“…[18][19][20][21][22] Generally, the most common important structure of a humidity actuator is bilayer, which consists of an active layer and an inactive layer that differ in their hygroexpansion properties, generating a bending motion. [23][24][25] The reported materials for active layer are electroactive polymers (EAPs), 26,27 liquid crystal elastomers (LCE), 28 hydrogels, 29 sodium alginate, 30 paper coated with MXene, 31 carbon nanotube, 32 graphene oxide (GO) 33 or toner, 34 etc. Ma et al 35 fabricated a humidity-responsive walking device using a poly(acrylic acid) (PAA)/poly(allylamine hydrochloride) (PAH) lm as the upper active layer and a UV-cured Norland Optical Adhesive 63 (NOA 63) as the underlying layer.…”