Why Is Spider Silk So Strong? Protein Assembled on the Way Out

Silk is a protein fibre and its properties come from how that protein is arranged rather than from any exotic chemistry. Regions of the protein chain fold into tightly packed crystalline blocks that resist being pulled apart and supply strength, while regions between them remain disordered and extend under load, supplying stretch. A material that is strong and one that is stretchy are usually different materials, and combining both is what makes silk remarkable, because toughness is the energy absorbed before breaking and depends on both together. By that measure dragline silk outperforms steel and most synthetic fibres, which is the basis of every comparison quoted about it and which is genuine provided the measure is stated.

An orb-weaving spider produces several silks from different glands for different purposes:

• •Dragline silk, the strongest, used for the frame and for the line the spider hangs from
• •Capture spiral silk, far stretchier and coated in sticky droplets
• •Attachment discs, gluing lines to surfaces
• •Wrapping silk for immobilising prey
• •Egg case silk, tough and resistant to damage
• •Each has a different protein composition and a different gland producing it

The production process is the part that has defeated imitation. The protein is stored in the gland as a concentrated liquid, held soluble by its arrangement and by the chemistry of the fluid around it, and it must not solidify there. As it passes down a narrowing duct, the acidity is lowered, salts are exchanged and the solution is subjected to shear as it flows, and those changes together trigger the protein chains to align and lock into the solid fibre, which emerges through a spigot the spider controls. The transformation happens in milliseconds, at ordinary temperature and pressure, using water as the solvent, which is a set of conditions no industrial fibre process comes close to matching.

The structure the silk is built into matters as much as the material. An orb web is a machine for absorbing the energy of a flying insect without breaking and without throwing it back out, which requires the capture spiral to stretch enormously and to dissipate energy as heat rather than storing it elastically, and the sticky coating holds the insect while the structure recovers. The radial threads carry the load to the frame. Different web designs solve different problems, with sheet webs, tangle webs and funnel webs each catching different prey in different places, and several species build no web at all and use silk only for draglines, retreats and egg cases. Spiders also recycle, eating the old web and reusing the protein, which is why webs are rebuilt so readily.

Producing silk at scale has been attempted repeatedly and the obstacles are specific. Spiders cannot be kept in density because most species are territorial and cannibalistic, which rules out the approach that works for silkworms, and the one famous textile made from spider silk required collecting from over a million wild spiders. Producing the proteins in other organisms, including bacteria, yeast, plants, silkworms and goats, has succeeded in making protein and has struggled to reproduce the spinning, since the protein is only half the problem and the assembly process is the other half. Progress has been substantial in the past decade, with several companies producing fibres and materials from engineered organisms, and the products remain expensive and generally short of natural performance.

Crystalline blocks in the protein supply strength while disordered regions between them supply stretch, and having both is what makes the material tough. A single spider produces several silks from different glands. The protein is stored as a liquid and solidified by changes in acidity, salts and shear as it flows out, in milliseconds at ordinary temperature, which is the part nobody has reproduced.