Actuating Fiber Based on pH-Induced Extension and Contraction of Positively Charged Polymers

Actuating fibers are characterized by their flexibility and capacity for further knitting and weaving. They exhibit rapid stimuli response, satisfying diverse practical applications. The pH-actuating fibers are typically prepared from weak negative polyelectrolytes, contracting in acid and extending in alkali, while weak positive polyelectrolytes can be used to construct fibers with inverse pH-actuating behaviors. Herein, we fabricate an “alkaline contraction and acid extension” actuating fiber, utilizing the biobased weak positive polyelectrolyte chitosan (CHI). To address the issues of CHI fiber dissolution at low pH values and its inherent brittleness, covalent cross-linking and toughening are introduced to produce the actuating fiber. The fiber performs work in response to pH variations with a maximum output strain of 62%, work density of 20 kJ/m3, and modulus of 215 MPa, which are all higher than those of human skeletal muscle, and can be assembled into devices with other kinds of fiber actuators to perform complex motions, holding a promising role in soft robots and biomedicines.