A method for studying swelling kinetics based on measurement of electrical conductivity

Abstract

A novel method was applied to the study of swelling kinetics of pH-responsive hydrogels. This technique is based on the pH-dependent electrical conductivity of these materials, which is measured by coating planar interdigitated electrode arrays with thin hydrogel membranes. To demonstrate the utility of the method, the swelling kinetics of a well-characterized pH-responsive hydrogel were studied. Cross–linked copolymers of 2-hydroxyethyl methacrylate (HEMA) with up to 20 mol% dimethylaminoethyl methacrylate (DMA) were studied as a function of copolymer composition in phosphate or triethanolamine buffer at buffer concentrations from 1 to 100 mM. The experiments consisted of measuring the change in electrical resistance of a hydrogel-coated electrode array following a small pH change in the external buffer medium. The characteristic response time to reach a new equilibrium following a pH change was proportional to the concentration of DMA within the polymer and was inversely proportional to the buffer concentration. The characteristic response times for devices tested in phosphate buffer were a function of the magnitude of the pH step, increasing from 2.6 to 5.6 min as the step size increased from 0.2 to 0.57 pH units. However, the response times for devices tested in triethanolamine were independent of step size. The observed dependences upon the values of the dissociation constant (pK_a) of the buffering ion, the apparent pK_a of DMA, and the pH of the external bath agreed with buffer-mediated diffusion–reaction theory, and as such this conductimetric method represents a powerful tool for the study of swelling kinetics of responsive hydrogels.