So the team tried a new idea: sponges. They bought some ordinary sponges from a hardware store and cut them into 1-centimeter circles, a size and shape they chose on a whim but later would discover were ideal for filling wounds. Then, they injected the bits of sponge into an animal injury. “The bleeding stopped,” says Steinbaugh. “Our eyes lit up. We knew we were onto something.” After seeing early prototypes, the U.S. Army gave the team $5 million to develop a finished product.

But kitchen sponges aren’t exactly safe to inject into the body. The final material would need to be sterile, biocompatible, and fast-expanding. The team settled on a sponge made from wood pulp and coated with chitosan, a blood-clotting, antimicrobial substance that comes from shrimp shells. To ensure that no sponges would be left inside the body accidentally, they added X-shaped markers that make each sponge visible on an x-ray image.