Capillary-driven microfluidic chips with evaporation-induced flow control and dielectrophoretic microbead trapping

Abstract

This work reports our efforts on developing simple-to-use microfluidic devices for point-of-care diagnostic applications with recent extensions that include the trapping of microbeads using dielectrophoresis (DEP) and the modulation of capillary-driven flow using integrated microheaters. DEP serves the purpose of trapping microbeads coated with receptors and analytes for detection of a fluorescent signal. The microheater is actuated once the chip is filled by capillarity, creating an evaporation-induced flow tuned according to assay conditions. The chips are composed of a glass substrate patterned with 50-nm-thick Pd electrodes and microfluidic structures made using a 20-μm-thick dry-film resist (DFR). Chips are covered/sealed by low-temperature (50 °C) lamination of a 50-μm-thick DFR layer having excellent optical and mechanical properties. To separate cleaned and sealed chips from the wafer, we used an effective chip singulation technique that we informally call the "chip-olate" process. In the experimental section, we first studied dielectrophoretic trapping of 10 μm beads for flow rates ranging from 80 pL s^-1 to 2.5 nL s^-1 and that are generated by an external syringe pump. Then, we characterized the embedded microheater in DFR-covered chips. Flow rates as high as 8 nL s^-1 were generated by evaporation-induced flow when the heater was biased by 10 V, corresponding to 270 mW power. Finally, DEP-based trapping and fluorescent detection of functionalized beads were demonstrated as the flow was generated by the combination of capillary filling and evaporation-induced flow.