# 4D Printing of Multicomponent Shape-Memory Polymer Formulations
## 2 Multicomponent SMPs  
SMPs are sometimes used in isolation as single materials; more often, they are combined with other functional materials. For example, polymers, metals, or other inorganic materials, in the shape of particles, fibers, and laminates, are combined to create multicomponent systems. Since these multicomponent systems are built up from discrete phases consisting of SMPs mixed with other polymers at the macroscopic or molecular level or reinforced with other domains (e.g., inorganic particle), they are particularly suited for the realization of multifunctionality [1; 34; 48; 49; 50]. Composites that consist of a polymer matrix with solid filler or reinforcement are common multicomponent systems to enable multiple functions in a single material [4; 51]. Active fillers such as iron oxide nanoparticles (IONP), carbon black (CB) and carbon nanotubes (CNT) are incorporated into SMPs using conventional processing techniques [36; 52; 53]. These fillers introduce new functions such as magneto-sensitivity, electric conductivity, or photosensitivity into SMPs along with thermal and structural functionalities [50; 51; 54]. Furthermore, when well-dispersed and compatible with the matrix, nanofillers provide much higher interfacial area per unit mass and much lower interparticle distances vs. conventional fillers, resulting in fundamental changes in polymer microstructure and behavior in the resultant nanocomposites. Additionally, high-aspect-ratio nanofillers are less readily damaged than their larger equivalents, providing the potential for much lower percolation thresholds in practice. However, being distinct phases, each constituent in such a multicomponent system remains recognizable and retains its characteristics, and well-defined adhesion levels between these phases are required for the successful realization of new functionalities [36; 55]. Other possible arrangements to enable SMP-based multicomponent systems are blends, interpenetrating polymer networks (IPNs), and macroscopic layered architectures [56; 57]. In the blending approach, instead of non-polymeric domains, SMPs are physically mixed with other functional polymers with or without chemical bonding. The polymer blend not only alters the thermal and mechanical properties of the base SMP but can also offer new functionalities not found in the original polymers. Here, the compatibility of the blend components is an important criterion in the development of high-performance multifunctional materials. However, most polymer blends are heterogenous systems, as sometimes, the structural differences of the individual polymers are more important than their composition. For instance, polyethylene and polypropylene technically have the same composition, since they are both saturated hydrocarbons with the same C:H ratios, but they do not mix [56; 57]. Another approach with similar spatial organization is an IPN, a thermodynamically stable arrangement of multicomponent crosslinked polymeric materials. An IPN is a combination of two or more polymer networks which are not covalently linked to each other and are obtained by two independent crosslinking reactions occurring sequentially or simultaneously to form two different networks. Numerous IPN-based SMPs have been investigated in recent years, and some have exhibited improved properties and new functionalities as well [57]. Among these different techniques, multilayer technology is considered one of the most efficient and practical owing to its simplicity and versatility [58]. SMPs and other functional polymers with desirable functionalities and architectures can be stacked together in a bi-/multilayer polymer system with a simple interface [59]. For example, combinations of SMPs with soft and rigid layers or layers with different coefficients of thermal expansion have been produced to enable novel shape change behavior [60; 61]. However, successful integration of different functions depends on the individual layers, layer geometry and the character of the interface between layers [58].