# 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
For example, when heating with a xenon light source with an intensity of 60 mW/cm\({}^{2}\) for 900 s, the unfilled TPU reached a maximum temperature of 29.3 \({}^{\circ}\)C, while TPU filled with 15 wt.% carbon black reached 40.5 \({}^{\circ}\)C. Thus, variations in filler content can be used to tailor the response of FFF produced objects. A content of 10 wt.% was selected for 3 DP because with this CB content, the filaments with desirable diameter  
Figure 4: (**a**) Temperature distribution of FFF-produced electro-responsive specimens with different infill orientations [100] (CC BY license); (**b**) temperature and shape of photo-responsive FFF-produced sunflower at different times after being exposed to 87 mW/cm\({}^{2}\) of light. Reproduced with permission from reference [32] copyright 2019, John Wiley & Sons; (**c**) shape recovery of a magneto-responsive ASD occluder. Reproduced with permission from reference [101] copyright 2019, John Wiley & Sons; (**d**) recovery of tracheal scaffold produced from a magneto-responsive PLA-Fe\({}_{3}\)O\({}_{4}\) composite. Reproduced with permission from reference [102] copyright 2019, Elsevier. could be extruded and was reliably printable by FFF. Using this material, a sunflower-like object was printed that opened when exposed to light (Figure 4b). Using solution casting, [PERSON] et al. [85] prepared mixtures of PLA and multiwalled carbon nanotubes (MWCNT) at concentrations from 0.1 to 0.6 wt.%; the material was then extruded into filaments and used to 3D print objects by FFF. The light source used in this investigation was a 275 W near infrared lamp. A systematic investigation showed that exposing films of the neat PLA to the infrared light for 15 s raised their temperature to approximately 90 \({}^{\circ}\)C, while the composites containing 0.6 wt.% MWCNT reached a temperature of approximately 170 \({}^{\circ}\)C; therefore, the photo-responsive nature of the composite was demonstrated. Rectangular actuators (60 mm \(\times\) 6 mm) were fabricated by printing two layers of the photo-responsive composite containing 0.5 wt.% MWCNT and a thickness of 200 um on top of a piece of paper with a thickness of 300 um. The actuators were fabricated with infill densities of 100, 66, 60, or 57%. It was observed that an infill density of 100% gave the longest response and recovery times. Infill densities between 57 and 66% gave similar response times, but an infill density of 66% gave the fastest recovery time. This highlighted the effect of infill density on the performance of photo-responsive SMPs. A flower-like structure was printed that opened and closed as the infrared light was turned on and off. [PERSON] et al. [101] added Fe\({}_{3}\)O\({}_{4}\) particles to PLA by solution casting and prepared filaments that could be successfully processed using conventional FFF equipment. The addition of Fe\({}_{3}\)O\({}_{4}\) particles made the resulting composite magneto-responsive, allowing for inductive heating by exposure to an alternating magnetic field (AMF). An AMF with a frequency of 27.5 kHz and field strength of 4 kA/m was used to investigate the magnetically induced shape memory behavior of PLA composites with different iron oxide concentrations. The PLA composite with 10 wt.% of Fe\({}_{3}\)O\({}_{4}\) experienced sufficient inductive heating to initiate shape recovery. All composites had \(R_{r}\) values above 94% despite adding up to 20 wt.% of magnetic particles. Using a composite containing 10 wt.% iron oxide, it was possible to print occluders to treat a congenital heart disease known as atrial septal defect (ASD). It was observed that the printed occluders could recover their shape within 16 s in the AMF described above (Figure 4c). In a similar study, [PERSON] et al. [32] prepared a PLA-15 wt.% Fe\({}_{3}\)O\({}_{4}\) composite filament via solution casting followed by twin-screw extrusion. Using this composite filament, tracheal scaffolds were produced by FFF.