Biomedical Engineering Reference
In-Depth Information
FIGURE 7-53
3D microelectrode
arrays. (a) Utah
electrode array.
(b) Utah slanted
electrode array.
(c) Umich array.
(Medscape 2008)
Researchers at the University of Utah-Salt Lake City and the University of Michigan-
Ann Arbor have devised methods by which complex three-dimensional microelectrode
arrays can be built. The Utah electrode array (UEA) and the Utah slanted electrode ar-
ray (USEA)are two examples of such neural interfaces and are shown in Figure 7-53a
and Figure 7-53b. A three-dimensional electrode array developed by researchers at the
University of Michigan was built using more conventional integrated circuit technologies,
shown in Figure 7-53c. It has multiple electrode sites that are distributed along a number of
electrode shanks, with the planes of the electrode shanks integrated into a single-electrode
array.
The UEA consists of 100 1.5 mm long microneedles that were designed to be inserted
into the cerebral cortex to a depth of 1.5 mm, the level of normal neural input to the
cerebral cortex. The electrodes of the UEA and USEA are built on a square grid with 400
μ
m spacing. A total of 100 gold bond pads are deposited on the back surface of these
arrays, and 100 thin insulated gold wires are bonded to these pads and to a percutaneous
connector for connection to external electronics. The tip of each microneedle is metallized
with iridium oxide to facilitate electronic to ionic transduction, and the entire array, with
the exception of the tip of each microneedle, is insulated with a biocompatible polymer.
The USEA also has up to 100 microneedles, but their lengths are graded from 0.5 mm
to 1.5 mm along the length of the array. The graded lengths of the USEA ensure that
when it is inserted into a peripheral nerve the electrode tips uniformly populate the nerve,
with most nerve fibers being no more than 200
μ
m away from an active electrode tip
(Medscape, 2008).
In the first animal trials, it was found that these arrays containing multiple electrodes
could not be pushed into the cortex as they just indent it (like a fakir on a bed of nails). A
high-velocity pneumatic insertion tool was developed that could insert the array in 200
μ
s,
corresponding to a velocity of 7 m/s.
7.9.9.2 Illinois Institute of Technology Research Group
IIT researchers lead by Philip Troyk started the development of a prototype intracortical
visual prosthesis in 2002. The system uses four 256-channel electric stimulators sitting on
a patient's skull under the skin to produce the tiny currents needed to drive 1024 miniature
electrodes implanted in the visual cortex. Small in size (25
6 mm), each ceramic-
encased stimulator module contains the electronics that control the 256 electrodes. The
implant is powered and communicated with using transcutaneous magnetic induction.
The approach to the design of most implantable devices has been to minimize the
functional requirements of the implant itself. This philosophy is based on a desire to
maximize the system reliability and operational flexibility. For any implant, the allowable
size must be defined relative to its function, and it is not necessarily true that increased
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