# 4D Printing of Multicomponent Shape-Memory Polymer Formulations
## 6 Summary and Outlook  
Four-dimensional printing is a modern state of the art technology that has received growing interest in recent years from the materials science, process technology, and mechanics communities [20; 136]. A major part of 4 DP is based on the 3D printing of SMPs, and tremendous progress has been achieved in this context in recent years [38; 61; 113]. The 4 DP of SMP multicomponent systems such as composites, blends, and layered 3D objects has enabled the fabrication of high precision complex prototypes with unique capabilities and offering multiple functionalities. These functionalities can be activated through external stimuli such as heat, light, electric current or magnetic fields and may be triggered simultaneously or in a sequential fashion [26]. The importance of 4D-printed SMPs is apparent based on the growing number of studies and research papers related to this subject that have appeared in the last decade [36]. This review has highlighted different 3D/4D printing methods and strategies investigated in recent years to print multicomponent SMPs formulations. Advanced fabrication methods for the printing of heterogeneous SMP-based structures have been discussed, including MEX techniques (FDM, DIW), VPP (SLA, DLP and MPP), and MJT. Up to this point, the materials selection, functional behavior and working principles of 4D printed trSMP multicomponent constructs have been presented. In this last section, the current limitations and underlying barriers to continued progress are identified and discussed. Although most of the printed trSMP-based systems are still in prototype form, they show strong potential to become functional objects with industrial applications in the near future [37; 60]. Different techniques have been used to print complex, multifunctional geometries impossible to achieve via conventional manufacturing techniques such as extrusion, injection molding or compression molding. These geometries enable complex actuation pathways and mechanical deformations following a predetermined program. Furthermore, the combination of shape memory with self-healing behavior, piezoelectric activity, magnetic sensitivity, electrical conductivity, and structural functionality makes such multifunctional constructs attractive in a broad range of use cases, including intelligent electronics, aerospace, robotics, biomedical, and day-to-day applications (textiles, ornamentals, and footwear) [101; 107; 115; 108]. Multicomponent approaches not only help to improve the mechanical properties of trSMPs but also allow for the introduction of new actuation methods such as remote triggering via magnetic field, electric current or light, thus increasing the versatility of such 4D objects [20; 21; 26]. Though the studies reviewed here demonstrate that significant progress has been made related to the 4 DP of trSMPs and composites, several challenges remain. For example, FFF, the most used technique for SMP 4 DP, results in anisotropic materials and introduces interfaces and internal defects where mechanical stresses concentrate, leading to delamination after repeated actuation. Likewise, residual stresses arising from temperature gradients during processing can overlap with the recovery stresses generated by the shape-memory effect, thus making the deformation of the polymer more difficult to predict and control. Furthermore, optimizing the printing parameters for a new formulation is always tedious and challenging. In the case of softer polymers, filament buckling occurs, while in the case of reinforced composites, material clogging jams the nozzle; in both cases, the printing process stops. The issue of filament buckling can be avoided by adding fillers or blending with other polymers to improve the filament stiffness [24; 44]. To avoid the clogging issue, the filler content can be reduced, flow enhancers can be added, and the printing parameters can be optimized to enable the smooth flow of thermoplastic material through the nozzle [87; 88].