Aerosol jet printing of advanced capacitive strain gauge for vibration monitoring of the human body

Abstract

Abstract Capacitive strain sensors are widely used in wearable devices to detect human movements and activities. Most previous studies have focused on improving the response of strain sensors under large strains while maintaining high transparency, but the high gauge factor, low hysteresis, and high reliability and stability during small deformations and vibrations are often neglected. To improve the sensing accuracy of capacitive strain sensors, novel double-layer capacitive strain sensors are proposed and fabricated with the aerosol jet printing method in this work. Using polydimethylsiloxane (PDMS) as the protective layer and polyimide (PI) and polyethylene terephthalate (PET) as the dielectric printing substrates, aerosol jet printing creates electrodes with a width between 80 and 100 µm, enabling fine vibration sensing in small areas. Sensors with different dielectric layering thicknesses and different electrode plate layering angles are printed and tested with a digital bridge (LCR meter), and the double-layer capacitive strain sensor structure with the highest gauge factor and response speed is determined. A comparative study between laser sintering and oven sintering of silver nanoparticle ink, the electrode printing material, is conducted, and the optimal sintering method is selected to ensure high transparency as well as bending durability. As application examples, the double-layer capacitive strain sensors are utilized in the measurement of several body parts, including the vibration of the throat during speech, the bending of the fingers, the abdominal breathing, and the fluttering of the eyelids. The results demonstrate excellent properties of the proposed flexible double-layer capacitive strain sensors in terms of improving gauge factor, reducing hysteresis, enhancing transparency, and maintaining high stability and reliability. They exhibit great potential for monitoring small vibrations in the human body, flexible robots, and precision instruments.