## 3 Fabrication Methods
### Other Emerging Methods  
Four-dimensional printing is a groundbreaking advancement in 3D-printing technologies that introduces the concept of self-transformation and functionality in printed materials. As illustrated in Figure 13a, 4D printing enables materials to change shape and perform specific functions in response to stimuli such as osmotic pressure, heat, current, ultraviolet light, and various energy sources. Unlike traditional 3D printing, which operates in three dimensions (\(x\), \(y\), and \(z\) axes), 4D printing adds a fourth dimension: time. This new dimension allows for dynamic and programmable behaviour in printed objects, revolutionising the design possibilities and applications of 3D-printing technology [195]. This fourth dimension allows for dynamic changes in the geometry of printed objects in response to external stimuli, thanks to the unique properties of smart materials. The design process of 4D printing enables the creation of objects that can undergo shape transformations and adapt to their environment, enhancing their functionality and versatility [196]. The development of 4D-printed devices involves the selection of various components, including printing technologies, materials, structures, stimuli, and applications. One of the key features of 4D designs is their ability to change shape, which can be achieved through three different approaches: using stimuli-responsive materials, employing a multi-material, or artificially inducing internal tensions [197]. The first approach, utilising stimuli-responsive materials, is widely popular and often considered the primary mechanism in 4D printing. In this technique, the 4D-printed structure initially possesses a temporary shape, and it transforms into its intended permanent shape when exposed to the appropriate stimulus. The second approach, known as multi-material, structures, leverages the unique characteristics of different materials, such as elasticity, swelling ratio, or thermal expansion coefficient, to achieve shape-changing capabilities. The multi-material structures combine many materials with varying properties, giving them special qualities, as discussed in 3D- to 6D-printing technologies. Different thermoplastics, polymers, and smart materials, including hydrogels and shape memory polymers (SMPs), can be combined with these structures. Combining these materials allows the production of objects with dynamic behaviours, such as the capacity to react to mechanical forces, temperature changes, and pressure, in addition to strength and durability. In biomedical applications, implants or devices must adjust to shifting biological conditions or mechanical stresses. The use of smart materials is the main factor driving the mechanism of form change in these multi-material structures. In 4D- and 6D-printing methods, these materials are designed to undergo controlled deformations in reaction to  
Figure 12: Spray coating process and semi-transparent capacitive-touch device [193]. external stimuli. SMPs, for example, can be configured to hold a given shape and then revert to that form in response to a given stimulus, like heat or light. In contrast, hydrogels can expand or contract in response to variations in temperature or humidity. The capacity of these materials to recall and return to their original form when necessary is a crucial component of this shape-changing mechanism. This characteristic is essential for medical applications, like the creation of implants that may alter shape in response to changes in the surrounding tissue or the patient's growth. The integration of smart material science and additive manufacturing techniques is the foundation of these technologies' entire operation. The first step in the process is 3D printing, which constructs an object layer by layer. With the introduction of other dimensions like time and multi-directional production, this develops into 4D and 5D printing.