Army scientists and engineers have developed a liquid body armor.
The key component of liquid armor is a shear thickening fluid. STF is composed of hard particles suspended in a liquid. The liquid, polyethylene glycol, is non-toxic, and can withstand a wide range of temperatures. Hard, nano-particles of silica are the other components of STF. This combination of flowable and hard components results in a material with unusual properties."During normal handling, the STF is very deformable and flows like a liquid. However, once a bullet or frag hits the vest, it transitions to a rigid material, which prevents the projectile from penetrating the Soldier's body," said Dr. Eric Wetzel, a mechanical engineer from the Weapons and Materials Research Directorate who heads the project team.
Reminds me of when I was a little kid playing with water and corn starch. The paste poured and your finger sank into it if you did it slowly. But fast movement made the stuff pretty rigid. Looks like the same concept here, but much more advanced.
Some links to scientific articles about this: