Spider silk has often been regarded as one of Nature’s “wonder materials”, with an unmatched combination of low density, high extensibility and enormous tensile strength, five times greater than that of steel. Historically, the physics of spider silk has been poorly understood, due to the complexities of measuring these fine fibres. But as efforts to develop a synthetic equivalent to spider silk intensify, the search to find the secret of the fibre’s extreme toughness has picked up pace.
Spider silk consists of long chains of repeating protein sequences, which are stored in the silk gland in a highly concentrated form. The silk gland also contains a high concentration of salt, which prevents the formation of threads inside the duct. It’s only when the protein molecules move into the spinning chamber – just before being used – that they start to orient into the long chains recognisable as spider silk.
In late 2011, a team at the Technical University of Munich were the first to demonstrate that during the spinning process, it was a change of pH, combined with a removal of salt that allowed the silk proteins to form into crystallites and assemble into cross-linked fibres. And it is this cross-linking that ultimately gives spider silk its high tensile strength.
A recent paper in Nature Materials has also added a major piece to the puzzle – a team from Stanford have become the first to measure the complete first-order mechanical response of spider webs, a measurement consisting of five independent elastic constants. And they have done this using a century-old technique called Brillouin spectroscopy. The team also directly measured a huge increase in the silk stiffness in the presence of high humidity – a property of spider silk called supercontraction that had been known but never previously measured. Their non-contact, laser-based technique also found some ‘unknown unknowns’ - the stiffness of a web was found to greatly vary between threads, junctions and glue spots, an unexpected result in what was thought to be a uniform fibre.
The human fascination with spider silks may stretch well beyond the scientific – webs are found everywhere from fashion to superheroes – but it is the silk’s impressive mechanical properties that interests materials engineers. By learning more about spider silk, we may be able to engineer new materials that mimic, or even go beyond, those that exist in nature.