A Gradient-Softening Strategy in Hydrogel Films to Optimize Balance between Interfacial Toughness and Structural...

Flexible hydrogel films are critical for flexible electronics and wearable devices, yet they face an inherent trade-off between interfacial peel resistance and structural integrity─an issue traditional methods (e.g., surface modification, uniform property tuning) cannot resolve. This study proposes a gradient-softening strategy to address this bottleneck. Hydrogel films with a through-thickness decreasing modulus gradient (0.24-0.034 MPa) were fabricated by adjusting cross-linker content (0.04-0.005 g) and water content (45-66 wt %): the surface layer maintained high stiffness for structural stability, while the substrate-adjacent layer softened to enable deformation and energy dissipation. Pure shear tests, multiangle (0°, 90°, 180°) peel tests, and finite element analysis (FEA) were conducted to characterize their performance and underlying mechanisms. The gradient-softening films exhibited a fracture toughness of 1253.5 J/m², which significantly exceeded that of low-stiffness (526.1 J/m²) and high-stiffness (762.1 J/m²) uniform films. They also showed superior peel resistance: 914.6 N/m (90° average peel strength), 538.6 N/m (180° stick-slip amplitude), and 1544 N/m (0° maximum shear strength)─2.1-3.6 times higher than uniform films. Deformation observations and FEA show that the high fracture toughness of gradient-softening films is converted into superior interfacial peel resistance through "stiff-soft synergy": the stiff surface layer provides structural load-bearing capacity to avoid excessive deformation, while the soft substrate-adjacent layer enables strain redistribution and crack blunting to alleviate stress concentration, collectively increasing the energy required for peeling and thus realizing enhanced antipeeling performance; a quantitative correlation between gradient ratio and peel resistance was also revealed. The films remained stable after a 6 day sealed storage and 1000 cyclic bending tests. This strategy provides a novel solution for flexible electronics. It can be integrated with advanced manufacturing (e.g., multimaterial 3D bioprinting) to develop biomimetic interfaces, driving progress in wearable sensing and soft robotics.