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Hydrogels and Other Ionic Conductors in ‘Piezo-Ionic’ Sensors, Actuators and Electrochemical Devices

Abstract : Ionic conductors offer exciting device possibilities, particularly with the advent of highly extensible and easily synthesized hydrogels. Following on from recent work on hydrogel “ionic skin”, ‘piezo-ionic’ sensors and ionic artificial muscle, we present example ionic devices, including stretchable and transparent capacitive sensor arrays, pressure sensors that generate currents upon stretching, actuators that bend or contract when ions are inserted, as well as stretchable electrochromic elements and batteries. Hydrogels and other ionic conductors offer advantages over metals and semiconductors in being intrinsically stretchable and non-absorbing of visible light. Their moderate to low moduli and non-linear mechanical properties can match those of tissue, and make them of interest for use in human interfacing devices. On the other hand, ions are much less mobile than electrons, ionic conductors don’t have bandgaps, and, unlike electrons and holes, anions and cations don’t recombine. The materials are typically wet, and properties can depend significantly on dimensions, pH and temperature. Given these drawbacks, how can we make practical use of ionic conductors? In order to avoid being limited by the low mobility of ions, we can take two approaches. One is to implement gels and other ionic conductors in devices that consume very little current – such as capacitive sensor arrays. Even in low current devices, dropping temperatures can dramatically and unacceptably increase resistance, so a careful consideration of RC charging time is important in the design. Another approach to avoid being limited by low mobility is to make the transit distances short. We present bending actuators that can operate hundreds of hertz. Relative speed of ion motion is important in ‘piezo-ionic’ sensors, where application of a pressure gradient leads to differential rates of ion motion between positive and negative ions – and the generation of current. NMR and electromechanical measurements suggest that in hydrogels ions move more slowly than solvent, perhaps hindered by the polymer structure. The small voltages that result are sufficient to stimulate nerves. Electrochemical devices combine electronic and ionic conductors – for example in batteries, electrochromics and ionic diodes. In such cases, the electrode material is not as stretchable as the hydrogel. We present a symmetric conducting polymer-based electrochromic element that can stretch and change colour, and battery that is robust under extension. If ionic devices dry out, they will slow or stop. One approach to avoid drying is to use salts that dramatically reduce vapour pressure – but this can come at the cost of conductivity. We instead demonstrate the use of non-volatile ionic liquids to keep a capacitive sensor array working for years without encapsulation. Alternatively, low vapour transmission rate elastomers can keep fast actuators and stretchable batteries functional for months or even years without preventing bending.
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Submitted on : Monday, April 4, 2022 - 2:46:20 PM
Last modification on : Saturday, July 2, 2022 - 6:30:03 PM


Distributed under a Creative Commons Attribution - NonCommercial 4.0 International License


  • HAL Id : hal-03629635, version 1


John D.W. Madden, Yuta Dobashi, Mirza Sarwar, Dickson Yao, Saeedeh Ebrahimi Takalloo, et al.. Hydrogels and Other Ionic Conductors in ‘Piezo-Ionic’ Sensors, Actuators and Electrochemical Devices. Materials Research Society Meeting, MRS Fall 2019, Dec 2019, Boston, États-Unis. ⟨hal-03629635⟩



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