ARTIFICIAL RETINA

The developments in biotechnology today are opening new avenues of research. From the beginning of the nineties, groups of researchers all over the world have been involved in the development of artificial, or electronic, retinas.

A high percentage (over 50 % in the civilized world) of visual disability or blindness is a consequence of pathologies which jeopardize normal functionality of the retina.
No medications or surgical interventions are available today to restore the compromised retinal tissue.  The potential but not yet realistic solution comes from staminal cells.
Even in an isolated way, it is understandable how everywhere in the world groups of researchers are involved, since the beginning of the nineties, in the development of artificial, or electronic, retinas; real substitutes for worn out and damaged retinas.
The impressive evolution in nanotechnologies represent a meaningful incentive for such research.
Complex problems remain and these will be better understood after the illustration of what are the physiological functions of the retina and what are the strategies employed to develop a substituting electronical support.

The retina is the internal bulbous membrane which transforms the photonic impulse, that is the light which arrives from the external world in bioelectric impulse. This process, indicated as transduction, is possible thanks to specialized cells called photoreceptors that have pigments (derived from vitamin A) which, while absorbing photons, activate the ionic canals of the celullar membrane exciting them. These cells are extremely sensitive considering that a single photon is able to activate the membrane. The excitement is therefore transmitted to a complex neuronal system (nervous cells) intra-retinal (bipolar and horizontal cells) where it undergoes codifications, translating the external world into luminous dots with varying contrast, colour, orientation and movement. The intermediate nervous cells are connected to another layer of cells called optic ganglions, which codify in a signal the frequence modulation through their axon, and send it to the geniculate body, first central station, which then passes it on to the visual cortex.

Each point of the retina communicates with a corresponding cortical area (V 1), forming a sort of shadow on the retina (retinotopic) at the cerebral level. The peripheral retina is able to transmit moving images, while the central retina provides the perception of details (discriminatory perception) and colours. The artificial retina is to completely substitute the role of photoreceptors, and partially the cellular system inserted between the photoreceptors and the optic ganglion cells. In certain pathologies and in particular in retinitis pigmentosa, the basic lesion involves photoreceptors and pigment ephitelium, and only later the more internal nervous cells. In such situations, an electronic device capable of picking up light to convert it in electrical stimulus allows restoring of the function of the retina, which is light perception. These are the concepts which have inspired, in the early nineties, researchers from the University of Illinois in Chicago, to replace the first artificial retina by putting together micro-photodiode array, connected with resistors, and inserting them in a sort of pocket developed surgically between the nervous retina and pigment epithelium. To give an idea of the level of miniaturization, let us precise that, in an area of two millimeters, are inserted 3,500 diode. Subsequent studies conducted by the German school (Prof. Eberard Zrenner Tubinga) have led to the development of micro-photodiode capable of  inducing positive or negative polarization with stimulation voltage, and distance between corresponding electrodes with bioeletrical characteristics, and with the density of interfaced retinal cells. The prosthesis was also designed with a porous surface in order to allow an optimal connection with retinal cells. The bio-compatibility at the experimental level provides quite satisfying results.

Really clinically acceptable then the artificial retina? There still subsists noteworthy reserve, most of all concerning the surviving capacity of the nervous retinal tissue, when the support of the coriocapillary and pigment epithelium begins to fail. A possible effect contributing to increasing the neuronal vitality is delineated in the use of growing factors particularly the neurotrophine (BDNF). At the experimental level, it has been documented that a direct action on the vitality of the retinal neurons, is able to inhibit apopthosis and stabilize its geniculate synaps. There is also some reserve on the visual capability, on the extent of the recovery, until it is verified. Perceptions actually obtained are limited to sensations of light and shade. Decidedly less attractive are the prostheses studied by researchers at the Boston Harvard Medical School in collaboration with the Massachussets Institute of Technology, and another research team coordinated by Prof. Rolf Eckmiller from the University of Bonn. They have developed prostheses conceptually different in the sense that they are external devices, which convert images from a photocamera in laser impulses, which activate an internal chip located on the internal side of the retina. The chip with its special electrodes transfers the impulse in ganglion cells.

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