Electronic eye

A retinal implant based on a microchip could help treat blindness
Electronic eye

MICROCHIPS form the basis of severaldevices, such as Min (magnetic resonance imaging) scanners and pacemakers, which have revolutionised medicaldiagnosis and treatment. Now, microchips may also provide relief to manypeople suffering from blindness.

Researchers working on the Projectfor a Retinal Implant, a joint effortof the Massachusetts Institute ofTechnology (MIT) and the HarvardMedical School, have completed experiments that provide hope that a retinalimplant may soon be a reality (Science, Vol 268, No 5211).

The scientists have finished a seriesof experiments in which prototype retinal implant components were used inrabbits. These devices were used todeliver tiny currents to the ganglioncells (nerve bodies in the retina that feedthe optic nerve which transmits theinformation to the visual cortex in thebrain) and a measurable responsewas detected in the visual cortex of the rabbits.

The components tested will eventually form a part of an artificial eyereplacement kit that will include aminiature camera mounted on eyeglasses, a laser beam to transmit the information and a microchip to convert light into electric pulses.

The camera will record the information in the visual field and digitise it for the laser to beam it to the retinal implant.

The retinal implant will be amicrochip with an electrode array toconvert the laser pulses into electricalsignals which then stimulate the ganalion cells to transmit the informationIn the brain.

Though this is the ultimate goal,ray of hope for the blindthere are still many hurdles to be crossedbefore it can be realised.

First, there is still some controversy on how many healthy ganglion cells are there in the retinas of blind human beings. After all, because it is the ganglion cells which are ultimatelyation transmitting the information, there should beenough healthy ones to dothe job.

Experiments conducted at the Johns HopkinsUniversity Hospital, Baltimore, have indicated thatthere may be as many as 70per cent healthy ganglioncells in patients with outer retinal degeneration. cone Even if the ganglion sule cells are healthy, the implant has to be such that the retina is not harmed physically. This tissue, says John Wyatt, an MIT CeC trical engineer, has the mechanical properties ofabout one ylayer of sticky wetKlccnex." And that doesn't make thingseasy, as the implant's electrode array,made of polymicle plastif 50 microns (amicron is a millionth of a metre) thick,has razor-sharp edges, Before insertion,therefore, the array is coated in siliconeand bent to match the retina's owncurve, so that it rests on the surfacewithout tearing it.

Finally, there is the most dauntingproblem of the chip communicating theinformation with the visual cortex in aform that can be recognised by thebrain.

The ganglion cells in the retinaarc highly specialised and diverse typesof these cells respond to differentaspects of the stimuli like colour, intensity and movement. To determineexactly how to stimulate the ganglionCells to Cause them to lire in an interpretable way is one of the majorunsolved problems.

Some 1.2 million people world-wide suffer from eye ailments thatlead to blindness because the visualreceptors in the retina are lost. Thoughthe day when all these problems aresolved successfully is still far off (somesceptics claim at least half a centuryaway), continuing research provides aray of hope.

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