An international team of researchers has invented new artificial muscles strong enough to rotate objects a thousand times their own weight, but with the same flexibility of an elephant's trunk or octopus limbs.
In a paper published online in Science Express, the scientists and engineers from the University of British Columbia, the University of Wollongong in Australia, the University of Texas at Dallas and Hanyang University in Korea detail their innovation. The study elaborates on a discovery made by research fellow Javad Foroughi at the University of Wollongong.
Using yarns of carbon nanotubes that are enormously strong, tough and highly flexible, the researchers developed artificial muscles that can rotate 250 degrees per millimetre of muscle length. This is more than a thousand times that of available artificial muscles composed of shape memory alloys, conducting organic polymers or ferroelectrics, a class of materials that can hold both positive and negative electric charges, even in the absence of voltage.
"What's amazing is that these barely visible yarns composed of fibres 10,000 times thinner than a human hair can move and rapidly rotate objects two thousand times their own weight," says UBC Assoc. Prof. John Madden, Dept. of Electrical and Computer Engineering.
Madden says, "While not large enough to drive an arm or power a car, this new generation of artificial muscles -- which are simple and inexpensive to make -- could be used to make tiny valves, positioners, pumps, stirrers and flagella for use in drug discovery, precision assembly and perhaps even to propel tiny objects inside the bloodstream."
Central to the team's success are nanotubes that are spun into helical yarns, which means that they have left and right handed versions, which allows the yearn to be controlled by applying an electrochemical charge, and to twist and untwist.
The new material was devised at the University of Texas at Dallas and then tested as an artificial muscle in Madden's lab at UBC. A chance discovery by collaborators from Wollongong showed the enormous twist developed by the device. Guided by theory at UBC and further experiments in Wollongong and Texas, the team was able to extract considerable torsion and power from the yarns.
The torsional rotation of helically wound muscles, such as those in the flagella of bacteria, has existed in nature for hundreds of millions of years. Many other natural appendages -- from the trunk of an elephant to octopus's powerful and limber tentacles -- also show how helically wound muscle fibers cause rotation by contracting against a boneless core.
The nanotube yarns are activated by charging them in a salt solution, much as a battery is charged. A breakthrough discovery came from former UBC PhD student Tissaphern Mirfakhrai -- now at Stanford -- who found that the deformation of the yarns is proportional to the size and number of ions inserted. A similar effect is seen in lithium ion battery electrodes used in portable electronic devices, but in yarns it is put to good use. The helical structure of the yarns makes them unwind as they accept charge and swell. They twist back up again when discharged.
"The discovery, characterization, and understanding of these high performance torsional motors show the power of international collaborations," says corresponding author Ray Baughman, Robert A. Welch Professor of Chemistry and director of the University of Texas at Dallas Alan G. MacDiarmid NanoTech Institute.
Support for this research includes a Discovery Grant from the Natural Sciences and Engineering Research Council of Canada.
To see animation of a potential application of the twisting actuator, visit: http://electromaterials.edu.au/news/UOW112032
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