A team led by Rod Ruoff of the Texas Materials Institute and the University of Texas at Austin have found that bilayer paper-like materials based on carbon nanotubes and graphene-based platelets strongly deform in response to stimuli such as humidity or temperature.

The researchers made their device from two neighbouring layers composed of overlapping and stacked graphene oxide platelets (the first layer) and criss-crossed multiwalled carbon nanotubes (the second layer). Each layer is about 10 µm thick and is formed by first filtering an aqueous suspension of MWCNTs and then filtering a similar solution of graphene oxide platelets. The finished device curls when humidity or temperature are increased (see figures) and the amount of curling can be controlled by varying the thickness of the MWCNT layer.

The bilayer material also has other interesting physical properties, says Ruoff. "For example, the surface of the graphene oxide layer is dark brown and electrically insulating while the surface of the MWCNT layer is black, shiny and electrically conducting." This electrical asymmetry could come in handy for applications such as wrapping, and as an energy storage material.

The actuators could also be used as gas sensors, say the researchers. And given the excellent electrical and mechanical properties of graphene, they might function as sensors, switches, MEMS and NEMS too.

The team now plans to understand how the actuators deform. Also on the to do list: study curling response times and how stable the devices are.

The work was reported in Small.