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Biophysics

New Twists in Spider Silk

Why does a spider rappelling from its silk remain motionless, rarely twisting or swinging? A recent CNRS study indicates that this could be due to the breaking and reformation of “sacrificial bonds.”

A spider's dragline silk is well known for its extraordinary physical properties: not only is it extremely ductile (stretching up to 40% of its length without breaking), it is also incredibly tough (five times as strong as steel). However, before these nanometer-sized threads can be employed in diverse applications such as medical sutures, parachutes, or body-armor, researchers have to fully grasp their complex properties. One of these properties is their capacity of oscillation absorption, as observed when a spider rappels down its silk. This proved of particular interest to a group of French physicists working on the measurement of the angular momentum of light. The two physicists, Albert Le Floch and Olivier Emile  from the CNRS Laser Physics Laboratory at the University of Rennes, assisted by zoologist Fritz Vollrath (Oxford University), set out to understand the torsion properties of primary dragline silk, only to unravel yet another reason why it is rightfully hailed as a super fiber.

 

In their experimentation, small plastic or copper rods, used to mimic the weight of the spider, were tied to different types of 10-cm threads. The rods were twisted through an arc of about 90 degrees, and their dynamic responses analyzed.1 When the synthetic polymer Kevlar was used, the rod was found to oscillate delicately around its original position, in a manner consistent with a weakly damped, torsion pendulum. In fact, the Kevlar behaved like a rubber band and didn't do much to dampen the result of twisting. In contrast, when a soft metallic copper thread was used, the rod slightly oscillated around a new equilibrium position, displaying the low resilience typical of high-energy dissipation. Even if it dampened the twisting much better than the Kevlar, the copper thread did not return to its initial shape, and after a few twists, became weak and brittle.

 

Compared to both these synthetic filaments, dragline silk from the common European garden spider Araneus diadematus showed high damping, hardly oscillating around the new equilibrium position. It also retained its twisting quality throughout multiple cycles. Most surprisingly though, the dragline was found to have an automatic “shape memory,” enabling it to effectively recover its initial form without any outside help, albeit slowly. This prompted the researchers to compare the silk's memory reaction to a synthetic thread with similar “shape-memorizing” attributes, a nickel-titanium alloy called Nitinol, used in the satellite antennae that are folded and then sprung back to their original shape in orbit. Indeed, the Nitinol thread oscillated weakly around an equilibrium position, but unlike the silk, it did not return to its original shape on its own, but needed to be heated to 90°C to do so.

 

“What is interesting here is the fact that the pendulum torsion gives us access to the dragline's relaxation, once excited. One possible interpretation of the 'auto-memory' effect is that inside the proteins, there are sacrificial bonds (of hydrogen or Van der Waals type) that are broken when the silk is twisted, and then reform themselves quite slowly,” explains Emile. This encouraged the Rennes team to further study silk analogue proteins (poly-L-alanine and poly-L-glycine), to see whether the reconstruction of sacrificial bonds could explain the draglines' behavior. “Right now we are trying to see if we can access the time constants and different levels of the proteins' folding. This could be particularly useful for applications in biochemistry of living organisms and in numerous protein-based systems.” It seems that spider's silk has yet more to unravel to the scientific world.

 

Marion Girault-Rime

Notes :

1. O. Emile, A. Le Floch, and F. Vollrath, “Shape Memory in spider draglines.” Nature. 440: 621. 2006.

Contacts :

Olivier Emile
olivier.emile@univ-rennes1.fr


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