# New Trends in 4D Printing: A Critical Review
## 1 Introduction  
Additive Manufacturing (AM) or 3D Printing (3 DP) is a digital manufacturing technology that provides nearly limitless potential to construct structures by precisely adjusting material properties, process parameters, and shapes [1]. The invention of AM techniques has pushed the limitations set by traditional manufacturing processes, leading the world into the \"third industrial revolution\", in which computers and automation work together to produce items quickly [2]. AM methods enable the construction of complex structures with little material waste. This digital manufacturing technique involves adding materials layer by layer to construct components directly from 3D model data. 3 DP has grown in popularity over the last two decades due to a variety of compelling benefits, including the capacity to make inexpensive, multipurpose items characterized by intricate structures [1]. 3 DP is a rapid and affordable manufacturing technique that finds a wide range of applications in biomedicine [3], aerospace [4], the automotive industry [5], robotics [6], smart textiles [7], soft electronics [8], and wearables [2].  
Recent breakthroughs in AM have offered revolutionary printing technologies for producing smart objects that can be switched between numerous configurations via environmental inputs [9]. This innovative approach, originally referred to as 4D printing (4 DP) by a research group at the Massachusetts Institute of Technology in 2013 [10], produces 3D-printed structures that can actively modify their configuration in reaction to an external stimulus or interaction mechanism; consequently, the mechanical condition of the 4 DP item may be observed to transition from static to dynamic [11]. Advanced research on 3 DP dynamic shape-shifting has recently received attention as the next significant advance in AM methods [12]. In contrast to typical 3D-printed components, which are frequently stiff and static [13], 4 DP is the next generation technology because of the merged dynamic behaviors inside the Smart Material (SM) [2]. 4 DP dynamic structures with changeable shape modifications have attracted industrial interest, since they can provide advanced features such as self-assembly, self-repair, and self-adaptability [14].  
4 DP can be defined as a combination of different factors, including 3 DP technology, SM, shape-shifting behavior, design for 4 DP, stimuli, applications, mathematical modeling, and finally the programming process [12; 14; 15]. Important elements to be considered in 4 DP are summarized in Figure 1. Each element is intimately connected to the others, and particular attention must be paid to the integrated framework design.  
The literature survey methodology employed in this review was structured by following the search, inclusion, and exclusion criteria listed in Table 1. The initial research returned 31,016 papers, but not all were concerned with 4 DP. The exclusion/inclusion criteria allowed us to filter the studies and increase their relevancy. As a result, 1204 articles were analyzed, and an in-depth analysis was undertaken, allowing a reduction to 197 papers. The analysis was focused on the applications, materials, stimuli, technologies, and programming strategies. The publication period was 2013-2023, and the number of papers is distributed as shown in Figure 2.  
\begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt}} \hline \hline
**Sources** & **Web of Science, Springer, MDPI** \\ \hline Publication period & 2013–2023 \\ \hline Keywords & 4D printing, additive manufacturing, smart materials, shape memory recovery, multi-material, shapeshifting \\ \hline \multirow{2}{*}{Scope} & Applications, materials, stimuli, technologies, programming strategies, limitations, accuracy, repeatability \\ \hline Publication type & Original research article \\ \hline \hline \end{tabular}
\end{table}
Table 1: Inclusion and exclusion criteria for the review research methodology.  
Figure 1: 4 DP structural elements.