Machines that move by bending in response to moisture can be made by exposing thin sheets of graphene oxide to sunlight.

A team led by Hong-Bo Sun at Jilin University in Changchun, China, focused sunlight on one side of graphene oxide paper. The ultraviolet radiation induced a reaction that negatively charged the surface so that it repelled water more strongly than the layer below, causing the paper to curl in seconds on contact with moisture.

Credit: Yong-Lai Zhang, Hong-Bo Sun/Jilin Univ.

The team fashioned a claw from the paper that closes when approached by a sweaty finger, and a paper robot (pictured) that crawls when the humidity in its environment is raised and lowered. The authors say that the material could be used in devices including sensors and smart textiles.

Adv. Mater. (2014)