By BRIAN WITTE, Associated Press Writer

BALTIMORE - Army scientists are working on a liquid body armor for clothing that stays flexible during normal use but can harden to stop a projectile when hit suddenly.

Researchers hope the liquid could be used in sleeves and pants, areas not protected by ballistic vests because they must stay flexible.

The liquid, hard particles suspended in a fluid, is soaked into layers of Kevlar, which holds it in place. Scientists recently had an archer shoot arrows at it to see how well the liquid boosted the strength of a Kevlar vest.

"Instead of the arrow going through the Kevlar, it is completely stopped by the Kevlar vest and sometimes just bounces right off," said Norman Wagner, a University of Delaware chemical engineering professor who is working on the project.

Vests treated with the liquid have also blocked stabs from an ice pick, and researchers are doing more tests to see if it can stop bullets or shrapnel, too.

The project, which has been under way for about three years, is a joint venture between the U.S. Army Research Laboratory and the University of Delaware's Center for Composite Materials.

Eric Wetzel, a mechanical engineer who heads the project team for the Army lab, said a "shear thickening fluid" is a key component of the liquid armor. Hard particles are suspended in the liquid, polyethylene glycol. At low strain rates, the particles flow with the fluid, enabling clothing to stay flexible. But when heavily strained, the particles become rigid.

"If it's impacted suddenly by a projectile or a knife, say, it rigidifies and somehow restricts the ability of the fabric to move," Wetzel said.

The transition happens very quickly, a millisecond or quicker.

Wetzel and Wagner are optimistic the liquid body armor will be useful to local police and prison guards and perhaps it could one day protect people in automobile and airplane crashes.