Sustaining redox-magnetohydrodynamics (R-MHD) microfluidics by switching oppositely-polarized permanent magnets: Synchronized activation and automation

Abstract

A transformative advance in redox-magnetohydrodynamics (R-MHD) microfluidics is demonstrated that indefinitely extends its pumping duration with a miniaturizable approach, while preserving its uniqueness as an internal, self-contained, on-device, active and versatile pump that can also propel fluid in a loop. R-MHD can address the need for fine-tuning microfluidics in micro total analysis systems (μTAS) for multiple functions in an automated fashion that conventional external pumps with channels and/or valves that determine direction cannot fulfill. In MHD, a body force produced by the cross product of ionic current between strategically-activated electrodes and magnetic flux from a permanent magnet or electromagnet drives the fluid. Conducting-polymer-modified electrodes (e.g. with poly(3,4-ethylenedioxythiophene), PEDOT), involve faradaic processes to convert electronic current in the external circuit to ionic current in solution, overcoming bubble generation and electrode corrosion that limited previous MHD microfluidic applications. PEDOT-R-MHD pumping operates with a wider variety of solution compositions and without redox additives. However, pumping stops after complete oxidation/reduction of redox sites in the PEDOT films. The new advance reverses current between PEDOT-modified electrodes to discharge/recharge the polymer while simultaneously swapping permanent magnets of opposite polarities to sustain a constant, unidirectional pumping speed interrupted with brief pauses and without inductive heating. Factors affecting fluid velocities are described, including positions across the magnet assembly, current magnitudes and synchrony with current reversal. A model system (microbeads in biologically-compatible phosphate-buffered saline) is used, which can be generalized more broadly to biological and environmental applications, where starting, stopping, and indefinitely sustaining pumping of a sample are important.