High-performance Super-capacitive Pressure Sensor with Both High Sensitivity and Wide Linear Sensing Range

Abstract

The popularity of wearable electronics in healthcare industry and soft robotics offer enormous application opportunities for capacitive type sensors, due to its merits of high reliability, rapid response and low power consumption. However, the existence of challenges, such as low sensitivity, narrow linear sensing range, low pressure-resolution and non-well-defined structure inevitably confines the extensive application of the capacitive type of flexible sensors. A key challenge in pressure sensor design lies in the trade-off between achieving high sensitivity and maintaining a wide linear sensing range. While high sensitivity is essential for detecting small pressure changes with high resolution, a wide linear range simplifies sensor calibration and ensures consistent performance across a broad range of pressures. To enhance the sensitivity of capacitive pressure sensors, the conventional dielectric layer was replaced with an electrolyte layer, resulting in a super-capacitive sensor. This design operates primarily through the electrical double layer (EDL) mechanism, which arises from the use of ionic materials. Since the EDL capacitance is typically 5–6 orders of magnitude higher than that of conventional parallel-plate capacitors, this thesis employs a polyvinyl alcohol (PVA)/phosphoric acid (H₃PO₄) elastomer as the electrolyte layer to enhance sensor sensitivity. The ionic conductivity of this material significantly amplifies the capacitive response. Moreover, its inherent flexibility and elasticity impart excellent mechanical compliance to the sensor, making it suitable for flexible and wearable applications. Another key factor of the sensor, wider linear sensing range, was acquired by the strategy of structure design for the electrolyte layer. In this thesis, different structures were proposed for the electrolyte layer of the super-capacitive pressure sensor. To be specific, the height-grading dome structure was introduced and optimized for the electrolyte layer, to obtain super-capacitive pressure sensors with higher sensitivity. Moreover, another design, hierarchical hemisphere structure, was brought into the electrolyte layer, successfully extending the linear sensing range as well, accompanied by the added effect from the curvy-surface top electrode. In addition to high sensitivity and wide sensing range, the super-capacitive sensors exhibit excellent repeatability, durability, and mechanical stability. These attributes enable reliable detection and monitoring of physiological signals from the human body. Furthermore, the sensors have been demonstrated as effective biomimetic electronic skin (e-skin) for soft robotics, highlighting their versatility in flexible and wearable pressure sensing applications.