# New Trends in 4D Printing: A Critical Review
## 6 Printing and Programming Strategies
### Multi-Material Approaches  
Due to the limits of a single SM's physical qualities, imperfect structures are frequently produced while printing with them. To promote the regulated shape-memory behavior of the printed structure, desirable thermomechanical behaviors may be achieved by mixing printing materials and using multi-material printing processes. Hinges, joints, bends [118], or twists [25] can be produced at the structure's interfaces by printing a mix of rigid and inactive materials with various thermomechanical characteristics. These features respond to the stimulus by producing differential stresses. Multi-material structures utilize the differences in material properties (i.e., thermal expansion coefficient, elasticity, swelling ratio, etc.) to produce a structural change. The combined printing of SMs with each other or with static ones adds different design properties when creating actuating parts. SM printed regions may serve as a component's hinge or actuation zone, while static material serves as the stiff framework that ensures the component retains its shape. Conversely to an overall transformation, only specific zones characterized by SMs react and actuate as exposed to stimuli [129]. Heterogeneous lattices were made in [52], combining several materials. It was found that the 2D and 3D open cell lattices may be swiftly and frequently actuated in a steady, well-controlled, and dependable manner by employing temperature as a stimulus. A diverse medium for the integrated design and production of challenging shape-morphing structures is also made available by this multi-material 4 DP lattice. The printing of multiple materials in a single component is an effective method to fabricate dynamic 4 DP structures. There are different approaches in the literature: some uses of blend materials are reported [145] where, to simulate sequential petal opening and sequential drug-releasing effects, samples with different PLA and polycaprolactone ratio composites were printed by using ink based on the DWI technique. The findings proved that these 4 DP composites can be employed in the manufacture of bio-inspired robots and biomedical equipment. In another approach, the SME in biodegradable blends based on PLA and polycaprolactone in different concentrations was studied in [146]. Regarding their results, the thermal degradation of PLA was improved by the polycaprolactone addition and the Tg decreasing from 67.2 to 55.2 \({}^{\circ}\)C.  
In bilayer multi-material printing, the shape-morphing ability of multi-materials typically depends upon the different reaction to the applied stimuli. Complex shape transformations can be designed based on this difference, e.g., by employing a combination of active and passive materials. Figure 7a-c shows the schematic of multi-material transformation before and after stimuli application. Numbers 1 and 2 indicate the passive and active layers, respectively. In Ref. [147], PLA was printed via Fused Deposition Modeling on a paper substrate with a thermo-responsive composite bilayer. The actuation capabilities were described and compared by tailoring the printing raster angle. As a result, the anisotropic stiffness of the method and the polymer layer's other characteristics were determined. The softening of the polymer layer above the Tg and the bilayer effect during the heating and cooling processes made the material a reversible soft actuator. A multi-layer membrane structure composed of alternating different materials can spontaneously provide a 3D geometry under heating and recover its original shape after cooling, due to the difference in the thermal expansion coefficient. In Ref. [117], the 4 DP self-morphing composite was characterized by two distinct parts: the bottom layer consisted of homogeneous resin, and the top layer was made of fiber-reinforced composites. The deformation was caused by the difference in thermal expansion coefficients between continuous fibers and the deformable matrix. These materials can provide the desired transformation in a controllable way.