The long and winding road to synthetic silk

While people have used the spun cocoons of the mulberry silkworm larva for millennia as the raw material for beautiful and lustrous cloth, scientists and engineers have been fascinated by the strength, low density, and ductility of this material. Despite this fascination we have not yet wound up with a clear understanding of the fundamental chemistry and physics that underpins silk's properties and so have been unable to make a material that mimics silk, although nylon comes close in some senses.

Now, researchers in the USA [Buehler et al., (2010) Nature Mater, doi: 10.1038/NMAT2704] have investigated the mechanism by which silks can reach very high strengths, yet retain high ductility. High ductility means that silk fails gracefully, step by step, rather than catastrophically breaking under excessive strain, a property of which engineers are naturally envious given that most materials with high strength, such as ceramics, are brittle. Avoid brittleness, as in many metals and alloys, and strength is reduced. The researchers now believe they have rationalized the silk paradox of how it is both strong and ductile, which offers new clues for making a silk biomimetic.

The researchers explain that silk uses surprisingly simple building blocks, unsophisticated amino acids and extremely weak hydrogen-bonds to build the protein structure of silk. “The exceptional strength of silks, exceeding that of steel, arises from β-sheet nanocrystals that universally consist of highly conserved poly-(Gly-Ala) and poly-Ala domains,” the team explains. Nevertheless, these are sufficient to achieve the material's superior properties. It is the arrangement of silk's building blocks into a hierarchical structure at defined length-scales that overcomes the apparent limitations of such simple starting materials of limited functionality.

The researchers carried out a series of large-scale molecular dynamics simulations, which can explain some of the experimental observations seen in studies of silk. For instance, by changing the dimensions of beta-sheet nanocrystals within the silk structure by just a few nanometres they observed severe changes in the strength and failure behaviour of the simulated silk. This, they say, suggests that there is huge sensitivity of mechanical performance with respect to structure. These nanocrystals when confined to a few nanometres are much stiffer and stronger than larger nanocrystals, but at the same time also much tougher. The studies also reveal how even weak hydrogen bonds between building blocks allow energy to be dissipated by a molecular “stick-slip” deformation that presents silk from snapping instantaneously under excessive load.

“Our findings explain how size effects can be exploited to create naturally inspired materials with superior mechanical properties in spite of relying on mechanically inferior, weak hydrogen bonds,” the researchers conclude. “This may lead us to a new paradigm in materials design in which we may no longer need strong building blocks to make our materials better, but are able to use inexpensive and abundant constituents,” says Buehler.