Dry-Printed Electrodes for Transcutaneous Electrical Stimulation of Innervated Muscles: Towards Wearable and Closed-Loop Neuromodulation
DOI: Pending DOI
Synopsis
Gel electrodes limit wearable transcutaneous electrical stimulation due to placement complexity, gel dehydration, and poor long-term stability. Soft screen-printed dry carbon electrode arrays offer a superior alternative. They were evaluated for accessory nerve stimulation in 12 healthy subjects, while recording muscle electrical activity and mechanical movement. Despite their high impedance, activation thresholds and discomfort levels were comparable to those of gel electrodes. Finite element analysis confirmed similar electric field distributions. Closed-loop facial stimulation was also demonstrated using dry electrodes for both recording and stimulation, enabling real-time symmetry restoration. These results support their feasibility for wearable, out-of-the-clinic neuromodulation.
Background
Transcutaneous electrical stimulation (TES) is a well-established tool in neuromodulation, used for pain management, post-stroke motor rehabilitation, and functional electrical stimulation (FES) [1]. Despite the growing interest in TES, several significant challenges limit its use in out-of-the-clinic settings. These include unpleasant sensations such as tingling and pain, the need for precise electrode placement, difficulties maintaining electrode stability, and optimizing stimulation waveforms [2].
Textile-embedded electrodes have been explored as an alternative to gel electrodes, but they require continuous hydration to remain effective [3]. Printed dry electrodes on soft polyurethane substrates were previously shown for biopotentials and bioimpedance recordings [4, 5]. However, their application for stimulation has been largely ignored, and their potential remains unexplored, especially regarding their ability to reach effective stimulation thresholds without causing discomfort.
The present study was motivated by a practical question: can printed dry arrays be used for effective neuromuscular stimulation? And if so, is the skin-electrode interface actually the limiting factor?
Methods
Electrodes: Dry arrays were fabricated by screen-printing silver traces and carbon contacts onto polyurethane (PU) films, passivated with adhesive PU, leaving the carbon sites exposed [4]. Standard disposable Ag/AgCl gel electrodes (1.5x2.0 cm) were used for comparison.
Neck stimulation (N=12): The accessory nerve was stimulated using charge-balanced biphasic pulses (cathode-first, 1 ms pulse width, 10 pulses at 1 Hz) incremented in 0.1 mA steps until maximum discomfort or overt muscle activity appeared. Upper trapezius sEMG, normalized to each subject's maximal voluntary contraction (MVC), and shoulder angular velocity (acromion-mounted IMU) were recorded simultaneously. Sigmoid fits to the resulting recruitment curves yielded EMG and movement thresholds (EMGTh, wTh); perceived discomfort was scored on a visual analog scale (VAS, 0-10) after each intensity step.
Impedance and modeling: Contact impedance (1-1000 Hz) was measured for each electrode and fitted to an equivalent circuit with tissue resistance (Rtissue), a constant phase element (CPEdl), charge transfer resistance (Rct), and a Warburg diffusion term (Wdiff). Electric field distributions were computed with SimNIBS using a realistic head model with an interface layer matching measured Rct values.
Closed-loop facial stimulation: A 14-channel dry array recorded sEMG from the left face; a 7-channel array on the right delivered stimulation (7 mA, 200 us, 100 Hz) over the zygomaticus motor point. Smile onset on the recorded side triggered contralateral stimulation automatically via threshold-crossing with slope confirmation.
Results
Stimulation thresholds and comfort: Overall, dry printed electrodes performed very closely to gel electrodes. No significant difference in muscle activation thresholds, 1.36 +/- 1.10 mA for dry and 1.83 +/- 1.05 mA for gel (p = 0.204-0.445) or angular velocity thresholds between the two electrode types. Pain ratings at threshold were in the mild range for both electrode types (mean ~3/10 on VAS), with no significant between-group difference (effect sizes 0.256-0.472). Neither electrode caused visible skin irritation. Notably, EMGTh was consistently at or below wTh across subjects, meaning isotonic muscle activity preceded observable shoulder movement, consistent with standard neuromuscular recruitment.
Contact impedance: At 100 Hz, dry electrode impedance averaged (7.15 +/- 6.11)x10^5 Ohm compared to gel ((3.65 +/- 1.96)x10^4 Ohm). Equivalent circuit fitting returned higher Rct and Warburg coefficient AW values for dry electrodes, although neither difference reached significance (Wilcoxon signed-rank, p = 0.052-0.910). The only parameter that was statistically different was the CPE exponent ndl, which was higher for gel electrodes (p < 0.01, effect size 0.560), indicating that gel behaves more like an ideal capacitor, whereas dry interfaces showed a more distributed, resistive character.
Finite element modeling: For dry electrodes (8 mm diameter, interface conductivity 4.6x10^-8 S/m), the modeled field at the skin surface was more concentrated than for gel (15x20 mm, 9.9x10^-8 S/m). A sensitivity sweep from 10^-10 to 10 S/m confirmed that electrode size, not interface conductivity, is the primary determinant of field strength, an important result given the large impedance spread across subjects.
Closed-loop facial stimulation: Left-side smile onset was reliably detected and triggered right-side zygomaticus stimulation within ~500 ms (dominated by Bluetooth latency). Evoked contractions were visible and produced facial expressions that mirrored the voluntary movement on the opposite side. The figure shows the electrode setup and representative sEMG traces across the three phases: neutral rest, voluntary unilateral smile, and stimulation-evoked symmetric response.
Discussion
Printed dry electrodes produced muscle activation comparable to that of gel electrodes during accessory nerve stimulation, despite their higher impedance. Finite element modeling indicates that the electric field distribution is governed primarily by electrode geometry rather than interface resistance, explaining why the elevated Rct of dry electrodes does not translate into higher stimulation thresholds under current-controlled delivery. The smaller electrode footprint produces a more concentrated field, which may further support effective neural activation.
From a practical standpoint, dry arrays offer meaningful advantages: rapid placement without gel preparation, a multi-channel layout allowing flexible selection of optimal electrode pairs without repositioning, and wireless operation that does not restrict movement during therapy, particularly relevant for facial expression training [5, 6].
The closed-loop facial demonstration serves as a proof of concept. Validation in a single subject is insufficient for clinical conclusions, and the ~500 ms loop latency, primarily due to Bluetooth transmission, requires further reduction for practical real-time symmetry correction. Additionally, this study focused exclusively on innervated muscles; patients with denervated facial musculature, the primary target population in facial palsy rehabilitation, require higher currents and longer pulse durations and must be studied separately. Long-term electrode durability under repeated stimulation also warrants systematic investigation beyond the preliminary stability data presented here.
Disclosures
This work was supported by the Israel Science Foundation (ISF, Grant No. 1355/17), the European Research Council (ERC, Grant Outer-Ret-101053186), and the Deutsche Forschungsgemeinschaft (DFG, Grant No. GU-463/12-1). The funding agencies were not involved in the design, acquisition, or interpretation of the findings or the writing of this manuscript. Yael Hanein declares a financial interest in X-trodes Ltd, which commercialized the screen-printed electrode technology used in this paper. All other authors have no other relevant financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.
References
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