Perhaps ranking behind only bullets and water, blood is one of those things you really don't want to run out of on the battlefield. But better battlefield medicine -- as well as some of the more malicious combat techniques employed by insurgent guerrilla fighters -- mean more soldiers are surviving their injuries, and that puts military blood banks in a bind. But a DARPA program launched in 2008 is coming to fruition, potentially providing medics an endless stream of universally accepted O-negative blood through a process known as blood pharming.