An unusually simple approach to artificial muscles – based on high-strength polymer fibres – has been developed by an international team of researchers. Rather than needing sophisticated or expensive materials, the muscles can be produced from simple polymers that are used to make fishing-line or sewing threads. When heated, these fibres can shorten or lengthen far more than biological muscle, and could be used for applications as diverse as temperature-sensitive window shutters, "smart" clothing and robotics.

Synthetic sinew

Materials that expand and contract in response to some form of stimulus are useful for robotics, where they are used to make "actuators" or artificial muscle fibres, and on smaller scales where they can produce sensors for lab-on-a-chip devices. Numerous designs for such materials have been developed, such as shape-memory materials – either metals or polymers that exist in two phases and can therefore suddenly shorten or lengthen at a specific temperature – or electroactive polymers – that change shape in response to an electric field – and electrochemically stimulated carbon-nanotube yarns. All of these, however, have limitations. Shape-memory metals can degrade after a limited number of cycles and show various amounts of hysteresis (a reluctance to change phase). Electric-field-driven polymers can require impracticably high changes in field. Furthermore, all of these materials – especially carbon-nanotube yarn – are highly specialized and can be expensive.

In the new research, Ray Baughman and colleagues at the University of Texas at Dallas, together with collaborators in Canada, South Korea, Turkey, China and New South Wales in Australia, took a simpler tack. Unlike most materials, polymer fibres tend to shorten when heated, because as the entropy of the system increases, the polymer chains become more disordered. On its own, this contraction is quite small – up to about 4% for a 250 K increase in temperature. More significant, however, is that when the chains become more disordered, the fibre becomes thicker.

Twisted tendons

Baughman's team worked out a simple way to use this radial expansion to amplify or, alternatively, to reverse the thermal contraction. The researchers started with a fibre consisting of highly ordered, linear polymer chains and then twisted it repeatedly, turning the chains into helices. When the heated fibre expanded radially, this increased the lengths of the individual chains' helical paths, thus creating a torque on the fibre as the increased tension in the polymer chains caused it to try to untwist.

Next, the researchers wound the twisted fibre into a coil. They showed that if the coil is wound in the same direction that the fibre had been twisted, the untwisting torque from heating causes the coil to tighten up and shorten by up to 49%. Even more remarkably, if the coil is wound the opposite way – i.e. one is wound clockwise and the other anticlockwise – heating wil cause the coil to lengthen by up to 69%. This compares favourably with human skeletal muscles, which can expand and contract by a maximum of about 20%, although muscles use energy many times more efficiently.

Pulling their weight

The fundamental mechanical properties utilized by the muscles – radial thermal expansion combined with axial thermal contraction – are found in many polymers that can be purchased in a hardware shop. To produce high-strength muscles, the researchers focused on strong polymers such as polyethylene fishing line and nylon sewing thread. Using these, they demonstrated their muscles in a variety of everyday applications, such as shutters that opened and closed in response to changes in temperature or textiles with coiled polymer threads interwoven so that the pores opened as the temperature increased. In the future, these could be used in clothing that allows more heat to escape if the wearer gets too hot. "We have a lot of improvements to make," says Baughman, "but already these muscles are ready to be deployed commercially. They're cheap to make – you don't have to make the precursor and you can make a hell of a lot of muscle very easily."

Richard James, an expert in shape-memory polymers at the University of Minnesota in the US, feels "it's not a huge breakthrough but it is very clever and I enjoyed reading about it". He believes the most valuable part of the research is the geometric effect, in which a coiled coil amplifies the strain produced, although he notes that a temperature change of more than 200 K is needed to produce the largest strains, which could be problematic in applications. "It's an interesting method," he concludes. "I can imagine people might combine this with shape memory as the fibres wouldn't have to be polymers."

The research is published in Science.