Electroosmotic-driven coiled hydrogel-based yarn muscles

Soft, biocompatible hydrogel fiber actuators are promising for advances in robotics, prosthetics, and bioengineering. However, they often face challenges such as slow and weak actuation due to their intrinsically low modulus and slow water diffusion. We report a twisted and coiled hydrogel fiber muscle, achieved by encapsulating hydrogel within a semipermeable polymer hollow tube. Driven by osmosis in an aqueous electrolyte, water was transported through the tube wall and absorbed by hydrogel, leading to fiber volume expansion and length contraction. The water transport rate was enhanced by turning the pore size and thickness of the tube wall through pre-stretching. Moreover, we boosted the contractile stroke rate and actuation stress rate to 16.8 % min-1 and 3.3 MPa min-1, respectively, using electroosmotic pressure, which are 7.3 and 6.2 times higher than driven osmotically. The inclusion of carbon fiber accelerates water evaporation, improving the cycle rate of the muscle. This study introduces a novel approach for creating high-performance hydrogel fiber muscles.