Bioinspired multifunctional gradient hydrogel composite fabricated via phase separation and in situ polymerization with exceptional mechanical, electrical, and adhesive properties

Hydrogels have demonstrated extensive applications in fields such as flexible sensors and electronic skin due to their exceptional flexibility and biocompatibility. However, conventional hydrogels face significant challenges in achieving a harmonious combination of mechanical properties, electrical conductivity, and adhesive capabilities. Herein, we propose a bioinspired gradient composite hydrogel that mimics the hierarchical epidermal-dermal-tissue structure of human skin, integrating mechanical support, conductive sensing, and interfacial adhesion functions. This hydrogel composite employs a triple-layered gradient architecture: (1) A mechanical support layer based on a salt-outing reinforced polyvinyl alcohol network achieves exceptional toughness (4.72 MJ·m⁻³), significantly enhancing fracture resistance; (2) A conductive sensing layer achieves high ionic conductivity pathways by adjusting dextran and ammonium sulfate concentrations; (3) An adhesive layer utilizing covalent crosslinking between poly(acrylic acid) grafted with N-hydroxysuccinimide ester and cutaneous amino groups, delivers superior tissue adhesion strength. The triple-layered hydrogel composite exhibits outstanding sensing capabilities and excellent robustness. The gel remains linearity and over 94 % resistance stability even after 1000 cycles of 20 % strain, which could strongly support the application of long term precisely detecting both strain and temperature variations. Overall, this gradient hydrogel provides a multifunctional interface for wearable electronics, intelligent interactive systems, and biomedical engineering.