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Front. Robot. AI | doi: 10.3389/frobt.2019.00127

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

 Jun Shintake1*,  Toshiaki Nagai1 and Keita Ogishima1
  • 1Department of Mechanical Engineering and Intelligent Systems, University of Electro-Communications, Japan

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 (≥ 10^4 cycles), and low hysteresis.

Keywords: stretchable, capacitive, Strain sensors, Auxetic structures, soft robotics, wearable devices

Received: 24 Jul 2019; Accepted: 08 Nov 2019.

Copyright: © 2019 Shintake, Nagai and Ogishima. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

* Correspondence: Prof. Jun Shintake, Department of Mechanical Engineering and Intelligent Systems, University of Electro-Communications, Chofu, 182-8585, Tōkyō, Japan, shintake@uec.ac.jp