Editorial: “Four-Dimensional (4D) Printing”

State Key Laboratory of Material Processing and Die and Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, China, Materials Research Group, Faculty of Engineering and the Environment, University of Southampton, Southampton, United Kingdom, Singapore Centre for 3D Printing, School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore

proposed a pre-strained strategy to accelerate the actuation of thermal responsive structures. A bilayer structure with one prestrained layer and the other non-pre-strained layer was integrally printed. Through experiments and finite element analysis, it was demonstrated that the pre-strain plays a key role in the deformation and also greatly accelerates the actuation of the bilayer structure.
Multi-material printing processes that can respond to diverse stimuli have become a research hotspot in 4D printing. Zhang et al. fabricated a typical hexagon-twist origami structure via multi-material printing technology. The printed structure has multi-stable features and the stiffness of the deformable structure is dramatically reduced under thermal triggering. The existing 4D printing technology is mostly subjected to only responsiveness to a single stimulus. To overcome this limit, Ren et al. proposed a 4D printing strategy with a dual stimuli-responsive shape-shifting structure that responds to both temperature and water. The structure was printed by direct ink writing (DIW) method to deposit a polyurethane elastomer material with water-swelling characteristics on a heat-shrinkage shape memory polymer material to form a bilayer structure. The multiple stimuliresponsive 4D printing method can effectively improve the intelligent responsiveness of 4D printed structures by combining multiple smart materials.
One of the research emphases of 4D printing rests with the achievement of the controllable changes in properties and functionalities of 4D printed components. To reflect this trend, this Special Issue has a collection of papers related to the changes in properties and functionalities based on sensing and energy conversion. Tang et al. summarized the recent advances of 4D printing towards soft tactile sensors, which combine the flexibility and the converting ability between mechanical forces and electrical signals. The review paper also discussed future perspectives for this emerging field. Liu et al. fabricated an underwater acoustic transducer by DIW using lead zirconate titanate (PZT) piezoelectric composites combined with furnace firing and resin impregnation. The electrical signal generated by the underwater acoustic transducer changed autonomously with the acoustic stimulation, which indicated the application mode of 4D printing in functional devices in the future.
In addition to artificial and active material and structural design of intelligent components, biomimetic design, which seeks inspiration from nature, is an extraordinary interesting method and worthy of in-depth exploration. Ren et al. published a review paper related to biomimetic 4D printing. By this novel method, they proposed morphing strategies and fabricated plant-inspired soft actuators. Plants can arouse inspiration because they exhibit ingenious adaptive motions that respond autonomously to environmental stimuli by varying cellular organization, the anisotropic orientation of cellulose fibers, and mechanical instabilities design, etc. This review summarized the morphing and actuation mechanisms of plants and concluded the recent development of 4D printed smart materials inspired by the locomotion and structures of plant systems.
4D printing is a rapidly growing research area. The collection of the papers in this Special Issue offers a snapshot of the attractive emerging field and future development trends. We hope this Special Issue can attract the attention of researchers from diverse disciplines. Finally, on behalf of the Editorial Board, we sincerely thank the contributors and staff at Frontiers, without whom this Special Issue would not have been possible.

AUTHOR CONTRIBUTIONS
KZ and CY write the first draft; YS and SY revise and finalize the manuscript.
Conflict of Interest: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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