# 4D Printing of Multicomponent Shape-Memory Polymer Formulations
## 3 Extrusion Based 4D Printing
### Direct Ink Writing
#### 3.2.1. DIW of Thermoplastics in Solution
The printed film was folded into a temporary spiral shape and placed in a tube, which restricted the full shape recovery upon heating. Exposing the tube to 30 kHz AMF, the spiral structure showed self-expanding behavior within 10 s as illustrated in Figure 8b. This restricted shape changing behavior could be useful in biomedical applications such as expandable stents (Figure 8b). A similar strategy was used to fabricate lightweight, highly conductive nanocomposites by incorporating silver coated carbon nanofibers (Ag@CNF) as hybrid fillers into PLA-DCM mixture (Figure 8c) [117]. One possible application for such composite materials could be smart grippers that open and close as electric current is applied (Figure 8d). The achieved nanocomposites could be printed in open air under ambient conditions to form freestanding coils (Figure 8e) and displayed a low percolation threshold (i.e., <6 vol%) and high electrical conductivity (i.e., >2.1 \(\times\) 10\({}^{5}\) S/m) without any post-treatment. Electric conductivity was confirmed qualitatively by incorporating an as-printed 3D cubic scaffold and a freeform spiral into a circuit with a red light-emitting diode (LED). The LED could be powered at a voltage of 2.5 V with a current of 100 mA, demonstrating utility immediately after printing as shown in Figure 8f. In another effort, the 3D and 4D printing of poly (D,L-lactide-co-trimethylene carbonate) (PLMC)/CNT nanocomposites with electro-responsive shape-changing capabilities was achieved by solution DIW [117]. To make PLA more useful for biomedical applications, the copolymerization of d,l-lactide and trimethylene carbonate was carried out, and the \(T_{g}\) of the resulting PLMC copolymer was tuned to 49 \({}^{\circ}\)C. Four-dimensionally printed structures displayed high electrical conductivity even when porous scaffolds were printed (Figure 9a,b) thanks to appropriate levels of filler dispersion (Figure 9c). The scaffolds  
Figure 8: (**a**) Schematic demonstration of 4D printing of a partially crosslinking PLA based SMPs via DIW and some of the printed structures; (**b**) demonstration of the restricted shape recovery process triggered by a 30 kHz alternating magnetic field and potential application of the 4D scaffold as an intravascular stent. Reprinted with permission from reference [115]. Copyright 2016, American Chemical Society, Washington, DC, USA. (**c**) Process of the preparation of solvent cast (SC) printable Ag@CNF/PLA based conductive polymer networks (CPNs) and a schematic demonstration of the SC printing process. (**d**) 3D printed smart gripper structure based on PLA/Ag@CNFs CPNs. (**e**) Optical and SEM images of a freestanding 3D printed spiral using PLA/Ag@CNFs ink. (**f**) High electrical conductivity of PLA/Ag@CNFs nanocomposites as the printed components lighted up an LED light under a 2.5 V voltage. Reprinted with permission from reference [116]. Copyright 2019, American Chemical Society. prepared with these composites displayed shape-changing behavior within 16 s following the application of 25 V, as illustrated in Figure 9d. Addition of UV photoinitiators triggered the hydrogen abstraction from PLMC and resulted a crosslinked system. UV crosslinking helped to significantly improve the mechanical strength and solvent resistance of c-PLMC/CNT, while the actuation temperature and volume electrical conductivity remained almost the same. The 3D printing of bone scaffolds based on waterborne shape-memory polyurethane (PU) inks was reported by [PERSON] et al. [118] The mixing of PU-based ink with polyethylene oxide (PEO) or gelatin improved the biocompatibility of the scaffolds. However, using gelatin reduced the recovery ratios and increased the recovery times (Figure 9e,_f_). The addition of superparamagnetic iron oxide nanoparticles (SPIO NP) to the printed inks promoted the osteogenic induction of the printed scaffolds. The printed scaffolds showed better shape fixity and shape recovery in water than in air (Figure 9g).