For over a decade, scientists have been growing organoids—small clusters of cells that mimic a particular organ—to serve as miniature biological models. Organoids of the brain have been used to study neurodevelopmental disorders; intestinal organoids, to model celiac disease; and lung organoids, to investigate SARS-CoV-2. Heart organoids have even been sent to space to test the effect of microgravity on cardiac muscle. But there's a tiny problem—the organoids can't grow any bigger than a sesame seed.