The fibers that make up blood clots are more elastic than rubber bands and stretchier than spider webs. They're even tougher than doctors suspected _ a discovery that could lead to improved treatment of heart attacks and strokes.

Understanding how much these fibers can be stretched before they break should point to better ways to bust up blood clots on demand.

Made of a protein called fibrin, the fibers are stretchier than any other naturally occurring ones, even super-stretchy spider silk, concluded researchers who rigged up a double-microscope to measure how tough the tiny strands _ 1,000 times smaller than a human hair _ really are.

The discovery, published in Friday's edition of the journal Science, goes a long way toward explaining blood clots'Jekyll-and-Hyde persona: You need clots to seal up wounds, prevent hemorrhaging and start the healing process. But abnormal clots can kill, blocking critical arteries to cause strokes, heart attacks or lung-clogging pulmonary emboli.

"It can be good and bad that they're so stretchable,"noted Wake Forest University physicist Martin Guthold, one of the lead researchers."When they do form in the bad places, it's kind of difficult to get rid of them. ... You can rip on them, and they will just stretch out."

Already, he's talking with the maker of a device that uses ultrasound to attack clots, with hopes of improving its effect.

It's an important finding, said Dr. Richard Becker, a cardiologist and hematologist at Duke University Medical Center and a spokesman for the American Heart Association.

Aside from better clot-busting treatments, the work could lead to better ways to prevent dangerous blood clots in the first place _ and, on the flip side, to help blood clot better in people with hemophilia and other bleeding disorders, he said.

"We need to find ways to protect those individuals. By understanding the fibrin architecture, we can use that science in a protective way,"said Becker, who wasn't involved with the research.

Blood clots are a mesh of fibrin fibers bonded to platelets, a sticky substance in blood. To heal a wound, those clots have to be both strong and flexible, to withstand the pounding of regular blood flow, explained study co-author Dr. Susan Lord, a pathology professor at the University of North Carolina, Chapel Hill.

But until now, the fibers'small size had prevented pinpointing just how strong they really are. UNC and Wake Forest scientists came up with a solution. They dyed fibrin fibers to appear fluorescent, and suspended them over one microscope. Then they balanced an atomic force microscope, which senses tiny surfaces using a special tip, over the first microscope. The second microscope's tip stretched the fibers while the scientists measured from below _ and watched as the toughest fibers stretched to over six times their original length before breaking.

On average, the fibers stretched to about four times their length. They also contracted back to their original size after stretching, elastic like a rubber band.

But natural fibers are far stronger than manmade rubber bands, Guthold said. Consider: Spider webs trap flying insects without breaking because the silk absorbs the insects'energy. Researchers have long tried to duplicate spider silk as they make artificial fibers to improve such products as bulletproof vests _ and the fibrin fibers proved even stronger than spider silk.

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