More than 1,000 people in the U.S. died while waiting for a liver transplant in 2018, partly because standard preservation methods can keep a donor liver alive outside the body for only about 24 hours. But now, in a feat of medical engineering, scientists have developed a machine that can keep a liver functional for a week or more. It has not yet been used for human transplants, but the technology represents a leap forward in the field of organ preservation.