Sensitivity Improvement of Highly Stretchable Capacitive Strain Sensors by Hierarchical Auxetic Structures.

Jun Shintake,Keita Ogishima,Toshiaki Nagai

doi:10.3389/frobt.2019.00127

Jun Shintake, Keita Ogishima + Show 1 more

Open Access

https://doi.org/10.3389/frobt.2019.00127

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Abstract

Highly stretchable sensors that can detect large strains are useful in deformable systems, such as soft robots and wearable devices. For stretchable strain sensors, two types of sensing methods exist, namely, resistive and capacitive. Capacitive sensing has several advantages over the resistive type, such as high linearity, repeatability, and low hysteresis. However, the sensitivity (gauge factor) of capacitive strain sensors is theoretically limited to 1, which is much lower than that of the resistive-type sensors. The objective of this study is to improve the sensitivity of highly stretchable capacitive strain sensors by integrating hierarchical auxetic structures into them. Auxetic structures have a negative Poisson's ratio that causes increase in change in capacitance with applied strains, and thereby improving sensitivity. In order to prove this concept, we fabricate and characterize two sensor samples with planar dimensions 60 mm × 16 mm. The samples have an acrylic elastomer (3M, VHB 4905) as the dielectric layer and a liquid metal (eutectic gallium-indium) for electrodes. On both sides of the sensor samples, hierarchical auxetic structures made of a silicone elastomer (Dow Corning, Sylgard 184) are attached. The samples are tested under strains up to 50% and the experimental results show that the sensitivity of the sensor with the auxetic structure exceeds the theoretical limit. In addition, it is observed that the sensitivity of this sensor is roughly two times higher than that of a sensor without the auxetic structure, while maintaining high linearity (R2 = 0.995), repeatability (≥104 cycles), and low hysteresis.

Highlights

Stretchable strain sensors, which detect large strains, are an important element for sensing deformations of compliant systems, to assess their status and performance control
We describe a method to improve the sensitivity of stretchable capacitive strain sensors
We demonstrate, that the sensitivity of the capacitive strain sensor can be improved by roughly two times, while maintaining the original characteristics, such as high linearity, repeatability, and low hysteresis

Summary

Introduction

Stretchable strain sensors, which detect large strains, are an important element for sensing deformations of compliant systems, to assess their status and performance control. These soft matter-based systems include robots and wearable devices (Polygerinos et al, 2017; Rich et al, 2018; Shintake et al, 2018a), with applications in exploration (Katzschmann et al, 2018), manipulation (Shintake et al, 2016), virtual reality (Suzuki et al, 2016), and human monitoring (Bartlett et al, 2016) to name a few. There are mainly two types of sensing modes available for highly stretchable strain sensors: resistive and capacitive modes. Resistive sensing is based on the piezo-resistive effect where strain causes deformation of the electrodes, and subsequent change in their electrical resistivity.

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