*2.1.1 Geometrical effect*

Geometrical effect means that the resistance change is caused by geometrical change, which is mainly due to Poisson's ratio (υ). Poisson's ratio (υ) is a fundamental parameter of materials, meaning that materials tend to contract in transverse direction of stretching when they are stretched. The resistance of a conductor is represented by:

$$\mathbf{R} = \rho \mathbf{L} / \mathbf{A} \tag{1}$$

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*Wearable Electromechanical Sensors and Its Applications DOI: http://dx.doi.org/10.5772/intechopen.85098*

*2.1.2 Structural effect*

*2.1.3 Disconnection mechanism*

mately estimated by Simmons's theory [14]:

*<sup>R</sup>*tu*nnel* <sup>=</sup> *<sup>h</sup>*<sup>2</sup> \_\_\_\_\_\_\_ *<sup>d</sup>*

separation of several crack edges.

**2.2 Wearable capacitive sensor**

crack propagation.

where ρ is the electrical resistivity, L is the length, and A is the cross-sectional area of the conductor. When strain or pressure is applied, the length increases and crosssectional area would be changed due to the shrinkage of materials, resulting in change of the resistance. Geometrical effect is usually limited compared to other factors.

Structural effect is defined as the change in the resistance caused by the structural deformations. This is usually observed in semiconducting materials. When strain or pressure is applied, the crystal structure especially interatomic space is changed, resulting in the change of the bandgap, which may increase the resistance of materials to few times [12]. For example, individual carbon nanotube (CNT) [13] shows ultrahigh resistivity change owning to their chirality and change in barrier height, respectively. However, compared with total resistance change, the part is usually low because strain applied on individual nanoflake is always small. In addition, the large elastic mismatch and weak interfacial adhesion strength between nanomaterials and polymers also make nanoflakes almost free from deformation.

The disconnection mechanism means that resistance change is caused by disconnection process between adjacent nanoflakes. It consists of three situations under different strains or pressures, which are contact area change, tunneling effect, and

> \_\_\_\_ 4*d <sup>h</sup>* <sup>√</sup>

\_\_\_\_\_

2*m*) (2)

When the applied strain or pressure is small, contact area changes between adjacent nanoflakes dominants. The electrons mainly pass through overlapped nanoflakes within the percolation conductive network. When the applied strain or pressure increases and fully pull some adjacent nanoflakes apart, the electrons still can pass through them because the distance between them is small enough. This phenomenon is called tunneling effect, and the distance is called tunneling distance. The tunneling resistance between two adjacent nanoflakes can be approxi-

> *Ae*<sup>2</sup> √ \_\_\_\_\_ <sup>2</sup>*m* exp(

capacitive change can be expressed by the classic equation:

where A, e, h, d, m, λ represent the cross-sectional area of the tunneling junction, single electron charge, Plank's constant, the distance between adjacent nanoflakes, the mass of electron and the height of energy barrier for insulators, respectively. It can be found that the distance between adjacent nanoflakes dominates the tunneling resistance. When there is no electron pass through by tunneling, the distance is defined as cut-off tunneling distance. The cut-off distance is usually several nanometers. When the applied strain or pressure is large enough, crack is formed, leading to rapidly increasing of resistance. Strain or pressure leads opening and enlargement of cracks, critically limiting the electrical conduction due to the

As **Figure 1b** shows wearable capacitive sensor is based on capacitance change of capacitor. Among different capacitors, the most popular architecture is the parallelplate configuration because it is easy to be fabricated and its model is simple. The

#### **Figure 1.**

*Schematics illustrating the different modalities of wearable electromechanical sensors. (a) Piezoresistivity, (b) capacitance, (c) piezoelectricity, and (d) iontronic.*

where ρ is the electrical resistivity, L is the length, and A is the cross-sectional area of the conductor. When strain or pressure is applied, the length increases and crosssectional area would be changed due to the shrinkage of materials, resulting in change of the resistance. Geometrical effect is usually limited compared to other factors.
