# New Trends in 4D Printing: A Critical Review
## 1 Introduction
#### Advantages and Challenges  
As a significant development in 3 DP technology, 4 DP is recognized as the next manufacturing evolution. 4 DP can provide flexible structures capable of geometrical transformation in response to an environmental stimulus regarding the characteristic behavior of the material. 4 DP offers the option to change the geometry of 3D-printed components over time. The new technique combines advanced material science, physics, and applications with 3 DP. 4 DP using Shape Memory Materials (SMMs) has a wide range of industrial applications, including aerospace [16] and biomedical [17] ones in addition to packaging, electronics [18], and textiles [7]. The self-operation characteristics of the 4 DP structures, including self-assembly, self-adaptability, and self-repair, have increased their viability in various applications [19; 20]. Self-assembled 4D structures can be designed as independent geometries; while exposed to stimuli they self-organize into an overall structure depending upon the interactions of the geometrical elements and the energy distribution given to the system. Such properties of 4 DP materials might be quite useful, with 4 DP being far more effective in large-scale businesses. It has potential aerospace applications, since in-space manufacturing is fundamental for future missions. 3 DP fabrication in space currently has cost, performance, and energy consumption issues. As a result, instead of employing 3D-printed materials, 4 DP could be employed to take advantage of the transformable nature of such products. 4 DP can provide solutions the manufacture of bridges, shelters, and installations, since it can provide self-reconfiguration in the case of weather interruption [21; 22]. Furthermore, for long-duration space travel to be viable, a paradigm shift in the design and production of space architectures is required. The In-Space Manufacturing (ISM) initiative aims to create technology for these exploratory missions [23]. Additionally, as components can be self-assembled in space, the aerospace industry can use 4 DP to manufacture self-deploying devices and elements related to air control, engine cooling, and other applications [24]. This capability can be used for the building of satellites and antennae [25]. It allows for reducing launch costs by providing flexible tools and avoiding resupply from Earth [26]. NASA is changing its manufacturing approach for space applications from Earth-based to exploration-based, with the potential goal to reach Mars by 2040. In addition, the ESA is planning the development of lunar-based regular missions. The crucial feature of 4 DP is its ability to achieve different shape-morphing behaviors. Other manufacturing techniques cannot, or scarcely can, encode the structures of materials. Figure 2: Distribution of reviewed articles related to 4D printing. For instance, the interior structure of an organ, where numerous cells are held in a multiscale structure, is what drives bioengineers to build tissues and organs through bioprinting. In [27], the authors used a biomimetic hydrogel composite for 4 DP that allowed for local properties. However, the effectiveness of this biomimetic 4 DP technique depends on its capacity to local regulate the orientation of cellulose fibrils inside the hydrogel composite to determine the elastic and swelling anisotropies. 4 DP offers various industrial opportunities. It has the potential to alter the way things are created by developing objects with programmable capabilities. Nevertheless, 4 DP remains in the testing and experimentation phase, and corporations still have a long way to go to make the technology commercially viable. However, as research proceeds, 4 DP's potential uses in a wide range of industries will become clear [28]. 4 DP has enormous potential for today's smart-goods production. The capacity to develop SMs that respond to external factors will lead to significant improvements for the aerospace industry. The defense sector may greatly benefit from 4 DP.