Recent progress in 4D printing of stimuli-responsive polymeric materials

4D printing is proposed based on the additive manufacturing of stimuli-responsive materials and structures, which can realize shape changing upon external stimuli. This article reviews the 4D printing methods and actuating performances of 4D printing structures based on shape memory polymers, hydrogels, liquid crystal elastomers, and electroactive polymers. This article shows that the shape morphing properties of single materials are limited, while 4D printing of composites can integrate the various driving modes of different smart materials. In the end, challenges facing 4D printing such as broadening the scope of smart materials, improving printing processes, the compatibility of printing different materials have been discussed.