# 4D Printing of Multicomponent Shape-Memory Polymer Formulations
## 3 Extrusion Based 4D Printing
### Fused Filament Fabrication (FFF)  
MEX using filaments as feedstock materials was first patented in 1989 by [PERSON] [78] and later commercialized as fused deposition modeling (FDM) by the company Stratasys, Inc. (Rehovot, Israel) As FDM is a registered trademark of Stratasys [79]; therefore, the alternative name fused filament fabrication (FFF) was introduced by members of the RepRap project [80]. Some alternative names such as fused layer modeling (FLM) [81] and layer plastic deposition (LPD) have also been proposed [82], but they are not popular. In this review, FFF will be utilized to refer to the most widely used MEX technique for processing thermoplastic-based materials. The versatility and low cost of the equipment and materials, the range of available materials, and the simplicity of operation and troubleshooting are all factors that contribute to FFF's popularity as a workshop tool in industrial and hobby settings alike [83]. Therefore, it comes as no surprise that there are numerous examples in the literature where FFF has been used for the 4D printing of multi-material trSMP systems. During the FFF process, the feedstock material is forced through a nozzle at temperatures at which it can flow. The generation of flow fields during both extrusion from the nozzle and deposition from a moving head onto the build platform or on top of the previously deposited layers can cause orientation of any anisotropic species present, from fillers down to individual macromolecules. When the material is rapidly (often directionally) cooled during the deposition process, residual texture and thermal stresses arising from both the flow and the cooling processes can result [84]. Shape memory behavior has been observed in thermoplastics available as filaments for FFF such as polylactic acid (PLA) [25; 83; 84; 85], polyurethanes (TPU) [26], acrylonitrile butadiene styrene (ABS) [30], and poly(vinyl alcohol) (PVA) [86]. Heating these thermoplastics beyond their \(T_{g}\) accelerates the stress relaxation process leading to self-deformation, which can be used to prepare self-folding origami by programming the deformation direction based on the deposition pattern of the extruded roads or strands. For example, some layers are printed axially, others transversally and yet others in an isotropic manner to realize twists and bends in selected sections of a printed part [30; 83]. In this review, the emphasis is placed on the 4D printing of trSMPs, which require thermo-mechanical programming to enable a shape-memory effect (SME). Therefore, it is better to deposit layers in a way that leads to isotropic properties in the deposition plane (e.g., alternating layers with \(+45^{\circ}\) and \(-45^{\circ}\) raster angles) at as high a density as possible to prevent delamination of the layers during the forced deformation [87]. The shape memory response of thermoplastics processed by FFF can also be changed by using different processing parameters, such as extrusion temperature and speed and layer height. It was observed that PLA processed at 210 \({}^{\circ}\)C and 40 mm/s and with a 0.3 mm layer height had a larger recovery ratio compared to when it was processed at 180 \({}^{\circ}\)C and 80 mm/s with a 0.15 mm layer height [88]. Additionally, greater shape recovery could be achieved when programming was performed at 85 \({}^{\circ}\)C compared to 65 \({}^{\circ}\)C [15]. On certain occasions, neat thermoplastics need to be modified to increase their printability, in particular to prevent geometrical distortions during FFF [88], and provide additional functionalities, such as enhanced thermal [89] or electrical conductivity [90], magnetic properties [91], mechanical reinforcement [92], or drug release capabilities [93]. Therefore, thermoplastic blends and composites have been prepared to further enhance the thermosresponsive shape memory capabilities of structures printed via FFF. On other occasions,the deposition of two or more distinct materials in different sections of a 3D object can help to realize the desired shape memory behavior.