Self-powered multifunctional monitoring system using hybrid integrated triboelectric nanogenerators and piezoelectric microsensors

Highlights: • A self-powered flexible multifunctional monitoring system with integration of TENGs and a linear pMUT array is proposed. • A PEI/GO-pMUT based relative humidity sensor is achieved with an excellent linearity and extremely high sensitivity. • A PEI-TENG based self-powered active CO₂ gas sensor is designed with humidity calibration for high detection accuracy. • Both the linear humidity-sensing mechanisms and CO₂-sensing mechanisms are explored. • This work demonstrates a new strategy to combine TENG and MEMS enabled devices for self-powered multifunctional systems. -- Abstract: Battery-less internet of things (IoT) devices and flexible electronics are attracting increasing attention worldwide for diverse applications ranging from human healthcare to environment monitoring. A flexible multifunctional monitoring system with the integration of triboelectric nanogenerator (TENG) for energy harvesting, piezoelectric micromachined ultrasonic transducer (pMUT) array and TENG sensor for simultaneous detection of multiple amenity parameters (i.e., temperature, relative humidity and CO₂ concentration) has been developed. With modification of polyethyleneimine (PEI) on the friction surface, the CO₂ concentration can be sensed by the electrical output of the TENG sensor in a wide range up to 12,000 ppm. To precisely calibrate the effect of humidity on the TENG sensor, pMUT array is integrated that can be potentially powered by another unmodified TENG through harvesting ambient mechanical energy. The pMUT array is coated partially with PEI/graphene oxide (GO) composite and adopted to simultaneously detect relative humidity using functionalized pMUT elements and room temperature using unfunctionalized elements. After optimization of PEI volume fraction in the composite, the pMUT humidity sensor exhibits an extremely high sensitivity (748 Hz/% RH) and relative sensitivity (290 ppm/% RH), an excellent linearity over a wide range, rapid response/recovery, small hysteresis, good stability and significant selectivity over CO₂ gas. Additionally, the pMUT temperature sensor shows an excellent linear response, which is much advantageous to eliminate the temperature interference of the humidity sensor due to the shared array structure. This work demonstrates not only a new and effective route of ultrasensitive and linear humidity detection, but also a new strategy to configure an "all-in-one" flexible self-powered multifunctional sensing system, which complementally combines the advantages of TENG and micro-electromechanical-system (MEMS) technology enabled devices and is highly promising to facilitate various applications.