**2.3 Wearable electromechanical sensors**

Various types of wearable electromechanical sensors have been developed by exploiting different sensing mechanisms such as piezoresistive, piezoelectric, capacitive and iontronic methods (**Figure 5**) [5, 42].

One of the simplest electronic devices is a piezoresistive, where a variation in resistance is recorded when the resistor is in contact with the human skin. In this piezoresistive mechanism, not only the value of the resistance but also several other parameters are assessed, including response time, recovery time and sensitivity. Since the resistors can be easily manufactured and characterized, they represent the most-widely-studied-device-structure for application detection, including flexible and wearable sensors [43].

Wearable capacitive sensors measure the change in the capacitance of a capacitor. The fabrication of these capacitive sensors is relatively simple (consecutive vertical stacking of metal/insulator/metal) and can detect various forms of mechanical force, such as strain, pressure, and touch.

Piezoelectricity defines as the accumulation of electrical changes in piezoelectric materials under mechanical stress. The piezoelectric-based sensors measure voltage change when electrical polarization induced by strain or pressure is generated.

Iontronic sensors are to form ionic-electronic interface at the nanoscale distance between the electrode and the electrolyte. When voltage is applied at each positive and negative electrodes, the corresponding counter ions accumulate at the interface of electrodes, resulting in an ultrahigh capacitance per unit-area. The capacitance of these iontronic sensors is at last 1000 times larger than that of metal oxide-based parallel plate capacitors. Thus, iontronic capacitance-type sensors are on the spotlight suitable for wearable electromechanical sensors due to its intrinsic high value of capacitance. The detailed working principle will discuss at Section 4 in detail.

Park et al. reported an array of ultra-thin consistent tactile detectors based on MoS2 film of 2.2 cm × 2.2 cm. The integrated sensor with a graphene electrode provided excellent mechanically flexible endurance and optical transmittance in the visible light wavelength range. The proposed sensor device exhibited high sensitivity, good uniformity, and linearity with an excellent endurance of 10,000 cycles. The ultrathin tactile sensor was made on a plastic substrate, providing high stability of its operation performance even on various substrates including leather and a fingertip [11].

Investigations into the body-sensory system and tactile perception of human skin significantly enhanced the operation performance of rubber-based medical

**Figure 5.**

*An illustrative diagram demonstrating different types of mechanical sensing methods.*

sensors and synthetic electronic skins [44–46]. Chun et al. demonstrated sensing device emulating hairy skin for multimodal detection. The thermal spray coating of graphene nanoplatelets (GNPs) enabled to fabricate the array of sensors and electrodes onto a flexible substrate. The percolation network array structure of GNP (5 × 5 cm2 , 16 pixels) was successfully able to detect the spatially applied pressures as well as locally varied temperature distribution, evaluating the human skin behavior. The microhairs in electronic devices were able to offer a mapping of electrical signals enabled by contactless air currents to identify the direction, angle of incidence, and intensity of the applied wind [12].
