# New Trends in 4D Printing: A Critical Review
## 7 Application and Case Studies
### Aerospace and Automotive  
The intricacy of the components and the challenges in assembling them are one issue in the aircraft sector. The high cost of replacing parts for airplanes and the interruption of the global supply chain are further issues. These issues are lessened by 4 DP technology, since it produces smaller assembly pieces and saves time. In space-limited aerospace applications, self-assembling is of great interest from a time and cost point of view [54]. In the aerospace area, the design phase has been challenging for engineers, based on aircraft specs and performance criteria. It is necessary to optimize aircraft design, and by reshaping SMs, it is possible to optimize structures, especially wings, for all flying conditions [177]. Engineers are significantly freer to ignore design constraints and expand the boundaries of \"trade-offs\" when employing proper Computer-Aided Design tools for modeling, simulation, and multi-objective design optimization based on SM requirements. 4 DP could aid in the formation of a self-deploying structure, which is useful in the aerospace sector. The authors of [174] created and tested a deployable morphing wing made of SMP composite. To achieve a higher degree of compression, the developed morphing wing underwent two deformations during deployment: wing shape recovery from the bending state and SMP filler shape recovery from the compressed state. This type of design can reduce the volume of the wing for storage in a small space. By 2025, 4 DP technology in aircraft is expected to have a market share of more than 25%, trailing only the military and defense sectors [178]. 4 DP might reduce the value of replacement parts by reducing their use and service requirements. 4 DP might aid in the formation of a self-deploying structure, which is useful in the aerospace sector. Airbus is collaborating with researchers to revolutionize the aerospace industry with a new generation of intelligent materials. In order to demonstrate the first programmable carbon fiber jet engine air inlet, they cooperated with MIT's Self-Assembly Lab [111]. In addition, using programmable materials, a research group at MIT demonstrated a non-mechanical morphing car airfoil [179]. Depending on the type of applications in the automobile sector, SMA and SMP components can be employed as thermal actuators in various temperature ranges. Many pre-commercial technologies, including smart vehicle lighting systems, fuel management, climate control, mirror adjustments, locking systems, suspension adjustments, and others, utilize SMMs actuators. Aircraft design and current morphing structures are linked in a unique way; for example, there will be an appropriate wing configuration for every mission based on power and velocity. It is feasible to create Morphing Aircraft Structures such as wings that alter geometry during flight using modern actuators and gadgets [180]. In 2016, Airbus introduced a bilayer structure containing multi-material for manufacturing a shell-like structural component [181]. The technique, in concept, appears to be 4 DP that can revolutionize large-scale manufacturing by promoting the ability to 3 DP entire airplanes. SMPs and SMAs can be merged in 4 DP to create active compliant hinges in small satellites, significantly increasing energy harvesting without adding mechanical complexity. An active compliant hinge with selectively changeable bending stiffness was created by [182] using fusing SMPs and SMAs. The hinge can be activated repeatedly in a slow and controlled way, it can retain any angular position between \(-90^{\circ}\) and \(+90^{\circ}\) and satisfies the stowing requirements of a 6U CubeSat with orientable solar panels. Shape-morphing methods have been employed in aircraft manufacturing to improve the aerodynamic performance, cost, drag, and noise [183]. For morphing actuators, NiTi SMAs have traditionally been widely employed.