## 3 Current Status of 3D Printing in Industry 4.0
### Technological Evolution in the Context of Industry 4.0
The most straightforward approach to achieving 4D printing is by 3D-printing a single smart material, a method that has recently attracted considerable interest from material scientists. The most commonly utilized smart materials for shape-shifting are shape memory polymers (SMPs) and liquid crystal elastomers (LCEs). Consequently, 4D printing can be realized by printing a single SMP for one-way actuation or an LCE for two-way reversible actuation [28]. 4D printing, achieved by 3D-printing smart materials like shape memory polymers (SMPs) and liquid crystal elastomers (LCEs), enables objects to change shape in response to external stimuli. SMPs are widely used for one-way actuation, while LCEs allow for two-way reversible actuation. The process typically involves programming SMPs at a specific temperature, fixing the shape, and then applying heat or light to recover the original form. Recent innovations in 3D printing techniques, such as UV-assisted direct ink writing (DIW) and magnetic field actuation, have expanded the applications of SMPs in biomedicine, flexible electronics, and soft robotics. SMPs can also facilitate multiple or automatic shape-shifting without the need for reprogramming. Using materials like Gray 60, researchers have demonstrated sequential shape recovery by programming two temporary shapes at different temperatures. Additionally, heat-shrinkable properties allow for complex shape transformations, with examples like foldable lattices and self-folding origami structures. These developments underscore the versatility of SMPs in 4D printing for creating dynamic structures. LCEs are another key material for 4D printing, offering reversible shape changes in response to heat. Integrating LCE films onto 3D-printed objects with conductive wires has enabled thermal actuation in devices like crawling robots. Recent advancements have focused on directly printing LCEs using high shear forces to align their molecular structures, leading to rapid and programmable shape transformations. Incorporating nanoparticles like gold and carbon nanotubes enhances LCE functionality, further expanding their use in 4D printing. Multimaterial 4D printing allows for the creation of structures with locally controlled properties, generating eigenstrains for shape changes in response to environmental stimuli. Systems like SMP composites and multimaterial SMPs enable multiple shape-shifting and complex designs. Techniques such as bilayer SMP composites and self-folding electronics have further advanced this field, with applications ranging from aerospace to biomedicine. These innovations highlight the potential of multimaterial 4D printing for developing complex, responsive structures. 4D printing has progressed from focusing solely on shape-shifting to also including changes in functional properties. This advancement allows for the integration of shape transformation with functional devices like electronics, while properties such as optics and conductivity can evolve alongside these changes. Time-dependent functions like tissue maturation, degradability, self-healing, and color shifting are now considered part of 4D printing. Recent developments in 4D bioprinting and self-healing materials highlight the expanding potential of multifunctional 4D printing [28].