Chemical and pH sensors based on the swelling behavior of hydrogels

Hydrogels show a strong swelling ability in dependence on pH value or solvent concentration in aqueous solutions. This behavior can be used for appropriate sensors if a suitable transducer transforms the volume change into an electrical output signal. In this work, piezoresistive sensors were used where the hydrogel led to a deflection of the silicon membrane within the sensor chip. This principle allows for a strict separation of the fluid from piezoresistors as well as from other electronic components at the sensor chip front side. Poly(vinyl alcohol)/poly(acrylic acid) (PVA/PAA) blend as well as poly(N-isopropylacrylamide) (PNIPAAm) have been applied and investigated for pH sensors and organic solvent concentration sensors, respectively. tput voltage versus pH value characteristics and the long-term signal stability of hydrogel-based sensors were investigated and the measurement conditions necessary for high signal reproducibility were determined. The influence of the preparation conditions of the hydrogel films on the sensitivity and response time of the chemical and pH sensors is discussed.