*2.1.2 Structural effect*

*Wearable Devices - The Big Wave of Innovation*

and even speech recognition.

**2.1 Wearable piezoresistive sensor**

structural effect, and disconnection mechanism.

facing the progress.

*2.1.1 Geometrical effect*

represented by:

new sensing materials. Structural design is also an effective strategy to improve the performance. Fabrication method is also the significant aspect. Many traditional techniques are utilized, such as screen printing, contact printing, electrospinning, and spray coating [10]. Moreover, wearable electromechanical sensor has been successfully demonstrated on a lot of applications, such as health monitoring, disease diagnosis, behavior correction, alarm of accident falls, human-machine interfaces,

The present chapter will discuss their basic working mechanism, fabrication methods, and applications of wearable electromechanical sensors and challenges

Firstly, we discuss the working mechanism of a wearable electromechanical sensor. Based on their working mechanisms, it can be classified into piezoresistive,

**Figure 1a** shows the mechanism of piezoresistive sensor. It transfers mechanical stimuli into resistance signal. The factors resulting in resistance change depend on the property of materials utilized and their structures, including 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

*R* = *L*/*A* (1)

*Schematics illustrating the different modalities of wearable electromechanical sensors. (a) Piezoresistivity,* 

**2. Working mechanisms of a wearable electromechanical sensor**

capacitive, iontronic, and piezoelectric sensor, as seen in **Figure 1** [11].

**76**

**Figure 1.**

*(b) capacitance, (c) piezoelectricity, and (d) iontronic.*

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.
