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Nanoconfinement controls stiffness, strength and mechanical toughness of β-sheet crystals in silk

Abstract

Silk features exceptional mechanical properties such as high tensile strength and great extensibility, making it one of the toughest materials known. The exceptional strength of silkworm and spider silks, exceeding that of steel, arises from β-sheet nanocrystals that universally consist of highly conserved poly-(Gly-Ala) and poly-Ala domains. This is counterintuitive because the key molecular interactions in β-sheet nanocrystals are hydrogen bonds, one of the weakest chemical bonds known. Here we report a series of large-scale molecular dynamics simulations, revealing that β-sheet nanocrystals confined to a few nanometres achieve higher stiffness, strength and mechanical toughness than larger nanocrystals. We illustrate that through nanoconfinement, a combination of uniform shear deformation that makes most efficient use of hydrogen bonds and the emergence of dissipative molecular stick–slip deformation leads to significantly enhanced mechanical properties. Our findings explain how size effects can be exploited to create bioinspired materials with superior mechanical properties in spite of relying on mechanically inferior, weak hydrogen bonds.

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Figure 1: Hierarchical structure of spider silk, simulation set-up and theoretical considerations.
Figure 2: Snapshots of deformation profiles and failure mechanisms of silk β-sheet nanocrystals at different sizes.
Figure 3: Size dependence of the stiffness, and bending versus shear contributions as a function of β-sheet nanocrystal size.
Figure 4: Strength, toughness, resilience and strain distribution in β-sheet nanocrystals as a function of crystal size.
Figure 5: Hierarchical effects in the architecture of spider silk nanocrystals.

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Acknowledgements

This work was supported by the Office of Naval Research (N00014-08-1-00844). Further support from the National Science Foundation (CMMI-0642545 and MRSEC DMR-0819762), the Army Research Office (W911NF-06-1-0291), DARPA (HR0011-08-1-0067) and the MIT Energy Initiative is acknowledged. B.I. acknowledges support from MIT’s UROP and the MISTI-Germany programme. This research was supported by an allocation of advanced computing resources supported by the National Science Foundation (TeraGrid, grant no. TG-MSS080030). Further simulations have been carried out at MIT’s Laboratory for Atomistic and Molecular Mechanics. The authors thank J. J. Connor and T. Radford at MIT for fruitful discussions.

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S.K. and M.J.B. designed the research and analysed the results. S.K., Z.X. and B.I. carried out atomistic and molecular simulations. S.K., Z.X. and M.J.B. wrote the paper.

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Correspondence to Markus J. Buehler.

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Keten, S., Xu, Z., Ihle, B. et al. Nanoconfinement controls stiffness, strength and mechanical toughness of β-sheet crystals in silk. Nature Mater 9, 359–367 (2010). https://doi.org/10.1038/nmat2704

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