Fabrication of gradient anisotropic cellulose hydrogels for applications in micro-strain sensing

A gradient anisotropic cellulose hydrogel was prepared by the diffusion of CaCl₂ solution. The degree of orientation of the cellulose chains decreased along the ion diffusion direction, and the birefringence of the highly oriented area was up to 1.323×10^-4. Importantly, we first propose and demonstrate the presence of sensitive region in the gradient anisotropy hydrogel. The sensitive region located in the order-disorder transition displayed large color variation with the optical path difference (R) range from 155 nm to 1200 nm, high sensitivity (1 % strain interval), low detection (minimum 1 % strain), good cycling ability of 50 times and frost resistance at -20℃. Based on this, the readable response colorimetric card was designed for micro-strain detection. The programmable Ca²⁺ diffusion design made it convenient to fabricate cylindrical and tubular hydrogels. This concept of sensitive region and this flexible strategy will broaden new horizons to materials that have excellent responsive properties for optical applications, sensors and multiscale bionics architectures.