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Molecular mechanistic origin of the toughness of natural adhesives, fibres and composites

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Abstract

Natural materials are renowned for their strength and toughness1,2,3,4,5. Spider dragline silk has a breakage energy per unit weight two orders of magnitude greater than high tensile steel1,6, and is representative of many other strong natural fibres3,7,8. The abalone shell, a composite of calcium carbonate plates sandwiched between organic material, is 3,000 times more fracture resistant than a single crystal of the pure mineral4,5. The organic component, comprising just a few per cent of the composite by weight9, is thought to hold the key to nacre's fracture toughness10,11. Ceramics laminated with organic material are more fracture resistant than non-laminated ceramics11,12, but synthetic materials made of interlocking ceramic tablets bound by a few weight per cent of ordinary adhesives do not have a toughness comparable to nacre13. We believe that the key to nacre's fracture resistance resides in the polymer adhesive, and here we reveal the properties of this adhesive by using the atomic force microscope14 to stretch the organic molecules exposed on the surface of freshly cleaved nacre. The adhesive fibres elongate in a stepwise manner as folded domains or loops are pulled open. The elongation events occur for forces of a few hundred piconewtons, which are smaller than the forces of over a nanonewton required to break the polymer backbone in the threads. We suggest that this ‘modular’ elongation mechanism might prove to be quite general for conveying toughness to natural fibres and adhesives, and we predict that it might be found also in dragline silk.

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Figure 1: Scanning and transmission electron micrographs of a freshly cleavedabalone shell, showing adhesive ligaments formed between nacre tablets.
Figure 2: Consecutive force–extension curves, obtained using an atomic force microscope, from pulling on a freshly cleaved abalone nacre surface.
Figure 3: Model of long polymers and force–extension curves for different kinds of polymers.

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Acknowledgements

We thank F. Lange and E. Kramer for help in understanding polymers, and H. Gaub and his group members both for helping to teach us how to unfold proteins, and for suggesting that organic components of abalone shells might be a good system in which to study polymer unfolding. This work was supported by the NSF (P.K.H.), by NSF through the Materials Research Laboratory, the Office of Naval Research, the Army Research Office MURI program, and the NOAA National Sea Grant College Program.

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Author notes

  1. Bettye L. Smith and Tilman E. Schäffer: These authors contributed equally to this work

Authors and Affiliations

  1. Department of Physics,, University of California at, Santa Barbara, 93106, California, USA

    Bettye L. Smith, Mario Viani, James B. Thompson, Neil A. Frederick, Johannes Kindt & Paul K. Hansma

  2. Department of Chemistry and Materials, University of California at, Santa Barbara, 93106, California, USA

    Galen D. Stucky

  3. Department of Molecular, Cellular, and Developmental Biology, University of California at, Santa Barbara, 93106, California, USA

    Daniel E. Morse

  4. Department of Molecular Biology, Max-Planck-Institute for Biophysical Chemistry, 37070 Gttingen, Germany

    Tilman E. Schäffer

  5. Department of Chemistry, The University of Texas at Austin, Austin, 78712, Texas, USA

    Angela Belcher

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  1. Bettye L. Smith
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  2. Tilman E. Schäffer
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  3. Mario Viani
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  7. Angela Belcher
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  8. Galen D. Stucky
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  9. Daniel E. Morse
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  10. Paul K. Hansma
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Correspondence to Bettye L. Smith.

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Smith, B., Schäffer, T., Viani, M. et al. Molecular mechanistic origin of the toughness of natural adhesives, fibres and composites. Nature 399, 761–763 (1999). https://doi.org/10.1038/21607

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  • DOI: https://doi.org/10.1038/21607

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