# 4D Printing of Multicomponent Shape-Memory Polymer Formulations
## 3 Extrusion Based 4D Printing
### Direct Ink Writing
#### 3.2.1. DIW of Thermoplastics in Solution  
When DIW is used with thermoplastics in solution, rapid solvent evaporation occurs during the printing process [114]. [PERSON] et al. reported solution-based DIW of a multifunctional particulate composites combining shape changing and sensing functionalities in a single printed structure [28]. The printing ink consisted of a blend of PLA and polyesteramide (PEA) dissolved in dichloromethane (DCM), with piezoelectric barium titanate nanoparticles (BTN: 0-30 wt.%) added as sensing elements. The composites were used to print different shapes, and the shape-memory and sensing capabilities were investigated. Shape memory investigations resulted in recovery ratios (\(R_{r}\)) of more than 95%. For all the composite samples, the piezoelectric voltage output of the materials was studied by applying a compressive impact force in the range of 0.05-2 N at 1 Hz at room  
Figure 6: (**a**) Printed structure with one layer of PLA and a second layer of TPU after heating the flower opens. Reproduced with permission from reference [108] copyright 2021, Elsevier; (**b**) geometry used in different layers of a 3D printed star fish object. (**c**) printed object with a conductive PLA composite and a color changing PLA composite as voltage was applied at different times. Reproduced with permission from reference [64] copyright 2021, Elsevier; (**d**) PLA gripper with silver nanowires (Ag-NWS) in the hinges to locally heat PLA for actuating capabilities. Reproduced with permission from reference [110] copyright 2020, Elsevier. temperature. The composite with 10 wt.% of BTN exhibited the highest piezoelectric output. The piezoelectric output voltage could be increased by poling the BNT in the printed specimen (Figure 7a) and by increasing the dynamic compressive force (Figure 7b). The combined effect of shape memory and piezoelectric sensing was also monitored in a rectangular printed film which was clamped at one of its edges to act as a cantilever, with silver electrodes deposited on both surfaces at the same edge (Figure 7c). This beam was then placed in an oven at 100 \({}^{\circ}\)C (higher than the \(T_{g}\) of the film). A 2 g calibration weight was placed near its free edge. Output voltages were measured across the electrodes as the beam softened above its \(T_{g}\) (\(\sim\)26 \({}^{\circ}\)C) and the weight eventually fell. As seen from the insets in Figure 7d, the beam deformed with the rapid change in stiffness under the influence of increased temperatures, with a proportional voltage output resulting. Solution DIW printing of PLA-iron oxide-based magnetic inks enabled the generation of 4D shape-changing architectures in custom-defined geometries [115]. To improve the shape-memory capabilities of PLA, a UV initiator was added to the PLA/iron oxide/dichloromethane (DCM) based ink. Exposure to UV irradiation and fast evaporation of  
Figure 7: (**a**) Piezolectric voltage output (root mean square, RMS) from poled and unpoled 10 wt.% BTN/3.6 wt.% PEA/86.4 wt.% PLA film sensors upon dynamic excitation at 1 Hz under compressive (impact) loads. Inset: (left) RMS voltage output from the sensors; (right) the test set up used. (**b**) Piezolectric response from poled 10 wt.% BTN/90 wt.% PLA subjected to dynamic compressive loads at 1 Hz; inset: microscopic images of the 10 wt.% BTN/90 wt.% PLA films revealing porosity and agglomeration at different magnifications. (**c**) Schematic of the test specimen and boundary conditions used for the final multifunctional sensor demonstrator. (**d**) Piezolectric voltage output from the multifunctional actuator deforming under a 2 g weight at 100 \({}^{\circ}\)C (insets: deformation of the material above its \(T_{g}\)). Reproduced with permission from reference [28] copyright 2020, Elsevier. DCM facilitated the formation of a crosslinked shape changing 3D architecture (Figure 8a). These nanocomposites could be magnetically and remotely heated by exposure to alternating magnetic fields (AMF).