# 4D Printing of Multicomponent Shape-Memory Polymer Formulations
## 5 Material Jetting (MJT) 4D Printing  
Material jetting (MJT) is another commonly applied 3D printing technique that can be used for the single-step fabrication of 4D multi-material objects [38]. In this technique, droplets of photopolymer material are jetted onto the build platform and each layer is immediately cured by UV light [76; 136]. The first ever MJT printers were introduced by Object Geometries Ltd. (Rehovot, Israel), which later merged with Stratasys Ltd. The nozzles of these printers are able to switch between different materials, including support materials  
Figure 14: Schematic of color and shape change of a constituent nanostructured element of a “disappearing” 3D printed structure composed of a shape memory polymer (SMP). The as-printed structure with upright grids (**left**) functions as a structural color filter that transmits only specific wavelengths of visible light. Deformation of the structures at elevated temperature flattens the nanostructures (**right**), rendering the part colorless; it remains in an invisible state after cooling to room temperature. Heating recovers both the original geometry and color of nanostructures, leading to a submicron demonstration of 4D printing. Reproduced with permission from reference [134], copyright 2021, Nature Communications (Nature Portfolio, Berlin, Germany). in the case of complex part geometries. The gel-like support material can be easily removed by hand or water jetting. At this time, MJT printers from Stratasys Ltd. are probably the most common commercially available jetting printers, with many authors using them to fabricate 4D multi-material systems based on trMSMPs [24; 137; 138]. [PERSON] et al. used an MJT printer (Objet 260 Connex, Stratasys, Edina, MN, USA) to directly print glassy SMP fibers in an elastomeric matrix [23]. Laminated structures with a range of fiber orientations could be printed and thermo-mechanically programmed to assume complex three-dimensional configurations including bent, coiled, and twisted strips, folded shapes, and complex contoured structures with nonuniform, spatially varying curvature (Figure 15a-h). The strips shown in Figure 15c-e had two layers one on top of each other: one with 25% fibers and the other one without. The fibers were oriented at 0\({}^{\circ}\) (Figure 15c), 90\({}^{\circ}\) (Figure 15d) and 30\({}^{\circ}\) (Figure 15e). The strips in Figure 15f-h had alternating sections side by side without fibers and with the fibers oriented at 0\({}^{\circ}\) (Figure 15f), 90\({}^{\circ}\) (Figure 15g), and 30\({}^{\circ}\) (Figure 15h). The original flat plate shape was recovered by heating the material again. The printed active composites could be integrated with other structural or functional components to create active devices. For example, hinges were directly printed in a two-layer laminated form: one layer of matrix material and one layer of an active laminate with a prescribed fiber size and spacing. The hinges were then connected to inactive (rigid) panels. These rigid panels were used as end tabs to apply mechanical loads. The hinges function occurs via a mechanism of programmed strain mismatch (eigenstrain) between the two layers that leads to constant bending curvature over the hinge region. The authors developed a theoretical model to describe the behavior of the printed active composite hinges and used it to design several active origami structures, including a self-assembling box, a pyramid and two origami airplanes (Figure 15i,j) [77]. Figure 15: Active composite laminates obtained by design of laminate architecture. (**a**) A two-layer laminate designed with one layer being a lamina with fibers at a prescribed orientation and one layer being pure matrix material is printed, then heated, stretched, cooled, and released. Upon release of  
the stress the laminate assumes a complex shape as a function of its architecture. When reheated it then assumes its original shape, a flat rectangular strip.