Compared with natural adhesives, man-made glues are puny: often coming unstuck and only working on clean dry surfaces, their uses are limited. Meanwhile, geckos’ sticky feet never become exhausted and barnacles weld themselves irreversibly to any wet surface; although the adhesive champions must be the spiders. ‘We're interested in the [spider web] glue because it is an incredibly tough material with the potential to inspire new glues with similar properties’, says Sarah Stellwagen from Virginia Tech, USA. Adding that spider glues are stretchy as well as sticky and that they work well in both wet and dry conditions, Stellwagen and her colleagues Brent Opell and Mary Clouse decided to find out what effects ultraviolet (UV) radiation might have on the glue. Knowing that UV radiation comes in two bands, UVA and UVB, both of which can have damaging effects, Stellwagen wondered whether spider web glue was enhanced or degraded by UVB and whether the glues of spiders that live in dark conditions was more vulnerable to UVB damage than the glues of spiders with sunnier lifestyles.
Fortunately, Stellwagen has been fascinated by spiders ever since her twelfth birthday when she became the proud owner of a pet tarantula, Clawdia; shooing arachnids away from their webs – to enable her to take the structures back to the lab – did not concern her. She also explains, ‘We needed full large webs with the largest droplets possible for testing and consistency, so we needed all of the spiders to be at the same life stage’. Working with Opell and Clouse, Stellwagen collected the webs of spiders that build their webs in locations ranging from full sunlight (the yellow garden spider) and bright and dark conditions (the orchard spider) to species that construct webs in varying degrees of shade: from the arrowhead spider, which prefers shady locations with occasional glimpses of sun, to the spined macrathena, which lives in dense shade and the nocturnal barn spider.
Back in the laboratory, Stellwagen extracted lengths of sticky spiral silk and exposed them to doses of UVB radiation (for 1–4 h). Photographing the glue droplets, Stellwagen compared the volume before and after UVB exposure to find out if the radiation had damaged the small molecules in the glue that absorb moisture from the atmosphere to keep the droplet hydrated. However, the droplets did not change size, so the radiation had not affected the molecules.
Next, Stellwagen measured the glue droplet's toughness – the amount of energy that the glue can absorb – by securing the droplet to a metal pin and slowly pulling it back. This time, UVB radiation had a clear effect. The glue of both the spined macrathena (dense shade) and the nocturnal barn spider was severely damaged by UVB – the toughness of both was reduced by at least half. However, the arrowhead spider's glue was unaffected by the radiation. Meanwhile, exposure to UVB was beneficial for the glue of spiders that lived in bright and less shady locations; the toughness of the yellow garden spider's glue increased more than three times after the highest UV dosage and the orchard spider's glue toughness doubled. Yet the orchard spider's glue was the only one that had become tougher after being kept in the dark, suggesting that it cures over time.
‘Spider glues are tailored to perform in specific environments in more ways than one: humidity, temperature and now UVB all result in performance differences,’ says Stellwagen, who is keen to find out how much UVB the most resistant glues can take before they begin to fail.
