The promise of “liquid body armor” incorporating shear-thickening fluid (STF) has been on the horizon for at least seven years, ever since the technology was first developed at the University of Delaware. Now, BAE Systems says we may see the first models in use within two years.
Liquid body armor is liquid only in that a thick fluid is used in combination with conventional Kevlar fabric. The liquid is actually a suspension of ethylene or polyethylene glycol and very fine silica particles. Alcohol is added to make the solution less viscous, and then it’s poured between layers of Kevlar. The alcohol evaporates, leaving the suspension behind.
To explain the way the technology works, imagine that you stack iron cannonballs as a barrier in the street. If you drive your car into the barrier at high speed, the car will be destroyed. But if you nudge the pile with the car, moving slowly, you can probably dislodge the pile and move through it. STF works similarly. If something compresses the fluid compartment slowly, the silica particles move out of the way. When something like a fast-moving projectile compresses the fluid compartment rapidly, the silica particles bind against each other and form a rigid barrier.
Viscosity of Applesauce
In a beaker, STF has the apparent viscosity of applesauce. You can stir it slowly, but if you try to move anything through the fluid quickly, it will seize up and hold the stirrer rigid for a moment before it returns to its viscous state. This mechanism is sufficient to rob any fast-moving projectile of its kinetic energy and stop it cold.
Penetration tests have shown that four layers of Kevlar impregnated with STF have more resistance to projectiles than ten layers without STF, and are considerably more flexible than the ten layer thickness. The downside is weight. Four layers of Kevlar and STF weigh two percent more than ten layers of Kevlar alone, although it’s only half as thick and slightly more flexible than four layers of dry Kevlar.
Uncomfortable and Stinky
There is no indication that the STF degrades the Kevlar over time, but wearing a protective garment like this might not be very comfortable, especially in warm climates. To keep the STF inside the Kevlar, any carrier is going to have to be impervious to the STF, and that may make it effectively sweat-proof, too. Body armor has never been comfortable when it’s hot, but most models allow for some “wicking” or other moisture transport to keep the wearer relatively dry of perspiration. Wearing one of these garments might be very uncomfortable, not to mention stinky.
STF-layered armor may be the first practical solution to a problem that has plagued the military as long as there have been soldiers: injuries to extremities. As many as 80 percent of combat injuries are to extremities, where there is typically no more protection beyond that of the uniform sleeve or trouser leg. It may now be possible to outfit combat troops with protective gear that is light and flexible enough to move in, while offering protection from wounds to arms and legs.
Will we see this in the public safety market? BAE Systems — the large holding company that has Safariland among its subsidiaries — has licensed the technology from the University of Delaware and has been working on development for several years. The latest report is that they hope to have a product ready for deployment within two years.
