While Kevlar® is the ‘gold medallist’1 of man-made fibres because of its bullet-stopping abilities, it’s overshadowed in many ways by the humble spiderweb. “Spider silk is stronger and more elastic than Kevlar®, and Kevlar® is the strongest man-made fibre,” according to Danish spider expert Fritz Vollrath.2 Dragline silk, the main support for its web, is a hundred times stronger than steel—a cable of this silk a little thicker than a garden hose could support the weight of two full Boeing 737 aircraft.3 It can also stretch to 40% of its length,1 while the flagelliform silk in web spirals can stretch to over 200%.3
The manufacture of Kevlar® requires harsh conditions, including the boiling of sulfuric acid and the leaving behind of dangerous chemicals that are expensive to dispose of.1 But spiders need only ordinary temperatures, and they use a much milder acid bath, which is produced by special ducts.2
Spiders can make silk at different speeds—up to 10 times faster when dropping to escape a predator—unlike most industrial chemical processes that would make ‘gunk’ if the speed was varied by that much. Spider silk is even environmentally friendly—spiders eat their own webs when they no longer need them.2
Spider silk owes its amazing strength and elasticity to its ‘complexity that makes synthetic fibres seem crude.’1 Man-made fibres are usually just simple strands of material, but a silk fibre has a core surrounded by concentric layers of nanofibrils (tiny threads). Some layers contain nanofibrils aligned parallel to the axis, while other layers contain nanofibrils coiling like a spiral staircase. The coiled ones allow the silk to be stretched, because they simply straighten up rather than break.
The nanofibrils themselves are very complicated, containing tiny protein crystals in an amorphous (shapeless) matrix of tangled protein chains. These nanocrystals contain electrical charges that stop the chains from slipping, so providing strength, while the amorphous material is rubbery and allows the fibre to stretch.
Some researchers have tried to make silk by forcing a solution of silk proteins, called spidroin, through tiny holes, but the fibres are less than half as strong as those produced by the spider. It seems that the spider produces the high complexity required by making the spidroin go through a liquid crystal phase, where rod-shaped molecules align parallel (Kevlar® manufacture also uses a liquid crystal phase).
Christopher Viney of Heriot–Watt University in Edinburgh believes that this enables them to flow more easily, thus saving energy.1 The liquid state also aligns the protein molecules so they can form the nanocrystals and coiled nanofibrils. This seems to occur in the spider’s long s-duct, where water is both squeezed and pumped out. This brings hydrophobic (water-repelling) parts of the proteins to the outside and forms the nanocrystals and enables the fibres to form.
[Addendum: An informative review article, Fritz Vollrath and David Knight, Liquid crystalline spinning of spider silk, Nature 410(6828):541–548, 29 March 2001, covers a number of important issues in detail, e.g., the high strength, stress v strain analysis, the composition of spidroins including some non-essential amino acids, liquid crystal spinning, the particular type of liquid crystal called the nematic phase where the rod-like molecules are aligned parallel to each other (the phase used in image display devices), the conventional external drawdown used in industrial spinning as well as the advanced internal spinning technology so far not duplicated in man-made processes, how we can learn much from the spider’s design, the hyperbolic geometry of the s-duct so that the material elongates at a constant rate, preventing disclinations (a weakening defect analogous to dislocations in solid crystals), the structural complexity of silk — even greater than previously thought. Alas, there is the usual homage to evolution as the designer without the slightest evidence. But Kevlar® expert Dr Patrick Young concurs with this Creation magazine article that spider silk is evidence of a Designer — see Interview.]