# 4D Printing of Multicomponent Shape-Memory Polymer Formulations
## 3 Extrusion Based 4D Printing
### Direct Ink Writing
#### 3.2.2 DIW of Liquid Pre-Polymers  
[PERSON] et al. reported the 3D printing of highly stretchable, shape-memory (SM) and self-healing (SH) elastomers via UV-light-assisted DIW printing [47]. An ink containing urethaneacrylate, linear semicrystalline PCL and fused silica nanoparticle (200-300 nm) was utilized for UV-assisted DIW printing of semi-IPN elastomer composites. PCL crystalline domains with \(T_{m}=55\)\({}^{\circ}\)C acted as switching domains for shape-memory experiments and as healing agents for self-healing behavior. The 3D-printed structures could be stretched up to 600% and showed interesting functional properties, such as high strain SM and SM-assisted SH capabilities (Figure 10a-c). Potential applications of the printed structures for biomedical devices, soft robotics, and flexible electronics, were proposed. Figure 9: (**a**) The usage of 3D-printed scaffolds as electrical components; (**b**) SEM image of the cut electrical component; (**c**) SEM micrograph of a nanocomposite cross-section; (**d**) shape recovery of the folded scaffold given an applied voltage of 25 V. Reproduced with permission from reference [117], copyright 2019, Elsevier. (**e**) Shape-fixity and shape-recovery ratios of freeze dried (FD) and 3D printed (3 DP) samples; (**f**) recovery time for each scaffold to reach the final recovery ratio; (**g**) programming and recovery of a PU scaffold in water at 50 \({}^{\circ}\)C. Reprinted with permission from reference [118], copyright 2018, American Chemical Society. [PERSON] et al. reported bio-based thermoset composites via 3D printing [119]. Renewable and eco-friendly epoxidized soybean oil (EBSO) was used as the matrix material. To tailor the mechanical properties, different concentrations of bisphenol F diglycidyl ether (BFDGE) were added into the matrix. Furthermore, carbon nanofibers (CNF) were used as a filler to render the printed structures electroactive. By varying the ratio of the individual components of the thermoset resin, mechanical, thermal (\(T_{g}\)) and transport properties could be tuned. The 3D printing of the composite resin resulted in complex origami-type multi-material architectures that exhibited programmable shape changes triggered by either direct thermal heating or electric (Joule) heating. The programming and shape recovery of a complex box-like printed structure is shown in Figure 10d. The UV-assisted DIW approach also enabled the 4D printing of epoxy based SMP IPN composites with high mechanical strength and thermal stability [120]. The base resin consisted of commercial photocurable acrylates, thermally curable epoxy and silica nanoparticles in different weight ratios. The silica nanoparticles were added to modulate viscosity and to realize shear-thinning behavior. In the first curing step, an acrylate network was formed during a UV-assisted printing process. The epoxy oligomer was thermally cured in the second step to generate an IPN composite (Figure 11a). A variety of complicated architectures such as lattices, gear wheels, swirl bows and locked structures were successfully printed. A lattice structure is shown in Figure 11b. These printed IPN composites exhibited clear shape memory effects, including complete shape change within 10 s at 104 \({}^{\circ}\)C (higher than \(T_{g}\approx\) 76 \({}^{\circ}\)C) along with enhanced mechanical toughness. Figure 10: (**a**) Thermally induced shape change of a printed hollow vase based on a semi-IPN elastomer (scale bars: 6 mm). (**b**) Pictures showing a SM-assisted SH process for a printed Archimedean spiral structure; insets are optical micrographs showing surface morphology with a scale bar of 1 mm. (**c**) Stress–strain curves for virgin, notched and healed semi-IPN elastomer samples; insets are micrographs of deformation and crack growth of notched samples clamped in the tensile tester. Reprinted with permission from reference [47], copyright 2018, American Chemical Society. (**d**) Resistive heating of a printed structure based on ESBO/BFDGE/CNF (5.6 vol% CNF).