Electromechanical Film Sensors An electromechanical film sensor is composed of two external layers that sandwich a third layer that is made of polypropylene layers with air pockets dispersed within it [32] (Fig. 3c). When external force is applied, the sensor creates an electric charge, meaning that they are typically passive in nature [32]. The charge generation mechanism is based on the mutual movement of static charges that were originally injected into the sensor during its creation [32]. This causes the air pockets to act as electrical dipoles that change with applied perpendicular pressure [32]. This sensing approach is used in the Emfit mattress [29], a commercially available product that has also been used for research purposes. Joshi et al. used the Emfit sensors, placed between the mattress and bedding (Fig. 2b) in an infant bed to monitor body movement, which was reported to be robust to noise, and performed well in clinical settings. Perez-Macias et al. used the Emfit mattress sensor placed under the thoracic area of a sleeping patient (presumably placed on top of the mattress) (Fig. 2b) to detect snoring events in a group of 33 adult subjects with an accuracy of 80%. Capacitive Pressure Sensors Capacitive pressure sensors are composed of two plates that can store electric charge separated by a material [45] (Fig. 3d). The charge between the two plates creates a potential difference that is dependent on the overlapping area (A) of the two plates, the permittivity of the material between the two plates, and the size of the gap between the two plates (d) [45]. Varying any of the factors can be used as the sensing foundation [45]. The potential difference is maintained by using an external voltage, meaning that capacitive PS are typically active in nature. The capacitance (C) of simple two-plate parallel sensor is given by: April 2021 Fig. 3. Sensor Types. (a) Wheatstone bridge with strain gauges [46]; (b) Diagram of flexiforce A401 sensor based on [47]; (c) Electromechanical film sensor based on [48]; (d) Parallel plate capacitor based on [49]; (e) Piezoresistive sensor based on [50]; (f) Fiber-optic pressure sensor; (g) Inductive pressure sensor based on [51]; (h) Proximity pressure sensors [52]; (i) Air pressure sensor based on [53]. (Photo credits: Wikipedia Commons.) IEEE Instrumentation & Measurement Magazine 17