Interfacial Water Confinement Couples Hygroactuation and Mechanical Robustness in Free‐Standing Biocomposites

ABSTRACT Water‐responsive (WR) materials that convert the chemical potential gradients of water into mechanical work are promising for soft robotics, adaptive systems, and sustainable energy technologies, yet achieving simultaneous high actuation performance and mechanical robustness remains challenging. Here, we report free‐standing silk‐cell biocomposite films in which regenerated silk fibroin acts as a mechanically robust matrix that effectively translates the WR stresses generated by autoclaved Bacillus subtilis cells into macroscopic actuation. Through blade casting followed by water‐assisted hot‐pressing, the optimized 75:25 silk‐cell composite achieved high ultimate WR stress of 55.0 MPa, free strain of 14.8%, and rapid response kinetics (dehydration and hydration time constants of 2.4 and 3.7 s, respectively), while maintaining outstanding mechanical properties, including Young's modulus of 6.8 GPa, tensile strength of 120.2 MPa, and toughness of 2.5 MJ m −3 . We found that the enhanced macroscopic performance originates from interfacial densification that suppresses mobile water, thereby improving coupling between dehydration‐induced deformation and mechanical stress transfer. These findings suggest interfacial water confinement as a key design principle for scalable hygroscopic biocomposites that combine high‐performance WR actuation with structural robustness.