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

Abstract

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 has improved the competitiveness of this modality of sensing. In this work, the first employment of helical auxetic yarns as capacitive sensors is presented. It is found that the response of the helical auxetic yarn capacitive sensors (termed as HACS) is dependent on the two main fabrication variables—the ratio of diameters and the helical wrapping length. Depending on these variables, sensors that respond to strain with increasing or decreasing capacitance values can be obtained. A greater auxetic character results in larger sensitivities accessible at smaller strains—a characteristic that is not commonly found when accessing high gauge factors. In addition, the highest sensitivity for auxetic capacitive sensors reported thus far is obtained. A mechanism of sensor response that explains both the variable capacitance response and the high gauge factors obtained experimentally is proposed.

Distributed sensing along fibers for smart clothing

“…We must first choose excitation frequencies—or, equivalently, manipulate the sensor’s specific resistance and capacitance—to maximize the discrimination between strains across each sensing region. We began an initial test by fabricating four 10-cm long HACSs with pitch of approximately 4 mm; this should yield GF ≈ 0.5 according to the model and results from our previous study ( 32 ). We measured the sensor response ΔCCfalse(0false) versus strain for each sensor independently and fit a linear model to obtain relaxed capacitance C (0), relaxed resistance R (0), and GF for each (Fig.…”

“…Thus, we desired our sensing fiber to have spatial specificity to strain at certain locations. We previously reported a previously unexplored capacitive fiber strain sensing modality ( 32 ) having some attractive features that can be used for distributed sensing applications. These are (i) leveraging the auxetic behavior of a helical yarn complex to achieve a higher sensitivity than expected for typical capacitive strain sensors; (ii) the ability to tune this sensitivity by manipulating the helical pitch; and (iii) high robustness to stress and suitability for a reel-to-reel manufacturing process.…”

“…( B ) Top: Geometric modeling of the HACS sensor low-sensitivity (II and IV: 3.5-mm pitch) and high-sensitivity (I, III, and V: 8-mm pitch) regions under relaxed and 30% strain conditions; Bottom: Photographs of the unstrained and strained sensor in the garment at the elbow joint. ( C ) Response versus strain plots (solid), GF (dashed black), and computational model predictions (round black marker) for the respective HACS sensitivity regions [model adapted from our previous work ( 32 )].…”

“…A very simple MLP model was compiled using Keras/Tensorflow with architecture consisting of an eight-unit input layer, two hidden layers of sizes (16,32) units and using tanh and rectified linear unit (ReLU) activation, respectively, and a four-unit output layer with linear activation. The second hidden layer had 𝓁 2 regularization with weight 1 × 10 −5 .…”

High-stretchability and low-hysteresis strain sensors using origami-inspired 3D mesostructures

“…As an alternative approach, structure designs capable of mechanical stretching can provide means of achieving stretchable strain sensing without relying on stretchable conductive materials. These designs include wavy structures, fractal designs, twisted or helical structures, open-mesh structures, and origami-and kirigami-inspired structures (15)(16)(17)(18)(19)(20)(21)(22)(23). For example, parallel-plate capacitive strain sensors using wrinkled ultrathin gold film electrodes exhibit high sensitivity and linearity with small hysteresis, but with limited stretchability (140%) (18).…”