76 S. Norman and R. Bellamkonda
visual system, so different methods have been devised to help patients with
different impairments. If the photoreceptive cells, i.e. the rods and cones which
are respectively sensitive to grayscale and colored light, are damaged, then an
electronic array to mimic their function can be placed over the optic nerve, forming
an artificial retina. An artificial silicon retina (ASR) implanted into the subretinal
space, between the retina and the optic nerve, of patients with retinal pigmentosa
improved patients’ general sensitivity to light, and some patients noticed an
improvement in the size of their perceived visual field.
65
The ASR in this study
was a 2 mm wide, 25 micrometer thick disk with an array of approximately
5000 microphotodiodes, which were 9 μm by 9 μm squares of iridium oxide
isolated from each other and bonded to the substrate. Flexible polyamide planar
arrays with oxidized iridium electrodes have also been implanted into the eye
on top of the retina; these epiretinal arrays deliver monopolar stimulation, and
experimenters showed that shorter stimulation pulses require less charge density
to elicit a response than longer pulses.
66
Parylene electrode arrays have been
fabricated for use atop the retina as well; the parylene is patterned on top of a
photoresist release layer, and platinum electrodes are patterned onto it.
67
While
retinal implants are promising and can allow patients to distinguish between high-
contrast objects and attain a wider visual field, the scenes created with a retinal
prosthesis are not as refined as those seen with a natural functioning eye, and
improvements must be made to create a device that adequately transposes a scene
from the external environment to the brain of a blind patient.
Another approach to visual restoration is the stimulation of the visual cortex
(V1) directly. V1 is located in the occipital cortex, at the most posterior portion
of the cerebrum. Studies have shown that blind patients, even those blind for
years, can perceive phosphenes, or small areas of light, when areas of V1 are
stimulated with an electrode array.
68
The stimulation arrays used in this study
were8by8arraysof1mm
2
platinum disk electrodes in Teflon ribbon cables,
and penetrating cortical arrays such at the Utah arrangement have been used
for recording studies of the visual cortex.
69
Microwire arrays, made from gold
and iridium wires coated in parylene, have also been used in V1 stimulation
experiments; these experiments show that less current is needed from penetrating
electrodes than from surface electrodes to generate phosphenes in a patient.
70
V1
stimulation is another promising technology for repairing sight, but the techniques
are not perfect and refinements are still needed.
4.2.7 Auditory Prosthetics
Auditory prosthetic devices are another MEMs application. If the auditory nerve is
intact and functional, a cochlear prosthetic can be used to return some hearing to a
patient. The main parts of a cochlear implant include a microphone for collecting
sounds, an external and internal processor, and a stimulation electrode array.
71
Cochlear electrode arrays contain sites for stimulation. The electrode array is
inserted into the cochlea and placed in close proximity to the auditory nerve. When
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