YOU won鈥檛 be able to swing between buildings on strands of spider silk any time soon, but an unexpected discovery has just opened a whole new range of applications for this super-tough material: it contracts and lengthens with changes in humidity, doing 50 times the work of human muscle for a given mass.
Spider silk鈥檚 tensile strength has already marked it out as a possible component in bulletproof vests and medical sutures. Now it could also play an active role in devices ranging from micro drug-delivery systems to green energy production.
of the University of Akron in Ohio and colleagues were studying a property called super-contraction when they noticed something unusual. Instead of shrinking dramatically, as normally happens to wet spider silk, it relaxed and lengthened. The key was to dampen the fibre rather than soak it. The silk contracted again as it dried. 鈥淲e started to think, 鈥榳ow, we might have a new type of biomimetic muscle here鈥�,鈥� says Blackledge.
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To see how powerful the effect was, the researchers extracted 5-micrometre-thick fibres from the golden silk spider. As the humidity decreased, each fibre could lift a 100-milligram weight. Blackledge calculates that a 2-centimetre-thick braid of fibres could raise a 2-tonne truck off the ground (The Journal of Experimental Biology, vol 212, p 1990).
鈥淎 2-centimetre-thick braid of spider silk fibres could raise a 2-tonne truck off the ground鈥�
With tight control over humidity, spider silk could be used to drive moving parts in a range of devices 鈥� from opening and closing microspheres that contain drugs, to creating artificial muscles for robots.
Spider silk might even provide green energy, says Blackledge. 鈥淭here are lots of environments where you get strong fluctuations in humidity. You can potentially develop a little silk motor that uses these to generate electricity.鈥�
鈥淭his is exciting stuff,鈥� says , a biomimetics expert at University of Bath, UK. He points out that as spider silk is biocompatible, you could use it for building actuators inside small surgical tools and implantable devices.
Producing enough silk will be a challenge, as spiders are too aggressive to be farmed. But there are other options: for example, genetically modified goats make spider silk protein in their milk, which can then be spun into silk fibres.
