Keep spinning, worms (Image: Yuji Sakai/Digital Vision/Getty)
The marvellous stuff that spiders and silkworms make has a big future in technologies from artificial corneas to brain implants
Chris
Holland kicks a bucket of rotten fruit. "It smells like death in here,"
he says cheerfully as a cloud of tiny fruit flies swarms up to its
fate. We're inside a glasshouse that would have any arachnophobe
quaking. As many as 100 giant spiders are hanging from the ceiling,
lurking on branches and lolling about on enormous webs. With their legs
outstretched, they are the size of an adult's palm, their bodies the
size of your thumb. Littering the webs are remnants of larger meals –
the sucked-out husks of bluebottles.
The
spiders are golden orb weavers and their bright yellow webs are some of
the largest and most impressive of any arachnid. For researchers who
study silk, like Holland at the University of Oxford, orb weavers are
ideal because of their large webs and the fact that they are easy to
handle. "They make great experimental subjects," says Fritz Vollrath,
who heads the Oxford group.
Weight
for weight, a typical spider silk is 20 times as strong as steel and
four times as tough as Kevlar. It is also extremely flexible,
stretching up to 50 per cent of its length without breaking. And it's
not just the silk's physical properties that are impressive. It elicits
no immune reaction in our bodies, it is biodegradable, and it is
produced at low temperatures and pressures relative to other polymers.
This is surely the kind of material that all of us, and not just our
favourite fictional superheroes, could put to good use.
The reason we don't is that spider silk is incredibly
labour-intensive to exploit. The only textiles made of spider silk that
we know of are a rug and cape on display
at the Victoria and Albert Museum in London. The rug is little more
than 4 metres square, but its production involved 80 people spending
five years collecting and weaving the silk from a million golden orbs.
Yet
the promise of silk is too inviting to ignore, which is why several
biotech companies and research groups have sprung up recently to
develop devices and applications using the stuff. Much of their
research has been done with silk from silkworms, which has a structure
similar to that of spider silk and is much more plentiful. Even so, to
make the most of silk's properties on an industrial scale we need to
understand how these creatures make silk and then copy the process
wholly or in part, to make new polymers. Here there has been
significant progress too.
Humans
have been trying to tease out silk's secrets for more than five
millennia. The Chinese were the first to cultivate silkworms and
enjoyed a monopoly on the resulting, much-sought-after cloth for more
than 3000 years, establishing the Silk Road trade routes throughout
Asia and Europe. Their selective breeding led to the Bombyx mori
silkworm, whose cocoons are far larger and easier to unravel than those
of wild counterparts. "These things are essentially the cows of the
insect world," says Holland. "They have been inbred to produce huge
quantities of silk."
A safe distance from the spider glasshouse, Holland
keeps a huge box of silkworm cocoons in his office. They look like
fluffy polystyrene packaging chips. "See if you can make a dent in it,"
he says handing me one. I can't, even though it is only half a
millimetre thick. The cocoon offers perfect protection from such
dangers as bacteria and hungry birds.
Before
they make a cocoon, the silkworms are fattened up on a diet of
mashed-up mulberry leaves. Eating almost continuously for a month, they
are about 10,000 times heavier by the time they are ready to pupate.
The worms then anchor a thread onto a convenient mulberry branch and
pull their heads back, reeling the silk from a tiny hole in their
mouths. As they move their heads in a figure-of-eight configuration,
they reel up to a kilometre of silk for their cocoon, starting with the
outermost layer and working inwards until they are locked in their
protective home.
Since Bombyx mori
silkworms are cultivated purely to satisfy our desire for their
shimmering wares, only a few are allowed to metamorphose into moths to
lay the next generation of eggs. The rest of the cocoons are collected
and plunged into boiling water. This kills the silkworms and removes
the sticky protective glue that holds the cocoons together. Workers
then scrape each cocoon to find the end of the filament, which is
threaded onto a bobbin. It takes several filaments wound together to
make yarn.
