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Bioactive conducting polymer-based neural electrodes
J. Goding, A. Gilmour, P. Martens, L. Poole-Warren, R. Green
University of New South Wales, Sydney, 2052, Australia;
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Neuroprosthetic devices, such as the bionic eye, augment lost or damaged
neurosensory or neurological function. These devices typically employ metal
electrodes to electrically stimulate the target neural tissue. However, metal
electrodes have reached a plateau and cannot meet the requirements of nextgeneration neuroprosthetics due to charge transfer limitations. Conducting polymer
(CP) electrode coatings offer significant improvements over metal electrodes;
however, in vivo performance is dominated by an inflammatory response which
impacts on intimate communication between electrode and neural tissue. Initial work
investigated the incorporation of bioactive factors within CP coatings in an attempt to
attenuate the inflammatory response and encourage neural cell interactions with the
electrode. Dexamethasone phosphate (DP) and valproic acid (VA), both antiinflammatory agents, with VA having additional neuroprotective effects, were
incorporated within poly(3,4-ethylenedioxythiophene) (PEDOT) as bioactive dopants.
The resultant bioactive PEDOT coatings were capable of significant attenuation of
inflammatory responses (reduced production of pro-inflammatory TNF-α) when
cultured in whole blood (Fig 1). However, incorporation of these factors resulted in
significant degradation of the CP mechanical properties, making them unsuitable for
use in implantable devices. This study investigated the fabrication of a novel
conducting hydrogel (CH) system in order to overcome the mechanical limitations of
bioactive CPs. Composite CH electrodes were produced by electrochemically
depositing PEDOT within a pre-formed hydrogel network. In order to encourage the
formation of totally interpenetrating polymer networks, poly(vinyl alcohol) (PVA) was
chemically modified to incorporate taurine dopant molecules as pendant functional
groups. The effect of taurine density on the growth of PEDOT within the PVA network
was assessed through characterisation of the CHs mechanical and electrochemical
properties (Figures 2 and 3). Degree of taurine substitution was found to be a critical
factor in the formation of PEDOT within the PVA network. Finally, DP and VA were
incorporated within CH coatings, and their biological properties assessed through
culture with primary mouse forebrain astrocytes. This work demonstrates that CHs
offer significant improvement over metal electrodes while overcoming the limitations
associated with incorporation of bioactive molecules within CP coatings.
[1]
[2]
[3]
[1] Concentration of pro-inflammatory cytokine TNF-α; [2] Elastic modulus overlaid on
topography of CH (atomic force microscopy using nano-mechanical mapping);
[3] Charge storage capacity of CH coatings over 800 cycles of cyclic voltammetry.