![]() Consequently, a smaller cavity height can be achieved at a lower DC bias voltage. This biasing configuration introduces an attraction between the membranes. In this design, a DC bias voltage is applied either to the top or the middle membranes, while the bottom electrode is grounded and shorted to the unbiased flexible membrane. In this work, CMUT and PMUT-based sensors alongside their structural mechanisms, device structures and implementation methods based on mass shift are discussed in detail. In these sensors, adsorption interactions between the gas molecules and the polymer occurs via physisorption that influences sensor response time during gas adsorption and desorption. This deflection creates a detectable shift in the frequency of their resonating plane due to the adsorption of the gas molecules on the sensing membrane, usually a polymer which is chosen based on the environment and analyte detection requirements. ![]() ![]() A common attribute of the mass loading effect is the implementation of the proposed capacitive and piezoelectric micromachined ultrasonic transducers, capacitive micromachined ultrasonic transducer (CMUT) and piezoelectric micromachined ultrasonic transducer (PMUT) structures, that are dynamically driven into mechanical deformation. In addition, MUT devices benefit from advanced micro fabrication technologies, and therefore, these micromachined-sensors can be fabricated in an array structure using various sensing materials in order to improve their selectivity in complex environments. MUT-sensors operate based on a change in the mass of their sensing components when used for gas sensing and volatile organic compound (VOC) detection. In an unconventional approach, these devices can be employed in gas sensing technology due to their unique structures that provide design flexibility and contribute to their potential high-performance capabilities. Amongst them, micromachined ultrasonic transducers (MUT) are shown as potential candidates in sensing applications. MEMS-based mass resonant sensors benefit from a low power consumption, a high selectivity and a low limit of detection (LOD) whilst being integrable in a wide range of applications. Amongst them, the mass detection method is reported as an emerging candidate due to its stellar performance in detecting low gas concentration levels. These gas sensors can employ various detection techniques using resistivity, optical properties, acoustic measurements and mass detection. Microelectromechanical system (MEMS)-based sensors are introduced as high-performance detectors due to their sensing capabilities at the micro and nanoscale levels and their potential for integration with wearable electronics.
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