Mikhail Kozlov and colleagues prepared graphene oxide nanoribbons by chemically unzipping multiwalled carbon nanotubes – a method that produces narrow ribbons with a fairly uniform size. Prepared nanoribbons were assembled into macroscopic "mats" by filtering a nanoribbon dispersion under vacuum. The researchers evaluated how good the actuators were by electrically heating the devices.

X-ray diffraction measurements revealed reversible changes in the interplanar spacing of the nanoribbons during heating. "These dimensional changes are associated with reversible adsorption and desorption of water molecules between highly hydrophilic graphene oxide nanoribbon layers," explained team member Mikhail Kozlov. "We believe this insertion/removal of water molecules in the nanoribbon structure is responsible for the actuation observed."

Rivalling SMAs
"The thermally driven graphene oxide nanoribbon actuators expand the family of materials that can be deployed as artificial muscles," Kozlov told nanotechweb.org. "After optimizing their structure and mechanical properties, the actuators might even be able to rival muscles based on commercial shape memory alloys (SMA)."

The new actuators could find use in electrically driven MEMS, which, along with graphene transistors, may become a complimentary part of emerging graphene electronics, he adds. The electrolyte-free nanoribbon devices produce enough force when they expand and contract to be able to control microgrippers, move components in micro-opto-electromechanical devices and open valves in microfluidic systems. They might even be used to make up the moving parts ("arms" and "legs") of future nanorobots.

The new work, which is reported in Chemical Physics Letters, comes from the same group that previously reported on elastomeric conductive nanocomposites for strain sensors, generating sound from carbon nanotubes and high-performance supercapacitors. It builds on earlier studies that showed that graphene itself can provide actuation.