Droplet-based interfacial capacitive sensing

This paper presented a novel droplet-based pressure sensor using elastic and capacitive electrode–electrolyte interfaces to achieve ultrahigh mechanical-to-electrical sensitivity (1.58 μF kPa−1) and resolution (1.8 Pa) with a simple device architecture. The miniature transparent droplet sensors, fabricated by one-step laser micromachining, consisted of two flexible polymer membranes with conductive coating and a separation layer hosting a sensing chamber for an electrolyte droplet. The sensing principle primarily relied on high elasticity of the sensing droplet and large capacitance presented at the electrode–electrolyte interface. A simple surface modification scheme was introduced to the conductive coating, which reduced hysteresis of the droplet deformation without substantially compromising the interfacial capacitance. Moreover, the major concern of liquid evaporation was addressed by a mixture of glycerol and electrolyte with long-term stability in a laboratory environment. Theoretical analyses and experimental investigations on several design parameters (i.e., the dimensions of the sensing chamber and the droplet size) were thoroughly conducted to characterize and optimize the overall sensitivity of the device. Moreover, the environmental influences (e.g., temperature and humidity) on the capacitive measurement were further investigated. Finally, the simply constructed and mechanically flexible droplet sensor was successfully applied to detect minute blood pressure variations on the skin surface (with the maximum value less than 100 Pa) throughout cardiovascular cycles.