4D Printing for Soft and Bio-Inspired Robotics: Progress and Challenges

4D printing, a term referring to 3D-printed structures with shape-shifting capability in response to external stimuli, such as thermal energy, electro-magnetic fields, and light energy, has attracted extensive research interest in recent years. Utilizing 4D printing, soft robotic components, including soft actuators, can be rapidly fabricated without the need for additional assembly of actuators. Although 4D printing technology has shown great potential in soft robotic research, certain challenges remain to be addressed, such as limitations of 3D printing devices, materials, and lack of modeling/control mechanisms, before its further applications in soft robots. In this paper, we first summarize the current 4D printing materials and approaches being applied in soft and bio-inspired robots. Then, current and potential applications of 4D printed soft robots are demonstrated. Finally, challenges of 4D printing to further boost soft robotic development are highlighted and an outlook for future research is presented.