The outer contours of this freshly printed ear may look complex, but they’re simple compared with the structure inside. The 3D printing system that crafted it, called integrated tissue-organ printer (ITOP), laces the artificial body parts with living cells, according to a study published online today in Nature Biotechnology. Researchers have printed with live cells before, but until now they only made tiny pieces of gelatinous living material, both because large structures tended to collapse and because the cells inside tended to die from lack of oxygen. Thanks to two innovations, ITOP can make life-sized body parts in which cells thrive. First, it interweaves a gooey, cell-friendly hydrogel with a stiffer substance that offers structural support. Second, it leaves tiny channels for oxygen to enter so that cells in the middle won’t suffocate. When researchers implanted ITOP-generated bone, muscle, and cartilage into rats and mice, the printed materials developed blood supplies and internal structures resembling those of natural tissue. The researchers are currently working with the Food and Drug Administration to set up human trials, with the ultimate goal of creating replacement body parts for people who need them.