## 3 Fabrication Methods
### DW Methods  
Three-dimensional printed designs utilise printing layer-by-layer and additive manufacturing methods (AMs). Three-dimensional printing (3 Dp) has various advantages as compared to other methods. It provides design flexibility and high resolution as well as low cost [163]. AM is an advanced technique that encompasses various methods such as fused deposition modelling (FDM), selective laser melting (SLM), electron-beam melting (EBM), aerosol jetting, inkjet printing, and more, which have become increasingly accessible in recent times. The new generation of AM techniques allows for the utilisation of a wide range of materials, including polymers, metallic nano inks, ceramics, nanocomposites, and alloys. These AM methods enable the production of flexible sensor devices. Notably, extrusion methods like FDM and FFF (fused filament fabrication) are commonly employed in AM for creating 3D designs. Some examples are depicted in Figure 8, including a rigid device shown in Figure 8a [164], a flexible device shown in Figure 8b [165], and a microsensor shown in Figure 8c [166; 167]. For the successful printing of complex and 3D surfaces, precise control of the laser beam is crucial. To achieve this, a shutter is typically placed in front of the nozzle to modulate the beam. Additionally, maintaining an optimal distance between the nozzle and the substrate is essential, typically ranging from 1 mm to \(\geq\)10 mm. Deviating from these specified boundary conditions can lead to undesirable overspray defects in the final design [168; 169]. Three-dimensional printing technologies have significantly impacted the biomedical field, emerging as an innovative method with diverse applications such as tissue engineering, organ fabrication, medicine, drug delivery, and more. In the biomedical domain, various 3 Dp methods are utilised, including FDM, selective laser sintering (SLA), and DIW. These methods make use of a wide range of materials, which are summarised in Figure 9 [164]. Figure 7: Flexible electronic: pen-based 4D printing [158]. Three-dimensional printing is utilised in the fabrication of various medical devices and structures, such as patient-specific orthoses, prostheses, craniofacial implants, and medical devices based on individual data. Three-dimensional printing utilises low-melting-point metal ink in an additive mode, offering versatile applications across various fields. In [170], liquids such as gallium, bismuth, and indium alloy were used to fabricate 3D devices. In the realm of 3D devices, certain inks such as Bi35 In48.6 Sn16 Zn0.4 exhibit unique characteristics wherein they do not absorb or release heat while behaving like metals during phase changes. Leveraging these novel ink properties, along with silicone rubber (a nonmetal), hybrid 3D-printed structures for wearable applications have been successfully fabricated. These structures possess remarkable mechanical and electrical properties while also exhibiting thermal conductivity. The exceptional insulating properties  
Figure 8: Flexible devices: (**a**) gradient echo device [164], (**b**) electroluminescence device [165], and (**c**) serpentine channel with integrated coverslips [167]. Figure 9: Fabricated devices using traditional 3D-AM methods. of the non-metal ink contribute to the durability and functionality of the 3D-printed designs, ensuring their effective performance even in demanding environments. Three-dimensional printing offers the capability to create a diverse range of antennas, encompassing polymer, metallic, ceramic, composite, and integrated antennas with multiple materials. For instance, an e-band lens antenna was designed using an automatic synthesis algorithm and realised through polyjet 3D printing technology using VeroWhite materials [171]. Furthermore, the field of electronics, sensors, and electrochemical devices has seen the utilisation of several conductive polymers and nanocomposites, enabling advancements in their respective applications [172].