Today, let's try to weave a magic web. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.
Next time you brush away a cobweb, consider what you're brushing. The diameter of a strand is around 1/10,000 of an inch. Human hair has 30 times that diameter and a thousand times the cross-sectional area. It's the difference between thread and rope.
The ancient Greeks applied cobwebs to wounds. That may sound spooky, but 19th-century doctors also studied treatment with cobwebs. Only in this century did we learn why it works -- that spiders coat their silk with antiseptic agents.
Entomologist May Berenbaum tells how the French scientist René Réaumur set out to find commercial uses for spider webs in 1710. That's the same Réaumur who looked at wasp's nests and suggested that we, like them, might learn to make paper from wood.
In the 19th century, astronomers did find a use for spider silk. They needed better cross-hairs on their telescopes. Spider silk proved to be the perfect material. By WW-II, gunsights and bombsights, range finders and transits, telescopes and microscopes were all using spider silk. Demand outran supply.
Now we're looking at the amazing structural properties of spider silk. The stuff is stronger than steel, yet it can stretch to 140 percent of its length. At the same time, it's inelastic -- it absorbs the energy used to stretch it and doesn't bounce back. A fly, caught in a web, cannot trampoline back off the web. The silk also stays tough at low temperatures.
But its properties vary. Spiders have six spigots for spinning silk, and they mix their fluids to regulate the composition. They can make one kind of silk for catching flies and another to shape a parachute that'll carry them away on the wind.
A spider might spend an hour weaving a web to trap bugs. A day later the gossamer cross members will be broken while the main guy lines stay intact. The spider eats the protein-rich broken strands. Then he spins new silk from the recycled strands.
And it's a material we want! We want to use it in airplanes and bridges -- clothing, body armor, and cable. We've tried to set up spider-silk farms. The trouble is, the spiders won't cooperate. They don't like being crowded. Put too many in a closed space, and they solve the problem by eating each other. If we're ever to have the stuff of spider silk, we'll have to synthesize it. And we haven't figured out how to do that.
So, for now, we let those ghostly strands twine about our imagination. Dryden once wrote,
Our souls sit close and silently within,
And their own webs from their own entrails spin
One day we'll learn just what it is that spiders from their own entrails spin. We are not likely to rest before we can utter the spell that will recreate -- that fairy-tale fabric.
I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.
Berenbaum, M., Spin Control. The Sciences, Vol. 35, No. 5, September/October 1995, pp. 13-15.
Preston-Mafham, R., and Preston-Mafham, K., Spiders of the World. New York: Facts On File Publications, 1984.
Kamel Salama, UH Mechanical Engineering Department, makes a useful caveat about this episode. It is that, on such a small scale, many materials show enormous strength, since they don't have the usual inclusions and imperfections of large specimens. Extremely small diameter steel wires, for example, will resist higher stresses than will spider silk, even though the breaking stress of larger steel specimens is less than that of spider silk. The challenge is thus not just to replicate the material, but also to produce it on a large scale without imperfections.
I am most grateful to Dr. Jimmy Schmidt, who first brought the Sciences article to my attention, and to Dr. John Rogers, Baylor College of Medicine, for additional advice on this episode.