Designing ultrasensitive mechanochromic pure cellulose nanofiber hydrogels via simple and feasible gas phase coagulation

Nanoellulose-based mechanochromic materials represent a frontier in sustainable optics, offering a renewable platform for advanced functional materials. However, the precise manipulation of cellulose nanofibers (CNFs) into intelligent optical systems remains challenging due to their inherent variability in size and the difficulty of achieving controlled orientation. Herein, we developed a straightforward and scalable strategy to construct mechanochromic CNF hydrogels (MCNFHs) with engineerable interference colors (ranging from yellow, red, blue, green) under compressive stress by exploiting their response to acidic vapor, that is, via simple and feasible gas phase coagulation. Remarkably, MCNFHs displayed excellent solvent resistance, mechanochromic reversibility and mechanochromic sensitivity, which could achieve the switch from black to yellow, red, blue and green under only a load of 0.36 N, a stress of 1.2 KPa, and a strain of 15 %, offering a robust platform for pressure sensing or micro-strains detecting. We also demonstrated that these new MCNFHs can be smoothly applied in optical coding systems for optical encryption. This work highlighted a versatile strategy for creating high performance CNF-based mechanochromic materials, without the need for complicated pre-assembly or precise manipulation of CNF, underscoring their potential in pressure sensing, encryption, and beyond.