Wonderful webs: Elastic data analysed

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  • Published: Feb 15, 2013
  • Author: David Bradley
  • Channels: Chemometrics & Informatics
thumbnail image: Wonderful webs: Elastic data analysed

Well spun

Photos courtesy of Yarger et al - Non-invasive determination of the complete elastic moduli of spider silks

The research offers an intriguing picture not only of one of nature's most fascinating structures but might also point the way towards novel "bio-inspired" materials.

Weight for weight spider silk is approximately five times stronger than steel and is tougher than Kevlar. It can be stretched to three times its original length, bent, moistened, dried and remains strong. Needless to say, materials scientists and engineers would like to emulate its characteristics. However the complexity of this protein-based material has presented a sticky problem that even the smartest polymer chemists are yet to solve. Spectroscopy is almost always a useful tool for investigating the intricacies of complex materials and scientists in the US have turned to a long-known, but little-used spectroscopic technique to investigate the nature of the spider's web. Brillouin spectroscopy is related to Raman scattering but probes lower energies and has higher resolution than its more well-known cousin. Brillouin scattering, named for Léon Brillouin, occurs when light in a medium interacts with time-dependent optical density variations, leading to a shift in frequency and path, the shift may be due to acoustic modes, such as phonons, magnetic modes, such as magnons, or temperature gradients.

The untangled web

Now, writing in the journal Nature Materials, Paul Akhenblit, Keri McKiernan and Jeff Yarger of Arizona State University together with post-doctoral researcher Kristie Koski who has since moved to the research group of Yi Cui at Stanford University, described how they could use this phenomenon to non-invasively and non-destructively for the first time examine the mechanical properties of an intact spider's web, as pristine as when first spun by the spider. They investigated major and minor ampullate spider silks from Argiope aurantia, Latrodectus hesperus, Nephila clavipes and Peucetia viridans species.

The study allowed the researchers to quantify the elastic response of spider silk based on five elastic constants that define the way in which it behaves under a combination of forces - pulling, twisting or shearing in any direction. This is the first time that all five constants have been determined in a pristine spider's web with previous studies measuring only one or two of the five constants at a time. Moreover, earlier studies were able only to obtain data for small sections of web, rather than the whole structure. Koski believes that understanding the complete properties of a spider's web as it exists in nature would be key to engineering novel bio-inspired materials.

“My goal is to study the nanostructure of silk to understand not just how spider silk behaves as it does, but also why it behaves in such remarkable ways in hopes of someday creating better man-made fibres,” Koski explains.

Scattering fibres

Brillouin spectra have previously revealed elastic tensors for proteins, collagens and muscle fibres. The Brillouin scattering of laser light from a spider's web varies depending on the forces present in the web. The spectrometer picks up these changes without the web needing to be touched. The gentle power of Brillouin scattering thus allows mechanical properties to be determined across the whole of the spider's web and picked out at precise spots such as silk intersections and glue spots. The technique, which was described by Brillouin in the early part of the twentieth century, circumvents the problem of having to grip and pull or twist single strands of spider silk. It is thus non-invasive and does not require breaking-strain tests to determine the mechanical properties of the silk.

The result is that Koski and collaborators are the first to quantify the complete linear elastic response of spider webs, testing for subtle variations in tension among discrete fibre, junctions, and glue spots for every type of deformation possible. It is a remarkable picture of the behaviour of one of nature’s most intriguing structures. Among the team’s findings is that stiffness of a web is not uniform. This is quite surprising given that spider silk is usually considered uniform. The team suggests this is a product of natural selection in that those webs with differential strength are better able to withstand wind and rain and to absorb the energy of a wriggling victim.

Another surprise emerged from studies of supercontraction at high humidity. When a spider's web gets wet in the rain or with morning dew, the silk absorbs water shrinking unrestrained fibres by up to half their normal length through molecular disorganization. Why such supercontraction occurs is not yet understood although scientists have theorised that it may simply be a by-product of the molecular structure of silk with no evolutionary adaptation. Conversely, supercontraction might be an adaptation akin to tightening the guy ropes on a tent, allowing the web to better withstand the elements by preventing heavy water droplets from dragging the fibres down. Indeed, Koski's work lends support to this, latter, more evolutionarily driven explanation as the team found that the spider silk, which is essentially a matrix of restrained fibres, stiffens at 100% humidity. It might also be that supercontraction assist the spider in tailoring the properties of its web during spinning by pulling and restraining the silk threads and adjusting the water content.

“The possibility of adjusting mechanical properties by simply adjusting water content is interesting from a bio-inspired mechanical structure perspective and could take us in interesting research directions as we try to invent new fibres,” Koski adds. "This extensive array of elastic information may greatly facilitate future modelling efforts aimed at understanding the interplay of mechanical properties and atomistic structure of silk," the researchers say. "Our hope is that by understanding the fundamental mechanical properties and the molecular level structure of spider silk, that we will be able to use this knowledge to create synthetic replicas of spider silk in the research laboratory and ultimately in the consumer market and health care industries," Yarger told SpectroscopyNOW.

Related Links

Nature Mater 2013, online: "Non-invasive determination of the complete elastic moduli of spider silks "

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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