4D printing: The spotlight for 3D printed smart materials

4D printing combines the typical 3D printing with “smart materials”, allowing 3D printed materials to undergo a structural change over time. Since its original concept was first introduced in 2013, 4D printing became an innovative research that has received more attention from scientists in different fields. This review summarizes the progress achieved in 4D printing technologies and their associated materials. First, the technology and process of 4D printing are overviewed, and then the structure and properties of smart materials utilized in 4D printing are analyzed in depth, including metamaterials, shape memory materials, hydrogels, and self-healing polymers. We systematically illustrate the morphing mechanisms of the 4D printed smart materials, and then critically discuss the stimuli that can trigger transformation in the 4D printed smart materials, including heat, light, moisture, pH, electric current, and magnetic field. For 4D printed smart materials, all the changes programmed in the materials follow a mathematical model that allows scientists to predict and design the desired behaviors of the structures, using parameters such as the material distribution and the spatial gradients of the metric tensor. We finally conclude with the discussion of future challenges and opportunities for this ever-growing technology. Overall, 4D printing can create dynamic structures programmed to be responsive to external stimuli in the environment, widening its use in a myriad of applications such as rapid prototyping, electronics, biomedicine, soft robotics, self-assembly structures, smart sensors, and dynamic actuators.