(a) Schematic demonstration of fabrication of torsional cotton yarn artificial muscle. (b)–(e) SEM images for (b) single, (c) two-ply, (d) two-ply self-balanced, and (e) three-ply cotton yarns. The twist densities for (b)–(e) are 1400, 1000, 1400, and 800 turns/m, respectively, when normalized to the non-twisted length, the scale bars are the same in (b)–(e). (f) Kinetics of the water absorption/desorption by a cotton yarn. (g) XRD (Cu Kα radiation) of cotton yarn before, after water absorption, and after water desorption, on exposure to a humidity of 100%.