Biomedical Engineering Reference
In-Depth Information
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Figure 8.7 Chemical
structure of
(A) polypyrrole and (B) poly(3,4-ethylene
dioxythiophene).
8.2.4.1 Neuroprosthetic Electrodes
Bioelectrodes are used in a wide variety of neuroprosthetic devices to
stimulate or record neuronal excitation, such as cochlear implants, deep
brain stimulators, and motor prosthetics. Current electrode designs typically
involve metal-to-tissue interfaces, resulting in poor long-term electrical sta-
bility and ecacy.
8,22
This poor stability greatly impacts upon a neuro-
prosthetic devices ability to chronically stimulate or record neural activity.
The root of this poor device performance is the inflammatory response at the
tissue-electrode interface. The inflammatory response is characterised by
fibrotic encapsulation of the device which results in an increase in inter-
facial impedance. Furthermore, the persistent presence of the device can
result in chronic inflammation at the interface, leading to local tissue
damage and neuronal death. The effect of the fibrotic encapsulation and
chronic inflammation is an increase in separation distance between the
electrode surface and the target cells for stimulation.
4,14,23
For stimulating
electrodes the combination of increased impedance and distance to target
cell population acts to decrease the quality of signal transmission, requiring
more charge to be injected (coupled with increased power requirements) in
order to successfully stimulate neuronal cells. However, metal electrodes are
severely limited in the amount of charge they can safely inject without
causing undesirable irreversible reactions at the interface, known as the
charge injection limit. As modern neuroprosthetic electrodes shrink in size,
the charge injection limit of metal electrodes is quickly becoming a key
limitation in the safe and ecacious operation of neuroprosthetics. Con-
ducting polymers have charge injection limits two orders of magnitude
larger than metal electrodes, with PEDOT having an injection limit of up to
15 mC cm
2
compared to just 50-250 mCcm
2
for bare platinum.
8
This
means that for the same stimulation threshold a conducting polymer elec-
trode can be significantly smaller than a traditional bare metal electrode.
.
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