# New Trends in 4D Printing: A Critical Review
## 6 Printing and Programming Strategies
### Print Strategies for Shape Memory Alloys  
SMPs have been widely utilized for 4 DP in the literature because of their outstanding manufacturability [9]. However, the low stiffness (less than 1 GPa), tensile strength (1-30 MPa), and Tg (\(\approx\)95 \({}^{\circ}\)C) limit the possible applications. Conversely, metallic SMAs exhibit much higher stiffness (200 GPa) and strength (1000 MPa) than SMPs. Additionally, metallic SMAs have a much larger transformation hysteresis, leading to a wider application temperature. Although there are many studies regarding SMA printability [168; 169], few studies are available regarding SME realization of 4 DP SMAs, and typically they involve TiNi (Ti-rich) and NiTi (Ni-rich) powders. One of the most popular alloys with the SME is nitinol (nickel-titanium, or NiTi), which has various uses in the aerospace, automotive, biomedical, and other industries. In [170], the authors used Ni-based SMA for fabricating and pre-programming a prototype of a crawling robot inspired by inchworms. The robot body is built as a monolithically flexible sheet that unites the \"torso\" and \"feet\" through the extraction and synthesis of the functional elements of the bionic device. Under the torso, an SMA spring is put together to mimic the abdominal longitudinal muscles. A foot layout was designed with \"one edge and two facets\", using silicone rubber as the high-friction surface and polytetrafluoroethylene as the low-friction material. This provides continuous movement of the body without jumping. A strip-type scanning strategy was employed to print samples and shorten the backfill  
Figure 9: 4 DP multi-material: direct 4 DP of structural parts with architecture-driven deformation modes (**a**) [151]; protein hydrogel cation programming and morphing through mechanochemical alterations (**b**) [163]; magnetic cantilevers sensitive to the magnetic field (**c**) [164]; shape-shifting cycle of a PLA rectangular multi-ply structure and TPU (**d**) [166]. time. A scanning angle of 67 was chosen for layer-to-layer stacking; investigations have suggested that this angle may lead to more random crystal orientations and refined grains. Although the authors introduce the sample as a two-way 4 DP structure, the two-way shape shifting is not achieved by 4 DP but by adding an SMA spring to the structure 1. A new approach using SMAs is presented in [171]. NiTi SMAs show excellent SME super elasticity and good biocompatibility, making them suitable for sensing, actuating, and biomedical applications. Bulk NiTi SMAs were fabricated via SLM with large elastocaloric effects, tailorable by varying the processing parameters of AM technology and heat treatment. In particular, the microstructure depends upon the laser power, the scanning speed and strategy, the layer thickness, and the hatch spacing [168]. As mentioned in Section 4.3, magnetic SMAs are another type of material used for 4 DP. The advantage of these materials over conventional SMAs lies in their ability to actuate by using a magnetic field. In [172], adapted 3 DP Ni-Mn-Ga powders were employed for creating net-shaped porous structures with good mechanical strength via binder jetting and sintering at 463 K for 4 h. Upon heating and cooling, printed components experience reversible martensitic change. The samples revealed a reversible magnetic-field-induced strain of about 0.01%, proving that magnetic shape-memory alloys are capable of being printed in 4D. Furthermore, controlling powder morphology can improve the final porosity of other metallic 3 DP technologies. Different studies have attempted to fabricate shape-shifting structures using Fe-SMAs. In Refs. [98; 173], a similar approach was applied for printing various complex 3D flower shapes and spring-like samples, lattices, and metamaterial structures via Laser Powder Bed Fusion. The strategy was characterized by 90\({}^{\circ}\) hatch bidirectional scanning and removing the primary bcc-\(\delta\) phase via heat treatment at 800 \({}^{\circ}\)C for 30 min.