Todd Blackledge is fascinated by spiders and their webs. `They are an incredible tool for exploring spider behaviour,' Blackledge explains and adds, `they capture the behavioural decisions that spiders make in response to natural selection in the environment'. But to understand how spider webs function, you have to understand the mechanical properties of the silk itself and how it is affected by environmental conditions. Blackledge and his colleagues, from the University of Akron, decided to focus on the effects of humidity on one of the web's major structural components; dragline silk. According to Blackledge, some spider silks contract dramatically when they get very wet and it wasn't clear why. He decided to focus on the effects of humidity on dragline silk produced by the golden silk orbweaver and made the amazing discovery that tiny humidity changes cause the silk to contract and relax, just like a muscle fibre (p. 1981, p. 1990).
However, Blackledge was initially interested in understanding how spider silks behave when exposed to dramatic increases in humidity. Curious to know what happens to dragline silks when they become wet and supercontract, Blackledge and Ingi Agnarsson carefully mounted samples of the silk in a tensile tester equipped with an environmental chamber and slowly increased the silk's humidity in 10% intervals while measuring the tension on the thread (p. 1981). Not expecting to see any change in the silk's length until they reached higher humidities, the duo were surprised to find that the silk relaxed as they increased the humidity, and contracted as the air in the environmental chamber dried. By cyclically raising and lowering the humidity in 10% steps, Blackledge and Agnarsson were able to make the silk extend and contract repeatedly, like a muscle. However, when they increased the humidity further, the pair saw the silk contract dramatically and irreversibly when the humidity reached 70%.
Wondering why the silk was able to contract and relax reversibly at low and high humidities, yet supercontract irreversibly at 70% humidity, Blackledge and Agnarsson monitored the amount of water lost and gained from the silk fibres as the humidity changed. They found that the fibre's water levels rose as the humidity increased, allowing the fibre to relax as it became softer and regions of the silk protein molecules became more mobile. However, when the humidity fell, the reverse happened; the fibre dried forcing mobile regions of the silk proteins to collapse together as the fibre contracted.
Curious to find out why the silk supercontracted irreversibly, Blackledge teamed up with Cecilia Boutry, Shing-Chung Wong and Avinash Baji to look at the silk's thermal stability and found that the silk was permanently changed after the supercontraction. Water molecules had been incorporated into the silk protein's structure. Blackledge suspects that at very high humidities, a glycine rich region of the silk protein becomes extremely mobile, breaking free of the bonds holding the region down. This allows another region of the protein to collapse together in the fibre, supercontracting the silk and trapping water molecules in the silk's structure.
Having figured out why the silk relaxes under some conditions and supercontracts under others, Blackledge was curious to know more about the silk's remarkable muscle-like behaviour (p. 1990). Teaming up with Ali Dhinojwala and Vasav Sahni, Blackledge and Agnarsson decided to measure the stresses generated by the contracting silk and to find out how much weight a contracting fibre could lift.
Drying a 5 μm thick fibre to force it to contract, the team measured stresses ranging from 20 mPa in moist conditions to 140 mPa when the air was almost dry. And the thread had no problem lifting weights up to 100 mg, which might not sound like much, but when Blackledge compared the silk's weightlifting prowess with that of a single human muscle fibre, the spider silk did as well as, and by some measures better than, the muscle fibre. What is more, the silk never tired and relaxed, unlike human muscle, which requires a constant energy supply to sustain a contraction.
Having realised that spider dragline silk can contract and relax rapidly as the humidity changes, Blackledge is keen to look at other species' silks. `There are over 40,000 spider species,' says Blackledge, `so we need to investigate cyclic contraction in other silks because there could be one that performs better.' And while Blackledge thinks it unlikely that spider silk muscles will ever power prosthetic limbs, he is optimistic that the silk's muscle-like behaviour could find applications in a wide range of new technologies, such as micro-mechanical systems and 21st century drug delivery systems.