As the delicate threads of a spider's web tremble in the breeze, it might not seem an obvious candidate for a new generation of sustainable materials. But a handful of scientists and new enterprises around the world believe that we could learn a great deal from the expertise of spiders and silkworms in the spinning of remarkable threads. Efforts are now looking seriously at what silk fibers could offer composite materials in engineering and biomedical applications.
On the roof of a building in the science area of the University Oxford sits a small greenhouse. Inside, flies buzz dizzily in circles to avoid numerous spiders’ webs strung from the walls and ceiling. On impressive yellow-colored webs, sit large female golden orb weaver spiders (Fig. 1). In a lab downstairs, meanwhile, is the fruit of their labor – a loose rope of soft, silky, and surprisingly yellow silk strong enough to bear the weight of a person.
As far as materials expertise goes, spiders have a head start on us. Silk is made up of protein molecules, reminiscent in structure of polymer–polymer composites. ‘Silk-based composites have undergone some 400 million years of research and development, field-tested by natural selection and evolution,’ points out researcher Chris Holland, formerly at Oxford but now at the University of Sheffield, who has worked on silk for the last decade. What spiders have come up with is impressive.
Silk possesses a unique combination of properties. Some silk fibers have mechanical properties that exceed Kevlar, one of the strongest artificial materials ever produced, by a factor of more than three , as well as nylon and high-tensile steel . As is obvious from watching a spider ride its dragline silk or a web bounce back after the impact of a fly, silk is also highly flexible and extensible. This ability of silk – to stretch to up to 50% of its length without breaking – is one of its key advantages because it allows the absorption of large amounts of energy. As well as these remarkable mechanical properties, silk is also biodegradable, bio-inert, and can be produced at low temperatures and pressures. The exceptional properties of spider silk – especially that spun by orb weavers – have been exploited as fishing lines and wound dressings dating back as far as Greek and Roman times.
This article appeared in the Nov/Dec issue of Reinforced Plastics.