4D printing: Fundamentals, materials, applications and challenges

4D printing refers to single-material or multi-material printing of a device or object that can be transformed from a 1D strand into pre-programed 3D shape, from a 2D surface into preprogramed 3D shape and is capable of morphing between different dimensions. Such transformations are facilitated by, e.g., heating, light, or swelling in a liquid, electrochemically and by programming different sensitivity to, e.g., swelling into various parts of...