Torsional actuation for different inserted twist and number of plies. (a) Diameter change of a cotton yarn with different inserted twist on exposure to 100% humidity. (b) Experimentally measured and theoretically calculated maximum rotation angles for cotton yarn muscle with different inserted twist. (c) Time dependence of rotation angle for torsional muscle prepared from different number of plies of cotton yarns, 1, 2, 3, 4, 5 are the number of the cotton yarns. (d) Maximum rotation angle and maximum rotational speed obtained from (c). The cotton yarns used for fabricating yarn muscles are 140 μm in diameter and 25 cm in length. The environmental relative humidity is 40%, and the room temperature is 25 °C. The isobarically loaded paddle weighs 0.47 g. (e) Schematic demonstration and (f), (g) sequential photos of a smart window by knitting a cotton yarn muscle through a textile. The muscles rotate and close the window on exposure to high humidity. Sequential photos show that the smart window can automatically close when moisture increases.