Thomas Wallmersperger

Coupled chemo-electro-mechanical formulation for ionic polymer gels––numerical and experimental investigations

Abstract Polyelectrolyte gels consist of a network of crosslinked polymers with attached electric charges and a liquid phase. Variations of the chemical milieu surrounding the gel or application of an external electric field lead to a change in the swelling ratio of the gels. This phenomenon may be used for contraction and relaxation of chemo-electro-mechanical actuators and in particular of artificial muscles. In this paper electrically stimulated polymer gels, placed in a solution bath, are investigated. For these gels, we present a volume- and surface-coupled chemo-electro-mechanical multi-field formulation. This formulation consists of a convection–diffusion equation describing the chemical field, a Poisson equation for the electric field and a mechanical field equation. The model is capable to describe the local swelling and deswelling of ionic polymer gels as well as the ion concentrations and the electric potential in the gel and in the solution. A chemo-electric simulation is performed for a gel fiber at a given electric field; the anionic and cationic ion concentrations as well as the electric potential inside and outside the gel are computed for a given number of bound anionic groups. The resulting increase in the concentration differences on the anode side of the anionic gel can be considered as an indicator for a higher swelling ratio of the gel fiber. Mechanical and electrical measurements on anionic polyelectrolyte gels are in good agreement with the numerical results. This demonstrates the validity of the employed numerical model.

Transport modeling in ionomeric polymer transducers and its relationship to electromechanical coupling

Ionomeric polymer transducers consist of an ion-conducting membrane sandwiched between two metal electrodes. Application of a low voltage ( 2% strain) due to the transport of ionic species within the polymer matrix. A computational model of transport and electromechanical transduction is developed for ionomeric polymer transducers. The transport model is based upon a coupled chemoelectrical multifield formulation and computes the spatiotemporal volumetric charge density profile to an applied potential at the boundaries. The current induced in the polymer is computed using the isothermal transient ionic current associated with surface charge accumulation at the electrodes induced by nonzero volumetric charge density within the polymer. The stress induced in the polymer is assumed to be a summation of linear and quadratic functions of the volumetric charge density. Euler-Bernoulli beam mechanics are used to compute the bending deflection of ...

THERMO-MECHANICAL BENDING OF FUNCTIONALLY GRADED PLATES

In this work the deformations of a simply supported, functionally graded, rectangular plate subjected to thermo-mechanical loadings are analysed, extending Unified Formulation by Carrera. The governing equations are derived from the Principle of Virtual Displacements accounting for the temperature as an external load only. The required temperature field is not assumed a priori, but determined separately by solving Fouriers equation. Numerical results for temperature, displacement and stress distributions are provided for different volume fractions of the metallic and ceramic constituent as well as for different plate thickness ratios. They correlate very well with three-dimensional solutions given in the literature.

Nanocarbon based ionic actuators—a review

Nanocarbons represented especially by carbon nanotubes (CNTs) and graphene have been of great interest during the last two decades, both from a fundamental point of view and for future applications. The most eye-catching features of carbon nanostructures (CNSs) are their electronic, mechanical, optical and chemical characteristics, which open a way for versatile applications. Among those future prospects, actuators are one of the promising technologies. Since 1999 when the first macroscopic actuator containing CNTs was reported, the interest of utilizing these materials as well as other CNSs in active systems has been triggered all over the world. This paper gives a thorough review as well as in-depth descriptions of the many aspects of nanocarbon-based actuators. The review covers aspects of worldwide research and development of nanocarbon ionic actuators up to 2012. Materials which are covered by this review include CNTs and their composites, carbon nanofibres (CNFs), graphene and its derivatives, microporous carbon materials (for example carbide derived carbons (CDCs) and carbon aerogels) as well as the possible combinations of these materials. The considered aspects cover the following fields: synthesis and characterization of the investigated materials, the actuation mechanism as well as modelling and simulation. Applications comprising system integration and device development are also reviewed within this paper. (Some figures may appear in colour only in the online journal)

High surface area electrodes in ionic polymer transducers: Numerical and experimental investigations of the electro-chemical behavior

Ionomeric polymer transducer (IPT) is an electroactive polymer that has received considerable attention due to its ability to generate large bending strain (>5%) and moderate stress at low applied voltages (±2 V). Ionic polymer transducers consist of an ionomer, usually Nafion, sandwiched between two electrically conductive electrodes. A novel fabrication technique denoted as the direct assembly process (DAP) enabled controlled electrode architecture in ionic polymer transducers. A DAP built transducer consists of two high surface area electrodes made of electrically conducting particles uniformly distributed in an ionomer matrix sandwiching an ionomer membrane. The purpose of this paper is to investigate and simulate the effect of these high surface area particles on the electro-chemical response of an IPT. Theoretical investigations as well as experimental verifications are performed. The model used consists of a convection-diffusion equation describing the chemical field as well as a Poisson equation d...

