# 4D Printing of Multicomponent Shape-Memory Polymer Formulations
## 1 Introduction
Here, the initial configuration was created by 3 DP, while the programmed action of the polymer created a time-dependent configuration--the 4D aspect. Since then, hundreds of publications related to 3 DP with smart, programmable, and stimuli-responsive materials, resulting in such 4D effects, have appeared in the literature, as shown in Figure 1. The number of publications and patents based on \"Web of Science\", \"scopus.com\", and \"SciFinder\" searches using the term \"4D printing\" and \"4D printing of SMPs\" have shown an exponential increase since 2012. SMPs are the most investigated 4D printed materials, and this can be attributed to their tailorable elastic properties and immense potential for technological applications [24; 25; 26; 27]. The increasing number of publications related to the term \"4D printing of SMPs\" is shown in Figure 1b. Various functions such as sensing, degradability, and self-healing have been combined with shape-memory capabilities in 4D-printed objects using different 3 DP techniques [24; 28; 29; 30; 31]. Furthermore, as SMPs can respond to various stimuli (such as temperature, humidity, electric current, magnetic fields, and changes in pH), this opens unprecedented opportunities for 4 DP [27; 32; 33; 34]. However, most current publications deal with the 4D printing of ttSMPs, with their shape-memory response activated by direct or indirect heating methods [32; 35]. With the growth in 3 DP technology, it is expected that SMP-based 4D objects responsive to other stimuli will also be developed, which will create a myriad of scientific and technological possibilities [36]. Recent advances in 3 DP have focused on printing of multicomponent SMPs to produce complex 4D objects with enhanced properties along with the shape-memory effect (SME) [16; 28; 37; 38]. Generally, in a multicomponent system, polymers are combined with metals or other inorganic materials to realize new functions or improve thermal or mechanical properties [1; 34; 39; 40]. The fabrication of such heterogeneous systems presents an exciting challenge for fundamental and applied research involving knowledge of physics, chemistry, mathematics, materials science, and process engineering [38; 41; 42; 43]. Based on growing interest in this area, we aim to review the recent advances in 4D printing of multicomponent SMP formulations with more than one discernible phase (e.g., SMP-based composites and blends), as each of the phases brings new functionality to the printed object. The existing surveys and reviews covering the subject of 3D and 4 DP of SMPs partially address the multicomponent aspect of SMPs [16; 38; 41; 44; 45]. Multicomponent SMPs have unique functionalities but complex molecular architecture, including phase-segregated  
Figure 1: Publication statistics on the topic of **(a)** 4D printing and **(b)** 4D printing of SMPs **(c)**. Data were collected from Scopus (Elsevier, Amsterdam, The Netherlands), Web of Science (Clarivate Analytics, London, UK), and SciFinder (Chemical abstracts service CAS, Columbus, OH, USA). (c) Patent statistics on the topic of 4D printing and 4D printing of SMPs. Data were collected from SciFinder. All searches were performed in June 2022.  
domains associated with different thermal transitions, as well as functional fillers and/or additives [1; 44]. Three-dimensionally printing these materials is challenging, and it is crucial to show the limitations and possible improvements in the context of different 3 DP techniques [46; 47]. Therefore, the present review article highlights current literature concerning the 4 DP of multicomponent SMP formulations using different AM techniques. The review starts with a general overview of multicomponent SMPs, fabrication strategies, limitations, and potential applications. The subsequent sections describe the different 3 DP technologies that are particularly applicable for the printing of multicomponent SMP formulations.