Reversible 4D printing

While research in 4D printing grows, a new demand—reversibility—has been raised for 4D printing to cater to real-life applications’ needs. Reversibility refers to the “two-way” shape memory that switches between two states by different stimulations. Most reported conventional 4D printing-related works only possessed “one-way” shape memory. The printed part must be programmed manually into one state and then is recovered to the original shape upon stimulation. Reversible 4D printing aims to achieve a stimuli-based transformation for both programming and recovery. Several methods and combinations of stimuli could achieve it. For example, it allows a 4D printed flower to simulate blooming and closing back into a bulb or function as a switch to connect and disconnect circuits using external stimuli. In the current chapter, the successful examples of reversible 4D printings are discussed, and their mechanisms are examined to provide an insight into how reversibility could be accomplished in 4D printing.