With the progress recorded in the development of 4D printing, 4D additive manufacturing looks promising for future work [26,27]. This technology can be successfully applied to expand several composite structures with shape memory, as in the case of 4D printing with “bi-stable” structures featuring intelligent responses [28]. Shape-memory polymer materials have additional functional capabilities that enable fourth-dimensional production, since these polymer-based materials are stimuli-responsive and have the advantage of modifying their shapes after printing has been complete [29]. In this context, several studies have investigated the use of these polymers in 4D manufacturing.

Bodaghi et al. [26] used double-layer encapsulated polycaprolactone (PCL) and thermoplastic (TPU) shape-memory composite structures 4D-printed for the first time. SME performance is studied by examining “fixity”, “shape recovery”, “stress recovery” and “stress relaxation” under flexural and compressive loading modes. On the other hand, the melting temperature of the PCL material, PCL and TPU influence the transition temperature, switching and net point, respectively. Taking into account the destruction of PCL, the dripping of this molten material and its contact with water, TPU encapsulates PCL, and this encapsulation offers a solution to the interlayer “bond/interface” while surpassing in the SME performance the bilayer printing of PCL-TPU. Subsequent experiments have shown that composites manufactured in 4D have a maximum stress recovery that does not change over time. The modulus of elasticity of TPU at the melting temperature of PCL is 16.5 times higher than that of PCL, because the latter has not been adapted to resist the release of TPU force, since the material has a behavior that is elastic in loading and recovery. Moreover, in the three bilayer and encapsulated structures, we find that the shape recovery values are 100%. In the compressive stress shape memory test, the highest temperature yielded a maximum stress value that did not decrease with time. Compared with extrusion-based SMP structures, the result of this work solves the problem of poor stress relaxation of previous SMPs. In [30], an adaptive metamaterial design with performance directly integrated into materials was investigated using FDM technology. The idea is to understand the thermomechanics of shape-memory polymers and the advantages offered by FDM for programming metamaterials that are self-folding. In this sense, five parameters that can influence material adaptation have been studied: “material”, “platform surface”, “relay-time” for printing each layer, “temperature”, “printing speed” and “liquefier temperature”. Given that the self-folding characteristic affects the change in shape and programming layer by layer, experiments have been carried out to determine how printing speed and liquefier temperature can affect this characteristic. In addition, a finite element (FE) formulation was used to provide a customized description of the materials in both the manufacturing and deformation phases. In this context, the combination of FE and FDM solutions was used to create straight or curved beams as structural primitives with the characteristics of being self-bending and self-winding. This 4D printing study demonstrated that adaptive metamaterials can be used to create prototypes capable of transformation in 2D or 3D and in several fields. This gives the advantage of designing and developing functional structures that feature “self-folding”, “self-coiling”, “self-conforming” and “self-deploying features in a controllable manner”. In [31], the parameters influencing 4D printing were studied, and these parameters mainly concern the design of the structure, the material, programming during printing and activation. In this context, FDM technology has been used with thermoplastic polymers as shape-memory materials (SMPs). These SMPs are printed in temporary shapes and then transformed into permanent forms under the effect of heat. In addition, material selection depends mainly on the shape-memory behavior of the filaments, while design is complex because of the freedom of design, and print programming depends directly on the printing parameters. For polymer activation, there are various methods, such as Joule or infrared activation, and these depends on the “amplitude”, “duration”, “support of the stimulus” and “stimulus environment”. In addition, it has been shown that activation parameters influence the transformation process, so a longer exposure time generates a greater amount of transformation, which reaches its maximum as the stress relaxes. For the activation temperature, the higher the temperature, the higher the velocity and the greater the transformation. Finally, the authors confirm that planning, comparison and presentation of the structured design of experiments offer the presented 4D FDM model advantages in terms of long-term time savings.