Biomedical Engineering Reference
In-Depth Information
systems as a dopant. The presence of the phosphate group allows the mol-
ecule to function adequately as a dopant, although a decrease in electrical
properties is observed compared to films produced using a conventional
dopant. Wadhwa et al. fabricated PPy/dexamethasone films capable of re-
leasing 16 mgcm
2
over 30 stimulation cycles, resulting in an estimated
concentration of 1 mM within 500 mm of the electrode surface.
36
This level of
release was successfully able to lower the number of reactive astrocytes and
inhibit microglial
d
n
3
r
4
n
g
|
2
growth when
cultured with
primary murine
cerebellum glia.
Thompson et al. physically entrapped neurotrophin-3 (NT3) within PPy/
pTS films.
37
It was found that 26 mm thick films were capable of releasing
approximately 5 ng cm
2
and 9 ng cm
2
, respectively, for unstimulated and
stimulated electrodes. Further studies demonstrated that when cultured
with primary rat auditory neuron explants, stimulated PPy/pTS-NT3 films
promote neuron survival and neurite outgrowth.
20,38
8.3.4 Limitations of Biofunctionalised Conducting Polymers
While it has been demonstrated that incorporation of bioactive molecules
within conducting polymer films shows significant promise in altering the
biological response to neuroprosthetic electrodes, there remain several
challenges and limitations to the use of bioactive conducting polymers. The
primary concern with incorporation of biomolecules is the negative impact
on the mechanical and electrical properties of the conducting polymer that
may occur. As stated previously, biofunctionality of a dopant molecule
typically comes at the cost of its ecacy as a dopant. Similarly, non-dopant
inclusions can have a significant impact upon the formation of the polymer
matrix. Green et al. reported a significant decrease in the nodularity and
surface area of PEDOT when doped with laminin peptide fragments com-
pared to PEDOT/pTS.
24
Doping PEDOT with laminin peptides resulted in
a four-fold decrease in charge storage capacity (200 mC cm
2
compared to
50 mC cm
2
) and was found to produce films more prone to delamination
compared to PEDOT/pTS. Generally, it has been found that incorporation of
larger molecules within conducting polymer matrices results in softer, less
adherent coatings.
Another consideration when incorporating larger molecules for drug re-
lease is that of mobility. Molecular weight and structure, as well as electro-
static interactions can affect the mobility of molecules within conducting
polymer matrices and this may limit the rate and total volume of release
such that a sustained therapeutic effect cannot be achieved. It is important
to note that size is not the only factor determining mobility release. Kontturi
et al. found nicotinic acid, a small anionic dopant, to be immobile within
PPy due to the nitrogen atom in the nicotinic acid extracting a proton from
the nitrogen in the pyrrole rings.
39
There are also application specific limitations to consider such as elec-
trode size and bioactive loading capacity. Modern neuroprosthetic devices
.
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