# New Trends in 4D Printing: A Critical Review
## 6 Printing and Programming Strategies
### Stress-Induced and Printing Strategies  
The properties of 3D-printed parts are markedly affected by processing parameters such as printing temperature, printing speed, layer thickness, etc. Therefore, by altering process parameters and print strategies, desired behavior can be programmed during fabrication. Consequently, stored energy regarding internal stress and strain can be released, allowing the shape recovery step [102; 136; 137]. The shape change is influenced by the residual stresses introduced into the thermoplastic materials throughout the 3 DP process. The thermoplastic materials are viscoelastic throughout the extrusion process, and the high temperature allows the polymer chains to stretch and align in the direction of the material flow via the extrusion nozzle [138]. Researchers have found that parameters such as heating and cooling temperatures, extrusion rate, printing speed, deposition path, and extrusion head diameter affect material microstructures such as crystallinity and anisotropy and mechanical properties [139]. Adjusting the design and process parameters can provide complex changes in the part structure when the memory shape physical principle is based on the releasing of mechanical stresses [2]. In AM, stresses are created by tailoring parameters such as strategy direction, infill type, road width, nozzle diameter, and platform temperature [140]. In Ref. [25], the influence of process factors such as printing speed, layer thickness, nozzle temperature, and printing pattern on shape-shifting behavior using FFF was investigated. By modifying such factors, the internal stress can be tailored, and many complex designs can be created [141]. The major advantage is that the programming phase is not necessary, since the ability is incorporated into the fabrication process. Nevertheless, considerable design and modeling are required to predict and control the deformation [92]. Works are now focusing on increasing the stress that can arise during the manufacturing process by printing on pre-strained substrates. A typical method is to employ a bilayer configuration characterized by layers having opposite, internal, directional stresses. This way, the generation of specific shape-changing response can be designed, e.g., bending or curling [2]. Stress-induced methods can be used to create 4 DP structures from single materials and those materials with no SME. A 4 DP method using FFF technology and, as the material, a single thermoplastic filament of Acrylonitrile Butadiene Styrene without an SME, is studied in [14]. A self-assembly function was obtained by alternating anisotropic regions and isotropic regions. Some layers were printed transversely and longitudinally, as shown in Figure 5a-c. However, as the authors reported, no reversibility could be achieved. To produce bilayer composites with a single material (PLA), the authors of [142] designed the tool path geometry by adjusting printing patterns per layer to create a differential expansion between layers. In this study, to create constraint layers and active layers via a single material, the authors used the printing angle as the main variable parameter; active layers were those printed at 90 degrees, and constraint layers at 0 degrees. The distribution  
Figure 5: 4D-printed cross-shape specimen route design (**a**); schematic of the heterogeneous lamination (**b**); shape transformation for different heating times (**c**) [14]. of the constraint layers and the active layers in the bilayer configuration considerably affected the shape change. When immersed in hot water, the constraint layer triggered the shape change. On the other hand, the active layers defined the main direction of curvature as they shrank along the direction of the print pattern longitudinally when heated. The result of this study shows that the proportions (length-to-width ratio) of the printed samples, as mentioned in [72], affect the bending angle. The bending angle increases as the length of the rectangles increases, while the width is kept constant.