Confined-space volume compression for ultra-thin and tough composite hydrogel films via enhanced hydrogen bond crosslinking

Hydrogels have gained attention as promising candidates owing to their favorable electrical conductivity and biocompatibility. Nevertheless, establishing the compatibility between hydrogel thickness and mechanical performance remains a significant challenge. In this study, a confined-space volume compression strategy was employed by encapsulating a thermoplastic polyurethane (TPU) nanofiber network within an in situ crosslinked dual-network hydrogel. An ultra-thin composite hydrogel film with a thickness of approximately 3.7 μm was fabricated for the first time. The resulting TPU-hydrogel film exhibited superior mechanical properties, including a tensile strength of 7.2 MPa, elongation at break of 1666%, toughness of 74.1 MJ·m⁻³, strong adhesion, excellent tear resistance, and high sensing sensitivity with a measurement factor of 2.93, while maintaining stability over 3000 loading–unloading cycles. Compared with conventional TPU-hydrogels fabricated without volume compression, the tensile strength and elongation at break increased by 1.35-fold. The conformal skin-integrated design enabled continuous, sensitive, and precise monitoring of motion and physiological signals, while the high-water vapor transmission rate improved breathability and comfort during extended wear. Deeply investigations demonstrated that increased spatial density of thermoplastic polyurethane (TPU) nanofibers offers a potential solution by restricting chain mobility, strengthening bond orientation, and enhancing hydrogen-bonding interactions, thereby improving load transfer and energy dissipation. At the same time, denser networks can promote ion transport pathways without compromising film thickness, facilitating the design of ultrathin structures while maintaining breathability. These findings establish a practical strategy for the development of ultra-thin, mechanically robust composite hydrogels with broad applicability in next-generation flexible electronic devices.