Mechanics-based design strategies for 4D printing: A review

Four-dimensional (4D) printing is an emerging technology that aims to endow the additive manufactured objects with the capability of time-dependent transformation. In particular, it provides a flexible and efficient manufacturing strategy by evolving flat precursory patterns into three-dimensional (3D) structures with complex geometry. To equip the precursory structures with desired transformation, it is required to use the principles of solid mechanics to intelligently pre-embed mismatch deformations that can be later activated by external stimuli. In this sense, 4D printing can be regarded as a mechanics-based manufacturing approach relative to conventional fabrication techniques. In this review, we classify the reported 4D printing works from the perspective of mechanics principle that yields 2D-3D transformation. We comprehensively comb the underlying interconnections among different 4D printing strategies, through which readers are expected to obtain a clear picture of how 4D printing is developed step by step. Finally, challenges of current 4D printing approaches are suggested from the aspects of mechanical design, printing techniques and material synthesis.