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.
(Theme music)
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.
The Engines of Our Ingenuity is
Copyright © 1988-1997 by John H.
Lienhard.
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