Surface Textures for Stretchable Capacitive Strain Sensors

Abstract

Advances in smart materials and electronics have enabled compliant sensing systems mimicking nature. In structural health monitoring (SHM) applications, the technology still lacks accuracy in tracking measurements over large geometries. Herein we report a facile fabrication for a compliant sensor based on an elastomeric capacitive sensor technology, augmented with a scalable surface texture inspired by various grid geometries. Texture designs are selected to direct, and therefore improve, the strain sensing capabilities and thus the sensor's gauge factor. We showcase selected designs based on an a detailed engineering analysis. The selected surface patterns are investigated under increasing strain targets and strain frequency sweeps. Results confirm that a stretchable thermoplastic composite sensor with a textured pattern embossed into the hyper-elastic matrix yields approximately 30% increase in the gauge factor and 35% in signal accuracy, great linearity up to 30% strain, and overall signal stability to empower SHM applications.

Numerical Investigation of Auxetic Textured Soft Strain Gauge for Monitoring Animal Skin

“…Briefly, the Poisson's ratio ν of an untextured dielectric film was measured with two digital cameras (FASTCAM SA-Z and Photron) by subjecting a specimen to a 30% uniaxial strain applied at 80 µm/s. Note that 30% is the maximum strain level used in this study, consistent with experimental tests conducted on free-standing SECs in [21] and with levels expected for the application [6], and that a characterization of the electromechanical model at higher strain levels is left to future work. A 7 × 7 mm 2 region around the transverse centerline was defined as the region of interest (red rectangle in Figure 3a) to extract the Poisson's ratio.…”

“…Previous work has demonstrated that the Poisson's ratio of the thin film was tunable by augmenting the sensor with a texture. In particular, a diagrid-like pattern was experimentally shown to enhance directionality and to improve the experimental gauge factor by 30% [21]. The current paper extends the previous study to auxetic patterns, known to yield a negative Poisson's ratio [22,23].…”

“…While the study presented in this paper is numerical, textured SECs (see Figure 1a) were studied experimentally in a previous work [21] to validate the numerical model, and their fabrication is therefore of interest. The SEC consists of a stretchable parallel plate capacitor, where a dielectric layer is sandwiched between two electrode layers.…”

“…DIC was used to calculate Poisson's ratio of the thin film, described in a previous work [21]. Briefly, the Poisson's ratio ν of an untextured dielectric film was measured with two digital cameras (FASTCAM SA-Z and Photron) by subjecting a specimen to a 30% uniaxial strain applied at 80 µm/s.…”

“…The employed numerical model of a free-standing SEC was validated in previous work on several non-auxetic texture patterns [21], and textured sensors were fabricated and tested on animal skin. Here, previous experimental results obtained from a diagrid-like pattern, constructed from intersecting diagonals and vertical reinforcements, are used to validate the numerical model of the SEC partially adhered onto skin.…”