Bioactive Conducting Polymers for Optimising the Neural Interface - Biointerfaces: Where Material Meets Biology - page 195

d n 3 r 4 n g | 2

Figure 8.3 Three-electrode cell for electrochemical deposition.

This oxidising potential causes the monomer unit to deprotonate and form
a radical ion intermediate capable of initiating polymerisation. As the poly-
merisation progresses the growing oligomers become insoluble and pre-
cipitate onto the working electrode, continued growth and cross-linking of
conducting polymer chains eventually forms a thin polymer film. The
thickness of the electrodeposited film is controlled by the total charge passed
through the electrode cell. Tallman et al. describe a model for the thickness
of deposited films, d, based upon charge passed, as seen in eqn (8.1):

d ¼ j t

F m EW

(8 : 1)

r

.

where j is current density (A cm 2 ), t is the deposition time (s), F is the
Faraday constant, r is polymer density and m EW is the equivalent weight of
polymer (molar mass of monomer plus associated dopant, divided by the
number electrons transferred per monomer unit during polymerisation). 6
Increasing the deposition charge, by increasing the current density or de-
position time will result in thicker, more textured films. As the electro-
deposition process continues the film develops a characteristic nodular surface
morphology, greatly increasing the films surface area as seen in Figure 8.4.

TIP: Films will initially deposit in a two-dimensional manner, but will begin
to shift to a three-dimensional nodular growth as film thickness increases.
Smooth two-dimensional growth can be encouraged through the use of low
current densities (i.e., slower deposition) and by limiting film thickness. The
use of poly(styrene sulfonate) as a dopant will also result in smoother films
compared to films doped with p-toluenesulfonate.

This nodular structure and high surface area are important to the elec-
trical superiority of conducting polymers over traditional metal electrodes as
this characteristic morphology is largely responsible for the high charge
storage capacities and low impedances observed in conducting polymers.