Characterization of Electromechanical Transduction in Polyelectrolyte Gels for Mechanical Sensor Applications
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1006-R07-07
Characterization of Electromechanical Transduction in Polyelectrolyte Gels for Mechanical Sensor Applications Katsiaryna Prudnikova, and Marcel Utz Center for Microsystems for the Life Sciences and Department of Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, VA, 22904
ABSTRACT We report a new experimental method for the characterization of the electromechanical properties of polyelectrolyte gels (PG). PGs have been studied extensively, but with limited success, as mechanical actuators. However, they are also promising as potentially biocompatible mechanical sensors. In order to integrate them into actual devices, their electromechanical transduction properties need to be characterized in a reproducible manner. We have therefore developed a technique to measure the mechanically induced change in electrostatic potential in PGs. The polyelectrolyte gel is subjected to a well-defined pressure gradient by placing a thin, flat sample on a substrate with integrated concentric Platinum electrodes and indenting it with a spherical indenter. The potential values at the electrodes are measured using a MOSFET operational amplifier circuit with an input impedance of 1014 Ω and an effective dynamic range better than 16 bit. This method can be directly used to quantify electromechanical coupling in polyelectrolyte gels.
INTRODUCTION Electroactive polymers exhibit a mechanical response to the application of an electric field, and vice versa. A number of recent studies have explored this property both theoretically and experimentally [1, 3-7]. For example, Doi et al. presented a semiquantitative picture of the response of the hydrogel to an electric field using Donnan theory [2] of osmotic pressure in ionic solutions [1]. Donnan theory describes an unequal distribution of diffusible ions between two ionic solutions separated by some restraint which prevents ionic components from moving from one phase to another phase and defines an osmotic pressure as: Π ion = RT
∑ (c N
j =1
gel j
− c sol j )
(1)
The electromechanical coupling in ionic gels arises because only the ionic groups attached to the cross-linked polymer backbone are coupled to the mechanical deformation of the gel, and not the free counterions. Depending on the osmotic pressure of the solvent, this leads to an imbalance in the ion concentration, and, in turn, to a pressure-dependent electrical potential. In addition, dynamic effects (coupled solvent and ion transport) may also play an important role in certain systems [3].
So far, interest in these materials has mainly been stimulated by their application as electrical actuators. The electromechanical effect is symmetrical, which also opens the possibility to use them as mechanical sensors. This, however, requires quantifying the transduction properties, which is not trivial, since it involves exposing the sample to a welldefined spatial pressure distribution, while the electrostatic potential needs to be monitored with precise spatial resolution. In this contribution,
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