## 3 Fabrication Methods
### Other Emerging Methods
The process of 6D printing combines the dynamic, stimulus-responsive properties of 4D printing with the structural benefits of 5D printing by employing five degrees of freedom to produce complex structures that can alter over time. Because of their intricate geometries, the printed items may now interact with their surroundings and have enhanced strength. This opens new possibilities in terms of self-healing, adaptation, and improved medical functions [139; 140; 141]. The third method involves intentionally inducing stress mismatching between layers, either during the design phase or during the printing process itself. By carefully modifying the design and printing parameters, complex mechanical changes can be achieved when the stimulus is applied, effectively releasing the accumulated mechanical pressures [196]. Polyurethane was employed as a material for designing shape memory polymers that exhibit a time-dependent change, triggering cellular response and morphological transformations [198]. Another approach discussed in [199] involves the development of a thermally responsive thergirper using biocompatible and biodegradable poly materials. This thergirper is designed to deliver drugs through its layers and pores. Four-dimensional printing has been implemented using various DIW fabrication methods, making it accessible and compatible with a wide range of materials. The use of 4D printing has enabled the creation of smart, multi-material designs that are highly flexible, deformable, and efficient, with a self-enhancing capability. Numerous examples demonstrate the ability to achieve temporary and recovered morphologies in 4D-printed structures through diverse printing techniques, as illustrated in Figure 13b [200]. In [201; 202; 203], researchers developed more sophisticated structures through the combination of 4 Dp materials. For example, these materials were made of nylon laminated with carbon fibre. They presented a broad range of frequency BW and radiation characteristics that can be utilised for 5G flexible and wearable devices. 4D printing is an innovation of a current 3D-printing method by incorporating time. However, 4D shapes can be deformed along with time upon the stimuli when they are exposed. Four-dimensional materials can respond to water, heat, magnetic and electric forces, light, and stress, as well as strain, pressure, etc. The research presented in [204] introduces a novel hybrid engineering process called 5D printing (5 Dp), as depicted in Figure 14a, which combines additive and subtractive manufacturing methods. This technique enables the printing of intricate structures with three movement axes (dimension: x, y, and z) and two rotational axes. Five-dimensional printing has found applications in creating artificial bones for surgical procedures and food packaging. The printer utilises a printing head and two rotational printing platforms to accommodate the curved surface of bones and provide enhanced strength to the implantable bone structures. Comparative analysis shows that 5D printing is four times stronger than traditional 3D printing and can withstand greater pressure. This capability makes 5D printing suitable for complex surgical components used in surgery, surgical planning, teaching, and learning. Additionally, 5D printing has potential applications in the food-packaging industry for creating intricate boxes, tableware, and other complex designs. Figure 14b showcases the ability of 5D printing to design sophisticated and curved shapes. The versatility of 5D printing extends to automotive and medical applications, as highlighted in [205]. The equipment utilised in medical research needs to be stronger and have more intricate shapes. Five-dimensional printing can produce any curved surface, which can be employed in automotive and medical devices. For instance, a concave type of cap cannot be 3 Dp; it requires a complex design, numerous fillers, and support. Five-dimensional printing facilitates printing by having the ability to print curved layers.