A bioadhesive-free, water-retentive, and rehydratable hydrogel for long-term dual-modal skin-interfaced bioelectronics

Hydrogels are promising materials for skin-interfaced bioelectronics due to their skin-like softness and high-water content. However, conventional hydrogels suffer from weak adhesion, rapid dehydration, deformation, and irreversible performance loss, especially on curved or mobile regions such as eyebrows, elbows, and fingers. These problems worsen in non-rehydratable systems that lose conformability and sensing stability after drying. To overcome them, we developed a bioadhesive-free, water-retentive, and rehydratable (BOWER) hydrogel based on thermoresponsive poly(N-isopropylacrylamide) (PNIPAAm) for long-term wearable sensing. While PNIPAAm provides soft skin adhesion near body temperature (~32 °C), its poor moisture stability was mitigated by integrating a composite electrospun nanofiber (NF) layer of hydrophilic polyvinyl alcohol (PVA) and polyethylene oxide (PEO) with hydrophobic waterborne polyurethane (WPU) on the top and sides, leaving the bottom uncoated for skin contact. This asymmetric design improved water retention from ~10 h to >14 h, suppressed dehydration under strain, and enabled full shape recovery and sensing restoration after rehydration. Incorporating conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)(PEDOT:PSS) imparted stable piezoresistive behavior with fast response and recovery (<4 s) during joint motion, facial expression (ΔI/I₀ ≈ 0.4), and respiration (~9 s). Maintaining a skin-like modulus (100-100 Pa) and scalability, the BOWER hydrogel offers a robust platform for multimodal, long-term wearable sensing.