Water-filled pads are built into the vest and evaporate through the membrane to help cool down the wearer. Behind the pad, a little fan adds more ventilation.
There were a few challenges in making this getup. First of all, existing fans weren't small enough to be added to a vest. On top of that, cool pads would leak because of stress caused by the outfit's strong material. So Empa made its own tiny fans and discovered a technique for welding the pads to the vests with diode lasers to keep the material flexible.
People sweat to help bring down their body temperatures. But that natural human function is impeded by a dense Kevlar flak jacket. While it might prevent someone from being fatally harmed by a bullet, it's like wearing a sauna. When you're overheated, you're sluggish, and that's not a state conducive to catching criminals. Donuts already do enough damage. Police in Zurich have tested the chilly vest on warm summer days, and they've given it their stamp of approval. And who knows? If police are comfortable out their doing their jobs, they may be less likely to arbitrarily tase you. [LiveScience]