Researchers are most familiar with the mechanics of dragline
and capture-spiral silk, which is sticky, extremely stretchy, and
tough. The properties of these two silks make webs effective for
trapping flying insects, explains Blackledge. The dragline frame
of the web absorbs the brunt of an insect’s energy. The capture-spiral silk absorbs some energy but sticks to and stretches
with the insect, so that it decelerates slowly and doesn’t bounce
off the web.
To learn about the lesser-known silks, Blackledge and his colleague Cheryl Y. Hayashi of the University of California, Riverside studied the five fibrous silks of the orb-weaving silver garden spider, Argiope argentata. They collected two of the silks
directly from the spiders and the other silks from webs, wrapped
prey, and egg sacs. They extended the fibers and measured the
silks’ mechanical properties using a
tensile-testing machine.
In the July 1, 2006 Journal of Experimental Biology, Blackledge and
Hayashi reported that the silks make
up a diverse toolkit of fibers “that seem
fine-tuned for particular ecological
functions.” For example, in keeping
with its prey-capturing role, the capture-spiral silk is 10 times as stretchy
as the other silks. Meanwhile, the tubuliform silk of the protective egg sacs is
the stiffest.
Furthermore, the researchers found
that aciniform silk, the threads that the
spider uses to wrap and secure freshly
captured—and still wriggling—prey, is
two to three times as tough as the other
silks, including dragline.
For a materials scientist interested
in a high-performance fiber that
absorbs kinetic energy, notes Blackledge, “the prey-wrapping silk may be
a better model to study than the
dragline.”
Blackledge is interested in the extent to which shifts in spider
behavior have influenced the performance of silks. “When the silk
is used in a new ecological context, what happens to the material
properties?” Blackledge asks.
ical properties. If the researchers increase the number of capture-spiral-silk motifs, for example, the elasticity of the fiber grows,
although not in direct proportion to the number of motifs.
While scientists know a lot about the sequence of individual
chains and a bit about the chains’ interactions with each other,
higher levels of structure are “basically completely unknown,”
Lewis notes. A silk thread contains hundreds of thousands of
protein chains, each of which folds on its own and also arranges
itself among other chains in the fiber, he says. He and his colleagues have begun nuclear magnetic resonance studies to explore
these structural details.
“The spider hasn’t given us all the secrets,” Lewis says.
GO WITH THE FLO W Silk’s transformation to a solid fiber from
a thick liquid containing primarily protein and water begins in specialized
glands, one for each type of silk. In each
gland, a structure called the tail
secretes the starting solution, or spinning dope, into a storage sac. When the
spider is ready to spin, the dope moves
into a duct. The diameter of the duct
narrows as it reaches a nozzle from
which the thread exits the spider.
To understand the characteristics
of the spinning dope, some scientists
have turned to rheology, the study of
how materials deform and flow. Silk
dope has properties intermediate
between those of typical liquids and
solids, explains Gareth H. McKinley,
a mechanical engineer at MIT. Such
viscoelastic materials are thick rather
than runny. They’re also elastic: After
being stretched, they return to their
original states. Silly putty and
uncooked egg white are two familiar
examples of viscoelastic materials,
McKinley says.
The handful of previous rheology studies of dope used samples that had been diluted to make their volumes large enough
to be tested. But machines that can work with small samples of
material are now available, notes Holland. Scientists can test
tiny amounts of silk dope that have been extracted from a spider. Reports on freshly obtained dragline-dope samples were
published last fall by an Oxford team, led by zoologist Fritz Vollrath and including Holland, and by McKinley’s team, which
includes Kojic.
An important concept in rheology is shear, the sliding motion
of adjacent layers of material. Silk dope experiences shear forces
as it moves through the spinning duct. McKinley’s group built
a microrheometric device that measures how the viscosity of
the dope changes in response to shear forces. The researchers
place the sample—a drop of dope the size of a pen tip—between
two plates. The lower plate remains stationary as the upper
plate moves back and forth. The machine’s action is much like
rubbing a drop of lotion between thumb and forefinger to gauge
its slipperiness, says McKinley.
The researchers found that the faster the upper plate moves,
the more readily the dope flows. Shear forces align the proteins in
the dope, Kojic says, “and as the proteins align, it becomes easier
for them to move relative to one another.” Adds McKinley, “Take
a big bucket of spaghetti. If you keep stirring it clockwise, it gets
easier because the spaghetti strands are lining up.”
This effect explains how the thick dope can progress through
the narrowing duct in an energy-efficient manner, Kojic notes. The
team calculated that overall, the viscosity of the dope decreases
10-fold as it flows through the duct.
BLACKLEDGE
GOTCHA — An adult female Argiope aurantia injects
venom into a honeybee caught in an orb web. The
spider first wrapped her prey in aciniform silk. She
will soon cut the prey from her web, bring it to the
center, and have her meal.
MATERIAL DIFFERENCE Silk’s mechanical properties primarily derive from two critical factors: the proteins that make up
the material and the spinning process that transforms the liquid
generated inside a spider into a solid fiber.
Randolph V. Lewis, a molecular biologist at the University of
Wyoming in Laramie, and his coworkers have determined the
amino acid sequence of several silks. They’ve found distinct amino
acid motifs that contribute to different silks’ properties.
For example, the two major proteins in dragline silk contain
frequently occurring stretches of the amino acid alanine. Lewis says
that these alanine repeats give the fiber strength by permitting
one protein chain to snap tightly to another, much as Lego blocks
combine. Minor-ampullate silk, which is not as strong as dragline,
has shorter stretches of alanines.
Meanwhile, capture-spiral silk has a motif, based on a sequence
of five amino acids that’s repeated up to 68 times in a row (SN:
2/21/98, p. 119). Lewis speculates that this sequence introduces a
series of spiraling molecular springs into the protein, which may
explain the silk’s extreme stretchiness.
Lewis’ group has built artificial silk genes and inserted them
into the common bacterium Escherichia coli to make proteins
that are shorter than the natural versions. The researchers add
the resulting proteins to organic solvents, spin this material into
fibers with a commercial spinning machine, and test its mechan-
