The route to synthetic silks

Another interesting silk composite, the sticky capture silks of Nephila and Araneus, are complex, albeit microscopic, mechanical windlass systems that make good use of the physics of biological micro-engineering (Fig. 8.3). In the windlass silk (which operates in the wet state) the elasticity is given by a combination of surface tension of the aqueous coat and recoil of the plasticised silk fibre [12-14] while adhesion is bestowed by a separate glycoprotein complex [12, 15]. 

The mechanical behaviour of radial and capture silks differs greatly. For example, the wet and soft sticky spiral of the Araneus diadematus garden spider absorbs energy by large extendibility circa 500%) of the wetted thread which develops substantial force only after 100-200% extension with 

2 Electron micrographs of Nephila dragline silk (A) longitudinal section of a fully urea-swollen fibre - note the numerous electron-light canaliculi and the electron-dark outer coat (bar 1pm)  

The functional and developmental details of the two so very different elastic recoil mechanisms of the two types of capture silk micro-machines are interesting and deserve deeper studies. However, at present we do not even understand the interaction of form and function in the much more typical spider silk fibre such as a dragline filament. Recent studies indicate that the toughness of spider dragline silk may depend on the complex hierarchical structure of the fibre [11] which in turn depends on a complex 

3 A capture thread of Araneus diadematus under increasing magnification. The windlass mechanism is seen in the lower picture where the core fibres after a large extension-contraction cycle have been reeled into a droplet (for details see ref. 12). 

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