Digital light processing 4D printing multilayer polymers with tunable mechanical properties and shape memory behavior

Four-dimensional (4D) printing has currently been renowned as a world-changing technology which has recently gained significant attention amongst researchers. It enables 3D printed objects to alter its shape, properties and functions through the application of external stimuli. However, the development of 4D printing has been impeded by the overexposure effect, which has made it difficult for photocuring to print gradient structures. In this paper, we propose a novel photocuring 4D printing strategy of gradient structures with tunable properties. Multi-walled carbon nanotubes (MWCNTs) are added to the intermediate layer (Fast Layer, FL) of multilayer shape memory polymers (SMPs) to reduce curing degree and absorb overexposure. The mechanical properties and shape memory behavior of multilayer SMPs can be tuned by adjusting the layer ratio of FL with low curing degree. Outcome of microscopic images, interlayer bonding force, and shore hardness originating from this study indicates that the printed multilayer SMPs possessed excellent interlayer bonding properties as well as obvious modulus differences between each layer. The average shape recovery speed (Sa) of multilayer SMPs was almost linearly related to the slice layer number of FL. The strategy was successfully demonstrated in the fabrication of multilayer SMPs with sequential recovery and multiple deformation behavior. An electrically responsive shape memory effect was also achieved by using embedded graphite electrothermal layer to exhibit its application in time-delay devices. The proposed strategy enables the application of multilayer SMPs gradient structures in soft robotics and flexible electronics.