The application of "stimuli-responsive" or "smart" cross-linked polymeric hydrogels in biochemical 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 in piezoresistive biochemical sensors 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 compounds and proteins in water was investigated at different temperatures. It has been demonstrated that the sensor's sensitivity depends on the polymer composition as well as on the polymer cross-linking degree. A higher sensitivity was observed for polyelectrolyte hydrogels with higher concentrations of ionizable groups. Gel stiffening was observed in the presence of protein in the solution by means of both a hydrogel-based biochemical sensor and an AFM cantilever. Not only the measuring principle was adapted to different species in aqueous solutions but also the implemented sensor has been used as an instrument for the investigation of the gel sensitivity to different external stimuli and for on-line monitoring of the gel behaviour kinetics. The measured kinetic curves have been analysed by means of appropriate models and some methods improving the properties of the chemical sensors have been proposed. The polymer film preparation conditions and measurement conditions, which are necessary for high signal reproducibility and high long-term stable sensor sensitivity, were determined.