SPIDER SILK is one of the strongest materials on Earth. Each fibre can
stretch by 40 per cent of its length and absorb a hundred times as much energy
as steel without breaking. Apart from catching flying insects—the purpose
it evolved for—it might be used to stop speeding bullets and tether
5-tonne satellites in space. But ask Christopher Viney, a chemist at Heriot-Watt
University in Edinburgh, why he has devoted much of his professional life to
spider silk and he’ll tell you that silk’s mechanical properties are only part
of the allure. The way spiders make it is just as important—and this is
what gets researchers like him excited.
The spider has evolved techniques way beyond those of even the most skilful
chemist. Its silk gland is a microscopic factory no wider than a human hair that
takes a gelatinous protein goo and sucks so much moisture out of it with a
miniature “kidney” you’d expect it to set like toffee. Yet somehow the spider
keeps this viscous mass flowing, and uses tricks of fluid dynamics to convert it
miraculously into a tough silk fibre.
If researchers like Viney could learn the spider’s secrets, they could do
more than simply make artificial spider silk: they should be able to improve on
it. Eventually, these secrets could revolutionise the chemical industry, leading
to a whole new class of eco-friendly techniques.
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Artificial fibres are typically spun under harsh chemical conditions. Kevlar,
for instance—the gold medallist of high-performance artificial
fibres—is spun from concentrated sulphuric acid heated to near boiling
point. Leftover chemicals are dangerous to handle and expensive to dispose of.
But the spider spins silk from protein and plain old water, at pH
levels and temperatures similar to those found in the human mouth. If chemists
mastered this remarkable chemical feat, they could devise new, green ways to
create and process their materials.
Despite years of study, spider silk is still an enigma. Under the microscope,
its fibres appear perfectly smooth. But soften them with solvent, peel back
their outer layers and you’ll find that each fibre exhibits organisation over a
fantastic range of scales from nanometres to micrometres—a complexity that
makes synthetic fibres look crude. Concentric layers of tiny threads called
nanofibrils enclose the fibre’s core. In some layers these nanofibrils point
straight along the fibre’s axis, while in others they coil around the fibre like
a spiral staircase.
This arrangement may help the fibre to absorb huge amounts of energy before
breaking. “If all the nanofibrils were parallel to the fibre, you’d pull on them
and they’d break when they reached their limit,” says Ron Eby from the
University of Akron in Ohio, who has used atomic force microscopy to study silk.
“But the angled nanofibrils have to be stretched until they’re straight, and
only after that can you break them.”
Hairy knuckles
Peer deeper inside a single nanofibril and an even greater complexity
emerges: nanometre-sized protein crystals are dispersed throughout an amorphous
matrix of tangled protein chains. Strong attractions between the charges on
these crystals seem to stop the protein chains from slipping past one another as
the fibre is pulled, says Lynn Jelinski, a polymer chemist at Louisiana State
University. Meanwhile, the amorphous, rubber-like material between the crystals
should extend elastically like a spring.
The more researchers learn about silk’s intricate structure, the more they
marvel at the spider’s ability to produce it so quickly and in such a tight
space. Take Nephila clavipes, a two-and-a-half centimetre, orange and
black, hairy-knuckled critter from Central America. Production of
Nephila’s unusually strong dragline silk (from which it dangles) begins in
the major ampullate gland, a structure the size of a pea, from which an aqueous
solution of silk protein—”spidroin”—is secreted at high
concentrations of almost 50 per cent. The sticky spidroin solution then passes
through a tapering silk duct just a twentieth of a millimetre across into a
section called the S-duct. After a total journey of two centimetres that takes
as little as a second, the spider can pull the silk from its spinneret, a
“nozzle” at its rear end, as a fully formed thread
(see Diagram).
Somehow, liquid spidroin is organised into a super-strong fibre during its
short journey. A clue to how Nephila achieves this feat was uncovered
in 1991, when Viney, then at the University of Washington and David Kaplan, then
at the US Army’s Natick Research, Development and Engineering Center at Natick,
Massachusetts, discovered that spidroin from its silk duct shows distinctive
patterns when examined in polarised light under an optical microscope. This
suggests that it is organised as a liquid crystal.