These fibres are not stored like a fishing reel inside the silkworm's body. Bombyx mori
produces its silk from two glands that add up to a third of its body
weight. The silk itself is a protein, made of a long chain of repeating
amino acids. Inside the glands the silk has the consistency of jelly,
but when it is time to make a cocoon, the jelly travels through
specially shaped ducts that align the protein chains and strip away
water molecules, converting it into solid fibres. These fibres are then
covered in sericin, a protein glue that binds the strands coming from
the two ducts before they leave the worm through a single "spigot" in
its head.
It is the structure of the thread that gives silk its impressive properties, which even let it protect soldiers' genitals (see "Military-grade smalls").
Silk is made up of protein crystals spread throughout a matrix of
haphazardly organised protein chains. The strong hydrogen bonding in
the tiny crystals gives the silk its strength, while weaker bonds in
the protein chains impart toughness.
If
silkworms are the industrial powerhouses of the silk world, then
spiders are its model citizens, which is why Vollrath's group studies
them so keenly. For a start, the dragline silk of a spider – the thread
it dangles from – is entirely uniform and round in cross section,
whereas silkworm thread varies in thickness with the stages of the
worm's figure-of-eight dance. What's more, spider silk is typically
four times as strong and can be extracted under carefully controlled
conditions. "We bring a spider down from the roof and either reel a
silk fibre from it, or extract the silk from a removed gland," Vollrath
says. "You can reel long lengths of fibre, controlling for humidity,
temperature and silking speeds."
Using
laser light and X-rays to study the molecular structure of the fibre,
his team can observe how different spinning conditions alter the
thread. This is essential if we are one day to create synthetic silk as
good as the real thing.
Until then, researchers have to make do with various
types of artificial silk, the most common of which is known as
"reconstituted silk". It is made by boiling silkworm cocoons to remove
the sericin coating and then soaking them in chemicals to break down
the protein chains into their original units. This is essentially
reverse-engineering the fibres back to their original state, says
Cedric Dicko at Lund University in Sweden.
The
silk solution can then be spun using a variety of techniques depending
on the type of thread required, or poured into moulds, cast into
transparent films, or freeze-dried to make a sponge or foam. The
advantage of reconstituted silk is that you can effectively reprogram
the silk, rather like uncooking a cooked egg, allowing you to make it
from scratch exactly as you want it, Dicko says.
Unfortunately,
reconstituted silk is far more energy-intensive to produce, and can't
match the real thing. No one understood why until Dicko and his
colleagues compared the structure of the jelly-like proteins from
reconstituted silk and a silkworm gland by bombarding them with
neutrons at the Institute Laue-Langevin
in Grenoble, France. The team showed that the processing involved in
making reconstituted silk destroys the structure and bonding of the
protein chains that gives natural silk its strength and toughness (Soft Matter, vol 6, p 4389).
The result is "like chicken wire", says physicist David Porter at the
University of Oxford, and has only one-fifth the strength of natural
silk.
An
alternative method is to genetically engineer other organisms to
produce the silk protein. In recent years, goats, potato and tobacco
plants, and Escherichia coli bacteria have all been enlisted,
having had the silk gene implanted into their genomes. But again, the
nanostructure of the protein is not preserved.
For
researchers who are interested in silk for reasons other than its
mechanical properties, this isn't too much of a problem. "If you need
high-strength, high-performance fibres then you need to be as close to
the native process as possible," says David Kaplan,
a bioengineer at Tufts University in Medford, Massachusetts, who has
been researching silk for more than 20 years. "For most applications we
are working on, you don't need to be near that. Most of what is needed
is covered by what can be made today." For Kaplan and his colleague Fiorenzo Omenetto, also at Tufts, the most enticing aspects of silk are its optical and biological properties.
