Characterization of Magnetic Bead-Based Microfluidic Mixing for Bio-Mems Applications

ABSTRACT

Achieving fast mixing in a microfluidic environment is a key challenge in designing a micro-total analysis system (μ-TAS) where the flow takes place at very low Reynolds numbers and mixing is dominated by molecular diffusion. Therefore, for high-molecular weight organic molecules, which have extremely low molecular diffusivities, biochemical reactions occur at a comparable slow rate unless some arrangement is made to improve mixing or the probability of intermolecular collisions between the reagents. Herein we show magnetic microspheres acting as mobile substrates in a microfluidic environment to promote biochemical reactions. The performance of a magnetic bead-based mixer in a pressure-driven flow through a microchannel is characterized for (i) immunochemical binding of a probe oligonucleotide on the functionalized beads, and (ii) hybridization of a target DNA strand with its complementary strand (probe oligonucleotide) immobilized on the beads. The probe oligonucleotide is labeled with a Cy3-fluorophore at the 3-prime end and biotin at the 5-prime end. A suspension of streptavidin-coated magnetic beads and the probe oligonucleotide solution are first passed through a microchannel for immobilizing the probe strands on the beads through streptavidin-biotin binding. The beads are magnetically guided in the transverse direction through the channel, using a suitably designed magnetic field, to maximize their contact with the oligonucleotides in the second stream. The extent of mixing and binding is quantified using fluorescence imaging (using a 543 nm He-Ne laser). Effects of different operating parameters on the overall binding rate have been investigated. Subsequently, the conjugate of bead and probe oligonucleotide are passed through a similar arrangement with a co-flow of the target oligonucleotides. The target oligonucleotides are tagged with quencher molecules (BHQ2) so that Cy3 fluorescence of the probe oligonucleotide is quenched in case a specific binding of the two complementary DNA strands take place. The extent of hybridization is studied in terms of the relative quenching of fluorescence. Results of this study are of direct relevance to the design and operation of magnetically assisted bio-MEMS devices.

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