Bioactive Conducting Polymers for Optimising the Neural Interface - Biointerfaces: Where Material Meets Biology - page 211

are tending towards the use of ever decreasing electrode sizes, with elec-
trodes as small as 5-50 mm in diameter being reported. At this scale, elec-
trode size will become the limiting factor in the total amount of bioactive
molecule that can be incorporated and presented/released to the sur-
rounding biological environment. Studies examining the release of en-
trapped NT3 from within 1.66 mm 2 PPy/pTS coated electrodes found a
passive rate of release of 0.03 ng per day and a stimulated rate of release of
0.1 ng per day. 20 When cultured with spiral ganglion neurons it was found
that only stimulated release of NT3 was capable of promoting neural
survival. However, the electrodes were depleted of NT3 after 21 days of
stimulated release after which no therapeutic effect was observed. Achieving
long-term therapeutic effect from microelectrodes will be an increasing
challenge as electrode size and hence volume for incorporation continues to
decrease. Despite the promise shown by bioactive conducting polymers, the
limitations discussed above demonstrate several critical shortcomings for
their use in neural interfaces.

d n 3 r 4 n g | 2

8.4 Modified Biofunctional Conducting Polymers
for Neural Interfaces
Several strategies have been developed to address the limitations associated
with bioactive conducting polymers. These approaches focus on either
modifying existing material platforms through the creation of conducting
polymer layered constructs and modification of conducting polymer struc-
tures or through creating new conducting polymer based composite
materials.

.

8.4.1 Structured Conducting Polymers
8.4.1.1 Bilayered Conducting Polymers
One simple method of addressing the mechanical shortcomings of bioactive
conducting polymers is the use of bilayered structures. This approach
combines two layers of conducting polymer; typically one will be a bioactive
layer providing biofunctionality and the other a conventional conducting
polymer layer providing electrical and mechanical stability. Utilising a pre-
layer of PPy/PSS underneath a bioactive layer of PPy/hyaluronic acid was
found to produce a coating electrically and mechanically superior compared
to a single component PPy/hyaluronic acid coating. 18 Alternatively, coating a
bioactive layer with a conventional conducting polymer can not only improve
electrical properties but can also be used to give a greater degree of control
over the release of biological factors. Massoumi et al. minimised the amount
of passive release of dexamethasone from within a PPy film by depositing a
layer of poly(N-methylpyrrole)/PSS on top of the PPy/dexamethasone layer. 40
However, the bilayered structure was also found to reduce the total release
volume of dexamethasone.