Piezoresistive chemical sensors based on hydrogels

Abstract

ABSTRACT Already eight years ago, the usage of piezoresistive sensors for chemical measurands was proposed at the Solid State Electronics Laboratory of the Dresden University of Technology [1] . Adding functionalised polymer coating which shows swelling due to chemical or biological values leads to a similar deflection of the thin silicon bending plate like for pressure sensors. The application of “stimuli-responsive” or “smart” cross-linked gels in chemical sensors is based on their ability to a phase transition under the influence of external excitations (pH, concentration of additives in water, temperature). Combining a “smart” hydrogel and a micro fabricated pressure sensor chip allows to continuously monitor the analyte-dependent swelling of a hydrogel and hence the analyte concentration in ambient aqueous solutions. The sensitivity of hydrogels with regard to the concentration of such additives as H + -ions (pH sensor), transition-metal ions, salts, organic solvents and proteins in water was investigated. It has been demonstrated that the sensor’s sensitivity depends on the polymer composition as well as on the polymer cross-linking degree. Time constants down to a few ten seconds have been found for thin hydrogel films deposited directly on the backside of the silicon bending plate. In order to achieve an optimum between sensor signal amplitude and sensor response time, the gel swelling/deswelling kinetics was investigated. Some methods improving the properties of the chemical sensors have been proposed. The long-term measurements have shown that the lifetime of piezoresistive chemical sensors can be prolonged up to several years provided that specific operation and storage conditions are fulfilled. Keywords: piezoresistive sensor, biochemical sensor, polyelectrolytic hydrogel, pH-sensitive, swelling behaviour