# New Trends in 4D Printing: A Critical Review
## 6 Printing and Programming Strategies
### Multi-Material Approaches
To generate a cyclopentanone vertical gradient in the SU-8 gel film, skin surface polymerization induced by heating was used. Using photopolymers, a hydrophilic/hydrophobic composite structure was developed in [156]. The rubbery nature allowed for a desirable actuation speed and force printable via DLP. The composite comprised a poly-ethylene glycol diacrylate hydrophilic layer that expands upon swelling in water and a poly-propylene glycol dimethacrylate hydrophobic layer acting as a soft support. The former was printed in two separate structures with different light patterns. The latter was injected and cured at the end of the fabrication of the total structure. Regarding the pattern of printing the material, different shape shifts, such as bending, swelling, opening, closing, and even sequential movement, were achieved. However, none of the abovementioned approaches are reversible, and for repetition, manual programming is necessary. For many technical and biological purposes, designing structures with significant reversible form changes is particularly desirable. However, the materials that can change their structure significantly and irreversibly are few. Hydrogels are typically soft, with a shear modulus ranging between 20 and 300 kPa. Combining SMPs with hydrogels is a solution to achieve the reversible shape change of structures with relatively complicated geometries. In Ref. [109], a hybrid DIW system was employed to fabricate in the same layer a soft elastomer, a glassy polymer, and Liquid Crystal Elastomers. The resulting 2D shape was able to achieve transformations into 2D configurations by heating and cooling. Using this approach, different structures were printed according to the monitoring behavior of auxetic structures; this showed that by fabricating the Liquid Crystal Elastomer pattern and modifying the elastomer and glassy polymer architecture, it is possible to obtain controllable geometry deformations in x- and y-directions. The multi-material was used to create active hinges to achieve reversible bending in a variety of applications: a box that can open and close, a pick-and-place soft robotic gripper, and a hand with five reversible actuating fingers to produce American sign language. In Ref. [157], the authors used bilayer printing and combined stimuli to achieve reversible 4 DP. The forward programming was obtained through asymmetric swelling with heated ethanol, and the recovery was achieved by dry heating. A different approach to pattern printing using hydrogel was introduced in [158]. In this work, a polystyrene pane was remotely stimulated via light emission, and a chitosan hydrogel ink was employed to manufacture actuating hinges. The actuation principle was the thermal stress gradient caused by the heating of the hinges, which converted infrared light into energy. Another method for manufacturing multi-materials is the combination of structures able to react to the same stimulus in different ways. A substantial part of the reason for the increased interest in biopolymeric gels is due to their great biocompatibility and rapid water absorption. Biocompatible gels, including protein-based gels, are studied comprehensively in the literature [159; 160; 161]. However, there are few studies regarding 4 DP with protein-based hydrogels. 3 DP protein-based hydrogels, as presented in [162], transform their shape with programmed motions when subjected to temperature, pH, and enzymatic degradation. Temp-Ink, pH-Ink, and Enz-Ink were used in methacrylated bovine serum albumin as building blocks. These shear-thinning gels are perfect for 3 DP of multi-layered stimulus-responsive hydrogels using DIW. An exclusive aspect of this approach is an enzyme-triggered shape change based on the breakdown of the bovine serum albumin network. By creating protein-based hydrogels that may reversibly change form in response to ambient temperature and pH and irreversibly change due to enzyme degradation, this technology highlights the complexity that can be added to 4 DP systems.