Bioprinting, illustrated here, is a promising technology for fabricating human tissues and organs. But unlike ordinary 3D-printed objects, which are ready immediately, bioprinted objects must undergo additional processes before they can be implanted. In bioprinting, a machine dispenses bioinks—materials made of cell aggregates. When deposited next to each other, the segments fuse and self-assemble into mechanically robust tissue. On page 38 , Ashkan Shafiee, Elham Ghadiri, and Robert Langer discuss the physics behind the self-assembly process. (Image by Ella Marus Studio/Science Source.)