Coupled chemo-electro-mechanical finite element simulation of hydrogels: II. Electrical stimulation

Certain polyelectrolyte gels are distinguished by a large swelling or bending capability under the influence of external physical, chemical or electrical stimuli. In this paper we investigate the mechanisms occurring in polyelectrolyte gels due to externally applied electric fields. By applying a coupled chemo-electro-mechanical model which is extended and predestined for electrical stimulation, we describe the concentrations and the electric potential in both the gel and the solution as well as the locally different swelling and shrinking in the gel. The local change of geometry is formulated by a local osmotic pressure difference between the gel and the solution next to the gel phase. In addition to this effect, the change of the local gel domain leads to a local variation of the concentration of bound groups and thus to a change of the local concentrations of mobile ions. As the focus of the presented work we demonstrate the superiority of the fully coupled chemo-electro-mechanical description compared to the previously developed one-way chemo-electric to mechanical coupled model. Finally, by a qualitative comparison with experimental results, the validity of the fully coupled chemo-electro-mechanical model for electrical stimulation is demonstrated.

Coupled multifield formulation for ionic polymer gels in electric fields

In this paper, electrolyte polymer gels, consisting of a polymer network with ionizable groups and a liquid phase with mobile ions, are investigated. For these gels, we present a volume- and surface-coupled multi-field problem involving chemo-electro-mechanics. First, we derive a convection-diffusion equation for the ion concentrations inside and outside the gel as well as a Laplace equation for the electric field. Second, an equation of motion in order to simulate the unsteady swelling-behavior of the gels, is presented. For the chemo-electro-mechanical coupling, the equations as well as the solution scheme, are given. For the numerical simulation, unconditionally stable, higher order accurate, conservative and implicit space-time finite elements with interpolations - continuous in space and discontinuous in time - are used. We investigate the anionic and the cationic ion concentrations for a given fixed number of bound anionic groups as well as the electric potential inside and outside the gel at a given electric field. The resulting increase in the Donnan potential difference on the anode side of the gel, which represents the higher swelling rate, is in good agreement with experimental results. This shows the validity and the potential of the model.

Thermodynamical Modeling of the Electromechanical Behavior of Ionic Polymer Metal Composites

Ionomeric polymer transducers are a class of smart materials which exhibit electromechanical coupling when subjected to low voltage (<5 V) excitation. Generally these materials are soft actuators exhibiting large bending strains (>5%) but correspondingly low force output. The mechanisms producing electromechanical coupling have so far not been completely understood. It is clear from experimental and theoretical investigations that diffusion and migration of ionic species within the polymer are the main cause for electromechanical coupling. For this reason we have developed a thermodynamically based mechanical model — using chemo-electrical inputs — which is able to predict the mechanical output i.e., deformation, bending, etc. for a given applied voltage to the IPMC strip. The chemo-electrical transport model is capable of computing the charge density profile in space and time as well as the current flux for applied electric fields. Based upon thermodynamic laws, the mechanical model has been developed to describe the strain within the material. The mechanical stress in this model is accomplished by two terms of the charge density, a linear and a quadratic one. The linear term represents the volume displacement caused by the charge migration while the quadratic term stands for the electrostatic forces caused by charge imbalances in the material. In this paper, numerical investigations of the electromechanical model as well as displacement measurements have been performed. A comparison of numerical and experimental investigations shows a very good correlation. This confirms the quality and the validity of the developed model.

Computational models of ionic transport and electromechanical transduction in ionomeric polymer transducers (Invited Paper)

A computational model of transport and electromechanical transduction is developed for ionomeric polymer transducers. The transport model is based upon a coupled chemo-electrical multi-field formulation and computes the spatio-temporal charge density profile to an applied potential at the boundaries. The current induced in the polymer is computed using the isothermal transient ionic current associated with surface charge accumulation at the electrodes induced by non-zero charge density within the polymer. The bending moment induced in the polymer is assumed to be a summation of linear and quadratic functions of the charge density. Euler-Bernoulli beam mechanics are used to compute the bending deflection of the transducer to an applied potential. Comparisons with experimental data demonstrate that this model accurately predicts the transition in the electrical current response from primarily capacitive at low frequencies to primarily resistive at high frequencies. Furthermore, the model exhibits good qualitative agreement with measured strain response of the transducer as a function of frequency. The electromechanical coupling model accurately reflects the nonlinear behavior of the material at low-frequency excitation and the relative decrease in the nonlinear response as the excitation frequency is increased. These phenomena are directly related to the asymmetric charge density distribution that develops in the polymer due to anion immobility.

Smart Hydrogel-Based Biochemical Microsensor Array for Medical Diagnostics

With the rapid development of micro systems technology and microelectronics, smart implantable wireless electronic systems are emerging for the continuous surveillance of relevant parameters in the body and even for closed-loop systems with a sensor feed-back to drug release systems. With respect to diabetes management, there is a critical societal need for a fully integrated sensor array that can be used to continuously measure a patient’s blood glucose concentration, pH, pCO2 and colloid oncotic pressure twenty four hours a day on a long-term basis. In this work, thin films of metabolite-specific or “smart” hydrogels were combined with microfabricated piezoresistive pressure transducers to obtain “chemomechanical sensors” that can serve as selective and versatile wireless biomedical sensors and sensor arrays for a continuous monitoring of several metabolites. Sensor response time and accuracy with which sensors can track gradual changes in glucose, pH, CO2 and ionic strength, respectively, was estimated in vitro using simulated physiological solutions. The biocompatibility and hermeticity of the developed multilayer encapsulation for the microsensor array has been investigated concerning the long-term stability and enduring functionality that is desired for permanent implants.