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.
Researchers 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.
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. This approach could be extended to other animals and
plants and could lead to a new class of bionic materials for ultimate applications.
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.