## 3 Wearable Sensors in Healthcare
### Electromechanical Biosensors  
The principle of sensing in electromechanical biosensors is fairly like that of electrochemical biosensors, though the generation of electrical responses is the variation between the two. This variation is primarily the result of the strain recorded due to an electrical bias or due to a mechanical force. Electromechanical transduction does not depend on molecule labelling, which gives it a significant advantage over electrochemical and optical sensors. It also facilitates a wide range of identification and quantification parameters for biomolecules. Accordingly, the most essential feature that dictates the finest operation of electromechanical biosensors is their capability to sense the physical variations that occur on the human skin surface at a macroscale, such as movements in the arm, leg and wrist or slight changes such as stifling or the stretching of the epidermis, which occurs during actions such as breathing [53]. Typically, electromechanical transduction mechanisms are based on any of the following: (a) piezo-capacitive, (b) piezo-electric, (c) piezo-resistive, (d) iontronic or triboelectric nano-generation (TENG) effects [40; 54] (Figure 5). Figure 4: Different wearable biosensor devices are categorized based on: (a) bio-analytes and (b) design and utility [39]. The function of a strain sensor is the quantification of the mechanical deformation concerning the corresponding changes in electrical signals. This can either be piezo-electric (when changes in surface potential due to polarization are captured) or piezo-resistive (when changes in the resistance produced by external forces are captured) [55,56]. The function of wearable strain sensors is the quantification of mechanical deformation for the corresponding changes in electrical signals. This could be in the form of capturing changes in surface potential due to polarization (also known as piezo-electric) or piezo-resistive, which means capturing variations in the resistance produced by exterior forces [53,55]. Capacitive wearable biosensors react to changes in capacitance and have superior sensitivity due to forces that cause geometrical deformations. However, surrounding noise is known to affect the sensors and may consequently impact their performance [57]. Iontronic wearable biosensors use a platform with super-capacitance. This is approximately 1000 times greater than a capacitor with a metal oxide platform. It forms an ion-electric interface between the electrolytes and electrodes, which causes a high capacitance per unit area and ion accumulation on the electrodes [54]. Triboelectric transduction operates on a simple principle known as frictional charges. This is a result of the interaction between two different materials. The principle was used in developing TENG [56]. Surface separation produces potential differences whenever friction is interrupted; hence, without the use of external power, this is used to develop sensors [58]. A flexible functional design that requires a high gauge factor is needed for the detection of minor movements by stretchable strain sensors, which arise on irregular skin surfaces. For example, a strain biosensor was designed by [PERSON], et al. [59] and was built by aligning a nanowire using a ratio with a high surface-to-volume ratio to monitor intangible human motion. In their study, they succeeded by achieving a high gauge factor of approximately 35.8 that could detect an incitement of deformation with less than 200\(\upmu\)m in under 230 ms. This achieved result was five times better in comparison with a comparable microwire-based biosensor [59]. Similarly, a stretchable ion-based biosensor was created by [PERSON], et al. [60]. It was built on surface strain redistributed elastic fiber (SSRE-fiber) where a wrinkle construct was used to improve the surface area together with an island bridge design [60].