Design Studies Towards Mixers and Pumps Based on Electrochemically Active Conducting Polymers for Microfluidic Devices

Abstract

This thesis concerns the design of mixers and pumps for microfluidic devices, based on the actuation and ion-exchange properties of electrochemically active conducting polymers (ECP). Electrochemical oxidation or reduction of these polymers is accompanied by ion-exchange with the electrolyte and an associated change of volume. The thesis proposes that this volume change can be utilised for the purpose of mixing and pumping, in which the electrochemical cell is formed within a microfluidic structure and the motion is induced in the electrolyte as a consequence of redox reaction of the polymer. The thesis falls into three parts. The first part explores theoretically the propagation of composition waves along a strip of ECP upon electrochemical stimulation, and the use of such waves as a means of mixing and pumping. For the theoretical description of the propagation of composition waves, an efficient solution of the electro-neutral Nernst-Plank equations in 2-D for electromigration and diffusional transport in the solution is developed. Under some circumstances, waves reflecting back from the end of the strip are predicted. We then demonstrate theoretically how such waves, associated as they are with expansion of the polymer, could be employed to enhance mixing or induce pumping in microfluidic systems. The anticipated effects of using ECP alone, based on known volume changes for ECP as a consequence of the redox reaction, seemed quite small. Therefore, the second part of the thesis explores the idea that, by using an ion sensitive polyelectrolyte gel as the electrolyte surrounding the ECP, a significant amplification of motion could be achieved. The hydrogel and ECP are assumed formed as a uniformly mixed composite. A 1-D model incorporating both diffusion and migration of ions and accounting for the space- and time-dependent variation in both ionic and electronic conductivity is set up and solved using the COMSOL multi-physics solver package. The importance for achieving a significant and long-lasting change in local ionic composition of the composite of the volume fraction of the two phases, the electronic conductivity contrast between oxidised and reduced states of the ECP, the ionic mobilities in the hydrogel and the ion-exchange capacity of the ECP are demonstrated. The final part presents experimental studies directed at depositing and actuating such ECP based composites in a miniature electrochemical cell.