Liquid crystals have ordered structures like solids, yet can flow like
liquids (“The world of liquid crystals”, New Scientist, 4 May 1991, p
25). They often form in solutions of rod-shaped molecules—at low
concentrations the rods arrange themselves at random like molecules in a liquid,
but at higher concentrations they begin to align. Spidroin molecules may
resemble balls made from tangled protein chains, but Viney suggests that these
balls stack up like abacus beads to form just the kind of rods found in liquid
crystals.
He and Kaplan now believe that this liquid crystal state helps the spider to
process the spidroin by lowering its viscosity. “The concentration of spidroin
in the gland is equivalent to the concentration of jelly in cubes, before you
add water,” explains Viney. “That stuff isn’t going to flow through a
microscopic duct.” But somehow the spider still manages to pull this viscous
mass through its narrow spinneret and turn it into silk.
The secret, Viney believes, is that the rod-shaped spidroin structures are
attracted to each other by weak forces: they can slide past one another quite
easily, keeping the viscosity of the spidroin low. This makes good sense. The
spider must produce its thread on a very strict energy budget, says Fritz
Vollrath, a zoologist affiliated with Aarhus University in Denmark and Oxford
University. Spiders can’t afford to waste energy. The liquid crystal state
lowers the viscosity of the silk secretions so the spider doesn’t work too hard
moving this concentrated solution through its narrow silk duct.
Spinning Kevlar
The liquid crystal state also organises the protein molecules so that they’re
properly aligned to form the tiny protein crystals and coiled nanofibrils that
give silk its remarkable properties. Some industrial processes, such as Kevlar
spinning, use a liquid crystal stage for a similar purpose.
But how does the spider transform this mix of water and protein into a fibre
that isn’t washed away by the rain? Viney believes that freshly made spidroin
balls are stable in water because their charged, water-loving parts are on the
outside, while their uncharged, water-hating segments are hidden inside. But
then, something extraordinary happens. The curled-up proteins must straighten
out, to reveal their water-hating interiors, and in the process transform into
insoluble nanocrystals. But what could coax the spidroin out of its stable ball
shape? “You’re telling the segments hiding inside the molecule, `Look, you’ve
got to go outside and expose yourself to water,” says Viney, “and they’re
saying, `Yuck, we don’t like this at all’.”
The trigger, Viney believes, could be friction. He suspects that friction
between the spidroin balls and the wall of the silk duct provides the force
needed to trigger a change in the spidroin’s conformation. This alters the
structure of the protein, bringing the water-hating protein segments to the
outside and allowing the fibre to form. Viney’s most recent data suggest that
the spider may even control the sensitivity of this trigger by adjusting the
concentration of calcium or zinc ions in the S-duct.
Vollrath, however, doubts that the liquid crystals are viscous enough to
experience significant frictional force as they pass through the duct. Instead,
he has found evidence that the spidroin balls may be destabilised by stretching
forces due to “elongational flow” as they pass through tapering sections of the
spider’s duct (Proceedings of the Royal Society of London B, 1 March
1999, vol 266, p 519).
Vollrath believes that a high rate of elongation may fold the spidroin,
“squeezing” the water out. He and his colleague David Knight believe they may
even have found an organ capable of controlling this process—tiny
finger-like projections called microvilli that line the interior of the S-duct.
Microvilli increase the surface area of the duct, increasing the absorption of
water. Vollrath and Knight believe the S-duct functions as a miniature kidney,
helping the liquid crystal to “collapse” into a fibre by pumping water out of
the spidroin.
In the past year, using lessons learnt from the spider, researchers have been
pushing ahead with plans to make their own silk. For example, Eby and a team at
Natick have been evaluating a technique known as electrospinning. By applying a
30 000 volt potential difference between the fine needle of a syringe and a wire
screen, they can produce a charged jet of spidroin solution. As the jet is
pulled toward the screen by electrical forces, the spidroin molecules stretch
and align, the solvent evaporates and dry fibres accumulate on the mesh.
