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. 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 is also excited that the threads used cost just US$5/kg. ‘They could be easily deployed in the developing world, children could make and use them,’ he tells Chemistry World.
The 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 polyethylene, 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 7.2 kg 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. As per Baughman, shape memory alloys also cost around £3000/kg, and are harder to scale-up. ‘We’ve gone up to 1mm diameter,’ he says. ‘The smallest is 25µm, but there’s no reason why we can’t go to 1cm diameter muscle and have the same contraction energy per unit volume.’ J Rossiter. a biomimetics and robotics researcher at the University of Bristol, UK, says that the resulting versatility to generate large strains or stress is ‘very important’. He also applauds the ability they’ve shown to operate robustly over millions of cycles. ‘I expect this to lead to practical artificial muscles in a relatively short time, even within five years,’ he says. ‘The challenge lies in control of thermal energy into and out of the material.’

Danni de Rossi from the University of Pisa, Italy, who worked with Baughman on nanotube based artificial muscles in 1999, is more cautious about the fibres’ potential. He suggests that electroactive elastomer artificial muscles being commercialised by California’s  SRI International are more likely to be successful than heat-responsive ones. ‘It’s excellent optimisation, but how many thermally activated switches are used by industry?’ he asks. ‘Almost zero.’  But the team has already exploited their muscles’ temperature response to replace motors that open and close windows to regulate building climate. They’ve also woven prototype textiles from them that could let more air through clothes when it’s warm. Baughman also hopes they could help the elderly in lightweight exoskeletons that amplify weak movements, or companion robots. With our small diameter polymer muscles, you can put as many as you want in a facial expression.’

SRI has patented a thin, flexible, smart material dubbed “artificial muscle” because it behaves much like a human muscle. Artificial muscle, available for license from SRI for a variety of application areas, uses a breakthrough technology called electroactive polymers. The material expands when exposed to an electric current and contracts when the electricity is removed, converting electrical potential energy into mechanical motion. Artificial muscle has the potential to fundamentally shift the way many types of industrial, medical, consumer, automotive and aerospace products are powered and operated. It offers significant advantages over typical electromagnetic-based technologies because it is much lighter, smaller, quieter and cheaper. It also offers more controllable and flexible configurations. Artificial muscle enables a wide variety of applications, including haptic displays to improve human computer interaction, adaptive optics, flat conformal loudspeakers, and, potentially, implantable active medical prosthetics. The technology has also demonstrated promise for a variety of actuator and electric power generation applications. Because of its inherent muscle-like characteristics, SRI’s artificial muscle can enable robots to mimic the dexterity and mobility of humans. It offers performance characteristics similar to those of natural muscle such as high strain, high peak power and high compliance. In addition to acting as a muscle-like actuator, artificial muscle can operate in reverse and generate power from being stretched and contracted. Compared to many other smart material technologies, the polymer materials used in artificial muscle are relatively inexpensive. Their high compliance allows artificial muscle to easily interface with human or other environmental sources of motion. Combined with its high energy output, these features make it attractive for a variety of energy harvesting applications, such as capturing the energy of ocean waves. In 2008, SRI first demonstrated a wave-powered generator that converts energy from ocean waves to electrical energy. In 2005, SRI spun off Artificial Muscle Inc. (AMI), to further develop the technology and introduce products based on EPAM. In 2010, AMI became a subsidiary of Bayer MaterialScience LLC.