# Recent Advances in Wearable Devices for Non-Invasive Sensing
## 2 Materials and Structural Design
### Materials
#### 2.1.1 Metal-Based Materials  
In this section, we introduce various metallic materials that show promise for use in stretchable and flexible electrodes. By using micro/nanofabrication methods, metallic materials can be deposited or printed on substrates to form electrodes. Inorganic nanomaterials can be synthesized in various dimensions, including zero-dimension (0D, e.g., metal nanoparticles), one-dimension (1D, e.g., metal or metal oxide nanowire/nanofiber), and two-dimension (2D, e.g., transition metal dehydroxylated nanosheet). Nanomaterials of different dimensions can be integrated with other materials to enhance their intrinsic properties. As shown in Figure 2a, 0D materials, such as silver nanoparticles (AgNPs) can be deposited in a flexible and stretchable matrix such as polyurethane to form a percolation network [21]. In this case, the electrical resistance is dictated by the intrinsic electrical conductivity of the nanomaterials and the junction-junction resistance between the individual nanoparticles. More importantly, the percolation network could absorb mechanical strain while ensuring electrical conductivity [22]. Cu-based core/shell NPs were used to fabricate electrodes with uniform electrical properties and high mechanical stability. In particular, the Cu\({}_{10}\)Sn\({}_{3}\) shell with a low melting point facilitated the facile triggering of an efficient densification reaction, thereby resulting in high electrical properties even on a vulnerable polyethylene naphthalate (PEN) substrate [23]. Nanowires (NWs) are 1D materials that are used extensively in conductors due to their high conductivity, simple solution synthesis, and device fabrication process. Nanofibers are also a kind of 1D material like NWs, but they have a higher aspect ratio than NWs, which can reduce the number of junction points between the nanomaterials. For example, silver nanofibers (AgNFs) have been used to replace rigid indium tin oxide (ITO)-based film because the AgNF network has good chemical stability, conductivity, transparency, and flexibility. With such characteristics, AgNPs are widely used with transparent or soft substrates such as polyethylene terephthalate (PET) and glass fabrics for the fabrication of flexible devices [24; 25; 26]. As shown in Figure 2b, a pressure sensor based on Au NWs identified the subtle difference in pulse, and it may become a wearable diagnostic device for the real-time monitoring of human health under various conditions [27; 28; 29]. 2D inorganic material atomic sheets are thin layered crystalline solids with the properties of interlayer van der Waals bonding and intralayer covalent bonding. The excellent properties of 2D crystals generate enormous benefits for traditional semiconductor technology and emerging flexible nanotechnology, especially with their ultimate thickness scalability [30]. For example, research involving 2D molybdenum disulfide (MoS\({}_{2}\)), which has a similar structure to graphite, has been studied actively. MoS\({}_{2}\) easily can be manufactured as a thin film, and it has excellent mechanical, electrical, and optical properties, suggesting the potential for 2D nanomaterials in various wearable sensor applications [31; 32; 33]. [PERSON] et al. described a high-density, hemispherical, bending-image sensor array that was designed using strain release devices and thin MoS\({}_{2}\)-graphene heterostructures at the atomic-level [33].