Sequential shapeshifting 4D printing: programming the pathway of multi-shape transformation by 3D printing stimuli-responsive polymers

Four-dimensional (4D) printing is an emerging technology that integrates 3D printing and stimuli-responsive materials to fabricate reconfigurable 3D structures. Broadly speaking, the printed structures possess the ability to evolve their shape, properties, and/or function over time in response to an external stimulus. Compared to common 4D printing, sequential shapeshifting 4D printing not only defines the initial and final shapes, but also controls the shape evolution rate and pathway, serving as a powerful tool for reaching complex target geometries. After a brief introduction of the basic concepts in 4D printing and sequential shapeshifting, this review presents the current advances in sequential shapeshifting 4D printing from the viewpoint of their working approaches and is divided in five categories including multi-material assembly, multi-shape material, geometrical design, localized stimulus, and combinations of these approaches. A variety of 3D printing techniques and smart materials have been utilized to achieve sequential shapeshifting and its applications, which are reviewed in detail. Finally, the potentials and the future directions for improvement are discussed.