Others, including Jelinski, are trying to spin silk by forcing a solution of
silk proteins through a microscopic aperture that resembles the spinneret of a
spider. A silk fibre coagulates in a solvent bath on the other side of the
aperture.
But even the best artificial silk fibres are less than half as strong as the
real thing. Jelinski suspects that they lack organisation on large scales, but
this may change if researchers can produce a liquid crystalline state during
artificial spinning.
While some researchers learn to spin silk into fibres, others are discovering
ways to improve on the properties of the natural material. Despite its strength
and resilience, the toughest silk—dragline silk—is far from perfect.
Get it wet and its fibres absorb water, swell and shorten to almost half their
original length, in a process called supercontraction. This is a big problem for
many applications.
Jelinski believes the problem lies in the nanofibril’s amorphous matrix.
Normally, proteins in the matrix are in a delicate equilibrium, balanced by
hydrogen-bonding between one another and any water molecules that are
present. Adding extra water unbalances this equilibrium and the silk fibres
shrink, she says.
Understand these interactions, says Jelinski, and you can tune the properties
of silk to maintain its strength and avoid supercontraction. Researchers are
already attempting to produce spidroin in the milk of transgenic goats
(This Week, 10 October 1998, p 11).
If they succeed, “tuning” the silk may be possible
by tweaking a gene or two. It may even be as simple as replacing one set of
amino acids with others that are slightly less hydrophobic.
Could silk be improved in other ways too? Yes, says Jelinski. “It’s up to us
to define what we need it for. If we need seat belts, for example, then we’d
want to optimise it to give a little bit, then stiffen.”
Viney believes understanding the chemistry of silk may ultimately yield a
still greater prize—a new class of environmentally friendly processing
technologies. “From an industrial point of view, it’s amazing that you can use
water as a solvent for something that ends up as being water insoluble,” he
says. “That’s one of the reasons we’re excited about it.”
Compare what the spider does with the process used to manufacture Kevlar, he
says. Making Kevlar requires complex organic chemistry, high pressures and hot
sulphuric acid. But clever physics like that used by spiders to spin silk could
serve the same purpose. “One could design a system, using conformational change
as a basis for solubility change, allowing you to use mild solvents like water,”
Viney says. “It’s something I’d like to see industry take up and run with.”
Such systems could use preprogrammed molecular jack-in-the-boxes that hide
water-hating, crystal-forming domains inside. Destabilise these molecules by
friction or elongational forces, et voilà!—the
crystal-forming domains pop out and the material self-assembles.
Kaplan, now at Tufts University near Boston, is already developing
recombinant spidroin proteins which incorporate steric triggers—bulky
projections which ensure that the protein coalesces out of solution only when
the manufacturing process is ready for it to happen. “We’ve shown that we can
control the formation of the [nanocrystals] using triggers,” he says. Kaplan
regulates his steric triggers by chemical oxidation and enzymatic activation,
but in principle, he says, you could design steric triggers to respond to any
signal—electric current, heat or simply light of a certain wavelength.
Studying how the spider makes its silk could give industry skills that it
could apply to similar materials. Spiders can produce silk over a 30 °C
temperature range—there are few industrial processes that can match that,
says Viney. Spiders also produce silk at different speeds. “The speed at which
they make a web is about one tenth the speed at which they spin silk when
they’re dropping down to get away from a predator,” says Viney. “There are very
few, if any, industrial processes where you can change a process variable by a
factor of 10 and still expect to get a sensible material at the end.” The secret
to this flexibility may be as simple as small imperfections in the structure of
the silk, he believes. Slight mismatches in the alignment of nanocrystals in the
protein chains may not significantly alter the physical properties of the silk,
but they mean that creating a fibre doesn’t depend on the time-consuming
business of building perfect crystals.
Spiders may even teach us something about how plants or our bodies are made,
suggests Kaplan. “Take cellulose and collagen, for instance,” he says. “There’s
certainly strong evidence that these go through liquid-crystalline phases. It’s
probably a fairly broad phenomenon in biology.” Embracing the spider’s
remarkable processing techniques will require time—at least five
years—but Viney, Kaplan and the others believe their efforts will be
richly rewarded.