The machine looks like the offspring of an Erector Set and an inkjet printer. The "ink" feels like applesauce and looks like icing. As nozzles expel the pearly material, layer by layer, you imagine the elaborate designs this device could make on gingerbread cookies. But the goo is made of living cells, and the machine is "printing" a new body part. These machines -- they're called three-dimensional printers -- work very much like ordinary desktop printers. But instead of just putting down ink on paper, they stack up layers of living material to make 3-D shapes. The technology has been around for almost two decades, providing a shortcut for dentists, jewelers, machinists and even chocolatiers who want to make custom pieces without having to create molds. In the early 2000s, scientists and doctors saw the potential to use this technology to construct living tissue, maybe even human organs. They called it 3-D bioprinting, and it is a red-hot branch of the burgeoning field of tissue engineering.