On the radio (Today Programme, BBC Radio 4) this morning was a piece on spider silk from tarantulas. Spiders and their silk are a perennial favourite with those among my discipline who wish to poularise it and make it accessible to the public. We are regularly told (and told in this article) that spider silk is “stronger even than than steel” and “very, very stretchy” and that “researchers envisage a huge host of commercial applications”. Unfortunately, researchers have been saying the same for the good old orb web spider and the host of commercial applications have yet to materialsise.

Orb Web Spider in its Web

I will return to the hype of things being “stronger than steel” in a bit but I will first comment on the commercial possibilities of spider silk. Silk is a protein based fibre and we know the sequence of amino acids in a large range of natural silks from arachnids and from insects. The bit of nature we find hardest to mimic is the action within the spinnerets of spiders (or silkworms/silkmoths) that converts the protein suspension into the fibres. This has prevented us from making artificial (synthesised) spider or silkworm silks and has also prevented us from processing natural silks into artificial fibres with the mechanical properties that rival those of natural silks. Before we get too fixated on spider silk, we should also consider why there is a major international textile industry that uses silkworm silk and not one that uses spider silk, despite man’s familiarity with both animals for millenia. The difference between these two sources of silk is that we have domesticated the silkworm/silkmoth and that it provides gramme quantities of silk in each cocoon/chrysalis. Spiders are, as yet undomesticated and we cannot produce commercial quantities of their silk. Hence most commercial applicatiopns of silks use insect derived material. However, spiders are still very good marketing material; e.g. the Oxford Biomaterials web site with its extensive use of spider derived product names. However, to the best of my knowledge all current commercial silk derived products from all manufacturers use materials (fibroin proteins) derived from silkworm silk. These are still wonderful products but spiders sound better from a marketing perspective.

I will now put my sceptical materials scientist hat on and talk about the strength claims of spider silk. Is spider silk stronger than steel? Well like all good scientific questions, the answer depends on how you frame the question. If you ask the question “Is spider silk stronger than the sort of steel you use to make car body panels?”, then the answer is yes. But if you ask “Is spider silk stronger than the sort of steel you use to make ultra-high strength steel wire?” the answer is no. You could also ask whether spider silk is stronger than silkworm silk? and the answer is probably no (Shao and Vollrath, Nature, 2002). Spider silk is strong (typically about 1 GPa tensile strength) but so are many other materials when they are manufactured as fibres with dimensions smaller than a few microns. Spider silk is about the same strength as carbon fibre, and about the same strength as humble polyethylene when it is drawn into a thin fibre. Steel, by the way, can have a strength in excess of 5 GPa when in highly drawn wire form.

This discussion returns to the essential core of the science of materials when applied to mechanical properties. Strength in particular is an extrinsic property, in that we can control its value by manipulating the structure of a material at the supramolecular scale or the scale of the microstructure. Spiders obtain the properties of their silk fibres through molecular manipulations that occur within the spinnerets induced by fluid shear. High strength steel wires are formed through plastic deformation, work hardening and crystal rotations, also induced by shear flow but now a completely different set of mechanisms are exploited to achieve high strengths. There is equal wonder in how we manipulate materials to achieve high strengths for engineering applications as there is wonder at the exploits of spiders.