**5. 4D Printing**

4D printing or smart printing has a unique basic characteristic that differentiates it from the static 3D-printing structures; 4D-printing materials are dynamic and able to have functionality [8]. The well-used definition describing the 4D-printing states "It is the evolution of a 3D printed structure either in shape, property, and functionality when it is exposed to external factors such as light [38], heat [39], pH [40], and water [41]".

*New Industrial Sustainable Growth: 3D and 4D Printing DOI: http://dx.doi.org/10.5772/intechopen.104728*

**Figure 11.** *3D vs 4D printing.*

4D printing can be defined as the best combination of a smart material, a 3D printer, and a well-programmed automated design (**Figure 11**) [8].

There are five factors that influence the 4D printing which are the additive manufacturing process, feedstock material, stimuli, interaction mechanism, and modeling [42].

According to F. Momeni and J. Ni, there are three laws that define the shapechanging behavior of 4D-printed objects [43]. The first law states that "all the shapes changing behaviors such as curling, twisting, coiling, bending, etc. of multimaterial 4D structures are due to the relative expansion between active and passive materials."


#### **Table 1.** *Types of materials used in 4D printing.*

**Figure 12.** *4D printed metamaterials reconfigurable object [48, 49].*

The second law states that "there are four physical factors behind the shape changing ability of all multi-material 4D structures i.e., mass diffusion, thermal expansion, molecular transformation, and organic growth."

The third law states that the "time-dependent shape-morphing behavior of nearly all multi-material 4D printed structures is governed by two "types" of time constants" (**Table 1**).

There are revolutionary applications associated with the 4D printing, such as biomedical applications of 4D printing in drug delivery, organ regeneration and transplantation, and tissue fabrication [44]. 4D-printed structures have great potential in soft robotics because of their capability to deform, adjust to environmental changes, and flexibility [8]. 4D-printed structures with smart materials can be used as self-evolving structures [45, 46], active origami structures [47], self-sustainable satellite manufacturing parts [8], sensors responsive toward moisture, temperature, pH, magnetic energy, etc. [8]. Despite diverse applications, 4D printing needs more research and development, especially in scaling it up. Commercializing the 4D printing is troublesome because of the high production cost, installation cost and material used and availability. Multi-materials printers could be a possible solution but need furthermore research (**Figure 12**).
