# New Trends in 4D Printing: A Critical Review
## 7 Application and Case Studies
### Soft Robotics  
This field is based on the use of low-compliance and deformable materials with 4 DP, allowing the possibility to design and fabricate internal structures such as channels and shafts that enable robots to achieve complex movements. Furthermore, the 4 DP multi-material allows soft robots to simultaneously manufacture integrated sensing and actuation [186]. The advantages of mechanotherapy for the treatment of various injuries or sick tissues are discussed in [187]. Further research should be done on these systems, which through stretching and compressive stress, could aid in cell regeneration. The use of SMP filament fibers for therapeutic compression and massage therapies was investigated in [177]. Through cyclic heat activation, massage was obtained by taking advantage of the embedded stress memory fiber. Additionally, the ability of the origami structure to absorb impacts is an intriguing topic for wearable technology for the elderly or others who are prone to falling. Impact absorption might significantly lessen the force placed on joints and knees for athletes and other active people, potentially reducing the risk of injury. Furthermore, the potential of multi-material devices can introduce new functionalities through a variety of means, such as multi-position actuation and multi-activation stimuli. Controlled microstructural architectures have demonstrated superior performance over conventional materials as rationally planned structures on the micro- or nanoscale because of their new optical, mechanical, or electrical capabilities [53]. A potential method that enables the production of many sophisticated 2D and 3D devices from particles, pillar plates, walls, or films is capillary-force-driven self-assembly. This can be employed for the development of microrobots and actuators [67]. A few mechanical metamaterials with intricate microstructures, including lattice structures and hierarchically architected metamaterials, have been successfully created via 3 DP. The configuration and spatial arrangement of the microstructures determine the mechanical characteristics of metamaterials, which implies that the qualities are fixed and irreversible after production. These characteristics are reversible thanks to 4 DP [26]. Smart devices able to form 3D origami and kirigami structures are applied in bioelectronics, metamaterials, micro robotics, and microelectromechanical systems [107]. Dual-photon lithography-based 4D microprinting, which was employed by [53], allows for the reversible and bidirectional self-assembly of systems. Using a variety of hatching distances, each liquid responsive micro-sheet was printed as two separate pieces with asymmetric crosslinking densities. The 4 DP curved microstructures spontaneously had regulated thickness, curvature, and smooth morphology when generated in n-pentane, which is difficult to create using the conventional 3 DP method. To produce a strain differential between the layers, the actuators in [152] were printed in two pieces: the bottom passive layer and the top active layer. The actuator was forced to bend upward because the top layer was designed to have a larger strain stored in the printing lines. To guarantee that no SME was added to the gripper or the passive components of the design, the same set of parameters was employed at the bottom layers of the actuators as for the gripper's body. Furthermore, 4 DP is the major driver for smart autonomous system creation. Reactive actuators fabricated through 4 DP can intrinsically respond to the environment. SMA's ability to reversibly change mechanical properties in response to heat stimulus has found applications in robotics. SMAs may be utilized as integrated sensors and actuators in thermomechanical systems, allowing them to track certain intended functionalities while sensing changes in external stimuli.