Spider-Man is returning to the silver screen. This event will be accompanied by a web of intrigue regarding whether the web-swinger and his web shooter mirror the performance of real spiders and real silk.
For several years, materials science has envied the ‘green’ processability and the tensile properties of major ampullate silk – the thread that spiders spin from secretions of their major ampullate glands, and use as the dragline on which they make controlled descents and ascents. This fiber exhibits impressive values of strength, initial stiffness, and toughness in tensile tests conducted at constant strain rate. However, such tests typically are completed within minutes; they do not duplicate realistic in-service load histories, and they do not adequately probe the long-term behavior of the sampled material. Mechanical testing regimes that are specifically designed to explore creep of, or stress relaxation in, silk are revealing a rather less promising outlook than constant strain rate tests: both effects are significant. This realization should not be surprising, since nature does not require major ampullate silk to carry high loads or accurately retain its length beyond a time scale of minutes or hours.
Major ampullate silk also suffers from significant moisture sensitivity. The load-bearing properties of the material are decreased, and creep and stress relaxation are greatly increased, by exposure to a moist environment. This, too, is not surprising, since the original functionalities that confer water solubility to the silk protein stored in the silk-producing glands are not discarded when the protein solution is spun into fiber. The protein chains are refolded into an arrangement from which they cannot as a whole resolubilize in pure water, but this does not prevent water from interacting with hydrophilic domains and enabling marked local changes in chain conformation and packing.
It is therefore apparent that native major ampullate silk has several Achilles' Heels, coincidentally befitting its many-legged source. The moisture sensitivity of dragline is readily demonstrated in a simple laboratory experiment [Viney, C., and Bell, F. I., Curr. Opin. Solid State Mater. Sci. (2004), in press] that can also be performed at home if the procedures are adapted to domestic circumstances as described here. The experiment works to best effect with long pieces of dragline: 20 cm or greater if available. These can be obtained by gently dislodging a spider that is crawling along the ceiling, and carefully collecting the thread on which it descends. Without stretching or otherwise deforming the silk, one end of the fiber is attached to a fixed support with a tiny dab of superglue. A small office staple, resting on a removable support so that the silk is initially straight but not stressed, is similarly attached to the other end. The support under the staple is then carefully withdrawn, so that the silk is now under load and hanging vertically, and the steam plume from a boiling kettle is directed intermittently towards the silk. The idea is to raise the ambient humidity without adding weight through excessive condensation on the sample, and without significantly heating the sample. The silk will first supercontract markedly as moisture plasticizes the least dense regions in the microstructure, allowing metastably extended chains in more ordered regions to randomize their conformation. After several minutes, the sample length will begin to increase slowly, as the weight of the staple causes the silk to creep. If heavier weights are used, the initial supercontraction becomes less marked, and subsequent creep becomes more rapid.
Unmodified dragline or an artificial analog would not be a good choice of material for replacing the steel cables and suspenders on the Brooklyn Bridge, given the propensity of the material to creep and the anticipated plasticizing effect of a heavy New York fog. Could our superhero find a plausible use for this stuff? He might try to capitalize on the fact that native dragline is essentially a ballistic material. Its propensity to creep, with the capacity for additional plasticization imparted by moisture, could be highly desirable in a material used to contain the solid fragments generated by catastrophic failure of, for example, an aircraft luggage container. That's more along the lines of what Spider-Man would get up to!
If Spidey's silk is like natural dragline, his attempts to descend rapidly and under control down the outside of a building, or to swing accurately from one building to another, could be thwarted easily. Doc Ock would simply need to conjure up a blast of moist air. Thus could a potentially successful web-swing be turned into a bad case of pavement rash. Most fortunately for Spidey, Doc Ock has something in common with the Green Goblin and any materials engineers who hope to use unmodified major ampullate silk as a critical component in a load-bearing structure: he didn't catch on to the idea that damp silk can sometimes wind you up, and will literally let you down.
[1] Christopher Viney is a founding professor of engineering at the University of California, Merced.