Ultrasensitive Self-Powered Pressure Sensor by Triboelectric Nanogenerator for Acoustic Sensing

With the rapid development of information technologies and the Internet of Things (IoT), various mobile and wireless wearable sensors convey different information from humans to many fields. A pressure sensor is one of the essential electronic devices for human-machine interfaces that enables monitoring of blood pressure, heartbeat, motion sensing, and acceleration. Capacitive, resistive, piezoelectric, and triboelectric pressure sensors are commonly used as electrical signal transducers responding to mechanical stimuli. Among those sensors, piezoelectric and triboelectric-based pressure sensors have been established as self-powered active sensors since they do not consume electrical energy during sensing. Through the mechanism of contact electrification and electrostatic induction, triboelectric nanogenerators (TENGs) effectively convert ambient mechanical energies into relevant electrical signals with the advantage of simple device structures with diverse material candidates. Furthermore, many studies in self-powered pressure sensors using TENG employed 3D microstructure geometries to enhance deformability and gap distance variation, create every new contacting surface, and facilitate a free-standing device structure without a spacer. However, the effect of deformation and contact area variation is not significant in a small pressure range (<1kPa), resulting in difficulties acquiring the low-level pressure signal such as blood pressure level or acoustic pressure with high frequency. In this work, we propose a strategy of using 3D electrodes embedded in an elastomer for TENGs with 3D microstructure geometry (micro-TENGs) that maximizes the transducing efficiency of mechanical deformation to the electrical output of open-circuit voltage and transferred charge density. Deformation of the micro-TENG induces a gradual configuration of electrical components of TENG in parallel and increases the electrical output, which is much higher than that of TENG with an external flat electrode. The 3D microstructure geometries were fabricated by mold casting of a PDMS elastomer using a commercial 3D printer (Formlabs). The 3D electrodes were prepared via spray coating of AgNW and PDMS embedding. Here, an easily processable one-step fabrication method of the 3D electrode also provided robust mechanical stability. FE-SEM images and EDS mapping confirmed the 3D electrode structure. By comparing the pressure sensing performance of the TENG on the 3D electrode with that of TENG on the external flat electrode, we could confirm the efficacy of the gradual configuration mechanism of micro-TENG on the dynamic pressure sensing. Furthermore, to find the optimal configuration of micro-TENGs and 3D electrodes, a finite element method (FEM) was conducted with various shapes, spacing, and sizes of 3D topologies. Finally, hemispherical micro-TENG and 3D electrodes showed the best sensitivity with high linearity and broad sensing range. Also, to explore the feasibility of dynamic sensing properties, the optimized pressure sensor was able to detect mechanical waves with frequency from 1Hz to 20kHz, which was confirmed by acoustic sensing. Here, the acoustic sensing characters (low-level pressure with high frequency) could be achieved with fascinating and straightforward features from 3D electrodes, unlike the existing acoustic sensing technology with a nano-meter vibration of a membrane.