New Advances and Opportunities of Magnetohydrodynamic Microfluidics
Authors: Nash Christena K. , Claycomb Adair , Khan Foysal , Jones Benjamin J. , Hutcheson Joshua , Muldoon Timothy J. , Fritsch Ingrid 
Published: February 2020

An important need in miniaturizing technology for applications in point-of-care medicine and environmental monitoring is the ability to manipulate small volumes of fluids to perform sample preparation and multiple steps for chemical analysis in an automated fashion. We are pioneering the use of magnetohydrodynamics (MHD) for this purpose, which offers combined advantages of other microfluidic methods: it pumps both aqueous and non-aqueous solutions; easily starts, stops, and reverses direction; finely tunes fluid speeds up to mm/s; offers flat flow profiles and stirring capabilities; and can pump in a loop. Probably most unique to MHD microfluidics is its programmability: magnitude, direction, and pattern of fluid flow can be altered without constructing new devices. In our presentation, we will report the most recent advances in development of MHD microfluidics technology.
MHD propels fluids when ions moving perpendicular to a magnetic field experience a force in the direction that is perpendicular to both, and impart momentum on the surrounding fluid. The relationship of the MHD force, FB
, on the ionic current density, j, and the magnetic flux density, B, vectors is expressed by the right-hand rule, j x B = FB
. We make use of individually-addressable microelectrodes patterned on a chip that is placed onto a magnet, to control the magnitude and direction of the ionic current in a solution through electrochemistry. Thus, the programmability of MHD microfluidics is realized.
Several advances have made MHD microfluidics more viable for lab-on-a-chip applications. Early studies in salt solutions reported complications from electrode corrosion and bubble formation (from solvent electrolysis).1,2 These problems resulted from inefficient conversion of electronic current in the external circuit to ionic current in the solution. Adding chemical species that are more easily reduced and oxidized (redox species) than the solvent and supporting electrolyte eliminate these problems,3,4 introducing the concept of redox-MHD (RMHD). Thus, the ability to tune fluid velocity by MHD could be investigated unhindered, leading to discovery of flat flow profiles and spiral stirring.5,6 The concentration of the redox species, electron transfer kinetics, and mass transport limit the maximum current, and therefore the maximum achievable fluid velocity. However, adding high concentrations of redox species to the solution is not always compatible with biological samples and detection schemes. Two new developments maintain the advantages of RMHD microfluidics and yet avoid the interferences that occur by adding redox species to a sample. The first modifies electrodes with redox polymers (e.g. a conducting polymer of poly(3,4-ethylenedioxythiophene), PEDOT) so that adding redox species to solution is no longer necessary. Immediate access to that charge at the electrode surface allows for high currents and therefore high velocities. The second sustains fluid flow even when the charge at the modified electrodes depletes, by a recharging process by manipulating both B and j sinusoidally and in synchrony (AC-RMHD).7 The frequencies are much lower than in prior AC-MHD studies in supporting electrolyte at bare electrodes where inductive heating is a problem.8

Our presentation will describe advances in RMHD and in AC-RMHD with the emphasis on applications involving analysis of biological samples. New approaches and materials for modifying the electrodes will be discussed. Enhancements in manipulating the fluid flow through electrochemical cell, electrode, and magnet design will be addressed. Also, recent developments will be described toward interfacing the unique properties of RMHD microfluidics (i.e. flat flow profile and pumping in a loop) with technology for cell identification of leukocytes in blood,9which is of interest for point-of-care monitoring of patients undergoing chemotherapy.
ACKNOWLEDGMENTS
Research was supported through the National Science Foundation (CBET-1336853) and the Arkansas Biosciences Institute, the major research component of the Arkansas Tobacco Settlement Proceeds Act of 2000.
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