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

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 (eta(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 eta(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 eta(c) was 54.3% for an mMB concentration of 1 x 106 beads ml(-1) (C1) and 41.6% for a concentration of 2 x 10(6) beads ml(-1) (C-2). These improvements were particularly significant at higher electric fields where the eta(c) with switching was, on average, similar to 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.