# New Trends in 4D Printing: A Critical Review
## 3 Design for 4 DP  
Currently, most of the research in 4 DP is concentrated on the shape-changing capabilities of SMs, such as the elongation, bending, corrugation, expansion, and curling of 4 DP materials. [15]. These basic transformations are all goals of shape-changing mechanisms [53]. They conceptually reflect the spectrum of changes through time and space to truly describe a change in shape, property, condition, or functioning. This is especially true for functions like helixing, buckling, curling, waving, etc. Such transformations provide an initial conceptual step for the definition and specification of smart devices, upon which further needs might be specified. The ideas and theories of transformation and reconfigurable systems emphasize these functions and fundamental transformations [54]. To incorporate 4 DP information into mechanical design, traditional product models must be updated. Given that this technology primarily enables shape, function, and property transformations, as well as multi-material fabrication, it is critical to reexamine traditional design and models in order to support multi-purpose knowledge and rules spanning space and time [53]. One issue with 4 DP is that research studies continue to rely on the \"trial-and-error\" method. A wide range of research initiatives have been sparked by the incorporation of AM in design and engineering, including topologically optimized solid or lattice, multi-material geometries, organic structures inspired by biotechnology, and multiscale systems [55; 56; 57; 58]. Except in deliberate instances, relatively little focus has been placed on designing and manufacturing more complicated 3D structures or folded or creased structures. Therefore, 4 DP requires math to design the material distribution and structure to produce the desired change in form, property, or effectiveness. To establish the links between the four basic elements--material structure, desired final form, material characteristics, and stimulus attributes--theoretical and numerical models must be constructed [25]. Researchers have presented models and simulations [7; 59] to better understand and anticipate the shape-shifting behavior of 4 DP objects [20]. Indeed, recent attempts have stressed the need to approach 4 DP from a design standpoint, introducing the concept of design for 4 DP. To achieve a desirable shape-changing and active structure, understanding the appropriate material distribution (passive and active) at the appropriate location plays an important role. In addition, the proper stimulus at the right time and location to alter the part's shape needs to be considered [60]. Conversely, the design of an active composite geometry to accomplish a goal shape change is difficult because it demands solving an inverse problem with spatially heterogeneous, markedly nonlinear material behavior within a complicated boundary problem. As a result, computational assistance and adequate geometric representation are required. The primary determinant of the precise form changes caused in an active material or composite is the spatial arrangement of both materials, commonly referred to as the material distribution. Conceptually, this offers a significant design advance in the form of a parameter that unlocks a virtually limitless range of options that were not previously possible for special standard materials. Inversely, this complicates finding the appropriate method or plan of action to achieve the optimum material distribution for a particular demand [61]. This inevitably results in two distinct design approaches: knowledge-based design, which is intuitive, and computational design, which may be contrary to intuition [62]. The former offers simple answers for fundamental demands that may then be compounded to address more complicated requirements by utilizing experience, knowledge, human intuition, and understanding of basic difficulties. Examples include simple patterns and multi-layer designs.