ABSTRACT

I. INTRODUCTION In this work we establish the technique of AC electrical impedance, coupled with the use of microfabricated, interdigitated, electrode arrays, as a means of effectively and efficiently studying the swelling behavior of polymeric hydrogels. The pHresponsive poly(2-hydroxyethyl methacrylate-co-N,N-dimethylaminoethyl methacrylate) [poly(HEMA-co-DMA)] hydrogels were used in this work for their ease of preparation, their moderate hydration (35 to 55% water by weight), and their inherent pH sensitivity. The hydrogels were cast as thin membranes onto the arrays and the devices were placed in temperature-controlled buffer solutions. Step changes in buffer pH were then produced while the device was interrogated with an AC voltage of appropriate frequency. The thin (3-10 Am) hydrogel layers respond quickly (minutes to hours) to pH changes (0.2-0.6 pH unit). This technique greatly reduces the time needed to perform swelling experiments on hydrogels and is well suited to studying the responses of broad classes of chemically responsive hydrogels. AC impedance studies were performed to assess the effect of copolymer composition, buffer ion plc, buffer concentration, and buffer composition. These studies clearly establish this technique as suitable and highly recommended for the study of swelling and deswelling kinetics of hydrogels. Hydrogel responses, previously found to be governed by the buffer-mediated diffusion and reaction of protons within the hydrogel, are consistent with established theory. A challenge in the use of these devices is found in the following two questions: Are the small perturbations in hydration and pH prescribed by mediated diffusion-reaction theory capable of producing swelling and deswelling changes that can alter ion concen-

tration and ion mobility within the hydrogel? Is AC electrical impedance sensitive enough to detect the changes in ion concentration and mobility attendant to swelling and deswelling of pH-responsive hydrogels? The chapter reports our work that answers both questions affirmatively.