Margarita Guenther

Hydrogel-based piezoresistive pH sensors: investigations using FT-IR attenuated total reflection spectroscopic imaging.

The strong swelling ability of the pH-responsive poly(acrylic acid)/poly(vinyl alcohol) (PAA/PVA) hydrogel makes the development of a new type of sensor possible, which combines piezoresistive-responsive elements as mechanoelectrical transducers and the phase transition behavior of hydrogels as a chemomechanical transducer. The sensor consists of a pH-responsive PAA/PVA hydrogel and a standard pressure sensor chip. However, a time-dependent sensor output voltage mirrors only the physical swelling process of the hydrogel but not the corresponding chemical reactions. Therefore, an investigation of the swelling behavior of this hydrogel is essential for the optimization of sensor design. In this work, Fourier transform infrared (FT-IR) spectroscopic imaging was used to study the swelling of the hydrogel under in situ conditions. In particular, laterally and time-resolved FT-IR images were obtained in the attenuated total reflection mode and the entire data set of more than 80,000 FT-IR spectra was evaluated by principal component analysis (PCA). The first and third principal components (PCs) indicate the swelling process. Molecular changes within the carboxyl groups were observed in the second and fourth PC and identified as key processes for the swelling behavior. It was found that time-dependent molecular changes are similar to the electrical sensor output signal. The results of the FT-IR spectroscopic images render an improved chemical sensor possible and demonstrate that in situ FT-IR imaging is a powerful method for the characterization of molecular processes within chemical-sensitive materials.

Ion-beam induced chemical and structural modification in polymers

In order to increase the sensitivity to moisture uptake of polyimide (PI) and polyethersulfone films applied in bimorphic humidity sensors 50, 130 and 180 keV boron ions with irradiation doses between 1013 and 1016 B+/cm2 were implanted. A complex investigation of the following features has been carried out: chemical changes in the surface regions by attenuated total reflection–FTIR spectroscopy, Raman spectroscopy and X-ray photoelectron spectroscopy (XPS); optical properties by spectroscopic ellipsometry; hardness and elastic modulus by depth-sensing low-load indentation technique; conductivity of modified polymer films. It could be shown, that the partial destruction of chemical bonding under ion bombardment leads to the creation of new amorphous and graphite-like structures, which increase the surface film conductivity by several orders of magnitude, and enhances the sensitivity of these polymer films to moisture uptake. The ion-beam irradiation destroys the anisotropic features of the refractive index of PI layers leading to its isotropization. Radiation-induced changes in the layer structure result in an increase of the hardness and elastic modulus of the modified layers up to ten and six times, respectively. The hardness and refractive index depth profiles were determined. The detectable effective modification depth estimated from the depth profiles is 250–300 nm at an ion energy of 50 keV and 400–450 nm at an ion energy of 180 keV.

Influence of ion-beam induced chemical and structural modification in polymers on moisture uptake

Abstract Polyimide thin films are a promising material for microelectronics and aerospace applications. In particular, they are sensitive to moisture and gas uptake. This leads to film swelling which may be monitored by a corresponding change in piezoresistance caused by plate bending of a polymer–silicon double-layer or bimorphic sensor, respectively. However, the expansion of the polymer, induced by moisture or gas uptake, can usually be influenced by surface ion-beam modification. By this, the selectivity to a partial gas may be enhanced or decreased. In this work, the influence of ion-beam induced surface modification on both polymer structure and moisture uptake of polyimide and polyethersulfone is investigated. To modify the polymer layer surface boron ions were implanted with energies from 50 to 180 keV and irradiation doses between 1013 and 1016 B+/cm2. It could be shown that increase of irradiation dose leads partly to a destruction of the imide and aromatic groups. The aromatic structure is degraded by hydrogen abstraction. This corresponds to the creation of a new amorphous and graphite-like structure, which increases the modified surface film conductivity by several orders of magnitude, and which decreases the Freundlichs coefficient of the moisture-uptake behaviour.

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.

Hydrogels for Chemical Sensors

A rapidly expanding field of on-line process monitoring and on-line control in biotechnology, food industry, pharmaceutical industry, process chemistry, environmental measuring technology, water treatment and sewage processing requires the development of new micro fabricated reliable chemical and biosensors that are specific for particular species and can attain the analytic information in a faster, simpler and cheaper manner. Using a functionalised hydrogel coating in sensors provides the possibility to detect, transmit and record the information regarding the concentration change or the presence of a specific analyte (a chemical or biological substance that needs to be measured) by producing a signal proportional to the concentration of the target analyte. In this chapter, we describe piezoresistive chemical microsensors for a comprehensive characterization of solutions, which could be embedded inside fluidic systems for real time monitoring of organic and inorganic contaminants.

