Biomedical Engineering Reference
In-Depth Information
It is these bipolaron sites which facilitate electrical conduction by pro-
viding a path for the movement of charge carriers along the polymer back-
bone when the material is exposed to an electrical potential. Electrical
conduction not only occurs along the polymer backbone but also between
neighbouring polymer strands.
3
Although not as well understood, con-
duction between polymer strands is critical to the observed macroscopic
conduction within conducting polymers due to their highly disordered
structure. The choice of dopant molecule plays a critical role in determining
the properties of the resultant conducting polymer film. Conventional
dopants, including perchlorates and sulfonated aromatic compounds such
as p-toluenesulfonate (pTS) and poly(styrenesulfonate) (PSS), are used due to
their eciency of oxidising the conducting polymer backbone.
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n
3
r
4
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2
8.2.2 Fabrication
8.2.2.1 Chemical Synthesis
Early research on conducting polymers primarily utilised chemical methods
for polymerisation. In chemical fabrication routes, the monomer is dis-
solved in an organic solvent and is added to a solution of mild Lewis-acid
catalysts.
3
The catalyst enables oxidative coupling of monomer units to form
polymer strands. Conducting polymer films can then be formed using
solvent casting techniques. While this fabrication route gives a high degree
of control and allows for high-volume fabrication of highly conductive
polymers on both conductive and non-conductive substrates, it is rarely used
for biomedical applications due to the required use of toxic solvents and
oxidative species. Additionally, compared to electrochemical methods of
synthesis, chemical methods are time consuming and typically require post-
fabrication processing. As such, the bulk of research concerning biomedical
conducting polymers utilises electrochemical deposition.
.
8.2.2.2 Electrochemical Deposition
Due to its relative speed, ease, reliability and degree of control electro-
deposition is heavily favoured for fabricating biomedical conducting poly-
mers.
4,5
Electrodeposition is generally carried out in a three-electrode cell as
seen in Figure 8.3.
The monomer unit is dissolved in the presence of a charged dopant
species, typically an anionic dopant. During electrodeposition an oxidising
potential, either galvanostatic or potentiostatic, is applied through the pre-
cursor solution.
TIP: The potentiostatic mode allows for control of oxidation/over-oxidation
whereas the galvanostatic mode allows for control over the kinetics of de-
position (growth rate and film thickness). It is the authors' opinion that
galvanostatic deposition produces more reliable samples.
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