# Recent Advances in Wearable Devices for Non-Invasive Sensing
## 3 Nano/Microfabrication Techniques  
Various nano/microfabrication techniques including physical vapor deposition (PVD), chemical vapor deposition (CVD), electrodeposition, lithography, and printing have been extensively studied and used for the fabrication of wearable devices. We recommend the readers refer to the following review papers to find more detailed descriptions on the processes and properties of the techniques [87; 88; 89; 90; 91]. PVD including thermal evaporation, electron-beam (e-beam) evaporation, and sputtering is widely used for the deposition of metal and metal oxide-based materials into thin films on substrates [92; 93; 94; 95]. Polymer solutions and dispersions of various conducting materials such as metal particles, metal wires, or graphene can be deposited onto diverse matrices and substrates using techniques such as spin-coating, vacuum filtration, and spraying [96; 97; 98; 99]. The adhesion between the deposited materials and the matrices is achieved by chemisorption or physisorption, and to enhance the adhesion and compatibility between them, surface treatments including O\({}_{2}\) plasma and silane coupling reaction are widely exploited [100; 101]. Electrodeposition is a process of forming thin films of metal-based materials or polymers by applying electric current onto a conductive material that is immersed in a solution containing precursors such as metal salts, monomeric or polymeric materials [102; 103; 104]. Processing temperatures, chemical resistances of substrates and the deposited materials, the required thickness and the uniformity of the deposition, and the geometrical properties of the substrate matrices are various factors to consider for the selection of deposition techniques. For instance, many flexible or stretchable substrates such as PDMS are susceptible to thermal expansion and contraction under temperature variations, which may cause damages on the deposited metal layers, thus favoring low-temperature processes [105]. For the patterning of deposited layers, photolithography that consists of UV exposure, wet etching or dry etching steps, and e-beam lithography are the most widely used techniques [71; 106; 107]. These techniques allow high-resolution patterning with nm scale feature sizes for e-beam lithography and tens of nm to \(\upmu\)m scale feature sizes for photolithography but have disadvantages of high-cost and small patterning area [90]. E-beam lithography provides low throughput compared with other fabrication techniques. Printing techniques such as screen printing, roll-to-roll printing, and inkjet printing are also extensively used for the nano/microfabrication of flexible and stretchable devices [108; 109; 110; 111]. In screen printing, inks are deposited and spread on to the mask that is placed and aligned onto a substrate, thus resulting in a selective and templated deposition of the ink onto the desired sites of the substrates [112; 113]. The screen printing technique is compatible with diverse matrices including textiles, elastomers, and paper, and the minimum feature sizes that can be obtained using the technique are usually tens of \(\upmu\)m, but highly dependent upon the composition of ink and particle sizes [114; 115; 116]. Direct writing and 3D printing enable the patterning of various materials onto substrates of diverse geometries and compositions [117; 118; 119]. [PERSON] et al. demonstrated high-resolution and reconfigurable 3D printing of liquid metal using direct printing as shown in Figure 4i[60]. The resolution of printing was controlled by the diameter of nozzles, and the minimum line width of 1.9 \(\upmu\)m was obtained reliably. The printed liquid metal could be lifted-up and relocated by the nozzle tip to form 3D structures. Soft lithography uses elastomeric molds and stamps that are patterned and engraved into desired patterns to produce patterns of various materials as shown in Figure 4h, which allows relatively simple and quick replication, large pattern areas, and low-cost processing [120,121].