# 4D Printing of Multicomponent Shape-Memory Polymer Formulations
## 3 Extrusion Based 4D Printing
### Fused Filament Fabrication (FFF)
#### 3.1.2 FFF of Polymer-Particulate Composites  
Particulates can bring additional functionalities and can enhance the printability and performance of SMPs; for example, they can be used to make electroactive [64; 98; 99; 100], photo-responsive [32; 85], and inductive [101; 102] materials for indirect heating of the trSMP matrix. They can also be used to improve fivity and recovery ratios by creating different morphologies in SMPs [103]. In this subsection, research work with particulate filled thermoplastic trSMPs is summarized. Using paper as a substrate, [PERSON] et al. [98] used a PLA/graphene based filament to print single layer structures that could be heated by passing an electric current through them. The shapes were programmed at 70 \({}^{\circ}\)C, and as the electric current passed through the composite and heated the structure, it recovered its original shape. Copper tape was placed at the extremities of the printed structure to create electrodes, and wires were soldered to the copper tape to pass the electric current through the printed structure. It was observed that the temperature of the composite and the angle of deformation of the printed structure were proportional to the voltage applied. A voltage of 50 V gave rise to a temperature of  
Figure 3: **(a)** Strips of PLA/TPU polymer–polymer composites, **(b)** room temperature shape programming of PLA/TPU composites, **(c)** shape recovery of PLA/TPU composites after heating to 70 \({}^{\circ}\)C. Reproduced with permission from reference [94] copyright 2014, John Wiley & Sons, Hoboken, NJ, USA; **(d)** fatty ratio (\(R_{f}\)) and recovery ratio (\(R_{r}\)) for TPU/PLA composites printed with different infill orientations (0\({}^{\circ}\), 90\({}^{\circ}\), and 45\({}^{\circ}\)) Reproduced with permission from reference [96] copyright 2021, Elsevier; **(e)** FFF produced PBS/PLA composite specimens after printing, programming in water at 85 \({}^{\circ}\)C, and shape recovery in water at 85 \({}^{\circ}\)C (scale bar 10 mm). Reproduced with permission from reference [97] copyright 2021, Elsevier. approximately 50 \({}^{\circ}\)C, and a voltage of 100 V increased the temperature to around 110 \({}^{\circ}\)C. As the number of heating cycles increased, the angle of recovery decreased [98]. Another example was the use of PLA filaments filled with 53 wt.% carbon black (CB), which were used to print structures with different infill orientations (0\({}^{\circ}\), \(\pm\)45\({}^{\circ}\) and 90\({}^{\circ}\)); the effect of Joule heating was investigated by applying 30 V (DC) for 20 min. It was observed that specimens with infill orientations of 0\({}^{\circ}\) and \(\pm\)45\({}^{\circ}\) could reach higher temperatures of around 46 \({}^{\circ}\)C compared to specimens with an infill orientation of 90\({}^{\circ}\), which could only reach 40 \({}^{\circ}\)C. However, the temperature distribution along the specimen was more homogeneous for the specimens with a 0\({}^{\circ}\) infill orientation [100] (Figure 4a). Therefore, the overall part design and infill orientation must be considered when generating structures for 4D printing with electroactive materials, since these two parameters can locally alter the response of such parts. [PERSON] et al. [32] prepared TPU-CB composites that could be processed by a commercial FFF machine. CB has a 92% photothermal conversion efficiency; therefore, addition of CB to a shape memory TPU can be used to fabricate photo-responsive shape memory materials. Exposure to sunlight with an intensity of 76 mW/cm\({}^{2}\) for 160 s was enough to recover the shape of a hollow cube frame with a side length of 30 mm. CB content was varied from 0.1 to 15 wt.%, and it was observed that the higher the content, the higher the temperature that was achieved.