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

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.