Advances in 4D-Printed Shape Memory Polymers from Materials, Mechanisms and Fabrication Techniques to Applications: A Comprehensive Review.

Shape memory polymers (SMPs) and their multifunctional composites have become important material systems for 4D printing because they can be processed into structures with programmed deformation and stimulus-responsive actuation. After being fixed in a temporary configuration, SMPs are able to recover their permanent shape when exposed to heat, electric or magnetic fields, and so on. This reversible shape change gives printed SMP structures functions beyond those of conventional static components, making them useful for biomedical devices, flexible electronics, and soft robotics. The performance of 4D-printed SMPs is determined not only by material chemistry but also by the design of the printed architecture and the manner in which external stimuli are applied. This review begins by covering the basic mechanisms that govern shape memory behavior in SMPs, including molecular switching, thermomechanical programming, and stimulus-controlled recovery. On this basis, representative printing methods for SMPs are compared, including stereolithography (SLA), fused deposition modeling (FDM), direct-write printing, and polymer inkjet printing. Recent applications are then considered in areas where programmed shape change has practical value, such as biomedical devices, soft robotics, and flexible electronics. The discussion also identifies unresolved problems in printing resolution, response speed, cyclic stability, structural design, and multifunctional coupling, which remain central barriers to wider use of 4D-printed SMP systems.