# 4D Printing of Multicomponent Shape-Memory Polymer Formulations
## 3 Extrusion Based 4D Printing
### Fused Filament Fabrication (FFF)
#### 3.1.4 Three-Dimensional Objects Produced by Multiple Extrusion FFF  
Multi-nozzle FFF printers can be used to print complex structures with variable stiffnesses and multiple functionalities by combining different SMPs and/or composites in one 3D object. Here, the printing parameters, the ratio between the different materials, and the geometry of each layer can be used to control the shape-memory response of these multi-material 3D objects. Some examples of multi-material printing of trSMPs are discussed in this section.  
[PERSON] and [PERSON] [108] prepared laminate structures with reversible shape-change properties by combining TPU and PLA using a dual-nozzle FFF machine. The printed laminates had layers of PLA that acted as an active material and layers of TPU that acted as a passive material, providing the structure with a reversible shape memory capability without an external mechanical programming step. The TPU used had a glass transition below 0 \({}^{\circ}\)C but a Shore hardness of 90 A, ensuring that it could be printed as a filament. As pointed out in previous investigations [83], the printing conditions used for depositing PLA can be used to control the amount of thermal expansion and contraction, in order to maximize bending deformation. By heating the bilayer system through the \(T_{g}\) = 63 \({}^{\circ}\)C of PLA to a higher temperature \(T_{h}\) = 90 \({}^{\circ}\)C and then cooling it to room temperature, a reversible bending behavior was observed. Furthermore, by changing the infill direction, PLA to TPU thickness ratio and overall geometry of the printed specimen, it was possible to control the achievable bending deformation (Figure 6a).  
[PERSON] et al. [109] used a triple nozzle FFF machine to fabricate 3D objects with filaments consisting of PLA and thermochromic pigments. The 3D objects had shape changing and color changing capabilities. For example, a multi-material flower was produced with orange and green petals that could open when heated to \(T_{r}\) = 80 \({}^{\circ}\)C, while at the same time, all of the petals turned yellow. Other fabricated structures changed color to camouflage certain information in printed letters or QR codes. [PERSON] et al. [64] combined an electroactive shape-memory composite with a color changing composite in a single 3D object. The electroactive composite was CB-filled PLA, while the color-changing composite was PLA containing a thermochromic pigment. A custom-built dual-nozzle FFF machine was used to prepare a multi-material starfish object that could change color from purple to pink by electric heating. The starfish consisted of three sections; the bottom and top sections were printed with the thermochromic composite, while the middle section was printed with the electroactive composite. The middle layer had a different inner structure as well, providing variations in electrical resistance and Joule heating capacity in different parts of the printed starfish (Figure 6b,c). A similar strategy was used with thinner structures resembling the wings of a dragonfly that could change shape and color as an electric current was applied. Using such an approach, control of both shape and visual appearance is possible, which could prove useful in combined sensing/actuating applications.  
Another possibility is to combine FFF with other extrusion-based deposition techniques as reported by [PERSON] et al. [110]. A gripper was printed with PLA and a suspension of silver nanowires (Ag-NWS) was deposited via syringe at the hinges of the gripper. After the suspension dried, the layer of nanowires was covered by a conductive paste and connected using electrodes and wires. Local Joule heating of the nanowires and supporting PLA film enabled indirect triggering of the gripper structure, as shown in Figure 6d.