# 3D and 4D Printing of Multistable Structures
## 4 Conclusions and Future Prospects
We first discussed three representative approaches in 3D printing of multistable structures. Then, we presented 4D printing of multistable structures designed for smart actuators and soft robots. Mechanical multistable structures have characteristics superior to conventional structures in several ways. They can allow reconfigurable shapes and remain in a stable state unless an external stimulus overcomes the structure's energy barrier. Moreover, mechanical multistable structures can enable more accurate motion control because motion can be directed to energetically favored stable states. Furthermore, it is possible to activate structures more rapidly via snap-through transitions. They can also show stimuli-active responses by adopting active materials into multistable structures. Therefore, multistable structures are expected to play an important role in 3D and 4D printing applications. Three-dimensional printing has also been considered for biomedical applications, such as scaffold [3; 4; 5; 6], splint [67], and stent [68]. Stimuli-responsive and biocompatible materials (such as hydrogels) were also 3D-printed for drug delivery [69]. Various 4D printing techniques could be helpful for releasing drugs at a precise target position. With the development of biocompatible printing materials, 4D printing is becoming attractive to bioprinting and biomedical applications [70; 71; 72]. Moreover, 4D bioprinting has an advantage in constructing 3D complex tissue structures based on responsive materials, which can reconstruct or alter their function upon external stimuli. In addition, 4D bioprinting can generate printed cell populations with programmable scaffold structures. It could be applicable to tissue engineering, such as dynamic tissue healing and regeneration for the injured human body. Multi-stable structures can be potentially useful for these biomedical applications too, because they can provide large-magnitude, highly controllable shape changes. Various actuation and reconfiguration schemes can be considered for these applications. Table 1 summarizes the multistable 4D printing schemes considered in this paper. It helps in understanding the current status and challenges for multistable 4D printing. By adopting smart materials in multistable structures, more advanced functionalities and enhanced controllability can be realized. Multistability could also help in increasing the magnitude of actuation force as well as the load bearing capacity. We also expect that the concept of 3D-printed multistable structures which we have discussed in this article could be extended to the micrometer scale too. Sub-micrometer patterns can be created using direct laser writing with two photon absorption [73; 74; 75; 76]. In addition, stimuli-responsive materials can be utilized in direct laser writing too [8; 77; 78; 79; 80]. This could lead to interesting research opportunities for micrometer-scale multistable structures, including applications in biomedical devices. These microscale multistable structures can be actuated by various external stimuli (heat, light, magnetic fields, etc. ),and this could be a very interesting way to realize more complicated functions at the micrometer scale. Therefore, we think there are abundant new opportunities to explore further. **Author Contributions:** All authors contributed to manuscript writing. All authors have read and agreed to the published version of the manuscript. **Funding:** This research was funded by National Research Foundation (NRF) of Korea (2018R1E1A2A02086050,) and Ulsan National Institute of Science and Technology through the Development of 3D Printing-Based Smart Manufacturing Core Technology Research Fund under grant 1.200069.01. **Conflicts of Interest:** The authors declare no conflict of interest.