Dec 12, 2012

Spider Webs: Strong as Steel

Covered with dew, a spider's web hangs from a branch like a jeweled necklace. The silk that forms the web is feather-light and looks fragile and delicate in the early morning rays of the sun. Yet this amazing material is actually stronger than steel and, at the same time, as elastic as a rubber band.

People have never been able to make a material with all the wonderful properties of natural spider silk. But now, using new techniques, researchers are trying to produce the silk commercially. If they succeed, spider silk may one day be used in everything from bullet-proof vests to stockings.

Silken Nets

Spiders are hunters that catch and eat insects. (Spiders themselves aren't insects. They belong to a group called the arachnids, which also includes scorpions and ticks.) All spiders have special glands that produce spider silk, which is a protein. The silk is spun by being forced out as a liquid through tiny fingerlike organs called spinnerets. The liquid hardens into fine, tough threads after hitting the air.

Most spiders use their silk to build webs that will capture their prey. And there are almost as many different kinds of webs as there are different kinds of spiders. Some webs consist of just a strand or two of silk; others are a jumbled tangle of threads. Some spiders spin broad sheets that hang horizontally in bushes and trees. Still others construct tunnel-like traps of silk.

The master builders of the spider world are the orb-weavers. "Orb" means "circle," and these spiders weave circular, wheel-like webs. Strong, dry threads, which act as spokes, run out from the center of the web and anchor it in place. The fine, sticky threads that spiral around the circle are coated with a sort of watery glue, to trap the spider's prey.

But web-building isn't the only use that spiders have for their silk-in fact, a few spiders don't build webs at all. Spiders can spin several different kinds of silk, each having a different purpose. All are made of the same basic protein, but they have different qualities.

After a spider catches an insect, for example, it wraps its prey in a special binding silk and stores it to eat later. Female spiders wrap their eggs in a soft, protective case made of another type of silk. When the baby spiders hatch, they spin long streamers of silk that catch the wind. The babies are lifted into the air and carried off, sometimes for many miles. Wherever they land, they make their homes.

All spiders also make a fine but extremely strong silk that they trail as a dragline- a sort of safety line. If you've ever startled a spider, you may have seen it drop to the ground and then, a moment later, scuttle back up its dragline to its web. Most of the cobwebs that people find in their homes are draglines that spiders have left behind.

Spider-Silk Stockings

The remarkable properties of spider silk have long fascinated people. The silk is amazingly strong and flexible. When a spider web stops a flying insect, it's the equivalent of stopping an airliner with a rope net. The silk doesn't break because it's stronger than the best steel wire and can stretch to nearly twice its length before it snaps. And it's practically rot-proof.

Spider silk is so amazing that people would like to produce it commercially, in the same way that the silk of another creature the silk moth - is used. Fibers produced by the silk moth caterpillar (or silkworm) are the basis of silk fabric. But raising spiders for their silk is a very different matter from raising silk moth caterpillars.

Unlike the caterpillars, which eat leaves, the spiders must have live insect food. Also, spiders attack each other as well as insects. And it would take many, many spiders to produce a useful amount of silk. By one estimate, 5,000 large spiders would have to spin day and night to make the silk for one dress. Thus in the past, spider silk has been used in only a few ways -- to make cross hairs in gunsights, for example.

Now, however, some scientists think there may be a way to produce spider silk commercially. The technique involves genetic engineering (altering the material within cells that carries inherited traits). With genetic engineering, bacteria might be programmed to produce spider-silk protein. The bacteria would be grown in big vats, and the protein would be extracted and spun into thread through mechanical spinnerets. If the plan works, spider-silk stockings may be the fashion craze of the future. (Grolier Book of Knowledge Encyclopedia)