From the Washington Post: "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. ... The possibilities for this kind of technology are limitless. Everyone has a mother or brother or uncle, aunt, grandmother who needs a meniscus or a kidney or whatever, and they want it tomorrow. ... The promise is exciting. The goal is not to squash that excitement, but to temper it with the reality of what the process is. ... The reality for now is that making such things as vertebral disks and knee cartilage, which largely just cushion bones, is far easier than constructing a complicated organ that filters waste, pumps blood or otherwise keeps a body alive. Scientists say the biggest technical challenge is not making the organ itself, but replicating its intricate internal network of blood vessels, which nourishes it and provides it with oxygen. Many tissue engineers believe the best bet for now may be printing only an organ's largest connector vessels and giving those vessels' cells time, space and the ideal environment in which to build the rest themselves; after that, the organ could be implanted."