Margarita Günther

Piezoresistive pH Microsensors Based on Stimuli-Sensitive Polyelectrolyte HydrogelsPiezoresistive pH-Mikrosensoren auf der Basis stimuli-sensitiver polyelektrolytischer Hydrogele

Abstract Stimuli-sensitive ionic hydrogels are suitable materials for the detection of pH, because they exhibit a tremendous volume phase transition in response to changes in pH. The combination of such hydrogels with a micromachined piezoresistive pressure transducer enables the assembly of a new class of pH microsensors. The hydrogel swelling behavior, the sensor properties as well as different sensor designs are discussed. In addition, the great capability of this sensor type towards its use for biomedical applications is presented. In particular, the properties of a sensor containing a HPMA/DMA/TEGDMA-hydrogel, which shows a sharp volume phase transition in the physiological pH range, are described.

Modeling of Temperature-Sensitive Polyelectrolyte Gels by the Use of the Coupled Chemo-Electro-Mechanical Formulation

Polyelectrolyte gels show adaptive viscoelastic characteristics. In water-based solutions they have enormous swelling capabilities under the influence of different possible stimulation types, such as chemical, electrical, or thermal stimulation. Possible applications for these intelligent materials can be either actuators, e.g., when used as artificial muscles or chemo-electric energy converters, or sensors, e.g., for measuring ion concentrations or pH-values of the solutions. In order to represent this active behavior, a coupled multi-field model is applied. The chemo-electro-mechanical multi-field formulation considers changes of concentrations, electric potential, or strains due to varying (initial) conditions. In the present work, a fully coupled 3-field formulation for polyelectrolyte gels using the Finite Element Method is applied. This formulation consists of a chemical, electrical, and mechanical field equation. The chemical field is described by a partial differential equation (PDE) first order in time and second order in space and includes diffusional, migrational, and convectional effects. The electrical field is represented by the Poisson equation, an elliptical PDE of second order in space. Finally, the mechanical field is described by a PDE of first order derived from the conservation of momentum. Due to the slow processes in time, inertia terms are neglected. The mechanical field is coupled to the chemo-electrical field by a prescribed strain stemming from an osmotic pressure term. A large dependency between the applied temperature and the actual swelling degree of the gel has been proven in experiments. In the present research, the thermal stimulation is investigated using two alternative approaches: The first is a straight forward modeling by modifying the actual temperature in the osmotic pressure term. As this approach does not lead to promising results, as a second alternative temperature-dependent material parameters obtained from experimental measurements are applied. The calibration of the derived simulation results is performed with experimental results from the literature. The Finite Elements introduced for the coupled multi-field formulation contain degrees of freedom for concentrations, electric potential, and mechanical displacements. They are adopted and applied in the commercial software package ABAQUS as a user defined element. For the numerical solution, the Newton-Raphson method in conjunction with the implicit backward Euler time integration scheme is applied.

Coupled chemo-electro-mechanical simulation of polyelectrolyte gels as actuators and sensors

Polyelectrolyte gels are ductile elastic electroactive materials. They consist of a polymer network with charged groups and a liquid phase with mobile ions. Changing the chemical or electric conditions in the gel-surrounding solution leads to a change of the chemo-electro-mechanical state in the gel phase: diffusion and migration of ions and solvent between the gel and solution phases trigger the swelling or shrinkage of the polymer gel. In case of chemical stimulation (change of pH or salt concentration), a swelling ratio of up to 100% may be obtained. Due to this large swelling ratio the gels exhibit excellent actuatoric capabilities. In this paper, a polyelectrolyte gel placed in a solution bath is investigated. The actuatoric and sensoric capabilities are described by a chemo-electro-mechanical model. The chemical field is represented by a convection-migration-diffusion equation while the electric field is described by a quasi-static Laplace equation. For the mechanical field a partial differential equation of first order in time is applied. Inertia effects are neglected due to the relatively slow swelling/shrinkage process. On the one hand, the coupling between the chemo-electrical and the mechanical field is realised by the differential osmotic pressure stemming from the concentration differences between gel and solution. On the other hand, the mechanical deformation influences the concentration of the bound charged groups in the gel. The three fields are solved simultaneously by applying the Newton Raphson method using finite elements in space and finite differences in time. The developed model is applicable for both, hydrogel actuators and sensors. Numerical results of swelling and bending are given for chemically and electrically stimulated polymer gels. In this paper we show the differences between the chemo-electric and the fully coupled chemo-electro-mechanical formulation for polymer gels in different solution baths. The inverse (sensor-) effect is demonstrated by the influence of the mechanical deformation on the gel, which results in a change of the chemical and electrical unknowns in the gel. The validity of the employed numerical model is shown by a comparison of the obtained results with experimental measurements.

Piezoresistive Chemosensoren auf der Basis von Hydrogelen (Piezoresistive Chemical Sensors Based on Hydrogels)

Abstract Hydrogele zeigen in Abhängigkeit von der sie umgebenden Flüssigkeit ein starkes Quellungsvermögen. Dies kann für die Schaffung chemischer Sensoren, z.B. für die Messung des pH-Wertes, ausgenutzt werden. Dafür lassen sich piezoresistive Sensoren nutzen, wobei das quellende Hydrogel zu einer Verformung der Silizium-Biegeplatte des Sensorchips führt. Es wurden die Ausgangsspannung in Abhängigkeit vom pH-Wert untersucht und die für eine hohe Signalreproduzierbarkeit erforderlichen Messbedingungen ermittelt.

Chemo-electro-mechanical modeling of pH-sensitive hydrogels

Hydrogels are viscoelastic active materials. They consist of a polymer network with bound charges and a liquid phase with mobile anions and cations. In water based solutions these gels show large swelling capabilities under the influence of different possible stimulation types, such as chemical, electrical or thermal stimulation. In the present work a coupled chemo-electro-mechanical formulation for polyelectrolyte gels using the Finite Element Method (FEM) is applied. In addition to the three given fields, the dissociation reactions of the bound charges in the gel are considered. Thus, we are able to model and simulate pH-stimulation and to give the different ion concentrations, the electric potential and the mechanical displacement. Depending on the initial conditions and the dissociation ratio, different kinds of stimulation cycles can be simulated. Concluding, the developed model is applicable for chemical stimulation and can model both, hydrogel actuators and sensors.

Thermo-chemo-electro-mechanical modeling of polyelectrolyte gels

Polyelectrolyte gels show adaptive viscoelastic characteristics. In water-based solutions they have enormous swelling capabilities under the influence of various possible stimulation types, such as chemical, electrical or thermal. In the present work a fully coupled 3-field formulation for polyelectrolyte gels using the Finite Element Method (FEM) is applied. This formulation consists of a chemical, electrical, and mechanical field equation. The mechanical field is coupled to the chemo-electrical field by a prescribed strain stemming from an osmotic pressure term. In experiments it has been proven that there is a large dependency between the applied temperature and the actual swelling degree of the gel. In the present research, the thermal stimulation is investigated. First, only the actual temperature is considered in the osmotic pressure term. Then, additionally, temperature-dependent material parameters obtained from experimental measurements are applied. The calibration of the numerical simulation is performed with experimental results available in literature.