A METHOD FOR STUDYING SWELLING KINETICS BASED ON MEASUREMENT OF ELECTRICAL-CONDUCTIVITY

A novel method was applied to the study of swelling kinetics of pH-res ponsive hydrogels. This technique is based on the pH-dependent electri cal conductivity of these materials, which is measured by coating plan ar interdigitated electrode arrays with thin hydrogel membranes. To de monstrate the utility of the method, the swelling kinetics of a well-c haracterized pH-responsive hydrogel were studied. Cross-linked copolym ers of 2-hydroxyethyl methacrylate (HEMA) with up to 20 mol% dimethyla minoethyl methacrylate (DMA) were studied as a function of copolymer c omposition in phosphate or triethanolamine buffer at buffer concentrat ions from I to 100 mM. The experiments consisted of measuring the chan ge in electrical resistance of a hydrogel-coated electrode array follo wing a small pH change in the external buffer medium. The characterist ic response time to reach a new equilibrium following a pH change was proportional to the concentration of DMA within the polymer and was in versely proportional to the buffer concentration. The characteristic r esponse times for devices tested in phosphate buffer were a function o f 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 t imes for devices tested in triethanolamine were independent of step si ze. The observed dependences upon the values of the dissociation const ant (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. (C) 1 998 Elsevier Science Ltd. All rights reserved.