**3.1 Carbon-based nanomaterials**

The carbon-based nanomaterials (CNM) discussed in this Chapter are materials with honeycomb lattice structures such as carbon nanotubes (CNTs), graphene

#### **Figure 6.**

*Typical soft electronic materials for wearable sensors. (A) Carbon-based materials. Adapted from Ref. [53]. (B) Inorganic structured materials. Adapted from Ref. [54]. (C) Polymer conductors and semiconductors. (D) Elastomers.*

#### *Advanced Materials and Assembly Strategies for Wearable Biosensors: A Review DOI: http://dx.doi.org/10.5772/intechopen.94451*

(containing graphene oxide (GO) and reduced graphene oxide (rGO)), etc [55]. CNMs contain superior properties such as good electrical conductivity, excellent mechanical properties, high chemical and thermal stability, low toxicity. These excellent properties of CNMs have drawn great attention in wearable electronics [56, 57].

Cohen et al. demonstrated a high elasticity strain gauge using the capacitive detection of carbon nanotubes-based parallel electrodes separated by dielectric elastomer as shown in **Figure 7A** [58]. The device relies on the Poisson effect, so that the uniaxial strain creates a scaled strain so that the two transmissive electrodes come closer together. Even in the 3000-cycle test with 3% strain, the sensor's capacitance did not decrease.

Crumpled/wrinkled skin-like sensor using graphene for noninvasive and realtime pulsed sensing operation was demonstrated by Yang et al. (**Figure 7B**) [59]. The modification of PDMS (Polydimethyloxane) substrate stiffness achieves the optimal balance between acceptable linearity and high sensitivity, further realizing beat-to-beat radial pulse measurements for people of various ages and before and after motion.

A strain sensor device using a fish-scale-like graphene layer embedded on an elastic tape was accomplished by Liu et al. as shown in **Figure 7C** [60]. This configuration enabled graphene to form adjacent overlapping layers, realizing overlapping areas through reversible slip and consequently change contact resistance. Due to the fish-scale-like structure, this strain sensor was able to detect both stretching and bending deformation, as well as high performances including high sensitivity, low detection limit, wide range of deformation, excellent reliability and stability were achieved. This strain sensor can be manufactured by stretching/exhausting the composite film of rGO and elastic tape, so the process is simple, inexpensive, and has excellent energy saving and scalability.

By using dry spinning, Ryu et al. reported that they developed a strain sensor with an extremely elastic behavior based on highly oriented CNT fibers as shown in **Figure 7D** [61]. As The device was made of a flexible substrate, capable of measuring more than 900% strains with superior sensitivity and exhibited rapid response and good durability. Such sensors should be used extensively in applications involving large variants, including soft robotics. These devices can be adapted for normal strain gauge applications.

#### **Figure 7.**

*Wearable biosensors based on carbon nanomaterials. (A) Design of a Poisson Capacitor. Adapted from Ref. [58] (From left to right) Schematic of our device geometry. SEM data demonstrating percolation of the CNTs within the electrode; scale bar is 500 nm. Close-up image of the sensing region of the device. (B) Schematic illustration of the pulse sensor. Adapted from Ref. [59]. (C) Fish-scale-like graphene-based (FSG) strain sensors. (From left to right) Photograph. Top view and cross-section view of SEM images. Adapted from Ref. [60]. (D) Schematic showing the morphology of a CNT fiber under strain; scale bar is 0.75 cm. Adapted from Ref. [61].*
