Biomedical Engineering Reference
In-Depth Information
With their characteristic conjugated backbone structure, intrinsically
electrically conductive polymers offer superior electrical properties for
charge transfer in biological environments when compared to bare metal
electrodes. They can also be incorporated within currently existing device
designs with little to no modification due to the simplicity of electro-
chemical fabrication methods. However, their greatest potential as electrode
materials and a significant advantage they have over metal electrodes comes
from their ability to accommodate biofunctionality through the incorpor-
ation of bioactive molecules. This chapter will review the range of systems
that have been developed with the common aim of producing high quality
interfaces between neuroprosthetic electrodes and the surrounding neural
tissue. Furthermore it will examine the current challenges and limitations
surrounding the use of conducting polymers and some of the proposed
solutions for optimising the neural interface.
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8.2 Conducting Polymers
8.2.1 Mechanism of Conduction
Electrical conduction within conducting polymers originates from the con-
jugated backbone structure.
1
This conjugated structure is characterised by
alternating single and double bonds between sequential carbon atoms as
seen in the chemical structure of polyacetylene in Figure 8.1.
In addition to the strong sigma (s) bond present between each carbon in
the polymer backbone, the alternating double bonds also contain a de-
localised pi (p) bond. Repeated conjugation along the backbone gives rise to
a conduction valence band within the conducting polymer. The incorpor-
ation of an ionic dopant molecule within the conducting polymer will oxi-
dise (p-dope) or reduce (n-dope) the polymer at the site of the p-bond by
removing or donating an electron.
2
This creates a free radical which pairs
with the dopant to form a polaron, which in turn can be further oxidised or
reduced to form bipolarons as illustrated in Figure 8.2.
.
Figure 8.1 Chemical structure of polyacetylene.
Figure 8.2 Bipolaron conduction along polypyrrole.
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