Pure Conducting Polymer Hydrogels Increase Signal‐to‐Noise of Cutaneous Electrodes by Lowering Skin Interface Impedance

Abstract

Cutaneous electrodes are routinely used for noninvasive electrophysiological sensing of signals from the brain, the heart, and the neuromuscular system. These bioelectronic signals propagate as ionic charge from their sources to the skin–electrode interface where they are then sensed as electronic charge by the instrumentation. However, these signals suffer from low signal‐to‐noise ratio arising from the high impedance at the tissue‐to‐electrode contact interface. This paper reports that soft conductive polymer hydrogels made purely of poly(3,4‐ethylenedioxy‐thiophene) doped with poly(styrene sulfonate) present nearly an order of magnitude decrease in the skin–electrode contact impedance (88%, 82%, and 77% at 10, 100, and 1 kHz, respectively) when compared to clinical electrodes in an ex vivo model that isolates the bioelectrochemical features of a single skin–electrode contact. Integrating these pure soft conductive polymer blocks into an adhesive wearable sensor enables high fidelity bioelectronic signals with higher signal‐to‐noise ratio (average 2.1 dB increase, max 3.4 dB increase) when compared to clinical electrodes across all subjects. The utility of these electrodes is demonstrated in a neural interface application. The conductive polymer hydrogels enable electromyogram‐based velocity control of a robotic arm to complete a pick and place task. This work provides a basis for the characterization and use of conductive polymer hydrogels to better couple human and machine.

One-Pot Synthesis of a Robust Crosslinker-Free Thermo-Reversible Conducting Hydrogel Electrode for Epidermal Electronics

“…The calculated specific capacitance area of each hydrogel variant is summarized in Figure S4, with G-PEDOT:PSS exhibiting the highest value, followed by G-TRG and G-MXene. The capacitive behavior of the hydrogel electrodes is thought to increase SNR, , as they are not expected to undergo electrochemical reactions at the electrode–tissue interface that could interfere with accurate recordings.…”

“…6 Furthermore, conducting hydrogels conform to the skin, reducing the gap between the electrode and the skin and minimizing skin−electrode contact impedance, hence enhancing the signal-to-noise ratio (SNR) of electrophysiological measurements. 7,8 However, several aspects need to be addressed for clinical and commercial translation to fully realize the potential of conducting hydrogels. The main challenge is optimizing the electrical conductivity such that it is comparable to that of metal electrodes while also ensuring long-term stability and biocompatibility.…”

“…Thus, they serve as a more biomimetic alternative for sensing, stimulating, and modulating electrical activity in biological tissues . Furthermore, conducting hydrogels conform to the skin, reducing the gap between the electrode and the skin and minimizing skin–electrode contact impedance, hence enhancing the signal-to-noise ratio (SNR) of electrophysiological measurements. , …”

Hydrophobic Eutectogels as Electrodes for Underwater Electromyography Recording

“…The micellization could enhance the ionic conductivity but disrupt the matrix's continuity, forming phase-segregated domains with a detrimental effect on recording the signal from the muscle to the electrode interface. 39 To prove our hypothesis, EMG studies were performed on the forearm using a two-electrode configuration (Figure S8A). First, we evaluated the impedance on the skin at 50 Hz, which was similar for all the eutectogels in a dry state (Figure S8B).…”

“…This behavior is expected to be more remarkable for longer, more lipophilic fatty acids (Figure B). The micellization could enhance the ionic conductivity but disrupt the matrix’s continuity, forming phase-segregated domains with a detrimental effect on recording the signal from the muscle to the electrode interface . To prove our hypothesis, EMG studies were performed on the forearm using a two-electrode configuration (Figure S8A).…”