Strain sensors based on conducting poly(acrylamide) hydrogels
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MRS Advances © 2020 Materials Research Society DOI: 10.1557/adv.2020.112
Strain sensors based on conducting poly(acrylamide) hydrogels Matthias Künzel1 and Marc in het Panhuis1,2,3 1
School of Chemistry and Molecular Bioscience, University of Wollongong, Wollongong, NSW 2522, Australia.
2 ARC Centre of Excellence for Electromaterials Science, AIIM Facility, University of Wollongong, Wollongong, NSW 2522, Australia.
3
Surf Flex Lab, Australian Institute for Innovative Materials, University of Wollongong, NSW 2522, Australia
ABSTRACT A simple model system towards an impedance-probing strain sensor based on conducting tough hydrogels is demonstrated. A poly(acrylamide) hydrogel, cross-linked with N,N'methylene-bis(acrylamide) was contacted with carbon fibres for electrical impedance analysis. The conductivity of the salt-containing hydrogel was determined to be 114 ± 10 mS/cm. Upon stretching the hydrogel samples to their fourfold initial length, the impedance response increased according to a power law. This was used to establish a sensing equation for the relation between the resistive component of the impedance signal and the applied mechanical strain under tension. This work contributes to the development of highly stretchable and soft strain sensors for applications in soft robotics.
INTRODUCTION Inspired by the exceptional inventiveness of nature to move, interact with matter and sensing changes in the environment, an entirely new field of research has grown in the past few decades. In “soft robotics” scientists try to mimic those concepts – established and perfected by nature – leading to the development of devices built from soft materials, which are capable to interact with their surroundings by responding to signals such as temperature, pH values, light or several types of mechanical deformation.[1, 2]
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One particular class of materials, which can readily be employed for these purposes, are hydrogels. Hydrogels are extremely hydrophilic polymer networks swollen to a high degree, of up to 99% (by weight) with water. Although the polymer fraction is only a small contribution to the overall material, it is critically decisive for its characteristic properties, such as toughness, elasticity or biocompatibility. Therefore, it is of importance to keep the requirements of the final application in mind, when designing a new hydrogel material.[3] The present work targets a mechanical tensile strain sensor based on measuring the impedance response of a hydrogel. For this purpose, the gel needs to be reasonably stretchable, tough and elastic, to resist the stress applied during stretching it and allowing returning into its initial state, without any persistent damage after the deformation. Furthermore, the material has to be conductive to assess the changes in the impedance r
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