The use of 3-D printers has opened up the possibility of on-demand implants, prosthetics, and medical devices. This week, scientists reported that they were able to 3-D-print the first stable ear, bone, and muscle structures out of living cells and implant them in mice. The results were published in Nature Biotechnology. Anthony Atala, the director of the Wake Forest Institute for Regenerative Medicine and an author on that paper, describes the challenges of 3-D printing living cells and how the technology could be used in bioengineering body parts.