# Recent Advances in Wearable Devices for Non-Invasive Sensing
## 2 Materials and Structural Design
### Materials
#### 2.1.2 Carbon-Based Materials  
Compared with other conductive materials, advanced carbon-based materials, such as carbon black nanoparticles (CBNPs), carbon-based nanofibers, graphene, and carbon nanotubes (CNTs), have unique advantages, including high chemical and thermal stability and good electrical conductivity [42]. It is also easy to functionalize carbon-based materials, which gives them great potential in wearable electronic products and applications [43; 44; 45]. Graphene is considered to be a desirable 2D material in the preparation of high strength and multi-functional hydrogels and elastomers due to its high modulus, high strength, large specific surface area, good electrical conductivity, and biocompatibility [46; 47]. Graphene oxide (GO) is synthesized by replacing the partial double bonds on the surface of a graphene sheet with oxygen-containing functional groups, such as the hydroxyl group (\(-\)OH), the  
Figure 2: (**a**) Transmission electron microscopy (TEM) image of Ag NPs. Scale bar, 50 nm (reprinted with permission from [21]. Copyright 2018 Publish by MDPI); (**b**) Scanning electron microscopy (SEM) image of silver nanowires, Scale bar, 1 um (reprinted with permission from [29]. Copyright 2018 IEEE); (**c**) TEM image of graphene oxide (GO). Scale bar, 200 nm (reprinted with permission with [34]. Copyright 2019 American Chemical Society); (**d**) SEM image of Ag flakes anchored with Ag Nanoparticles/amine-functionalized multi-walled carbon nanotubes (NH\({}_{2}\)-MWCNT) hybrid material (reprinted with permission from [35]. Copyright 2017 nanoscale); (**e**) SEM image of the PPy nanowires. Scale bar, 200 nm (reprinted with permission from [36]. Copyright 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany); (**f**) Optical image of the poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) electrode on cotton textile. Scale bar, 250 um (reprinted with permission from [37]. Copyright 2019 IEEE); (**g**) TEM image of a poly (vinylidene fluoride-co-hexafluoropropene) (PVDF-HFP)/PEDOT nanofibers (NF). Scale bar, 200 nm (reprinted with permission from [38]. Copyright 2018 Springer Nature); (**h**) Optical image of LM/CNT composite using CNTs decorated with Pt nanoparticles. (reprinted with permission from [39]. Copyright 2019 American chemical society); (**i**) High conductivity and surface tension of Galinstan (left) and optical image of a device after Galinstan injection (right). Scale bar, 250 um (reprinted with permission from [40]. Copyright 2014 Springer Nature); (**j**) Stretched and tough hydrogels. The stretched to 17 times its initial length (right) (reprinted with permission from [41]. Copyright 2012 Springer Nature). carboxyl group (\(-\)COOH), and the epoxy group (\(-\)C\(-\)O\(-\)C) as shown in Figure 2c [34; 48]. Therefore compared with graphene, GO can be dispersed more stably in a variety of organic solvents, thereby providing the conditions for the preparation of various high-strength, multi-functional composites [46]. CNTs have hollow, nano-tubular structures that are obtained by curling single or multi-layer graphene nanoplates. CNTs can be ideal wearable sensor materials due to their excellent electrical and mechanical properties and their availability [49; 50]. [PERSON] et al. proposed a scalable strain sensor by coupling overlapped CNT bundles with silicone elastomers. The sensor showed excellent sensing performance with a broad sensing range of over 145% strain, ultrahigh sensitivity (maximum gauge factor of 42,300), high repeatability, and durability [51]. CNT can be used extensively in other wearable healthcare systems in hybridization with various materials. For example, CNT is used as a 3D printable material with high conductivity and low-temperature processability by forming a carbon-metal hybrid combined with Ag nanoparticles, Ag flakes, and a thermoplastic copolymer, as shown in Figure 2d [35]. CNT can be used in other wearable healthcare systems.