All-Lignin-Based Hydrogel with Fast pH-Stimuli Responsiveness for Mechanical Switching and Actuation

Fast pH-stimuli-responsive hydrogels have high practical value in multiple fields, including actuators, sensors, and separations. However, with existing hydrogels, it is very difficult to achieve a combination of low cost, fast pH response, and good mechanical performance. Lignin, the most abundant aromatic renewable biopolymer in nature, has a unique pH-stimuli-responding property, which has long been used in the dissolution and precipitation of lignin in the traditional pulp and paper industry, though it has not gained appropriate attention for other applications. Herein, a low-cost and simple all-lignin-based hydrogel was prepared, which exhibited excellent pH-stimuli-responsive performance between softening/enhancement and straight/bending shapes. The alternation cycle can be completed in just 1 min and repeated many times. Moreover, a smart hook and a fluidic pH-controlled switch were also prepared from a representative strip-shaped and square-shaped sample, respectively. These new findings will initiate a new design for not only smart materials but also lignin-based materials.