3D printing has been used in the health care field to make prosthetic limbs, custom hearing aids, dental fixtures, and other helpful tools for patients. The printing technology is now being used to create more complex structures, even human tissue. Bio-printers, for example, form human tissue using a "bio-ink" made of living cell mixtures. The structure is then built with the bio-ink, layer by layer, to build tissue. Eventually, researchers hope to use the bio-printing technology to create and replace organs.