The scientists who discovered that natural silks get stronger the colder they get, have finally solved the puzzle of why.

The interdisciplinary team examined the behaviour and function of several animal silks cooled down to liquid nitrogen temperature of -196 ^oC. The fibres included spider silks but the study focused on the thicker and much more commercial fibres of the wild silkworm Antheraea pernyi.

In an article published today in Materials Chemistry Frontiers, the team was able to show not only ‘that’ but also ‘how’ silk increases its toughness under conditions where most materials would become very brittle. Indeed, silk seems to contradict the fundamental understanding of polymer science by not losing but gaining quality under really cold conditions by becoming both stronger and more stretchable. This study examines the ‘how’ and explains the ‘why’. It turns out that the underlying processes rely on the many nano-sized fibrils that make up the core of a silk fibre.

In line with traditional polymer theory, the study asserts that the individual fibrils do indeed become stiffer as they get colder. The novelty and importance of the study lies in the conclusion that this stiffening leads to increased friction between the fribrils. This friction in turn increases crack-energy diversion while also resisting fibril slippage. Changing temperature would also modulate attraction between individual silk protein molecules in turn affecting core properties of each fibril, which is made up from many thousand molecules.

Importantly, the research is able to describe the toughening process on both the micron and nano-scale levels. The team concludes that any crack that tears through the material is diverted each time it hits a nano-fibril forcing it to lose ever more energy in the many detours it has to negotiate. And thus a silk fibre only breaks when the hundreds or thousands of nano-fibrils have first stretched and then slipped and then all of them have individually snapped.

The discovery is pushing boundaries because it studied a material in the conceptually difficult and technologically challenging area that not only spans the micron and nano-scales but also has to be studied at temperatures well below any deep-freezer. The size of scales studied range from the micron size of the fibre to the sub-micron size of a filament bundle to the nano-scale of the fibrils and last but not least to the level supra-molecular structures and single molecules. Against the backdrop of cutting edge science and futuristic applications it is worth remembering that silk is not only 100% a biological fibre but also an agricultural product with millennia of R&D.

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