An improved protocol is proposed for preparation of a humiditysensitive soft actuator through the layer-by-layer assembling of weight-ratio-variable composites of sodium alginate (SA) and poly(vinyl alcohol) (PVA) into laminated structures. The design induces nonuniform hygroscopicity in the thickness direction and gives rise to strong interfacial interaction between layers, making the actuator have directional motility. A mathematical model reveals that the directional motion is driven by the chemical potential of humidity, and its energy conversion efficiency from humidity to mechanical work reaches 81.2% at 25 °C. By coating with CoCl 2 , the composite film of SA@PVA/CoCl 2 can act as a warning sign that provides reminder information to prevent people from slipping or falling by a conspicuous red sign during a high-humidity environment. When the film is involved in a bidirectional switch, it is capable of turning on/off light-emitting diodes by humidity, showing promising potential in control over humidity-dependent devices.

A smart bilayer of AG@CNT/AG composed of carbon nanotubes (CNTs) and agarose (AG) is developed.

This work reports a soft bilayer actuator that demonstrates controllable anticlockwise/clockwise wiggling and forward wriggling motions in response to acetone vapor.

The evolutionary mechanism of solid-to-hollow sodium alginate hydrogel is reported, which provides a promising concept or method basis for the preparation of hollow hydrogels with sophisticated geometries.