UBC Reports | Vol. 50 | No. 3 | Mar.
4, 2004
UBC researcher freeing industry from a web of problems
By Michelle Cook
People have been using silk for more than 2,000 years but
scientists are still trying to unravel the mystery of its
strength and flexibility. One question that continues to stump
them is why spider silk contracts to almost half its size
when wet.
In a recent study, UBC physicists found the answer, and it
could help synthetic fibre manufacturers create better artificial
silk.
“One of the things that the people making silk-inspired fibres
should be able to control is supercontraction,” says Carl
Michal, an assistant professor who conducted the study with
PhD candidate Philip Eles.
“Can you remove it? Can you enhance it? Do you need it at
all? Can you tailor a material with that sort of property?
My feeling is ‘yes’ and this study identifies what parts of
the silk’s molecules [called polymers] are responsible for
the supercontraction. We’ve laid the groundwork for the people
developing these fibres to be able to control that.”
Spider silk is one of nature’s material marvels. Lightweight,
biodegradable and five times stronger by weight than steel,
it is one-tenth the width of a human hair but can snag a bee
flying at 32 km/hour without breaking.
For decades scientists have been trying to duplicate these
remarkable properties with a view to developing higher performance
sporting equipment, stronger nets and parachutes, more protective
clothing for police and military personnel, and improved sutures,
bandages, artificial tendons and ligaments.
While researchers already know a lot about spider silk’s
molecular architecture, opinions differ on why it supercontracts
when it comes into contact with water.
Spider silk undergoes an interesting transformation when
it gets wet. As it soaks up water, it swells in diameter but
shrinks to almost half its length.
Popular theory is that this supercontraction tightens up
a web weighed down by rain or morning condensation, helping
it to keep its shape. For this reason, some researchers claim,
supercontraction lasts for as long as conditions require it
to – that is, as long at the silk is wet. But other researchers
suggest that wet spider silk only shrinks to a certain point
then stops, and it has nothing to do with a sagging web.
Using a technique called nuclear magnetic resonance (NMR),
Michal and his research team studied the dragline silk of
the golden orb-weaver, a fist-sized arachnid from the tropics
that produces honey-coloured silk.
The orb-weaver uses its dragline as a frame for spider webs
and it also allows the spider to dangle and plummet down to
nab prey.
Dragline silk is made up of long polymers. When dry, the
polymers are solid and stationary. UBC researchers found that
when the silk absorbed water, it underwent a large-scale,
rapid molecular transition with some regions remaining solid
and other others collapsing into a rubbery state.
Picture strands of dry spaghetti that, when tossed into boiling
water, collapse and become pliable but don’t fall apart completely.
Silk molecules react in much the same way — only in room
temperature water.
“What we saw was molecules transforming from completely stationary
and static to liquid-like and rapidly tumbling,” says Michal.
“There was no in-between, no slow motions at all, and that
had not been recognized before.”
Michal says the UBC findings support earlier studies, providing
the clearest evidence to date of how supercontraction occurs
at the molecular level.
He hopes the research, funded by the Natural Sciences and
Engineering Research Council of Canada, will guide industry
in its ongoing quest for better synthetic silk.
“For a lot of the applications for silk-inspired fibres,
you really wouldn’t want supercontraction,” Michal says. “Silk’s
combination of strength and stretchiness make it fabulous
for something like a seatbelt, but you don’t want a seatbelt
to shrink in the rain or when you spill coffee on it.”
But will artificial silk be as strong as the real thing if
its ability to supercontract is removed?
Michal thinks so, but that’s for industry to figure out.
“The fibres that people are developing aren’t as good as
what the spider makes yet. They’re making progress and I’m
sure there will be trial and error as to how you remove some
properties without affecting others, but in some sense that’s
an engineering problem that industry has experience in solving.”
