Thin and Small Pressure Sensors Based on Ion-Infiltrating Piezo Capacitive (i-PIC) Film for Monolithic E-Skin Integration

Flexible pressure sensors are essential elements for realizing artificial electronic skin, human-machine interfaces, and smart healthcare. The major development goal of the flexible pressure sensors has been improving sensitivity, and that was mostly achieved by introducing microstructures in a pressure-sensing film that can involve a large deformation when pressed. The strategy was successful, especially when it collaborated with an ionic gel film having a huge capacitance density of over 1 μF, and a capacitance change with pressure reached over 1000 times.<br/>Despite the success in sensitivity, the microstructures in the pressure-sensing film implied serious drawbacks in the flexible pressure sensors in terms of system integrity and scaling-down. The fabrication processes of the conventional sensing film and the sensor involved a molding process to make the microstructures and a laminating process for “bottom electrode/ sensing film/ top electrode” structure, thus it was very hard to secure reproducibility and collaborate with conventional processes of semiconductor devices or circuits. The scaling-down of the sensor was limited as well due to the feature size of the microstructure and the laminating processes, while a smaller pressure sensor is beneficial for many E-skin applications. These are significant drawbacks for flexible pressure sensors to be integrated into an E-skin system. To overcome these limitations, a high-performance flexible pressure sensor through monolithic stacking and micro-patterning processes, which are compatible with conventional fabrication processes, is needed, ensuring not only the integrity of a single sensor but also scaling-down and integration into a system for future applications.<br/>In this study, we propose a new piezo-capacitive pressure-sensitive mechanism of a gradual electrical double layer (EDL) modulation based on an ion infiltrating piezo-capacitive (i-PIC) film, in which a very small amount of ionic liquid is included in a polymer network. The i-PIC film shows very small capacitance of about 100 pF/cm² that is of a neat polymer film without pressure due to the very small ion concentration. Its capacitance substantially increases with pressure near 1μF/cm², because the pressure causes thickness reduction and it causes a relative increase of the ion concentration and finally the gradual generation of EDL in the film. The i-PIC film with an optimal ion concentration of 0.2 mg/mg in the polymer film exhibited a high sensitivity of 3.94 kPa⁻¹ over a wide pressure range of up to 1 MPa with a very high linearity of 0.999. Additionally, due to the absence of surface texturing, it showed excellent mechanical durability with identical characteristics for over 10,000 repeated pressure and release cycles for pressures of up to 1 MPa. The i-PIC film shows excellent properties with a thickness of below 10 μm and is fabricated by one-step spin-coating directly onto a bottom electrode without any molding or laminating processes, thus is compatible with monolithic fabrication processes. The i-PIC film involves very small pores of a few μm, thus the almost identical performance and high uniformity of the flexible pressure sensor was maintained for a scaling-down below 100 μm, and this small size is enough to mimic the detection resolution of human skins that is about 50 ppi.