## 3 Fabrication Methods
### DIW Method
It encompasses a wide range of areas, including materials innovation and transformative manufacturing approaches [152; 154]. Recently, a novel approach has surfaced for creating electronic circuits using handwriting, where an array of writing instruments, including brush pens, pencils, fountain pens, and ballpoint pens, are employed to directly write on dielectric substrates and create electronic devices. This alternative method has gained attention as a promising way to integrate traditional writing tools with electronic functionalities. This technique of direct writing with a pen offers a convenient and rapid method for depositing electrically conductive materials on substrates, benefiting new biomedical and electrochemical sensing applications, electronic components, and energy storage devices. Pen-based writing enables end users to create and deploy sensor devices on-site as per their requirements. Additionally, when combined with printing methods, pen writing offers simplicity, cost-effectiveness, mass  
Figure 5: Writing electronics used in the literature: (**a**) brush pen, (**b**) fountain pen, (**c**) pencil, and (**d**) ball pen [146]. production capabilities, and high-resolution printing, known as the pen-analogue writing method. This method primarily encompasses applications like electrohydrodynamic and micro-plasma-based drop-on-demand (DoD) techniques. An example of a precise and versatile pen plotter, the AxiDraw machine, is demonstrated in Figure 6, which is used to design various items such as postcards, invitations, or drawings [155]. The researchers have used hand-writing electronics (HWE), which can provide the same benefits as printed electronics (PE) at a lower cost, making it simpler to prototype and create do-it-yourself (DIY) electronics. Figure 6 demonstrates mechanical printing processes, in contrast to conventional direct painting or writing methods, which enhance the accuracy of liquid metal-based flexible electronic circuits, a method that can be utilised for healthcare applications and wearable devices [156; 157]. A variety of electronic devices use silver nanoparticle ink [158]. For example, to print on a mould surface, an automated micro-pen type dispensing device (such as Nordson Pro4 EFD) was utilised. After printing, the mould and printed traces were subjected to a 30-min sintering process in an ISOTEMP 282A vacuum oven [158]. The printed traces were positioned at a specific distance from the injection point on the mould surface to investigate the impact of polymer pressure on the interlocking and adhesion of ink/polymer. Each trace was printed in an inline form, measuring 20 mm in length and 50 um in height. As discussed in detail in [158], a pen-based 3D printer provides a direct 3D structure, but using a pen in space and on a 2D surface is not convenient. Recently, researchers created a pen-based 4D printing (4 Dp), which includes dipping the devices in a monomer solution to turn 2D prints into 3D shapes. One material used in 4 Dp is MXene inks, which are produced from titanium carbide (Ti3C2) sediments that resemble clay. Ti3C2 inks can be utilised for stamping, printing, painting, and writing [159]. A pen-type setup was utilised to design glass nozzles using a micropipette puller (Sutter-based P-1000 instrument, as the company is located in Novato, CA, USA) that provides varied apertures, as shown in Figure 7[160]. Direct laser writing (DLW) offers the capability to create precise patterns without the need for masks or chemicals. In a specific application, DLW was employed to fabricate a three-dimensional (3D) print of reduced graphene oxide (RGO). The researchers utilised a programmable approach that involved scanning a laser beam voxel-by-voxel, with each voxel measuring approximately 300 nm in diameter. By focusing the laser on concentrated flakes of graphene oxide (GO), the overlapping RGO flakes formed a 3D-printed structure.