Analytical Modeling and Experimental Validation of Triboelectric Behavior in Kirigami Flexible Capacitive Sensors

Flexible capacitive sensors have attracted extensive attention in recent years, especially in their application for biomedical electrophysiological sensing, as they improve comfort and flexibility while being more robust to some types of motion artifacts (MAs). Due to their contactless nature, they are still susceptible to triboelectrification which, because of their flexibility, appears to be stronger and more unpredictable compared to their rigid counterparts. In this work, we propose a novel analytical model to predict and physically justify the triboelectric behavior of flexible capacitive sensors applied to nonflat surfaces. In particular, we consider the general case of an electrode conforming to a spherical surface, which loses contact because of a transversal motion. The model takes into account both the effect of the triboelectric voltage and the varying coupled capacitance, describing the different phases of the movement. Finally, electrical measurements were performed on the sensor, reproducing the same setup and dynamics in the laboratory. The results were compared to the analytical model and discussed: both the analytical and experimental results exhibit similar trends and voltage characteristics, with spike duration for each speed of 4.1, 2.1, and 0.9 s for the modeled effect and 4.6, 2.5, and 1.1 s for the corresponding experimental results. The presented analytical model was revealed to be accurate in describing the MAs caused by the considered motion and represents an important tool for describing and predicting similar artifacts for flexible capacitive sensors.