Webs of orb spiders are elastic and strong because of sacrificial bonds that break and then reform

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Spider capture silk is the silk used to form the spiral in webs built by orb weaver spiders. It is a remarkable material, as strong as Kevlar yet elastic at the same time. Orb weaver capture silk has a tensile strength of 1 GPa but can stretch to 500-1000% its original length before rebounding perfectly to regain its original properties. This is very important in spiral webs, which must be able to survive insects flying into them at speed without breaking and must also not become baggy and loose after being stretched.

Spider silk is made up of long protein molecules called fibroin. Parts of the molecule are disordered, however in others it folds neatly into pleated sheets called beta-sheets. The sheets are held in place by networks of very weak bonds between hydrogen and oxygen atoms. The physical shape of the protein means that it is most likely to adopt the configuration that forms the greatest number of these weak bonds, however, because they are so weak, they are easily broken.

Even paper can be made elastic by folding it into a concertina shape. In the same way, the pleated sheets of the silk protein pull apart, for instance because of impact by a flying insect. This causes the weak hydrogen bonds to break and allows the silk fiber to stretch. However, it most easily adopts the folded conformation and so once the energy of the impact has dissipated, it goes back to its original shape, allowing the hydrogen bonds to reform.

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References

Journal article
Molecular nanosprings in spider capture-silk threadsNature MaterialsJanuary 1, 2003
Becker N, Oroudjev E, Mutz S, Cleveland JP, Hansma PK, Hayashi CY, Makarov DE, Hansma HG

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