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Tim Dees
Editor-in-Chief
Officer.com

One of my last columns when I was the technology editor for another law enforcement magazine dealt with the development of “liquid body armor.” This innovation has surfaced again in a report from Business Week, and with all the attention it’s getting, the new invention is apparently not as well-known as I thought.

The substance being called “liquid body armor” is actually an enhancement to existing soft body armor materials, such as Kevlar, Spectra fiber, and Dyneema. It’s a surprisingly simple compound made of polyethylene glycol and silica. Polyethylene glycol (PEG) is contained in human waste, but is also used in laxatives and in some surgical applications. Silica is, basically, sand. The silica particles used in this new compound are micron-sized, meaning that you would need not quite four million of them, laid side by side, to make an inch.

When the silica particles are mixed into the PEG in the appropriate ratio, the result is a thick, mucous-like mixture that has the apparent viscosity of a milkshake. You can stir it with plain old muscle power, as long as you stir slowly. If you speed things up, the liquid seizes and becomes rigid for a moment, easily long enough to stop your momentum. The mechanism is pretty easy to understand. Stirred slowly, the silica particles flow around the stirrer and allow it to pass through the mixture. But as the speed of the stirrer increases, the silica grains can’t get out of each other’s way fast enough, and the mixture seizes until the flow is restored. This is called a “shear-thickening liquid” (which Business Week called “sheer thickening”) or STF, the shear being the movement of the object through the liquid.

The body armor application comes when the STF is added to standard body armor made of soft fibers. Ballistic fiber can be cut with a pair of scissors, but it is much more difficult to break, having a tensile strength greater than an equivalent strand of steel. This helps stop blunt objects like bullets, but is not as much help with cutting instruments or needle-like weapons (like ice picks) that can focus the force on a much smaller surface area and push the fibers out of the way at the same time. With the STF mixture added to the armor, a fast-moving piercing object is slowed considerably, robbing it of its cutting force and giving the wearer time to react without being wounded. The applications for street police officers and military combatants are obvious, but it’s even better news for prison and jail officers, who face more cutting and piercing instruments in the hands of their ethically different, morally challenged clients of the correctional system.

The actual application may not require the STF to remain in the thick liquid state. An abstract of a paper published on the web site of the Nano Science and Technology Institute (and available for only $185.00 — let me know if it’s a good read) indicates that the STF is impregnanted into the ballistic fiber, and then the ethanol in the mixture is evaporated, reducing the liquid volume. The result is an armor system that is much more penetration resistant than ballistic fiber alone, while adding only about 20 percent to the weight of the system.

Will this be available soon? I wouldn’t count on an STF-enhanced vest being in your Christmas stocking, at least not this year. But it wouldn’t surprise me to see STF body armor products being rolled out at IACP in 2007.