Cheap polymers twist into superhuman muscles

24-Feb-14

An international team has unmasked the hidden superpowers of humble sewing thread and fishing line. Twisting nylon and polyethylene into coils, they made artificial muscles that can lift loads over 100 times heavier than human muscle of the same length and weight.1 They could replace motors in many uses, particularly robotics, and enable new technologies, such as smart clothing, says Ray Baughman from the University of Texas, Dallas. He’s also excited that the threads used cost just $5/kg (£3/kg). ‘They could be easily deployed in the developing world, children could make and use them,’ he tells Chemistry World. Baughman’s team previously made artificial muscles from twisted carbon nanotube yarns that expanded or contracted when electrical charge passed through them. But their yarns were expensive and could only make small muscles. Worse still, twisted muscles responded more powerfully than untwisted yarn, but they didn’t know why. Nylon and polythene, by contrast, were cheap to investigate. In a thread these polymer chains line up lengthways, and contract in this direction when heated, while also expanding sideways. In coiled up threads, the two differing responses drive a twisting action that gives them their previously unrealised power. Baughman compares this to how metal springs move, their wire twisting when stretched or compressed, only in polymer coils the motion is temperature-driven. The coils expand or contract on heating depending on the direction they are spun in. The many muscles the researchers made include four twisted polyethylene fishing lines in a silicone tube. Pumping alternating hot and cold water down the tube raises and lowers a 7.2kg weight twice a second. That shows these fibres avoid the hysteresis disadvantage seen in rival artificial muscle candidates, shape memory alloys, which creates a delay before movement.