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Spiders sprayed with water containing carbon nanotubes and graphene flakes have produced the toughest fibers ever measured, say materials scientists.
Spider silk is one of the more extraordinary materials known to science. The protein fiber, spun by spiders to make webs, is stronger than almost anything that humans can make.The dragline silk spiders use to make a web’s outer rim and spokes is amazing stuff. It matches high-grade alloy steel for tensile strength but is about a sixth as dense. It is also highly ductile, sometimes capable of stretching to five times its length.This combination of strength and ductility makes spider silk extremely tough, matching the toughness of state-of-the-art carbon fibers such as Kevlar.So it goes without saying that the ability to make spider silk even stronger and tougher would be a significant scientific coup. Which is why the work of Nicola Pugno at the University of Trento in Italy and a few pals is something of a jaw-dropper.They have found a way to incorporate carbon nanotubes and graphene into spider silk and increase its strength and toughness beyond anything that has been possible before. The resulting material has properties such as fracture strength, Young’s modulus, and toughness modulus higher than anything ever measured.
Schematic illustration of the experimental procedure
(a) Sampling site and experimental box for the collection of (b) neat dragline silk samples and, after spraying (c) nanotubes or (d) graphene aqueous dispersions, of treated dragline silk samples. (e) Collection of silk in the form of a single fibre, composed of multiple threads, from which multiple samples have been obtained by fixing the ends of the fibre to 15 x 10 mm2 “c” shaped double adhesive cardboard holders and (f) cutting the fibre into four shorter pieces, (g) mounted on nanotensile machine clamps (h) for nanotensile tests and (i) for measuring the fibre cross-sectional surface area with a FESEM. Scale bar 5 µm
The team’s approach is relatively straightforward. They started with 15 Pholcidae spiders, collected from the Italian countryside, which they kept in controlled conditions in their lab. They collected samples of dragline silk produced by these spiders as a reference.The team then used a neat trick to introduce carbon nanotubes and graphene flakes into the spider silk. They simply sprayed the spiders with water containing the nanotubes or flakes and then measured the mechanical properties of the silk that the spiders produced.For each strand of silk, they fixed the fiber between two C-shaped cardboard holders and placed it in a device that can measure the load on a fiber with a resolution of 15 nano-newtons and any fiber displacement with a resolution of 0.1 nanometers.The results make for impressive reading. “We measure a fracture strength up to 5.4 GPa, a Young’s modulus up to 47.8 GPa and a toughness modulus up to 2.1 GPa,” say Pugno and co. “This is the highest toughness modulus for a fibre, surpassing synthetic polymeric high performance fibres (e.g. Kelvar49) and even the current toughest knotted fibers,” they say.
In other words, giving spiders water that is infused with carbon nanotubes makes them weave silk stronger than any known fiber.The work raises some interesting questions. For a start, exactly how the spiders incorporate carbon nanotubes and graphene flakes into their silk is not clear. The team use spectroscopic methods to show that the carbon-based materials are present in the fiber but are unable to show exactly how.One possibility is that the silk becomes coated with these carbon-based materials after it is spun. Pugno and co cannot rule this out but say it is unlikely because the resulting structure would not have the strength they measured. “Such external coating on the fibre surface is not expected to significantly contribute to the observed mechanical strengthening,” they say.Instead, the team say it is more likely that the spiders ingest the water along with the carbon-based materials and these are then incorporated into the fiber as it is spun. So the nanotubes and graphene end up in the central part of each fibere where they can have the biggest impact on its strength.The team have even simulated the resulting molecular structure and say that the mechanical properties are in good agreement with the experimental results.There are challenges ahead, of course. Nobody has discovered an efficient way to harvest spider silk, although not for lack of trying. So an important future step will be the development of such a technique that can work on an industrial scale. That would open the way to widespread applications in everything from tissue repair to garment design.This isn’t the first time that researchers have attempted to modify spider silk. Various groups have added metallic elements by placing the silk in the appropriate vapor. In this way they have significantly increased the strength and toughness of the silk, although never to the extent that Pugno and co have managed.Which is why their work is impressive. The extraordinary properties of spider silk are the result of 400 million years of evolution. So such a significant improvement is clearly something special.And the technique’s simplicity suggests that a similar approach could be used on other organisms. “This new reinforcing procedure could also be applied to other animals and plants, leading to a new class of bionic materials,” they say.Reference: Silk Reinforced With Graphene Or Carbon Nanotubes Spun By Spiders arxiv.org/abs/1504.06751More information:www.unitn.it - www.technologyreview.com - www.cornell.edu