Silk of the Darwin's bark spider is twice as strong as other spider silk due to extreme extensibility combined with high strength.

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Man-made structural fibers are generally coils of simple, homogenous strands. In contrast, spider silk fibrils are constructed with alternating nano-segments that are either extremely flexible (amorphous glycine-rich matrices) or extremely strong (crystalites made of anti-parallel pleated beta sheets). Dozens of fibrils come together to form each thread. As a result, the fibers are nearly as strong as Kevlar yet much more stretchable and tough.

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“Combining high strength and elasticity, spider silks are exceptionally tough, i.e., able to absorb massive kinetic energy before breaking. Spider silk is therefore a model polymer for development of high performance biomimetic fibers…We examined the biomechanical properties of silk produced by the remarkable Malagasy ‘Darwin’s bark spider’ (Caerostris darwini), which we predicted would produce exceptional silk based upon its amazing web. The spider constructs its giant orb web (up to 2.8 m2) suspended above streams, rivers, and lakes. It attaches the web to substrates on each riverbank by anchor threads as long as 25 meters. Dragline silk from both Caerostris webs and forcibly pulled silk, exhibits an extraordinary combination of high tensile strength and elasticity previously unknown for spider silk. The toughness of forcibly silked fibers averages 350 MJ m3, with some samples reaching 520 MJ/m3. Thus, C. darwini silk is more than twice tougher than any previously described silk, and over 10 times better than Kevlar®. Caerostris capture spiral silk is similarly exceptionally tough.

Conclusions: Caerostris darwini produces the toughest known biomaterial. We hypothesize that this extraordinary toughness coevolved with the unusual ecology and web architecture of these spiders, decreasing the likelihood of bridgelines breaking and collapsing the web into the river.” (Agnarsson et al. 2010:1)

Journal article
Bioprospecting finds the toughest biological material: extraordinary silk from a giant riverine orb spiderPLOSSeptember 16, 2010
Ingi Agnarsson , Matjaž Kuntner, Todd A. Blackledge

“Spidroin I have previously been characterized by NMR and x-ray diffraction. They were found to be predominantly in b-sheet conformation and to organize into crystallites… These crystallites are interconnected in an amorphous glycine-rich matrix” (Du et al. 2006: 4528)

Journal article
Design of Superior Spider Silk: From Nanostructure to Mechanical PropertiesBiophysical JournalSeptember 2, 2006
Ning Du, Xiang Yang Liu, Janaky Narayanan, Lian Li, Matthew Lek Min Lim, Daiqin Li

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