Biomedical Engineering Reference
In-Depth Information
Figure 11.1. Illustration of the epiretinal prosthesis.
(epiretinally) and fully integrated with photosensitive imaging and microchip
stimulating components. It is capable of sensing and processing environmental
visual information and transforming that information into a patterned stimulus on
a microelectrode array. Electrical connection to the retinal bipolar and ganglion
cell layers is accomplished by positioning the array in close proximity (within
20m) to the retina. The leads and track lines of stimulating electrodes are
individually electrically isolated and coated with biocompatible polymer coatings
to protect them from intraocular fluids. When energized, the prosthesis can
bypass damaged photoreceptor cells and reinitiate threshold membrane potentials
in retinal neuronal cells to restore the visual neural pathway.
The rationale for the retinal prosthesis arises from previous experimental
analysis of the morphology and electrical response of retinas damaged by degen-
erative disease. In these eyes, photoreceptor damage is extensive but transsy-
naptic neuronal degeneration is limited. Initial research with retinal electrical
stimulation in patients by Humayun et al. [2] showed that viable neurons could
be electrically excited using small extracellular currents delivered by hand-
held electrodes placed on the retinal surface. Performed in blind human volun-
teers in the operating room, these short-term tests showed that retinal electrical
stimulation resulted in the perception of light in patients who were blind.
More recently, a functioning 16-pixel microelectrode array has been chronically
implanted on the retina of a patient by Humayun et al. [3]. In the laboratory,
external digital imaging and data processing devices were linked with the internal
prosthesis and, when the electrodes were current pulsed, the patient perceived
light. He could also discern large print letters and everyday objects, such as a
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