Saturday, October 2, 2010

Stronger than steel...

The strength of nature can be an impressive thing. Biologists, physicists, engineers and chemists alike often spend a lot of time just trying to figure out what makes things as strong as they are. What makes the shell of a clam rigid and tough? What makes the silk of a spider pliable yet strong?

This is an area of research called biomaterials. The study of biological substances and their physical properties in terms of measurements like strength and stiffness.

Recent research by scientists at the University of Puerto Rico has revealed that the toughest material on the planet is spider silk. In particular, the trophy goes to the web-spinning silk of the Darwin's bark spider, which lives on the island of Madagascar. This spider spins enormous webs that extend across rivers. Therefore, they must stretch and contract as the trees (to which they're anchored) move in the wind.

Spider silk, in general, is amazing stuff. It is a protein. It is both strong, meaning it resists breaking, and it is elastic, meaning it can deform and then recover its shape. Many materials have to trade off these properties. A substance can be very strong, like steel. But, steel is not elastic. If you bend steel, it will not return to its original shape. Spider silk, on average, has the same tensile strength as steel. But at the same time, it is very ductile, and can stretch about one and a half times its own length before breaking.

The bark spider of Madagascar spins fibers that are stronger than the strongest known man-made substance, which is Kevlar. Kevlar can resist about twice the force of steel. This is why they make bullet-proof vests from the stuff.

Spiders also can change the properties of their silk, by changing the water content of the silk. Most spiders can also weave more than one kind of silk, generally speaking there is strong silk that creates the support for the web, and sticky silk that catches the prey in the web. When you put these two abilities together, you end up with about a dozen distinctly recognizable kinds silk that can be produced by just one spider depending on the job at hand. The silk used to wrap up prey is even stronger than the silk used to support the web, and the silk used to form egg sacs is stronger still. Therefore, both of these silks are stronger, on average, than steel. At the other end of the spectrum, many spiders, particularly those that have just hatched, can extrude long, very thin strands of gossamer silk used for ballooning to new locations to settle and build their own webs.

The impressive properties of spider silk make it popular for study by engineers hoping to mimic Mother Nature. Unfortunately, it is not possible to create spider ranches so that the spiders can do the work for us. Spiders are not like docile cattle, making them extremely poor candidates for domestication. Spiders are aggressive and will eat one another, making it inadvisable to keep many spiders together in the same space. Reproducing the properties of the silk with man-made mimics is the only viable option, though scientists have created transgenic goats that will produce spider silk (I’ll save the ethical debate about that sort of process for a later article!).

For right now, the score is still Mother Nature 1, Humans 0 in terms of who can make the stronger substance.

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