Spider silk's flexibility makes webs super-strong

A computer model demonstrates how a single thread in the web takes the strain of an impact.

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A spider web's ability to adapt to different levels of stress is the key to its remarkable stability, say scientists.

As well as seeing how much strain natural webs could take, researchers used computer simulations to find out how the silk structures responded.

Webs stood up to a variety of stresses, including hurricane-force winds.

The team report their findings in the journal Nature.

They discovered that a spider web's design, and the unique properties of its silk, allowed just a single thread to break so the rest of the web remained unharmed.

"It is stunning because, in fact, engineered structures don't behave that way," explained Dr Markus Buehler from the Massachusetts Institute of Technology, US, who led the study.

"If a building, a car or an aeroplane is exposed to large mechanical stress, it typically breaks as a whole and the entire structure becomes dysfunctional."

The expert in molecular mechanics and his team studied the webs of a variety of species including European garden spiders (Araneus diadematus) and orb weavers (Nephila clavipes).

By investigating the silk on a molecular scale, the researchers found they could explain the behaviour of the web as a whole.

Spider silk facts

Argiope bruennichi spider in web (c) Francesco Tomasinelli & Emanuele Biggi
  • Spider silk is five times stronger than steel of the same diameter
  • Each thread is finer than a human hair and is able to keep its strength below -40C
  • The faster and tighter the strand is drawn, the stronger the silk

Each individual thread of silk, Dr Buehler explained, could be "sacrificed" to maintain the overall structure. The key to this ability lies in the fact that the silk "changes" as it is tugged at.

"When a filament is pulled, the silk's unique molecular structure unfurls as stress increases, leading to a stretching effect," the researcher said.

This change occurs in four stages: In the first phase the entire thread is pulled taught; it is then "drawn out" and stretched as the proteins making up the thread "unfold".

In the third stage, the thread goes through a "stiffening phase" that absorbs the greatest amount of force.

There is then one final phase just before the silk breaks, which Dr Buehler called "stick-slip". He compared it to pulling on a piece of sticky tape in an effort to break it; a great force is needed to break the thread because the proteins are being held together by "sticky" hydrogen bonds, he explained.

The "slipping and sticking" occurs because although the force breaks the bonds - some of them reform. This process repeats, with fewer and fewer of the bonds sticking back together, until none remain and the thread breaks completely.

"The durability of the web is not just controlled by how strong silk is, but also, how its mechanical properties change as you stretch it," said Dr Buehler.

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