A wearable body temperature sensor would allow for early detection of fever or infection, as well as frequent and accurate hassle-free recording. This thesis explores the design of a body-temperature-sensing device inkjet-printed on a flexible substrate. All structures were first modeled by first-principles, theoretical calculations, and then simulated in HFSS. A variety of planar square inductor geometries were studied before selecting an optimal design. The designs were fabricated using multiple techniques and compared to the simulation results. It was determined that inductance must be carefully measured and documented to ensure good functionality. The same is true for parallel-plate and interdigitated capacitors. While inductance remains relatively constant with temperature, the capacitance of the device with a temperature-sensitive dielectric layer will result in a shift in the resonant frequency as environmental or ambient temperature changes. This resonant frequency can be wirelessly detected, with no battery required for the sensing device, from which the temperature can be deduced. From this work, the optimized version of the design comprises of conductive silver in with a temperature-sensitive graphene oxide layer, intended for inkjet-printing on flexible polyimide substrates. Graphene oxide demonstrates a high dielectric permittivity with good sensing capabilities and high accuracy. This work pushes the state-of-the-art in applying these novel materials and techniques to enable flexible body temperature sensors for future biomedical applications.

The repeated usage of frying oil is hazardous due to the degradation caused by chemical reactions, which happen while heating. The total polar compounds and the free fatty acids are the main two chemical parameters affected by frying. These parameters increase significantly with the use of oil for frying and are reported as reasons for causing serious illnesses like heart diseases. For this purpose, sensor systems for oil quality assessment are necessary. In fact, changes of the composition due to frying leads to variation of its dielectric parameters. This can be measured using a capacitive sensor and the measurement of its impedance change. The main challenge thereby is that the impedance changes are very small and stray capacitances have a big influence on the measurements. In this context, this work proposes a sensor system with high accuracy able to detect the small changes that occur in the resistance and capacitance under influence of stry capacitances. Theoretical and simulation studies are carried out for different cap acitive sensors as well as meas urement procedures of its cornp lex imp edance. The sensor should provide a high sensitivity to relative perrnittivity and the electrical conductiv ity, and at the same time a small size and a high reproducibility. Interdigital electrodes sensor with a suitable design fulfils all these requirements. A deep consideration of stray capacitances is needed to realize an accurate sensor system. For t hese reasons, the design of the measurement circuit is crucial within this work. We propose, a measurernent circuit based on a combinat ion of the method of capacitance to voltage conversion and the phase shift measurement method. By cornbining both rnethods together it is possible to rneasure accurate ly the complex irnpedance of edible oil. Experimental results show that measurement systern is capable to detect small changes of dielectric parameters, which are correlated to the chemical parameters. / Die mehrfach wiederholte Verwendung von Frittieröl ist aufgrund der Qualitätsver­ schlechterung, die während des Erhitzens auftreten durch chemische Reaktionen verursacht wird, gefährlich für die Gesundheit. Die totale polaren Kompon enten und die freien Fettsäuren sind die zwei wichtigsten chemischen Komponenten, die wesentlich durch das Braten beeinflusst werden. Diese Komponenten erhöhen sich signifikant mit der Wiederverwendung von Bratöl und verursachen u. a. ernste Herzkrankheiten. Diese Arbeit zielt darauf hin, ein mobiles, kostengünstiges, einfach zu verwenden­ des Sensorsystem für die Abschätzung der Ölqualität zu entwickeln. Das System charakterisiert die Veränderung der elektrischen Parameter des Öls durch Messung der Änderung seiner komplexen elektrischen Eigenschaft en. In dieser Arbeit wurde ein Sensorelement mit interdigitalen Elektroden entwickelt, der eine hohe Empfindlichkeit auf die relative Permittivität und die elektrischen Leitfähigkeit des Öls hat und dabei einer hohe Reproduzierbarkeit erzielen kann. Es wird ein Messverfahren vorgeschlagen, das auf der Wandlung in einer Spannung und einer Phasenverschiebung basiert. Sowohl durch theoretische Überlegungen als auch durch Simulationen konnte belegt werden, dass die Kombination beider Metho­ den eine akkurate Messung der Komplexem Imped anz hochdielektrischer Materia lien ermöglichen kann. Experiment elle Ergebnisse zeige n, dass das Messsystem in der Lage ist , kleine Änderungen der dielektrischen Parameter zu erfassen, die mit den chemischen Ölparamtern stark korrelieren.