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Posted: Oct 14, 2011
Researchers invent carbon nanotube artificial muscles with the strength, flexibility of elephant trunk (w/video)
(Nanowerk News) 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 today on Science Express ("Torsional Carbon Nanotube Artificial Muscles"), 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.
A forest of multi-walled nanotubes are pulled and twisted into yarns.
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 Assoc. Prof. John Madden, UBC 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.
Here is an animation that shows potential application for the artificial muscle:
In the animated video above, you first see a few bacteria like creatures swimming about. Their rotating flagella are highlighted with some detail of the flagella motor turning the "hook" and "filament" parts of the tail. We next see a similar type of rotating tail produced by a length of carbon nanotube thread that is inside a futuristic microbot. The yarn is immersed in a liquid electrolyte along with another electrode wire. Batteries and an electrical circuit are also inside the bot. When a voltage is applied the yarn partially untwists and turns the filament. Slow discharging of the yarn causes it to re-twist. In this way, we can imagine the micro-bot is propelled along in a series of short spurts.
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.