Bistable Hydrogel-based Sensor Switch for Monitoring Relative Humidity

This contribution presents a hydrogel-based bistable sensor switch with a switching hysteresis for the measurement of the relative humidity via a moisture-sensitive swellable material in ambient air. The sensor enables the continuous inline monitoring and control of the relative humidity, for example in buildings and containers. Since the required energy is taken from the environment, the sensor does not need any internal electrical energy supply for the monitoring of the relative humidity and the operation. To protect the sensor from arcing and damage the contacts have to close or open very fast, i.e. the sensor has to show a switching hysteresis. This contribution deals with the principle concept of a sensor switch and the implementation of such a switching hysteresis based on a mono-/bimorph beam partly covered with hydrogel. For that, it provides a simple mathematical model to describe the switching in dependence on the geometry parameters and the material properties of the sensor. The switching hysteresis is implemented by a pre-deflection due to an axial force and by buckling resulting from the hydrogel swelling. To prove the switching hysteresis a macroscopic sensor switch was studied and, from the results, the mathematical model was validated. Based on the obtained results a miniaturized silicon-based sensor switch is proposed.