Scientists have developed a new 3D printing technique that can create soft materials programmed to either stretch or shrink when heated. Researchers have long been interested in soft materials that respond to temperature. However, 3D printing these materials has faced an important limitation. In a common technique called extrusion-based 3D printing, soft material is pushed through a small nozzle to create individual lines, or filaments. To understand why this happens, the researchers combined 3D printing experiments with measurements of how the ink flows, X-ray techniques that reveal molecular structures and computer simulations of molecular behavior.