# 4D Printing of Multicomponent Shape-Memory Polymer Formulations
## 3 Extrusion Based 4D Printing
### Fused Filament Fabrication (FFF)
#### 3.1.3 FFF of Continuous Fiber-Reinforced Composites  
Continuous fiber-reinforced composites can be used to provide lighter, stiffer parts with greater energy absorption capabilities as compared to those provided by particulate-filled composites. FFF is a convenient way to prepare complex geometries with continuous fiber-reinforced thermoplastics using special FFF devices that allow for the co-extrusion and co-deposition of fibers and thermoplastics [104]. Besides mechanical reinforcement, electrically conductive continuous fibers can be used to heat trSMPs [104; 105; 106; 107; 33]. In this section, examples of continuous fiber-reinforced thermoplastic structures that change shape due to Joule heating are discussed. [PERSON] et al. [104] investigated the Joule heating of 3D printed structures consisting of continuous carbon fibers (CF) and PLA or PEEK produced by a modified FFF machine. Bar specimens were prepared with one layer of neat polymer and a second layer of polymer reinforced with the continuous CF. The FFF specimens showed a linear relationship between electric power and temperature. The data showed that there were two distinct slopes: one steeper from room temperature to \(T_{g}\) and another one less steep from \(T_{g}\) to \(T_{m}\), indicating a diminution of the thermal conductivity of the respective polymer matrices. Specimens with a PLA matrix needed 0.45 W of electric power to reach PLA's \(T_{g}\) of 57 \({}^{\circ}\)C and 2.4 W to reach the onset of melting (\(\sim\)155 \({}^{\circ}\)C), while specimens with a PEEK matrix required 1.6 W to reach PEEK's \(T_{g}\) of 143 \({}^{\circ}\)C and 5.1 W to reach the onset of melting (\(\sim\)282 \({}^{\circ}\)C) (Figure 5a). Bilayer specimens fixed as cantilever beams bent up as the temperature increased and down as the temperature decreased due to the different stiffnesses and thermal properties of the layers. The bending degree was proportional to the power applied and followed a similar trend to that of power vs. temperature. Nevertheless, not all specimens were able to fully recover from such deformations following heating and cooling. The authors attributed this to the softening of the polymers and increased shear stress at the fiber-matrix interface at temperatures higher than \(T_{g}\). This study demonstrated that in such printed bilayer structures, it is possible to control the deformation by controlling the amount of applied power. [PERSON] et al. [33] reported the effect of printing parameters on the mechanical properties of continuous CF reinforced PLA specimens produced by FFF. Increasing the extrusion temperature promoted more complete densification, resulting in higher bending performance and lower surface roughness of the printed components. In contrast, higher printing speeds caused weak interfacial bonding, resulting in a poor mechanical response. By increasing the amount of deposited continuous fiber and using a deposition angle parallel to the length of the bending specimen, the bending strength of FFF specimens could be increased. The Joule heating and shape recovery of specimens with the highest bending strengths was evaluated, and it was observed that applying 5 V for 75 s increased the temperature to 75 \({}^{\circ}\)C and induced recovery of 95% of the original shape. Here, fiber breakage and delamination were considered as major obstacles to 100% shape recovery (Figure 5b). The infill geometry used in continuously reinforced structures was also investigated in PLA-CF composites. [PERSON] et al. [106] prepared horseshoe lattice structures with different sinusoidal wave amplitudes (Figure 5c). It was observed that all metamaterial structures had the ability to fix their temporary shapes and freely recover their original shapes. Of particular interest was the fact that the sinusoidal wave amplitude affected the shape recovery and shape fixity ratios of the prepared specimens. A larger sinusoidal wave amplitude gave the highest shape recovery ratio (\(R_{r}\)) and the lowest shape fixity ratio (\(R_{f}\)). [PERSON] et al.