Two papers on spider silk have coincidentally come out within a week. One announced an environmentally friendly way to replicate its remarkable features, while the other reveals one of its secrets. Together, the research could take us into a world of stronger, lighter, and more flexible wires with a multitude of uses.
The capacity of spider silk to capture prey and transport the spiders remarkable places has been celebrated in legend and popular culture. Until recently, it was something of a mystery to scientists. Although efforts have been made to replicate the remarkable combination of properties of this material, they have been expensive, consumed plenty of energy, and produced toxic by-products.
In the Proceedings of the National Academy of Sciences, Cambridge University chemists and architects announce a hydrogel whose polymers are cross-linked to provide great strength, while also having spaces that keep it light. The product, they claim, “exhibits better tensile and damping properties than conventional regenerated fibers, such as viscose, artificial silks, and hair.” Best of all, it can be spun at room temperature, a feat spiders developed tens of millions of years ago, but we have struggled to match.
The team created a hydrogel, which as its name suggests is mostly water – 98 percent in fact. The rest is silica and cellulose, widely available from rocks and plants respectively. When fibers are drawn from the hydrogel, they form threads just a few microns across – much thinner than human hairs. The water evaporates slowly when in a container, but disappears within 30 seconds from the thin strands, leaving the strong but stretchy fiber behind.
"Although our fibers are not as strong as the strongest spider silks, they can support stresses in the range of 100 to 150 megapascals, which is similar to other synthetic and natural silks," co-author Dr Darshil Shah said in a statement. "However, our fibers are non-toxic and far less energy-intensive to make."
Shah admits the team has yet to produce something as good as real spider silk, but he regards what has been produced as suitable for cases where strength to weight ratios are important, such as climbing ropes or aerospace.
Nevertheless, work will continue on further improvements, and some hints may come from an explanation in Applied Physics Letters of how spiders manage to avoid spinning helplessly on the end of their threads.
Most materials twist when used as a dragline for a falling object, which causes the object to spin uncontrollably. The fact that this does not happen to spiders has puzzled physicists for decades.
"Spider silk is very different from other, more conventional materials," said Dr Dabiao Liu of Huazhong University of Science and Technology in a statement. "We find that the dragline from the web hardly twists, so we want to know why." Replicating this feature on helicopter rescue ladders could be a lifesaver, but the authors also think it could be useful in such unexpected places as violin strings.
Liu and co-authors suspended washers from multiple strands of collected spider silk inside a cylinder, making a torsion pendulum, and filmed what happened with a high-speed camera. The silk absorbed three-quarters of the torque that would cause other thin strands to spin the washers, dampening oscillations in a way that allows spiders on the end of threads to maintain their orientations.
Although the paper does not provide a complete answer as to how the silk does this, the authors are confident the answer lies in the way amino acids are combined into a mix of structured sheets and disorganized looping chains. Twisting forces make the sheets stretch and warp the links in the chains. The authors think that the sheets quickly recover, while the chains do not, and this combination serves to dampen oscillations in the silk. Nevertheless, there is still work to be done.
"This spider silk is displaying a property that we simply don't know how to recreate ourselves, and that is fascinating," said co-author Professor David Dunstan of Queen Mary University of London.
