Ultra-flexible and tough iridescent cellulose nanocrystal-hydrogel membrane towards sensing, actuating and thermal management.

Cellulose nanocrystals (CNCs) exhibit iridescent structural colors benefitting from the cholesteric helical structure generated by evaporation-induced self-assembly (EISA), showing broad applications in fields such as sensors, anti-counterfeiting and actuators. However, CNC membranes suffer from brittleness due to excessive hydrogen bonding. Current strategies struggle to achieve a harmonious integration of mechanical properties and structural color of CNC membranes. Inspired by the color-regulating mechanism of chameleon skin, this study presents a novel bilayer Janus CNC-hydrogel membrane integrating high flexibility, extraordinary toughness, structural color, sensing capability, actuating performance and thermal management behavior via solution-casting. Benefitting from the strong interfacial interactions (hydrogen bonding and electrostatic forces) between the layers, the membrane shows exceptional flexibility (227 % elongation at break) and toughness (6.88 MJ m), which exceed the flexibility of most CNC composites in reported literatures. The membranes demonstrate potential applications in strain sensing, artificial muscles and thermal management. This work advances CNC composite design by balancing mechanical performance with structural color integrity, paving the way for multifunctional integration and commercialization.