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

Firstly, we discuss the working mechanism of a wearable electromechanical sensor. Based on their working mechanisms, it can be classified into piezoresistive, capacitive, iontronic, and piezoelectric sensor, as seen in **Figure 1** [11].

#### **2.1 Wearable piezoresistive sensor**

**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, structural effect, and disconnection mechanism.
