Researchers at the NanoTech Institute of the University of Texas, Dallas, have made a new type of actuator, or artificial muscle, from graphene oxide nanoribbons.

The name graphene is derived from graphite, which consists of several graphene sheets stacked together. From an electronics point of view, graphene does everything a carbon nanotube can do, only better. Graphene has wonderful properties such as very high speed, very low power consumption; enormous strength (strongest material known) which is higher than stainless steel and high thermal properties that conduct heat better than other materials.

Nanoribbons are made from graphene, a 1-atom-thick sheet of carbon atoms. Graphene nanoribbons (GNRs) are materials with properties distinct from those of other carbon allotropes. The all-semi conducting nature of sub-10-nm GNRs could bypass the problem of the extreme chirality dependence of the metal or semiconductor nature of carbon nanotubes (CNTs) in future electronics.

Making nanoribbons

Researchers at the NanoTech Institute of the University of Texas, Dallas 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.

Reversible actuation

The actuators were able to expand and contract by up to 1.6% of their original size when electrically heated. Study revealed that there are reversible changes in the interplanar spacing of the nanoribbons upon heating and these changes happen due to reversible adsorption and desorption of water molecules between highly hydrophilic graphene oxide nanoribbon layers. This insertion and removal of water molecules in the nanoribbon structure is responsible for the actuation observed. Such thermally driven graphene oxide nanoribbon actuators can lead to a family of materials that can be deployed as artificial muscles. They provide a maximum work capacity of around 40 J/Kg, which is similar to that of natural muscle, and compare well to many existing electrochemically driven actuators on the market today.

By optimizing the structure and mechanical properties, the actuators might be able used to replace the artificial muscles developed based on commercial shape memory alloys (SMA). The new actuators could find use in electrically driven MEMS and along with graphene transistors, may be a complimentary part of graphene electronics. These actuators can be used in micro grippers to produce controllable force when they expand and contract, move components in micro-opto-electromechanical devices and in micro fluidic systems. They might even be used to make up the moving limbs of future nanorobots.