For the direct measurement of chemical concentrations of substances in process liquids, sensors need to be fast and reliable. We investigated the usability of hydrogels as sensitive coating materials for piezo-electric thickness shear mode resonators. Hydrogels are cross-linked polymer networks that show well-defined shifts of their degree of swelling upon changes of environmental parameters such as ionic concentrations, concentrations of organic solvents or temperature. These changes of gel properties are then transferred into an electrical signal. The novel sensor solution exhibits response times of only few seconds and matches the reliability of innovative process measuring technology. A method for its application specific adaption is presented. It is based on optimizing the interrelation of the sensor structure, the manufacturing technology for the hydrogel layer, and the processing of the measured data. The performance of the method is illustrated with the example of a PVA/PAAhydrogel coated sensor which monitors the concentration of a cleaner used in surface technology. The developed measurement device for this application was approved with a measurement error of less than 5 %.

Abstract. The volume production of industrial hydrogel sensors lacks a quality-assuring manufacturing technique for thin polymer films with reproducible properties. Overcoming this problem requires a paradigm change from the current recipe-driven manufacturing process to a specification-driven one. This requires techniques to measure quality-determining hydrogel film properties as well as tools and methods for the control and optimization of the manufacturing process. In this paper we present an approach that comprehensively addresses these issues. The influence of process parameters on the hydrogel film properties and the resulting sensor characteristics have been assessed by means of batch manufacturing tests and the application of several measurement techniques. Based on these investigations, we present novel methods and a tool for the optimization of the cross-linking process step, with the latter being crucial for the sensor sensitivity. Our approach is applicable to various sensor designs with different hydrogels. It has been successfully tested with a sensor solution for surface technology based on PVA/PAA hydrogel as sensing layer and a piezoelectric thickness shear resonator as transducer. Finally, unresolved issues regarding the measurement of hydrogel film parameters are outlined for future research.

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