Flexible wearable biosensors from poly (ionic liquid) for real-time signal monitoring.

Introduction: Modern wearable electronics demand materials that are simultaneously stretchable, conductive, and environmentally robust. Hydrogels meet some of these requirements but dehydrate or freeze easily. To overcome these limitations, we prepared a poly-ionic-liquid (PIL) ionogel that integrates high elasticity with stable ionic conductivity, aiming to enable reliable, skin-compatible strain and biopotential sensing.

Methods: 1-Vinyl-3-butyl-imidazolium hexafluorophosphate and 1-butyl-3-methyl-imidazolium hexafluorophosphate were mixed at optimized mass ratios, followed by N,N'-methylenebis-acrylamide (cross-linker) and Irgacure-2959 (photoinitiator). The homogeneous precursor was UV-cured for 6 min to obtain a PIL ionogel (PIL-1 - PIL-4 series). Structural, thermal, mechanical, rheological, adhesive, and electrical characteristics were analysed by FT-IR, SEM, TGA/DSC, uniaxial tensile testing, rheometry, 90° peel tests, and real-time resistance measurements. Applications were evaluated by attaching the gel to human joints and by recording EMG/ECG signals.

Results: The UV one-step process yielded a dense multi-cross-linked network that combined covalent and ionic interactions. The optimised sample (PIL-2) showed a fracture stress of ∼390 kPa with 320% elongation, sustaining a 500 g load without failure. It retained mass and softness after 30 days and adhered strongly (up to 90° peel strength >4 N) to glass, metals, and skin-even underwater. Electrical tests gave a gauge factor of 1.94 (0-100%), 3.98 (100-200%), and 4.04 (200-320%), with 400 ms response and 500 ms recovery. The gel monitored finger (30°/90°), wrist, and elbow motions reproducibly, functioned as a bioelectrode capturing stable EMG/ECG with clear PQRST waves, and reliably transmitted Morse code via hand gestures.

Discussion: The solvent-free PIL ionogel couples mechanical toughness, wide-range elasticity, and stable ionic pathways, outperforming water-rich hydrogels in thermal/long-term stability. Its strong, humidity-tolerant adhesion eliminates extra fixatives, while rapid, high-gain strain transduction and low-impedance skin contact enable multimodal biosensing. These attributes position the material for next-generation flexible electronics, real-time health monitoring, and gesture-based human-machine interfaces.