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| Carbon nanotube yarned |
Scientists have created an artificial muscle fiber 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 multifluidic lab on a chip devices,or could be used as a propulsion system for microscopic robots as suggested by the researchers.Artificial muscles are
typically made from polymers and metals that change size and shape. But to be truly useful, these materials need to twist or rotate when an electric current is applied, and very few such materials created so far can do this.
The new carbon nanotube spuns into a yarn as it can produce much torque or twisting force as a commercial motor.Carbon nanotubes have only recently been identified by scientists; they are "straws" made only of atoms of carbon linked together in hexagons. They have remarkable physical properties - being more than 100 times stronger than steel.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. 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. 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, 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 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.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.
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