# 4D Printing of Multicomponent Shape-Memory Polymer Formulations
## 2 Multicomponent SMPs
Modern 3 DP techniques provide a growing range of possibilities to print multicomponent systems (i.e., composites, blends, IPNs or layered structures) at different dimensional scales [21; 30; 46]. For example, SMP-based multicomponent systems have been used for the 3 DP of responsive mechanical devices, where part geometry has been scaled down to micro- and nanometer levels to form metamaterials [62; 63]. However, among the listof widely adopted 3 DP techniques, few have been used or customized to be suitable for the 3 DP of multicomponent SMPs formulations. These mainly include MEX-based 3 DP (e.g., FFF and DIW), VPP (e.g., DLP, SLA, and MPP) and MJT [38]. Here, the selection of a 3D printer is related to its suitability to print either an ink, liquid resin, or filament. Furthermore, the printer should allow for the adjustment of key parameters affecting material deposition and solidification, such as printing speeds, delivery volumes, material, and chamber temperatures [37; 41; 42]. A schematic demonstration of some of most used 3 DP techniques along with their capacity to print different multicomponent SMP formulations is shown in Figure 2.  
MEX printers are commonly used to print SMP particulate composites or blends. Additionally, by using print heads with multiple nozzles, MEX systems can simultaneously print several materials (i.e., layered structures), for example, multi-color objects or complex objects with dissolvable support structures [64; 65]. Nevertheless, there are some limitations in MEX printing, as the printed parts have mechanical anisotropy in the z-direction (vertical direction) and produce parts with stepped structures at their surfaces [37; 66]. Furthermore, a lack of strong inter-layer bonding can lead to mechanical failure of the printed object, particularly after repeated shape changing cycles [16; 67]. Nevertheless, the printing parameters and the material characteristics have a significant effect on the fatigue behavior of the printed objects [68]. VPP, on the other hand, is limited to the printing of low-viscosity (<10 Pa s) photopolymers that tend to have poor aging characteristics and limited thermal stability but can provide objects with good resolution and surface finish [69]. The VPP of high-viscosity resins is challenging, as the vertical detachment of the crosslinked layer from vat film can occur. Nevertheless, there are some reports about the heat-assisted VPP of highly viscous photopolymers, as heating can reduce the viscosity [70]. VPP techniques have also been used to print multicomponent SMP resins with electric or magnetic functionalities [71; 72]. Here, careful control over the ratio of the reinforcement and matrix is required as high amount of reinforcement can absorb and scatter the light, resulting in partially crosslinked systems with poor mechanical properties [73]. Nonetheless, by controlling the exposure time or by using multiple vat systems with different photopolymers, multi-material printing can also be performed. However, changing materials during printing significantly slows the printing process since a wiping/cleaning step is needed to avoid cross-contamination [74]. Compared to VPP, MJT printing has an intrinsic capability to print multiple materials simultaneously, given the ease with which multiple ink reservoirs and multiple nozzles may be implemented as in a conventional color ink jet printer [75; 76]. The multi-material printing of a variety of SMP resins via the MJT method is reported [31; 77]. Such multi-material systems provide novel shape memory functionalities along with tailorable thermal and mechanical properties of the printed objects. Anisotropic polymer chain orientation, complex geometries that resemble natural structures, and combinations of different materials in different layouts bring more flexibility to accomplish complex actuation not possible with other shaping methods such as injection molding, extrusion, and thermoforming.