Leveraging Conducting Polymers and Hydrogels for Direct Current Stimulation


The tunable electrical properties of conducting polymers (CPs), biocompatibility, fabrication versatility, and cost-efficiency make them an ideal coating material for stimulation electrodes in biomedical applications. Several biological processes like wound healing, neuronal regrowth, and cancer metastasis, which rely on constant electric fields, demand electrodes capable of delivering direct current stimulation (DCs) for long times without developing toxic electrochemical reactions. Recently, CPs such as poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT/PSS) have demonstrated outstanding capability for delivering DCs without damaging cells in culture while not requiring intermediate buffers, contrary to the current research setups relying on noble-metals and buffering bridges.
However, a clear understanding of how electrode design and CP synthesis influence DCs properties of these materials
is still needed. This study demonstrates that various PEDOT-based CP coatings and hydrogels on rough electrodes can
deliver DCs without substantial changes to the electrode and noticeable development of chemical byproducts depending
on the electrode area and polymer thickness. A comprehensive analysis of the tested coatings is provided according to the desired application and available resources alongside a proposed explanation for the observed electrochemical  behavior. The CPs tested herein can pave the way toward the widespread implementation of DCs as a therapeutic stimulation paradigm.

A2.4Spot-cast PEDOT:PSS hydrogel (hPEDOT)

The coating with PEDOT:PSS hydrogel (hPEDOT) required an initial surface functionalization step with polyurethane (PU) of the electrodes, which was applied to both SIROF and LIG unless otherwise stated. The process started with surface treatment of the electrodes in air plasma at 100 W for 5 minutes to functionalize them with hydroxyl (-OH) groups (Femto, Diener Electronic, Ebhausen, Germany), followed by their immersion in a (3-Aminopropyl)trimethoxysilane (APTMS, 1% v/v) solution (Sigma Aldrich, MO, USA), which adds amine (-NH2) groups to the electrode’s surface. After 60 minutes in APTMS, the electrodes were cleaned with DI water, dried, and subsequently coated with hydrophilic polyurethane (PU, 1% w/v) (AdvanSource HydroMed D3, USA) dissolved in ethanol (95% EtOH) using a dip-coater (ND-R Rotary Dip Coater, Nadetech Innovations, Spain).