Continuous Microfluidic Pump Involving Conducting Polymer Modified Redox-Magnetohydrodynamics (R-MHD)
Authors: Khan Foysal Z , Parette David N. , Fritsch Ingrid 
Published: February 2020

Continuous microfluidic pumping is an important requirement for numerous applications, such as lab-on-a-chip separation and imaging cytometry. To satisfy this necessity we have developed a strategy to indefinitely push fluid in a single direction by Redox-Magnetohydrodynamics (R-MHD). R-MHD is a phenomenon where fluid flows between electrodes by a net body force (
FB

), followed by the equation
FB = j

× B
, where
j
and
B
are the ionic current and magnetic flux densities, respectively.
1,2 Ionic current density (
j
) converts from electronic current through the redox reaction of chemical species (K3Fe(CN)6 / K4Fe(CN)6) or electrode confined conducting polymer (CP) whereas magnetic field (
B
) comes from a permanent or electromagnet. CP, such as Poly(3,4-ethylenedioxythiophene) or PEDOT can be electropolymerized on the electrode surface with good spatial resolution, adherence, uniformity, and controlled thickness.3 These characteristics along with high electronic conductivity and capacitance have made them perfect candidates as redox center in R-MHD pumping. CP generates high ionic current density (j) in the electrolyte solution and avoids the interference issue of solution redox species.4-6

R-MHD pumping has unique advantages; such as flat flow profile, valve and channel less device, tunability, portability, circular flow capability, and low voltage requirement.6 Therefore, it is of interest in different microfluidic applications including chemical analysis, mixing, synthesis, detection, imaging cytometry and separation. To satisfy these growing needs, achieving longer pumping with a high speed is a primary requirement. Deposition parameters such as types of monomers, the number of redox centers or polymer loading, choice of deposition method, and electrolytic strength of the pumping solution affect both the current and charge density of polymer films that limits pumping speed and duration. PEDOT deposited by the potentiostatic technique, propylene carbonate solvent, and TBAPF6 electrolyte exhibited the best combination for maximum electrochemical response and mechanical stability. The improved polymer showed 80% retention of its capacity even after 500 pumping cycles.7,8

Though the optimized PEDOT film could pump for 211.70 ± 8.0 sec with a moderate 49.40 ± 6.0 µm/s fluid speed through a cross section of 760 µm × 3 mm pumping region, the fluid can only flow in one direction until the charge in the PEDOT film is consumed (discharging). Polymer film recharges by applying a reverse current but owing to have the same magnetic field orientation underneath the electrodes, the fluid flows in the reverse direction. This periodic reversed flow was useful in image cytometry application of leukocytes in post cancer monitoring.5,9

Some microfluidic applications such as on-chip separation require even longer pumping, at a high speed, and in a single direction. To address this limitation, we built a raspberry pi-controlled translational device that synchronizes magnetic field with opposite bias current. The continuous change in current bias recharges the polymer film after the initial discharge but the simultaneous position switching of two permanent magnets with opposite field directions maintains a unidirectional MHD force. This programmable R-MHD offers several advantages over AC-MHD approach, such as higher magnetic field, simplified instrumentation, and less fluidic disruption.10 Results will be reported on a newly developed smaller device where galvanostat can trigger a translational stage with opposing orientation of magnets through a single board computer so that, magnets can be switched automatically when a target voltage is reached or within a wait time (open circuit potential).

References

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