nerve impulses, which are then processed by the retina and sent though nerve fibers to the brain. The nerve fibers exit the eyeball at the Blind spot and reach the brain through the Optic nerve. The further anatomical details can be found elsewhere.
The Retinal Implant Functionality
To help visually impaired patients, a visual prosthesis could be designed to be placed in the eye, either under or on the retinal surface, sub retinal areas, etc, in the optic nerve, or in the visual cortex of the brain. Each approach has certain advantages and disadvantages.
Based on the above principal idea, the light is sensed by a large assembly of photo-receptor diodes integrated on a Chip connected with the electrodes for the electrical power. This chip collects the information of the sketched image fall on it and triggers the family of cells grown on this chip. The function of this chip/implant is the same as the retinal photoreceptor cells which are connected with the bipolar cells and ganglions and transmitting the photo signal to the respective part of the optic nerve [Ref. 4]. It is not easy to explicate the phenomena for the transmission of the sensed sketch produced in the chip of photoreceptor diodes to the grown cells of the retina, and from them to the optic nerve. Biologists and chemistry people are trying to resolve these phenomenons into a simple, explainable and electronically feasible for the best possible performance of the retinal implants.
Conclusion and Future work
There are certain flaws for the In-vivo application of retinal implants. Bio-compatibility, one of the first main issue, of an Implants means especially the metal electrodes and the whole chip (the electronics) must be able to resist the body environment. The electrolytic properties of the blood, pH and ionic behavior of the In-vivo environment are some crucial parameters, nowadays faced by the scientists. The compatible implants should also be longtime susceptible to the animals as well as to the human beings.
Similarly, the diffusion of liquids through the sealing and packed retinal implant is also a great challenge that limits the working period (life) of an implant. For the commercial available synthetic materials, (plastic foils, etc) the permeability rate of water vapors is 5*10-3 g/m2/day. Scientists are seeking the best possible material with the least permeation which would in turn enhance the life time stability and bio compatibility of the implant in the body.
Besides the state-of-the-art device fabrication improvements, a visual prosthesis must receive two types of inputs, information about the visual signal from the retinal implant and the power to run this whole electronic assembly. It is detrimental for the purposes of long-term implantation to have wires penetrating the body or imbedded batteries that could corrode and have to be replaced if not properly sealed as mentioned above. Alternatively, one can send the visual signal and power to the implant without the use of wires. Wireless communication can be accomplished with laser light or radiofrequency transmission.
Before performing experiments on the human eye, it is necessary to perform laboratory experiments to determine safe methods of device implantation and electrical stimulation of the retina. These studies are performed in vitro (i.e. outside the animal; using tissue preparations) and in vivo (i.e. in the living body). In order to carry out such experiments, approval must be obtained and granted from committees that monitor experimentation with animals and humans from all the over the world.