Design and fabrication of a new 3D AC-electroosmotic micropump

Abstract

Integrated Microsystems, MicroTAS or the Lab-on-a-chip, require integrated fluid handling. Advances in microelectronics fabrication processes have allowed the miniaturization of fluid handling devices such as micropumps. In biomedical technology, pumps for handling extremely small fluid amounts become more and more important where microsystems for biological analysis routinely use solid-state electrokinetic micropumps.

AC Electrokinetic micropumps in particular AC electroosmosis pump can be used to pump fluids using planar electrodes which induce electrical forces on the fluid. However, planar electrodes have limited pumping capability of the micropump.

In this thesis a new design for the AC electroosmotic is introduced. The new AC electroosmotic design presents the transition from planar microelectrode arrays to planar with High Aspect Ratio (HAR) pillars in order to increase the surface area of the electrodes. The physical mechanism of AC electrosmosis is the motion of induced Electrical Double Layers on microelectrodes driven into motion by the electric field generated by the electrodes. Since AC electrosmosis is a surface driven effect, increasing the surface area increases the power coupled into the fluid movement. By taking the channel volume and filling it with conductive pillars, the surface area therefore increases, but the volume remains the same, increasing the drive per unit volume. This will have the effect of increasing the pressure generated by the pump.

To explore and realize the proposed pumping principle we attempted to benefit from available expertise of Professor Marc. J. Madou who specializes in Bio-MEMS field and microfabrication techniques. Prof. Madou and his team at UC Irvine have been able to construct large dimensions of high-aspect-ratio carbon pillars made out of pyrolyzed SU-8 using Carbon-MEMS process. This conversion of polymer to a conductive-polymer technique was adopted and applied to our proposed smaller dimension of 3D-electrodes design. The current planar electrodes designs studied previously were made out of gold and it is desired to make the pillars out of gold also. However due to some microfabrication limitations, and since gold pillars undergo chemical reactions involving dissolution and redeposition, pyrolyzed pillars are suitable for our process. Although pyrolyzed SU-8 pillars are less conductive than the gold, but they are perfectly polarisable, which is ideal for AC-electroosmosis. In this particular area of interest, we have investigated with the collaboration of Prof. Madou and his team the fabrication of high-aspect-ratio carbon pillars with different aspect ratios and dimensions and introduced them to AC-electroosmosis pumping. Carbon electrodes were successfully and generate local fluid and drive fluid, where the new 3D-AC-electroosmosis micropump has shown an increase of 5 times to previous planar electrodes design.

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Identifiers

ORCID for Nicolas Green: orcid.org/0000-0001-9230-4455

ORCID for Hywel Morgan: orcid.org/0000-0003-4850-5676

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