This summer, scientists demonstrated the first functional replacement lung in a rat. The technique follows what's becoming a standard practice in regenerative medicine, but the lung is the most complex organ copied so far. Biomedical engineer Laura Niklason's group at Yale University removed a rat's lungs and stripped away the cells, leaving an inanimate scaffold of collagen and other connective tissue. They seeded the scaffold with malleable lung cells harvested from other rats, which multiplied to form new airways and blood vessels. After just eight days of growth, the lungs were transplanted into yet other rats, where they oxygenated blood 95 percent as well as a normal, healthy organ.