New analysis of the structure of silks explains paradox of super-strength

[bananas]

http://www.scienceblog.com/cms/new-analysis-structure-silks-explains-paradox-super-strength.html

New analysis of the structure of silks explains paradox of super-strength

CAMBRIDGE, Mass. -- Spiders and silkworms are masters of materials science, but scientists are finally catching up. Silks are among the toughest materials known, stronger and less brittle, pound for pound, than steel. Now scientists at MIT have unraveled some of their deepest secrets in research that could lead the way to the creation of synthetic materials that duplicate, or even exceed, the extraordinary properties of natural silk.

Markus Buehler, the Esther and Harold E. Edgerton Associate Professor in MIT's Department of Civil and Environmental Engineering, and his team study fundamental properties of materials and how those materials fail. With silk, that meant using computer models that can simulate not just the structures of the molecules but exactly how they move and interact in relation to each other. The models helped the researchers determine the molecular and atomic mechanisms responsible for the material's remarkable mechanical properties.

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"In most engineered materials" ? ceramics, for instance ? "high strength comes with brittleness," Buehler says. "Once ductility is introduced, materials become weak." But not silk, which has high strength despite being built from inherently weak building blocks. It turns out that's because these building blocks ? the tiny beta-sheet crystals, as well as filaments that join them ? are arranged in a structure that resembles a tall stack of pancakes, but with the crystal structures within each pancake alternating in their orientation. This particular geometry of tiny silk nanocrystals allows hydrogen bonds to work cooperatively, reinforcing adjacent chains against external forces, which leads to the outstanding extensibility and strength of spider silk.

One surprising finding from the new work is that there is a critical dependence of the properties of silk on the exact size of these beta-sheet crystals within the fibers. When the crystal size is about three nanometers (billionths of a meter), the material has its ultra-strong and ductile characteristics. But let those crystals grow just beyond to five nanometers, and the material becomes weak and brittle.

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[ck4829]

1. Ah, that must be the thing with superheroes and capes!