Biomedical Engineering Reference
In-Depth Information
as protein deposition). he tissue conducts currents in the form of ions, and the electrode conducts
current in the form of electrons, so the current needs to be transduced either through capacitive
currents or through faradaic currents from redox reactions at the electrode surface. A fundamental
problem in electrode technology has been the few options available: gold, tungsten, platinum, and
iridium—which have a well-deined chemistry but are diicult to “improve.” Daryl Kipke's group
from the University of Michigan (Ann Arbor) has taken a radical turn and has reengineered the
surface of the electrodes with nanotubes made of conducting polymers, such as poly(pyrrole) and
poly(thiophene) (
Figure 8.5
). hey demonstrated that these nanotube-based electrodes signiicantly
enhance the quality of the recording signals, showing that the signal at day 49 ater surgery is virtu-
ally the same as immediately ater surgery, and its impedance has a lat spectrum over more than four
orders of magnitude in frequency (whereas the traditional metal electrodes decay linearly).
8.2.2 The Utah Electrode Array
he UEA, a 10 × 10 array of penetrating silicon needle electrodes (1.5 mm tall, 400 μm spacing),
is the brainchild of Richard Normann's group at the University of Utah. It is also available in a
coniguration in which the needles have a range of heights, from 0.5 to 1.5 mm. (he Michigan
group has produced an array similar to the UEA with integrated ampliiers.) Perhaps the biggest
success of the UEA was a study performed by John Donoghue (who studied with Kensall Wise)
at Brown University in 2006, which showed that the UEA allowed a tetraplegic patient to com-
municate via a brain-computer interface for 9 months (
Figure 8.6
).
UEA
Local field potentials while subject moves
cursor to center of screen
a
b
e
2.5
50
20-trial average
40
2
UEA
1.0 mm
c
d
30
1.5
20
1
10
0.5
0
Trial 3
Trial 4
160
Electrode 33
Electrode 22
f
0
Two cells
Trial 8
Day 90
after
implantation
100
-160
Electrode 34
Electrode 95
0
-100
-0.5
0
0.5
1
1.5
Time (s)
1.6 ms
FIGURE 8.6
The.Utah.electrode.microarray.used.as.a.brain-computer.interface.for.the.paralyzed..
(From. Leigh. R..Hochberg,.Mijail.D.. Serruya,. Gerhard. M.. Friehs,.Jon.A.. Mukand,.Maryam. Saleh,.
Abraham.H..Caplan,.Almut.Branner,.David.Chen,.Richard.D..Penn,.and.John.P..Donoghue,.“Neuronal.
ensemble.control.of.prosthetic.devices.by.a.human.with.tetraplegia,”.
Nature
,.442,.164-171,.2006..
Reprinted.with.permission.of.the.Nature.Publishing.Group.)
