Noninvasive Material Thickness Detection by Aerosol Jet Printed Sensors Enhanced Through Metallic Carbon Nanotube Ink

Demand for cheaper and more functional sensors continues to rise in an era when data can be used to improve health, safety, and efficiency in daily lives. In this paper, we present a fully printed sensor capable of noninvasive material thickness detection. By applying an oscillating signal between two millimeter-scale electrodes, the fringing electric field is measurably perturbed by a material placed directly on top of the electrodes, leading to a linearly varying capacitance with change in the material’s thickness. We simulate this electric field perturbation and experimentally demonstrate the linear correlation between capacitance and overlying material thickness. Various parameters, from sensor size and structure to substrate and ink materials, are studied to optimize the performance of the printed sensors. Sensors made of metallic carbon nano-tube ink yield the best sensitivity, exhibiting a capacitance change of 26 fF per mm thickness of rubber—ten times more sensitive than devices composed of silver nanoparticle ink. Finally, we demonstrate an effective application of the sensors in automobile tires. By applying the sensors directly beneath the tread (within the tire), mm changes in the tread depth are able to be detected in a 99% confidence interval. These findings provide a straightforward, low-cost approach for monitoring mm changes in material thickness using noninvasive, printed sensors applicable to innumerable Internet-of-Things (IoT) applications.