Suitably formulated hydrogels exhibit changes in swelling with changes in glucose concentration.

Sensors that exploit pH-sensitive hydrogels for measuring concentrations of glucose in aqueous solutions are undergoing development. Because the underlying chemical and physical principles are also applicable to sensing of biochemicals other than glucose, it is expected to be relatively easy to modify the glucose sensors to enable detection of such biochemicals.

Deflection of a Microcantilever that had been coated with a hydrogel containing glucose oxidase was measured as a function of glucose concentration. In this experiment, the deflection was measured optically, but in a fully developed sensor as envisioned, the bending of the microcantilever would likely be measured via the piezoresistive effect.

The term “pH-sensitive hydrogels” denotes hydrogels that exhibit structural and hydration properties that change with changing concentrations of H+ ions in aqueous solutions. For the purpose of sensing glucose or another chemical species, a pH-sensitive hydrogel can be modified by the incorporation of active chemical components (e.g., enzymes) for catalyzing chemical reactions that lead to pH changes that increase with the concentration of that species. Then the change in a conveniently measurable physical property (e.g., a change in size as a measure of a change in swelling in response to a change in pH) can be used as an indication of the concentration of the species of interest. pH-sensitive hydrogels that have been modified or formulated to exhibit such changes in response to glucose or other chemical species of interest are sometimes called “smart gels.”

The development work thus far has focused on techniques for stable, efficient immobilization of enzymes and on transduction techniques for measuring small changes in gel size or water content. In one of two transduction techniques that have been investigated, a smart hydrogel is immobilized on one side of a microcantilever, the bending of which is taken as an indication of swelling of the hydrogel. The other transduction technique is based on fluorescence resonance energy transfer: The basic idea is to incorporate, into a hydrogel, fluorescent molecules, the spectral properties of which shift because of energy-transfer changes induced by gel swelling. Efforts to demonstrate sensing of glucose by use of both transduction techniques have been successful (for example, see figure). In addition, this development work has contributed to knowledge of the molecular structures and other properties of hydrogels.

This work was done by Michael J. McShane of Louisiana Tech University for the Army Research Laboratory. For further information, download the free white paper at www.defensetechbriefs.com under the Materials category. ARL-0006

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