Review of 4D printing materials and their properties

Since its introduction, the 3D printing technology has been widely used in fields such as design, rapid prototyping, and biomedical devices, owing to its advantages of inexpensive, facile embodiment of computer 3D files into physical objects. Later, 4D printing was introduced by adding the temporal dimension to 3D. Stimuli such as heat, humidity, pH, and light trigger the actuation of printed objects without motors or wires. Smart materials that respond to external stimuli are good candidates for 4D printing. In this paper, we review the recent research on 4D printing, and categorize it with respect to the activating stimuli. The mechanical properties of 4D printing materials are mentioned as well. Finally, the future of 4D printing is discussed.