Efficient capture of magnetic microbeads by sequentially switched electroosmotic flow—an experimental study

Abstract

Magnetophoretic separation is a commonly used immunoassay technique in microfluidic platforms where magnetic microbeads (mMBs) coated with specific epitopes (antibodies) entrap target pathogens by antigen-antibody kinetics. The mMB-cell complexes are then separated from the continuous flow using an external magnetic field. The goal of this study was to design and test a microfluidic device for efficient separation of fluorescence-tagged mMBs driven by electroosmotic flow (EOF) under steady (time invariant) and switched (time varying) electric field conditions. The EOF was driven at electric fields of 100-180 V cm −1. The mMBs were captured by a neodymium (NdFeB) permanent earth magnet. The capture efficiency (η c) of these mMBs was improved by sequential switching of the applied electric field driven-EOF. The fluorescent images of the captured mMBs, obtained using an inverted epifluorescence microscope, were quantified using image processing tools. In steady EOF, induced by constant electric field, the number of captured mMBs decreased by 72.3% when the electric field was increased from 100 V cm −1 to 180 V cm −1. However, alternating the direction of flow through sequential switching of EOF increased the η c by bringing the escaped mMBs back to the capture zone and increasing their residence time in the area of higher magnetic fields. The average increase in η c was 54.3% for an mMB concentration of 1 × 10 6 beads ml −1 (C 1) and 41.6% for a concentration of 2 × 10 6 beads ml −1 (C 2). These improvements were particularly significant at higher electric fields where the η c with switching was, on average, ~70% more compared to flow without switching. The technique of sequential switching demonstrates an efficient method for capture of mMBs for application in magnetophoretic immunoassay.

Analysis of Magnetic Microbead Capture With and Without Bacteria in a Microfluidic Device Under Different Flow Scenarios

“…This study provides improved analysis of the capture efficiency of the device designed previously by our group and published in Das et al [20]. It is important to efficiently capture the mMBs in order to produce accurate and reliable results with the greatest sensitivity possible.…”

“…The previous studies have shown how multiple passes can increase capture of mMBs using the implementation of a switching protocol to return initially uncaptured mMBs to the device [5,20]. It was hypothesized that the switching protocol integrated into this device will return the mMBs to the area of higher magnetic field, allowing for more mMBs to be captured, and thus increasing the capture efficiency.…”

“…Properties. The microfluidic device was created through standard soft lithography technique with polydimethylsiloxane (PDMS) according to the procedure presented in Das et al [20]. The resulting device consists of PDMS bonded to a glass slide to create a 50 mm channel with a cross section of 50 lm  50 lm with 6 mm diameter wells at each end of the channel.…”

“…The mMBs used in the experiments were 2.8 lm diameter Dynabead M280 Sheep anti-Rabbit IgG mMBs tagged with an Alexa Fluor 488 Rabbit anti-Mouse IgG fluorescent marker, as shown in Fig. 1(a), following the procedure in Das et al [20]. The microbeads were run through the system at concentrations of 2 Â 10 6 and 4 Â 10 6 beads/mL.…”

“…The use of EOF allows greater control over the magnitude and direction of the flow [19]. A previous study in our group, by Das et al, analyzes a device utilizing an EOF-driven switching mechanism and permanent Neodymium (NdFeB) magnet perpendicular to the channel to improve mMB immobilization [20]. The Das et al study characterized the capture of mMBs due to fluorescent intensity of captured mMBs at different mMB concentrations and flow rates in comparison to a standard fluorescence of known concentration samples.…”