The 2.5-ounce syringe slides deep into a injury, such a bullet track, and deposits pill-size sponges that soak up blood and rapidly expand to stem bleeding from an artery. Each sponge is coated with chitosan, a substance that clots blood and fights infection. The FDA says the sponges are safe to leave in the body for up to four hours, allowing enough time for a patient to get to an operating room. To ensure they don't get left inside a wound, X-shaped markers make each sponge visible on an x-ray image.