The concept of 4D printing isn't a new idea, but processes that use new types of environmental stimuli keep on appearing. 4D printing is usually defined as 3D printing with materials that change shape in response to light, heat, water, air pressure, or other factors. It usually combines 3D printing with shape-memory materials.
The latest is a new process developed by a team at Lawrence Livermore National Laboratory (LLNL). Researchers there have 3D printed shape-shifting structures that can fold and unfold themselves, or expand and contract in size, when prompted by changes in electricity or heat. The primary shapes they created were printed with shape-memory polymer inks developed by the team using a direct-ink writing 3D printing process.