HACS: Helical Auxetic Yarn Capacitive Strain Sensors with Sensitivity Beyond the Theoretical Limit.

The development of flexible strain sensors over the past decade has focused on accessing high strain percentages and high sensitivity (i.e. gauge factors). Strain sensors that employ capacitance as the electrical signal to correlate to strain are typically restricted in sensitivity because of the Poisson effect. By employing auxetic structures, the limits of sensitivity for capacitive sensors have been exceeded, which improves the competitiveness of this modality of sensing. We present the first employment of helical auxetic yarns as capacitive sensors. We found that the helical auxetic yarn capacitive sensors (which we term HACS) response was dependent on the two main fabrication variables-ratio of diameters and helical wrapping length. Depending on these variables, we could obtain sensors that responded to strain with increasing or decreasing capacitance values. We found a greater auxetic character resulted in larger sensitivities accessible at smaller strains-a characteristic that is not commonly found when accessing high gauge factors. In addition, we obtained the highest sensitivity for auxetic capacitive sensors reported thus far. We propose a mechanism of sensor response that explains both the variable capacitance response and the high gauge factors obtained experimentally. This article is protected by copyright. All rights reserved.