Biomedical Engineering Reference
In-Depth Information
implants. In the case of electrodes implanted directly in the visual cortex,
pain around the implant occurs due to the irritation of the meninges by the
electrode current. In order to alleviate this pain, smaller electrodes and
reduced currents must be used. The main disadvantage of cortical implants is
the risk of surgical complications during the implantation procedure. The
advantages of this strategy are that the electrodes are protected by the skull and
the possibility of completely bypassing the damaged retina or optic nerve.
Placing electrodes around the optic nerve allows the stimulation of the visual
cortex using a video device situated outside the brain. This strategy requires the
optic nerve to be intact. However, there is a risk of infection from the
external wires.
With regard to retinal prostheses, there are mainly three types of approaches
that can be employed. These are subretinal microphotodiode arrays, epiretinal
and subretinal electrode arrays.
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As a result of the direct contact of the
electrodes with such a sensitive neural tissue, biocompatibility considerations
become extremely important. Complete hermeticity of the encapsulated
intraocular components and special materials that do not corrode or release
toxic contaminants must be used.
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The implanted electrodes must be covered
by a capacitive layer with very high resistance. Tissue reactions from mild
formation of loose vascularized tissue to newly formed tissue covering the
electrodes raise the impedance, which strains the already challenged power
supply. Since the retina itself is approximately 250 mm thick, such growth
cannot be tolerated. The central vision is given by the macula area of only
1-2mm in diameter, usually the first target of the degenerative processes. The
presence of the prostheses itself can speed up the deterioration by limiting the
nutrient and the oxygen supply to the retina. In particular, the tissue reaction
involves a cascade of events at the molecular level, mainly the production
and release of neuroinflamatory cytokines—interleukins, tumor necrosis
factor-a (TNF-a), monocyte chemotactic protein-1 (MCP-1), platelet-derived
growth factor (PDGF).
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Electrochemical impedance spectroscopy methods
are able to detect and quantify the process with high accuracy.
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Finally,
retinal detachment, cataract formation, changes in the retinal pigment
endothelium, infection and local inflammation due to surgical insertion are all
potential risks which can endanger the peripheral vision that patients still
retain.
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Scientists around the world are trying to circumvent all these apparently
insurmountable diculties and restore at least partial functional vision to the
blind in order for them to regain some independence and at least spatial
orientation and feature recognition for everyday life. Epiretinal arrays of
microelectrodes can be connected to an exterior camera that captures the
images and subsequently converts them into an electrical stimulation pattern
imposed on the remaining functional retina. The array is essentially a readout
chip coupled to the ganglion cells. The extraocular unit is sutured to the outside
of the eye onto the sclera. Radiofrequency transmitter coils for power and data
are mounted on the patient's glasses and receiving coils are implanted on the
temporal side of the eye.
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