# 4D Printing of Multicomponent Shape-Memory Polymer Formulations
## 1 Introduction  
Shape-memory polymers (SMPs), whose ability to recover from a temporary programmed shape to a permanent shape in response to an external stimulus, have drawn increasing attention because of their scientific and technological significance [1; 2; 3; 4]. Typical stimuli to trigger the shape transition in SMPs include heat (i.e., thermo-responsive), light (i.e., photo-responsive), chemicals (i.e., chemo-responsive, e.g., water/moisture, pH change), or mechanical loading (i.e., mechano-responsive). Thermo-responsive SMPs (trSMPs) have been most extensively studied because of their tailorable transition temperature (\(T_{r}\)) and the convenience with which their shape change may be triggered [5; 6]. The activation of trSMPs can be carried out by either directly, by increasing the environmental temperature (\(T_{e}\)), or indirectly, by Joule heating (i.e., passing electric current), inductive heating (i.e., exposure to an alternating magnetic field: AMF), or photo-thermal effects (i.e., exposure to light) [7; 8; 6]. At the molecular level, trSMPs consists of net points and a switching domain. The net points can either be covalent crosslinks (SMP networks) or physical intermolecular interactions such as hydrogen bonds or crystallites (thermoplastic SMPs) and act as memory components of the network. On the other hand, the switching domains are flexible polymer chains associated with a thermal transition (glass transition temperature, \(T_{g}\), or melting transition, \(T_{m}\)) and are responsible for the shape change process. To observe the shape-memory effect (SME), a thermomechanical programming procedureconsisting of deformation to a temporary shape at elevated temperature (\(T>T_{r}\)) and subsequent cooling under constraint to a low temperature (\(T<T_{r}\)), is required. The recovery to the original shape takes place by reheating the polymer to an elevated temperature (\(T>T_{r}\)). The quantification of the SME is carried out by determining the shape-recovery (\(R_{r}\)) and shape-fixity ratios (\(R_{\beta}\)), as commonly described in the literature [9; 10; 11]. A combination of SMPs and other functional materials is helpful to improve properties and increase relevance in technological applications. Such multicomponent SMPs provide high deformation capability, improved recovery force, faster shape recovery, and better shape fixity/recovery ratios [1; 12]. The excellent performance of multicomponent SMPs has found potential utility in the aerospace, biomedical, electronic, and textile industries [13; 14]. However, conventional processing techniques are limited in their ability to produce complex structures with precise dimensions useful for the aforementioned fields [15; 16]. For this reason, additive manufacturing is being investigated. Additive manufacturing, colloquially known as 3D printing (3 DP), has attracted increasing interest of academic and industrial researchers and engineers due to its appealing features, such as prototyping freedom and the ability to realize complex structures at high resolution [17; 18; 19]. The 3 DP of smart, deformable materials has resulted in the creation of a new term, \"4D printing (4 DP)\", which refers to the ability of materials to alter their form after they are printed, with time as the fourth dimension that is controlled through the design of dynamic structures [16; 20]. Four-dimensional printing provides additional functional capabilities and performance-driven utility in a broad range of domains, such as aerospace, biomimetic, biomedical, electronic, acoustic, textile, fashion, mechanical, defense, retail, and food industry applications [21]. The term \"4D\" in combination with \"printing\" emerged for the first time in 2012 in a conference proceeding, where it referred to a 4D barcode (color + 2D + time) [22]. However, the first journal publication related to the 3 DP of photo-polymeric inks with shape change capability appeared in 2013 [23].