Every year, thousands of people die waiting for a new heart or kidney for transplant. 3D printing jelly-like biocompatible ‘inks’ laced with living cells could create new organs — from a pancreas to a retina — on demand.

Scientists have already used 3D bioprinting to build new cartilage and skin for lab rats. Cartilage is the easiest tissue to work with, since it doesn’t need a blood supply, so in a few years, people with bad knees might go to the hospital and have new joints printed directly into their bodies. Hollow tubes — arteries or urethras — will come next, followed by hollow organs, such as bladders. Harvard’s Jennifer Lewis has 3D printed tissues with vasculature and kept it alive for weeks. That points the way to building complex organs, with many types of cells interspersed with blood vessels. Then when someone needs a new heart, finding one will be as easy as pressing ‘print.’