The perfect scaffold
Omenetto,
an optical physicist, got involved five years ago when Kaplan asked him
to laser-cut a film of artificial silk to create synthetic corneas. The
material was like nothing Omenetto had seen before. "It looked
unusually, amazingly transparent," he recalls. And it was something of
a eureka moment for Omenetto. He has since developed ways to pattern
silk films and has made holograms, lenses, sensors and diffraction
gratings from the stuff (Nature Photonics, vol 2, p 641).
Silk's
failure to produce an immune response, plus the fact that it
biodegrades, mean it has a huge range of possible applications in the
body. In 2004, Kaplan demonstrated that stem cells are happy to grow
around the stuff, so a silk sponge could in theory be used as a
scaffold to help mend broken bones or torn muscles (Biomaterials, vol 23, p 4131).
Omenetto
and Kaplan have also shown that enzymes and proteins continue to
function when embedded in silk, and that the material can be engineered
to release its payload after a delay lasting anything from a few
seconds to a year, just by tweaking how fast it will dissolve. That can
be done during the production process, making silk a great candidate
for novel drug delivery systems, says Omenetto. "You could imagine
having a cup that stores antibiotics. You could pour water into it and
dissolve the first layer of the cup to take your medicine." Other
colleagues at Tufts are looking into silk implants doped with drugs for
people who are on long-term medication (Expert Opinion on Drug Delivery, vol 8, p 797).
Together with John Rogers,
a materials scientist at the University of Illinois at
Urbana-Champaign, and others, the Tufts researchers are also developing
"meltable electronics" designed to become part of the fabric of living
tissue. Last year, they demonstrated that silk could be used to deliver
ultra-thin electronics directly onto the surface of the brain, a
capability which could one day be used to diagnose epilepsy or improve
brain-computer interfaces. Silk films offer a much more useful surface
on which to embed electronics than traditional silicon wafers as they
can conform to the contours of the brain without damaging tissue. The
idea is that once in place, the silk is dissolved with salt water and
broken down by the surrounding tissue. Capillary forces between the
silk and brain tissue help the electronics to wrap around the brain (Nature Materials, vol 9, p 511).
For
Kaplan, the most interesting discovery of the last two years is that
silk can become sticky at the flip of a switch. "It's an unusual
property you don't find in other polymers. You can turn silk into a
sticky gel that adheres to tissue just by switching on an electric
field. Reverse the field and you can turn it back into an unsticky
solution," he explains. This property was totally unexpected. "After
all these years, silk is still surprising and impressing me."
The
team is still working out exactly what drives this reversible
stickiness but they can already see an application. "A situation that
springs to mind is that the gel could be pumped into someone who is
bleeding en route to hospital. When they arrive, it could be reversed
so the surgeon can open the person up and get to the site of the
injury," says Kaplan.
So
in years to come, we may be grateful to silkworms for something a lot
more precious than our silk shirts and underwear. We may be thanking
them for saving our lives.
Military-grade smalls
To
you, silk underwear might be a little touch of luxury. Not so for the
British, American and Australian troops in Afghanistan. For them, they
are an essential piece of kit. "The first thing a soldier tends to ask
when they have been injured is 'are my bits OK?'," says armour
scientist Simon Holden. "Damage to their groin area can really affect
their quality of life, both physically and psychologically."
Of
all the fabrics to fashion underpants from, it seems a traditional
favourite is best. "Silk is lightweight so it doesn't impede a
soldier's movement, and it can stretch a lot before it yields," says
Holden, who is at the Defence Science and Technology Laboratory
in Porton Down, UK. Because it can absorb an impressive amount of
energy, silk helps reduce the severity of injuries from bomb blasts,
especially the small-scale devices often used against troops in
Afghanistan. Small pieces of shrapnel are unlikely to pierce the silk,
and if they do embed themselves in the groin, the silk is pulled in
with them. This means fragments can be extracted simply by pulling on
the surrounding material.
http://www.newscientist.com/
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