04-05-2011, 03:59 PM
ABSTRACT
MEMS technology has been attractingincreasing attention in several medical fields forits possible applications in curing diseases,which were considered intractable until fewyears ago. A typical application, as example,regards the MEMS based cochlear implant,which is meant to give hearing to deaf peoplethrough a wireless communication device.Moreover, MEMS accelerometer are widelyapplied to detect movement, heat flow, skintemperature, heart rate, etc...Among all these applications, one inparticular attracts the attention of researchers: themicrochip retinal implant. Extended studies havebeen conducted in several places of the world(USA, Japan, Australia, etc…) in order todevelop a microchip able to stimulate damagedretinal cells. This is very important for patientsaffected by Retinitis Pigmentosa (RP) and Agerelated Macular Degeneration (AMD).In this research, the structure of the eyeand the basic principle of the eye’s view will beinvestigated. The focus will be in particular onthe function of the retina on the process of sightand the characteristics of the macula, the part ofthe retina, which gives the highest resolution forthe images. The possible diseases and abnormalretinal conditions will also be briefly considered,especially the aforementioned RetinitisPigmentosa and Age related MacularDegeneration.In the second part of the project, thepossible approaches to cure the retinaldegeneration will be analyzed (i.e.: epiretinal andsubretinal implant). In particular the study willfocus on the microdevices applied in both theaforementioned approaches. On one side theepiretinal system is based on a retina encoder, atelemetry page link and a stimulator device. On theother side the subretinal implant is based on asilicon microchip of 2mm diameter, whichcontains microscopic solar cells calledmicrophotodiodes. These are designed to convertthe light energy from images intoelectrochemical impulses that stimulate theremaining functional cells of the retina inpatients with AMD and RP. Moreover, it isdesigned to produce visual signals similar tothose produced by the photoreceptor layer inorder to subsequently induce biological signalsin the remaining functional retinal cells.The research will also point out the mainaspects in the microfabrication of thesemicrochips and the possible alternatives in theprocess. Another important issue is thebiocompatibility of the various componentinvolved in the fabrication, with a particularconcern for the biocompatibility of silicon.Finally, some important results from theapplication of these new techniques will beaddressed. In fact, pre-clinical laboratory testingshowed that animal models with the retinalimplant responded to light stimuli with electricalsignals and sometimes brain wave signals. Theinduction of these biological signals indicatedthat visual responses had occurred. Besides, thesubretinal device has been implanted in somepatients with RP to study its safety andfeasibility in treating retinal vision loss. Untilnow no patient has shown signs of implantrejections, inflammation or other problems.
INTRODUCTION
Retina PhysiologyThe human eye is composed by severalparts but the most important is definitely theretina, which converts light information intoneural electrical signals. These signals aretransported to the visual cortex of the brain bythe optic nerve. The visual cortex then decodesthe neural signals into a meaningful image.The retina is composed of approximately126 million photoreceptors (size: 2-3 μm), whichprovide an analog electronic signal to theattached bipolar neural cell layer. The bipolarcells convert the signal into electrical pulse train.Signal processing and convergence is performedin all neural cell layers comprising horizontalcell, bipolar cells, amacrine cells and ganglion.Approximately one million axons of the ganglioncells form the optic nerve, which extends into thevisual cortex of the brain. The light passesthrough the eye and through the 200 μm thinretina neural layer before reaching thephotoreceptors (Fig. 1 in next page).
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