Thrombin, on the other hand, is already used by the body to stop bleeding. Civilian hospitals also use it already, but it's in liquid form so sponges must be soaked immediately before they are applied to the wound, making them impractical for the battlefield. MIT's sponge instead uses a spray-on biological nanoscale coating using alternating layers of thrombin and tannic acid, which results in a film that contains a large amount of functional thrombin with a shelf life that makes it feasible to pack them into the field. Both substances are already FDA approved, the researchers say, which means the sponges could quickly find their way into wider use.