Stimuli-responsive hydrogels are called "smart" materials because they autonomously respond to environmental stimuli. For example, pH-responsive hydrogels swell at lower pH levels and deswell as the pH increases. Hydrogel-based sensors could prove beneficial for providing continuous monitoring of bioreactors. The motivation of this project is to create a hydrogel-based sensor that can be used for bioreactor monitoring to help researchers monitor bioreactor conditions. The magnitude of the swelling/deswelling behavior can be measured by placing a sample of the hydrogel in a piezoresistive sensor. The degree of swelling/deswelling is directly proportional to the change in pH of the aqueous solution in which it is placed. In this project, an initial characterization of the hydrogel response was performed, followed by an analysis of the hydrogel components and optimization of the hydrogel response based on those components. The longevity of the hydrogel response was tested in terms of shelf life and response after multicycle testing. A hydrogel sample was then synthesized in situ in a microsensor and tested to determine the ability to transport hydrogels and how the miniaturization of the sensor may affect the stimuli response. In all experiments, the response time and magnitude results were compared to determine the effect of the noted changes on the kinetics of the swelling behavior of the material in order to find the optimal composition, thickness, and device specifications that will yield the desired response time and sensitivity.
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