UBC Reports | Vol. 50 | No. 3 | Mar. 4, 2004

Solving the Mysteries of Spider Silk

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."