Abstract: Shape memory polymer (SMP) refers to a class of stimuli-responsive polymer materials that can sense the change of the external environment and respond to this change, and adjust its state parameters to return to the preprogrammed state. Compared with shape memory alloys and shape memory ceramics, SMPs have been demonstrated great potential application in the fields of medical devices, flexible electronics, textiles, information carriers, aerospace and soft robotics etc., due to large recoverable strain, tunable response temperature, diverse stimulus methods, multiple material attribute, and versatile shape memory effects. However, the original shape of the SMPs or the permanent shape after the recovery is relatively simple, and the molding process is highly dependent on the mold and is greatly limited by the demolding process. Thus, its shape is difficult to be complicated, which is difficult to further meet the demand for the complexity of intelligent structures in emerging high-tech fields. Aiming at above problems, domestic and foreign researchers have carried out some innovative research in this field. The current research hotspots mainly focus on the following two aspects: (1) The dynamic covalent bonds are embedded into thermosetting polymers and then solid-state plasticity is fulfilled by the rearrangement of the cross-linked network, leading to geometrically complex permanent shapes. This type of the shape memory polymer is also called thermadapt SMP; (2) The complexity of material geometry can be achieved by 3D printing (Three-dimensional printing) technology to print intelligent materials, and the new type of printing technology is termed as Four-dimensional (4D) printing. At present, the most widely used smart materials is the SMP for 4D prin-ting. In recent years, combined with the latest 4D printing technology, the geometrically complex shape memory materials with diverse stimulus responses and shape memory effects are prepared by researchers using the SMP as research objects and using fused deposition modeling, stereolithography, direct ink writing and PolyJet as the 3D printing methods. Based on different driving mechanisms, thermal-responsive, photo-responsive, electrical-responsive, magnetic-responsive, and water-responsive 4D printing SMPs have been successfully designed and prepared, enriching the types of stimulus response with complex geometric shape memory materials; in terms of the deformation mode, the design, preparation and application of 4D printed shape memory materials with double-shape shape memory effect, two-way shape memory effect, multiple shape memory effect and sequentially gradient 4D printing SMPs are realized. This paper mainly reviews the printing method, driving mechanism, deformation mode and application of 4D printing SMP. Finally, the problems of 4D printing SMP are summarized in current research and the development direction is prospected in the future.
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