72 S. Norman and R. Bellamkonda
one side and a silicon–aluminum compound traverses the wafer, essentially p-type
doping columns in the silicon with aluminum. The wafer is polished and electrical
contacts placed where the aluminum squares were previously. The n-type areas
between the p-doped squares are ground down with a dicing saw, creating the
pillars. The pn junctions formed electrically isolate each electrode from the next.
The electrodes are then sharpened with a chemical etching process in a mixture
of hydrofluoric and nitric acids. First, the electrode tips are put into an etch bath
that is agitated with a stir bar. Afterward, the electrodes are etched in a bath with
no stirring, cleaned, and coated with gold and platinum. Polyamide is then used
to insulate the entire electrode, except for the recording tips, and the electrode is
complete.
27
Newer electrodes are fabricated slightly differently; some use glass
around the base of each electrode, instead of the pn junction, for electrical insula-
tion, and the insulating coatings on the electrode shafts are different as well.
30
A pneumatic insertion device is used to place the Utah array into the cortex.
26
Utah arrays have shown incredible longevity in the clinical setting–primate exper-
iments using the Cyberkinetics 100-electrode array have documented recordings
for over 1.5 years.
31
4.2.2 Michigan Probes
The Michigan probes are a planar array of batch-fabricated silicon electrodes.
Unlike the Utah array, which has recording sites at the end of each probe, the
Michigan array has multiple electrodes on each shank (Fig. 4.4). Today there are
many custom-fabricated varieties of the electrode, but two of the most common
are a four-shank, four electrodes per shank configuration
32,33
(shown in Fig. 4.4),
and a one-shank, 16 electrodes per shank configuration.
34
The Michigan array
Figure 4.4. The four shanks of a Michigan probe, top; closer image of the four electrodes
per shank, bottom. The darker electrode sites in the lower picture have been treated
with the conductive polymer poly(3,4-ethylenedioxythiophene) (PEDOT).
32
Reprinted with
permission from the Journal of Neural Engineering,Ludwiget al., 2006, Vol 3, Issue 1,
pp. 59–70. For color reference, see page 259.
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