Bioactive Conducting Polymers for Optimising the Neural Interface - Biointerfaces: Where Material Meets Biology - page 206

8.2.5 Limitations of Conducting Polymers in Neural
Interfaces
Conducting polymers have been shown to have clear advantages over metal
electrodes in the in vitro setting. However, in the in vivo environment this
benefit is reduced, largely due to the effects of the inflammatory response on
the neural interface. This was exemplified in a study by Abidian et al. in
which gold electrodes coated in conducting polymer nanotubes were im-
planted in the barrel cortex of rats. 19 Prior to implantation, coating the
electrodes resulted in the impedance decreasing by two orders of magnitude
and the charge transfer capacity increasing by three orders of magnitude.
Upon implantation the measured impedance of the electrodes rapidly in-
creased from 841 7 to 908 5kO for uncoated electrodes and from 17 4
to 87 8kO for coated electrodes (measured at 1 kHz). By day 8 of im-
plantation, impedance had risen to 1250 3kO for uncoated and 546 30 kO
for coated electrodes, before finally settling out to 980 15 kO for coated and
521 18 kO for coated electrodes by day 49 of implantation. Overall, the
benefit gained from conducting polymer coatings decreased from nearly
three orders of magnitude to less than a two-fold increase. This demon-
strates that the biological response and the quality of the neural interface
established are key to the functional performance of the conducting polymer
electrode. One approach to overcome this limitation is the biofunctionali-
sation of conducting polymers via the incorporation of bioactive molecules
within conducting polymer films.

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8.3 Biofunctionalisation of Conducting Polymers
The main goal of incorporating bioactive molecules is to elicit a desirable
cellular response which will result in the establishment of high quality
neural interfaces. 4,14 Two common approaches to this include attenuating
the inflammatory response using anti-inflammatory drugs and encouraging
neuronal survival and intimacy at the interface through the use of growth
factors and cell adhesion molecules.

8.3.1 Incorporation of Bioactive Molecules
There are two main routes through which bioactive molecules can be in-
corporated within conducting polymer films. These are electrostatic in-
corporation of charged molecules as bioactive dopants, or through physical
entrapment of neutral or weakly charged molecules as non-dopant inclu-
sions. 3,27 Less commonly used methods of incorporation include physical
adsorption and covalent attachment to the surface of conducting polymer
films. The method of incorporation is typically determined by the properties
of the bioactive molecule in question as well as the desired means of bio-
logical presentation.