## 3 Fabrication Methods  
Fabrication methods play an important role in the accuracy and precision of the measurements and impose various limitations on the cost and fabrication time. Various fabrication methods have been utilised for wearable devices, which are summarised in [145]. There is a growing need for affordable and readily accessible fabrication methods that enable the mass production of wearable and flexible electronic devices [11; 12; 13]. DW is an AM method that directly transfers onto a substrate material and that can be used in various industries, including microelectronics, optics, and biomedical engineering. DW is a flexible fabrication method that eliminates the need for moulds or masks and allows for exact material deposition on substrates. Because DW makes high-precision patterning possible, it can be applied to the production of microdevices, PCBs, and biosensing. With the help of the extrusion based DW assembly technique, a large variety of inks may be patterned with feature sizes as small as 250 nm in both planar and three-dimensional forms (3D). However, compressed air is utilised in this procedure to force inks with regulated rheological characteristics via a single nozzle (diameter: 1 to 500 \(\upmu\)m). The DW assembly uses a regulated printing speed and pressure that are dependent on the rheology of the ink and the diameter of the nozzle to deposit inks at room temperature or in an appropriate coagulation reservoir. DW assembly has, nevertheless, been used to handle a broad variety of inks, including colloidal suspensions and gels, nanoparticle-filled inks, polymer melts, fugitive organic inks, hydrogels, sol-gel, and polyelectrolyte inks. While some inks, like polyelectrolyte inks, must be written into reservoir-induced coagulation in order to facilitate 3D printing, other inks, like sol-gel inks, can be printed directly in air and offer superior control over the deposition process, allowing the ink flow to be repeatedly started and stopped during assembly by layer-by-layer extrusion of viscoelastic inks. DIW, a subset of DW, enables the creation of 3D structures with unique shapes. Applications of DIW include energy storage devices, soft robotics, and tissue scaffolding. DIW, called direct-write assembly, is an extrusion-based 3D AM printing method in which liquid or semisolid colloidal inks are dispensed via small nozzles under controlled pressure and flow rates. Several DIW methods are available that can make 3D patterns inside the materials. They can be divided into the following two categories: droplet-based techniques like ink-jet printing and hot-melt printing, and filamentary-based techniques like micro-pen writing, robocasting (or robotic deposition), and fused deposition. Numerous ink patterns have been used, such as concentrated polyelectrolyte complexes, highly shear-thinning colloidal suspensions, colloidal gels, polymer melts, and diluted colloidal fluids. These inks solidify via solvent or temperature-induced phase changes or by liquid evaporation gelation. It is possible to create 3D structures with high aspect ratios (like parallel walls), spanning features, or continuous solids by carefully adjusting the ink composition, rheological behaviour, and printing conditions. Because the \begin{table}
\begin{tabular}{p{142.3 pt} p{142.3 pt}} \hline \hline
**References** & **Various Research Areas That Used Wearable Devices in the Literature** \\ \hline
2022 to 2023 & Soft materials: Enabled electronic for medicine and human interfaces, and soft, skin-integrated multifunctional microfluidic systems \\ \hline
2013 to 2023 & Liquid metal-based electronic: Sensing and multi-site sensing capabilities \\ \hline
2022 to 2023 & Deep learning-based: Blockchain and artificial intelligence technology \\ \hline \hline \end{tabular}
\end{table} 
Table 2: _Cont._later structures require self-supporting elements to bridge gaps in the underlying layer or layers, they present the most difficulty for ink designers.