# 4D Printing of Multicomponent Shape-Memory Polymer Formulations
## 6 Summary and Outlook
Furthermore, as FFF deals with highly viscous fluids, the nozzles used are the largest of all those used in the context of AM-based materials deposition more generally; as a result, poor surface quality is common, and post-treatment is required to improve it [140]. New libraries of SMPs and composites combining improved properties, excellent printability and retained shape memory response are required to overcome these challenges and enable the generation of high-quality parts. The adaptability of DIW to various materials makes it the second most commonly used MEX technique for 4 DP. Various trSMP multicomponent systems, including functional composites and IPNs, have been printed using DIW to produce smart 4D objects [111; 115]. Here, moderate ink viscosities and shear-thinning behavior are required [111]. DIW has been used to print a range of polymer systems, including both thermoset and thermoplastic composites [115; 116]. To enable the printing of thermosets, time consuming post-processing steps are often required, however, while in the case of thermoplastic systems, the polymer must be dissolved in a volatile solvent which should evaporate at just the right rate during the printing process. The latter (solution) method has enabled the printing of multi-material trSMP parts with magnetic, electric, and piezoelectric functionalities [28]. Nevertheless, DIW has relatively low printing speeds since parts are built by depositing strands of materials, and like FFF, it produces 3D structures with low surface quality due to the size of the nozzles used [111]. Furthermore, DIW, like FFF, is often used to print trSMPs with amorphous switching domains, which only enable a one-way shape-memory effect. The use of trSMP with crystallizable switching domains would enable reversible bidirectional SME. However, an alignment of polymer chains would be required to achieve well-oriented crystalline domains [141]. Such type of polymer chain orientations has been used for 4D printing of liquid crystalline elastomers to enable reversible movements in these elastomers, where the drawing force imposed by the movement of the nozzle in the extruded printing process was able to align the mesogen units along the specific printing path [142]. In contrast with MEX techniques, VPP techniques are restricted to photosensitive systems, and printing is carried out layer-by-layer using some type of light source [69]. Though the printing of complex 4D objects based on trSMP multicomponent systems such as composites containing carbon and silica nanoparticles has been reported, VPP is inherently more suitable for single phase materials with photo-sensitive moieties [72]. In the case of composites, fillers concentrations must be optimized to avoid shielding effects that could interfere with photoinduced polymerization or crosslinking [71]. Likewise, while VPP has enabled the multi-material printing of complex microstructured trSMP-based parts using different vats filled with different photosensitive formulations, switching between them reduces the speed of an already slow process (thanks to its high resolution) still further [125; 143]. Furthermore, it is inherently difficult to incorporate crystalline segments because oligomers based on such segments are solid at ambient temperature. One way to make these oligomers printable is to heat the vat above the melting point of the crystalline segments. Nevertheless, controlling unwanted thermal polymerization can be a challenge. Finally, while VPP enables the printing of both macro- and microstructured smart 4D objects with high levels of precision and surface finish, there remains a need for a greater variety of photosensitive formulations yielding materials with broader ranges of properties/offering new functionalities. Compared to MEX and VPP, MJT is very well suited for manufacturing 3D objects with very fine features based on different materials [75]. Different compositions can be readily placed in different locations, bringing additional functionalities.