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Magnetophoretic immunoassay is a technique for detection and isolation of target cells, pathogens, and biomolecules. In this method, target antigens bind to specific antibodies coated on magnetic microbeads (mMBs) and are separated by an external magnetic field for further analysis. For improving the sensitivity and performance of magnetophoretic immunoassay in microfluidic devices, the first important step is to improve the capture efficiency (CE) of mMBs. The use of electroosmotic flow (EOF) can overcome the problem of pressure penalty, and increase the residence time of mMBs by altering the flow direction to improve the CE. This study was conducted to show that sequentially switched EOF can improve the CE of mMBs. The first aim of this study was to simulate the capture of mMBs using a permanent magnet in sequentially switched EOF. In the computational model two conditions of EOF, with applied electric fields of 150-450 V/cm, were studied to evaluate how residence time could be increased with flow reversal and improve the CE. In the control condition (without switching), an electric field without reversal of polarity was applied across the microchannel. During `switching’ condition, the electric potential at the inlet and outlet of the microchannel was switched, causing reversal in flow direction. From the computational results it was concluded that CE was function of the momentum of mMBs and magnetic field strength. By switching the electric field, CE increased from 75% (without switching) to 95% for lower electric fields (150-200 V/cm) and from 35% to 47.5% for higher electric fields (400-450 V/cm). The CE was lower at higher EOF electric fields because the momentum of mMBs overcame the magnetic force. Switching led to improved CE of mMBs due to decrease in EOF velocity during flow reversal and increased residence time of mMBs in the region of stronger magnetic field.The second aim of the study was to design and fabricate a microfluidic experimental setup to assess the concept of switching. The separation of fluorescence-tagged mMBs in EOF was studied under steady and switched electric fields of 100-180 V/cm. The mMBs were captured using NdFeB magnet. The CE of mMBs was improved by sequentially switching the electric field. Under steady electric field, CE of mMBs decreased by 72.3% when the electric field was increased from 100 V/cm to 180 V/cm. Alternating the direction of EOF through sequential switching increased the CE by bringing the escaped mMBs back to the capture zone, increasing their residence time in the area of high magnetic fields. The average increase in CE was 54.3% for mMB concentration of 1 × 106 beads/ml (C1) and 41.6% for a concentration of 2 × 106 beads/ml (C2). These improvements were particularly significant at higher electric fields where CE was ~70% higher with switching. The improvements in CE with sequential switching, particularly at higher electric fields, make sequential switching of EOF an efficient mMB separation technique for use in high throughput devices. The technique can be considered for further development towards integration into microfluidic assays. |
