Biomedical Engineering Reference
In-Depth Information
8.4.1.2 Nanostructured Conducting Polymers
Several studies have examined the effect of structuring conducting polymer
coatings at the nanoscale. This typically involves the use of a nanotemplate
during electrodeposition to either impart nanoporosity or to create nano-
wires or nanotubes. Kang et al. created nanoporous PPy/PSS coatings loaded
with nerve growth factor (NGF) by electrodepositing onto electrodes coated
with polystyrene beads.
41
Electrodes were soaked in tetrahydrofuran after
deposition in order to remove the polystyrene template. The nanoporous
PPy/PSS coatings had improved electrical properties due to increased surface
area. The nanoporous electrodes were also capable of releasing greater vol-
umes of NGF as a result of increased surface area and the continuous por-
osity acting as a low-resistance diffusion pathway. Luo et al. utilised a similar
system to create dual-release electrode coatings.
42
Fluorescein, a model drug
dopant, was incorporated within nanoporous PPy as a dopant. The coating
was then subjected to an ethanol soaking process to make the film hydro-
phobic. The electrodes were then incubated with a solution containing
dexamethasone phosphate, allowing the drug to absorb to the nanoporous
structure. This method of incorporation significantly increased the loading
of dexamethasone and allowed for the electrically controlled release of two
different molecules.
A major limitation to the use of template systems is the required use of
solvents to remove the template. Kim et al. used alumina templates to create
conducting polymer nanowires.
43
The use of hydrofluoric acid or sodium
hydroxide to remove the template resulted in a decrease in conductivity from
30 S cm
1
to 7 S cm
1
and 0.01 S cm
1
, respectively.
43,44
Abidian et al. developed a system for creating conducting polymer
nanotubes without the need to use organic solvents to remove the nano-
template.
45
The nanotemplate was formed by electrospinning dexa-
methasone-loaded poly(lactide-co-glycolide) (PLGA) nanofibres onto the
electrode surface. Electrodeposition of PEDOT around the PLGA fibres
resulted in the formation of PEDOT nanotubes ranging from 100 to 600 nm
with wall thickness varying from 50 to 100 nm. Delivery of dexamethasone is
accomplished through both the passive degradation of the PLGA fibres and
through the active electrical stimulation of the PEDOT nanotubes. It was
found that under passive conditions less than 25% of incorporated dexa-
methasone was released over a 54 day period. Periodic electrical stimulation
was found to result in a cumulative release of approximately 80% of the
incorporated dexamethasone. No indication was given as to the bio-
functionality or therapeutic effect of released dexamethasone, however this
level of release is expected to be therapeutically significant.
46
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8.4.2 Conducting Polymer Composites
Another approach to overcoming the limitations of bioactive conducting
polymers is the creation of conducting polymer composites. The aim of
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