The new muscle-like structures will be important for applications that require mechanical movement and where volume is limited, says Geoff Spinks from the University of Wollongong in Australia. Making such actuators on the nanoscale is a challenge because conventional devices are too complex to be easily downsized and, to boot, perform significantly worse as they are made smaller. Nano-actuators and motors will be crucial in future micromachines and are already being employed in zoom lenses for digital cameras and vibration alerts for mobile phones, to name but two examples.

The muscles, made by a team led by Spinks and Ray Baughman of the University of Texas at Dallas, are composed of thin carbon nanotube threads, or "yarns". Carbon nanotubes themselves are hollow cylinders of rolled up carbon. Key to making the torsional structures is to twist the carbon nanotubes as they are made into a thread, explains Spinks. "The twisting produces a helical structure of intertwined carbon nanotubes," he told nanotechweb.org.

The researchers take lengths of the nanotube thread and partially immerse them in an electrically conducting liquid (or electrolyte). They then hold each end of a thread firmly and connect one end of it to a power supply, like a low-voltage battery. When the power is applied, the thread absorbs some of the liquid and swells up. The pressure subsequently produced by the swelling causes the twisted structure to partially unwind, creating a rotating action similar to that seen when stretching a helical spring. The structure can be made to rotate in the opposite direction by decreasing the applied voltage.

Baughman and colleagues observed the rotation by attaching a plastic paddle to the thread. They found that they could produce rotations of about 250° per millimetre of yarn thread length. This value is roughly 1000 times larger than those observed in previous torsional artificial-muscle systems that are based on ferroelectrics, shape-memory alloys or conducting organic polymers, says Spinks. And that is not all: the output power per yarn weight already rivals that of conventional, macroscale electric motors.

"Carbon nanotubes, which are normally stiff and strong and that have been made more flexible by spinning them into yarns, are ideal for making such muscle-like structures because they have good electrical conductivity," he added. "Our work now also shows that we can efficiently charge the thread with just a few volts of electricity, and that the threads are strong enough to sustain large weights – for example, the plastic paddle we attached is nearly 2000 times heavier than the thread itself."

The twisting motion observed in the new nanotube muscles is like that seen in elephant trunks, octopus limbs and human tongues, to put things in context.

Lab-on-chip and micro-robots
The structures could be useful in applications like microfluidic pumps, valve drives and mixers. Indeed, the set-up made by researchers (a plastic paddle attached to the rotating yarns) is in fact a simple mixer in its own right. Mixing fluids on the micro- and nano-scales is difficult but will be crucial for lab-on-chip diagnostics, for example. "Other likely applications are difficult to predict, but we are fascinated by the possibility of using our torsional muscle like a flagellum that would propel a micro-robot in the same way it propels a bacterium," said Spinks.

The team – which also includes scientists from the University of British Columbia in Canada and Hanyang University in Seoul, South Korea – now hopes to study the muscle-like structures in more detail and optimize yarn geometry. It also hopes to produce even better performing carbon nanotube torsional muscles by adjusting twist angle and diameter.

The current work is reported in Science.