Why all the torque leaves Spirder-Man unfazed

SPIDER- MAN never twists like a helpless mountain climber when he dangles from one of his web lines-and the answer lies in the unique molecular structure of his spider silk, say French scientists.

In addition to be being famously strong, the proteins that make up spider thread have incredible torsional qualities, according to a paper, which appeared on Thursday in Nature, the British weekly science journal.

The threa both damps and resists torsional force, and after it is twisted returns to the same position, like special alloys with socalled shape memory, it says.

A team led by Olivier Emile of the Laser Physics Laboratory at University of Rennes, western France, used a small rod to represent the weight of a spider.

They successively tied the rod to a thread of Kevlar, the strongest synthetic polymer, to an ultra-fine soft copper thread, and to dragline silk from the European garden spider (Araneus diadematus). Each thread was then twisted through an arc of 90 degrees, with the rodsuspended from the bottom.

With the Kevlar, the rod twisted back an forth vigorously in response to the movement of the thread. In contrast, the copper was a good damper, stopping the twisting oscillations quickly. But it became brittle after only a few twists.

Spider silk performed best of all. The suspended rod barely budged after the thread was twisted and, unlike the copper, the silk retained its strength throughout the experiments.

The team found a similarly stellar performer in the nickel-titanium shape-memory alloy nitionl, a thread of which was tested using a weight of 3g.

The difference, though, is that Nitinol is expensive and after being bent and twisted at ambient temperature has to be heated to 90C to recover its original shape.

"By contrast, the spider has evolved a shape memory material that needs no external stimulus for total recovery," say Emile's team, admiringly.

The scientists suspect the no-twist silk evolved to give the spider a protective advantage. By not twisting at the end of his thread like a climber swinging from a rope, an abseiling spider is less conspicuous to predators.

The silk's torsional secret could lie in proteins called poly-L-alanine and poly-L-glycine, say the French team, speculating that the coiled double-helix molecule of DNA, the code of life itself, may also have a tiny torque effect.