Ultrastrong spider's silk no longer a huge stretch

Army, firm create man-made fiber

January 18, 2002|By Michael Stroh | Michael Stroh,SUN STAFF

Stronger than steel and lighter than cotton, spider silk has long been the envy of engineers and a source of frustration for scientists trying to conjure it in the lab.

But in a breakthrough that could pave the way for new ultrastrong and wear-resistant materials, scientists with the U.S. Army and a Canadian biotech firm have created a man-made spider silk that's almost as good as the real thing.

"We didn't believe it at first," says Steven Arcidiacono, a molecular biologist at the U.S. Army Soldier Biological Chemical Command in Natick, Mass. "There have been so many disappointments."

The feat, described today in the journal Science, was accomplished by inserting spider silk genes into cells of hamsters and cows and squeezing the resulting proteins into a gossamer fine thread.

People have been trying to produce mass quantities of spider silk since the late 19th century. But early "spider farms" quickly flopped. Unlike docile plant-munching silkworms, spiders, entrepreneurs soon discovered, are territorial carnivores.

"It's like farming tigers, you put them in a big pen and they eat each other," says Jeffrey Turner, chief executive officer of Nexia Biotechnologies Inc., the Montreal firm that partnered with the Army to create the silk.

Efforts to re-create silk in the lab also progressed slowly. In the 1960s, scientists began to unravel some of spider silk's secrets. Biochemical studies revealed that the fibers were composed of long, repetitive chains of amino acids, which researchers speculated gave the fibers their strength and flexibility. But scientists "really couldn't do anything with that knowledge," says Arcidiacono.

It wasn't until the first spider silk gene was decoded in 1990 that efforts to make spider silk from scratch started to heat up.

The Nexia-U.S. Army team used silk-making genes isolated from two species of orb-weaving spiders, Araneus diadematus and Nephila clavipes.

Orb weavers, the spiders responsible for the cobwebs found in gardens and haunted houses, make silk so rubust they're known as "the Arnold Schwarzeneggers of the spider world," says Turner. Genetically, they're also some of the most-studied spiders around.

Orb weavers spin seven varieties of silk, each designed for a specific function, says Cheryl Hayashi, a biologist at the University of California at Riverside. Webs, she says, require three silks alone.

Nexia scientists chose to make "dragline" silk, which spiders use to frame webs and as a lifeline when they bungee jump. The strongest of the seven silks, dragline has the kind of technical specifications that make engineers drool: Ounce for ounce, it's three times tougher than Kevlar, the man-made fiber used in bulletproof vests, and five times stronger than steel.

"A fiber the size of our thumb would have a breaking strength of 300,000 to 500,000 pounds," notes Turner. A fiber that thick, on paper at least, could lift more than two fully loaded Boeing 737s.

Although plane lifting isn't one of the applications scientists imagine for artificial spider silk, they do talk about wear-resistant clothing, stronger surgical sutures, artificial tendons and ligaments - even the world's toughest fishing line.

The other thing that impresses engineers about spider silk is how it's made. Unlike man-made fibers such as Kevlar, which require intense pressure and poisonous sulfuric acid to produce, spiders manufacture silk at room temperature, under normal pressure, and using little more than proteins and water. In other words, spider silk is environmentally friendly.

To make silk, Nexia scientists first inserted silk-making genes into cells from hamsters and cows, which squirted out silk protein about the thickness of maple syrup. The Army team, which has been working on spinning techniques for nearly a decade, then set about turning the sap into silk.

In reality, spiders don't spin, they squeeze. Liquid silk protein is forced through sets of nozzles called spinnerets and emerges as wisps of steely fiber. The Army team tried to mimic nature by filling a syringe with the protein solution and squeezed it through a tube. The tube - called a Charlotte after the spider in the book Charlotte's Web - was so narrow "you could barely see the hole," says Arcidiacono. The thin filament that emerged was then stretched, something that spiders do with their legs. Stretching, he says, helps "all the silk molecules to line up" and imparts strength.

In tests, the man-made fiber turned out to be a little stretchier than the real thing and not quite as strong. It's unclear whether the spinning technique or genetic recipe is to blame. The Nexia team used only one of the two genes spiders use to make dragline silk.

Still, "they're substantially better than anything we've made," says Randolph Lewis, a molecular biologist at the University of Wyoming who decoded the first spider silk gene. "They're halfway there."

Nexia is working on figuring out how to spin spider silk in commercial quantities. Their factory is likely to be a flock of goats genetically engineered to carry the spider silk genes and manufacture the protein in its milk. Nexia scientists say they've begun work on extracting the silk proteins from goat milk and spinning it.

"Spiders have been doing this for 400 million years," says Turner. "The spider has a lot of tricks, and obviously we have a lot to learn."

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