Carbon nanotubes give artificial muscles a new twist

Scientists have created an artificial muscle fibre that can twist at high speeds in both directions along its axis while carrying a heavy load. The torsional micromotor, based on spun threads of multiwalled carbon nanotubes, could be used for pumping or mixing fluids in microfluidic lab on-a-chip devices, the researchers suggest, or could be used as a propulsion system for microscopic robots. 

The research team, led by Ray Baughman of the University of Texas at Dallas, spun twisted yarns from a ’forest’ of aligned nanotubes, each of which was around 10nm in diameter. In this way the team created helical yarns of around 10?m in diameter and several centimetres long. When the yarn is placed in an electrolyte with a counter electrode and a current applied, the yarn spins in one direction for multiple revolutions before a limiting rotation results. The rotation can be reversed by changing the applied voltage. 

’When charge is electrically injected into the nanotube, ions migrate into the nanotube structure to balance the electronic charge,’ says Baughman. This ingress of ions causes the yarn’s volume to increase, accompanied by a shrinking in its length. This mechanical distortion causes torsional rotation because of the helical structure. 

The team, which included researchers from the universities of Wollongong in Australia, British Colombia in Canada and Hanyang in South Korea, showed that the yarn could be reversibly rotated for more than 40 turns at a speed of nearly 600 revolutions per minute. The yarn was also able to rotate an attached paddle, weighing 2000 times more than the yarn itself. 

’The demonstrated rotation of 250 degrees per millimetre of muscle length is more than a thousand times that of previous torsional artificial muscles, based on shape memory alloys or conducting polymers,’ says Baughman. ’The output power per yarn weight is comparable to that of large electric motors.’ 

Experts on molecular actuators are divided on the merits of the work. Tony Ryan, of the University of Sheffield in the UK, says: ’It is a spectacular result combining really neat mechanics and electrochemistry,’ and predicts applications in microelectromechanical systems and microfluidics.  

Eugene Terentjev, of the University of Cambridge in the UK, is less convinced. ’In such a twisting motion of a thin fibre, the actual exerted forces are very low and so the total work output is low too,’ says Terentjev. ’Therefore, the only good use of such a motor is to be a switch: to shut or open channels. In fact, there are many - really, a lot - of alternative ways to deliver such a switch actuation.I don’t think this will be an effect we will hear about in technology applications.’ 

Simon Hadlington