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Printing isn't the only promising new technique for creating entire organs. It might one day be possible to grow them in situ. In December, scientists in Israel reported that they had managed to grow miniature but fully functional kidneys by implanting fetal pig or human cells into immunodeficient mice (NatureMedicine, DOI: 10.1038/nm812). But growing organs from scratch will take much longer than printing them, Mironov says. “Patients don't always have the luxury to wait.”

The printers are adapted by washing out the ink cartridges and refilling them with suspensions of, say, cells. The software that controls the viscosity, electrical resistances and temperature of the printing fluids is reprogrammed and the feed systems altered.