From Hydrogels to Cosmetic Contact Media: Honeycomb-Structured Reusable Textile Electrodes for Wearable TENS

Abstract

Textile-integrated electrodes are promising for wearable transcutaneous electrical nerve stimulation (TENS), but their practical use is limited by high dry skin-electrode impedance, mechanical instability, and insufficient durability after washing and deformation. Here, we report reusable screen-printed textile electrodes based on thermoplastic polyurethane/carbon composites and a stretchable honeycomb architecture for low-impedance skin-interfacing TENS applications. Three carbon skin-contact layers: graphite (G), carbon black (CB), and graphite/carbon black hybrid (G:CB) were integrated with a silver current-delivery layer and transferred onto an elastic polyamide/elastane textile. The honeycomb design significantly improved electromechanical stability, reducing strain-induced resistance changes by approximately one order of magnitude under tensile loading. Under 1000 cycles of 20% strain, CB-based electrodes showed the highest stability, with a final relative resistance increase of 52.1% and an absolute resistance below 0.5 Ω. After 10 washing cycles, resistance increased but remained partially recoverable by post-treatment heat pressing, indicating reconfiguration of the conductive network within the thermoplastic matrix. All carbon-based layers were noncytotoxic toward L929 fibroblasts in an indirect MTT assay. Scanning electron microscopy (SEM) and contact profilometry revealed filler-dependent surface morphologies, with surface roughness decreasing from CB and reaching the lowest values for G. In vivo skin-electrode impedance measurements showed that dry interfaces were strongly influenced by surface contact conditions, whereas hydrogel, glycerin-based cream, and oil-in-water (O/W) emulsion substantially reduced impedance across 0.1–200 Hz. Importantly, low impedance was retained after washing and cyclic deformation when a contact medium was used. These results highlight a synergistic design strategy combining material composition, stretchable architecture, and interface engineering to enable robust, reusable textile electrodes for wearable electrotherapy. Notably, the demonstrated compatibility with simple, nonspecialized contact media suggests that common cosmetic formulations can effectively serve as impedance-reducing interfaces, opening a practical pathway toward accessible, user-friendly TENS systems without reliance on dedicated hydrogel electrodes.