Introduction: Conductive hydrogels, comprising a conducting polymer, e.g. poly(3,4-ethylene dioxythiophene) (PEDOT), grown within a hydrogel matrix to form a composite system, have emerged as a promising new class of materials to functionalize electrode surfaces for enhanced neural interfaces and drug delivery. The conducting polymer offers favourable electrical properties and the capability of drug delivery, while the hydrogel forms an aqueous matrix for the integration of biomolecules. This provides mechanical properties that are more similar to brain tissue. Shortcomings of such systems are delamination from the connection surface and the lack of suitable patterning methods for confining the gel to the electrode site. We present a novel conductive hydrogel that can be covalently bound to the substrate electrode and can be patterned with a photolithographic process. This hybrid material forms a true interpenetrating network of both components and shows good electrical properties, which are needed for recording and stimulation of neural tissue. Materials and Methods: The proposed system is composed of the conducting polymer PEDOT and the hydrogel p(DMAA-co-PSS-co-BP), which is a copolymer consisting of repeating units of hydrophilic and biocompatible dimethylacrylamide (DMAA), anionic styrene sulfonate (PSS) and UV-crosslinkable benzophenone (BP). The hydrogel was deposited onto microfabricated probes with patterned indium oxide (IrOx) electrodes via a dip coating procedure. Prior to this, the IrOx was functionalized using UV-reactive silane to enable covalent bonding, thus facilitating adhesion between the hydrogel and the surface. The hydrogel film was cross-linked and patterned by UV-light exposure through a photomask. PEDOT was then galvanostatically grown within the hydrogel mesh. Since the counter-anion PSS was provided by the hydrogel network, no additional supporting electrolyte was used, enabling a homogenous growth of the polymer throughout the charged scaffold. The hybrid material was electrochemically characterized by means of cyclic voltammetry (CV) and impedance spectroscopy (EIS), and compared to both the bare IrOx sites and the plain hydrogel. Results and Discussion: The conductive hydrogel overall compared well to the IrOx in terms of signal transmission properties. The hybrid coating displayed an increased charge storage capacity compared to both the plain hydrogel and the indium oxide surface. Furthermore, the impedance of the conductive hydrogel was lower compared to the plain hydrogel, confirming that a PEDOT network was successfully formed. Thus, the presented material has the electrical properties suitable for neural communication electrodes, can be patterned by a photolithographic process and can be covalently bound to the substrate electrode. Conclusion: The conductive hydrogel presented here efficiently addresses many of the challenges found with other conducting polymer/hydrogel based systems, and thus shows great promise for a wide field of future applications. Characterization of biocompatibility, using in vitro cell culture models, and mechanical properties, using AFM, are needed to further validate the advantage of this material.
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