Deposition of PZT Thin Films on Copper-Coated Polymer Foils—Challenges and Perspectives

In this work, the synthesis of (111)-textured PZT on Cu-coated polymer substrates is demonstrated by means of a RF-modulated plasma-jet system comprising a hollow cathode for reactive sputtering. Ion bombardment during film growth and lead stoichiometry are shown to be key parameters for the improvement of the PZT thin film quality.

Force-compensated hydrogel-based pH sensor

This paper presents the design, simulation, assembly and testing of a force-compensated hydrogel-based pH sensor. In the conventional deflection method, a piezoresistive pressure sensor is used as a chemical-mechanical-electronic transducer to measure the volume change of a pH-sensitive hydrogel. In this compensation method, the pH-sensitive hydrogel keeps its volume constant during the whole measuring process, independent of applied pH value. In order to maintain a balanced state, an additional thermal actuator is integrated into the close-loop sensor system with higher precision and faster dynamic response. Poly (N-isopropylacrylamide) (PNIPAAm) with 5 mol% monomer 3-acrylamido propionic acid (AAmPA) is used as the temperature-sensitive hydrogel, while poly (vinyl alcohol) with poly (acrylic acid) (PAA) serves as the pH-sensitive hydrogel. A thermal simulation is introduced to assess the temperature distribution of the whole microsystem, especially the temperature influence on both hydrogels. Following tests are detailed to verify the working functions of a sensor based on pH-sensitive hydrogel and an actuator based on temperature-sensitive hydrogel. A miniaturized prototype is assembled and investigated in deionized water: the response time amounts to about 25 min, just half of that one of a sensor based on the conventional deflection method. The results confirm the applicability of t he compensation method to the hydrogel-based sensors.

Modeling and Simulation of Hydrogels for the Application as Bending Actuators

Polyelectrolyte gels show a quite large swelling or bending behavior under external physical, chemical, thermal or electrical stimulation. In this paper the bending actuation of polyelectrolyte gels under applied electric fields is studied and the mechanisms occurring in polyelectrolyte gels due to the applied stimulus are investigated.In the present research, a complete formulation for describing actuation of polyelectrolyte gels in a solution bath is presented. First, the kinematics, the balance laws of continuum chemo-electro-mechanics and the constitutive equations of the involved fields are given.Then, in the numerical simulation part, the changes of the mobile concentrations, of the electric potential and the displacements in the gel domain are shown. It is demonstrated by a comparison between a chemo-electrical and a chemo-electro-mechanical test case, that the full coupling also leads to a change of the concentration of bound groups resulting in alterations of the local concentrations of the mobile ions, of the electric potential and again of the local gel displacement. It will be shown, that the presented coupled multi-field model is predestined for investigating electrical stimulation of hydrogels, and for simulating hydrogel bending actuation.

Modeling of nonlinear effects in pH sensors based on polyelectrolytic hydrogels

pH-sensitive hydrogels are capable of reversibly converting chemical energy into mechanical energy and therefore they are widely used as sensitive materials for pH sensors. However nonlinear effects such as hysteresis and drift are observed in the swelling behaviour of the polyelectrolytic hydrogels complicating the calibration procedure for the pH sensor and affecting the signal reproducibility. In the present work, in order to realize a pH sensor with a high signal reproducibility and high long-term stable sensor sensitivity, the complicated kinetics of gel swelling/deswelling processes is analysed and the origin of the hysteresis nonlinearities is elucidated. It is found that the long-time drift in the sensor characteristic is caused by the drift of hydrated ions and water into the gel or out of the gel in dependence on the pH range of the solution and on the chemical reactions which occur in the gel during the swelling or shrinking processes. The rate of the water drift is determined by the change rate of the concentration of ionized groups which increase the gel hydrophilicity and consequently the gel swelling.

Smart hydrogel based microsensing platform for continuous glucose monitoring

In this paper, we present preliminary results showing the response of glucose-sensitive hydrogels, confined in micro-pressure sensors, to the changes in environmental glucose concentration. The glucose concentrations were incrementally varied between 20 and 0mM in 0.15M PBS solution at 7.4 pH and bovine serum at 7.4 pH at room temperature and response of the sensor was recorded. The micro sensors demonstrate a response time of 10 minutes in both PBS and serum. Tissue response after 55 days of subcutaneous implantation of a EtO sterilized sensor in mice is presented. The preliminary analysis of the surrounding tissue shows inflammation which is believed not to interfere with the sensor performance.