Continuous dielectrophoretic sorting of liquid beads

Abstract

We report the concept of continuous sorting of liquid beads using dielectrophoresis (DEP) in a microfluidic device. Liquid beads are liquid droplets encapsulated in a hard polymer shell, which provides a unique core-shell structure crucial for applications such as digital polymerase chain reaction (dPCR), digital loop-mediated isothermal amplification (LAMP), drug delivery, cell cultures and microreactors. A population of consistent microscale liquid beads are required for these applications. We utilised microfluidic methods to continuously produce beads with a narrow size distribution. However, this technique requires a precise control of flow rates to prevent the formation of core-less beads. We generated monodispersed liquid beads with trimethylolpropane trimethacrylate (TMPTM) shell and hydrofluoroether (HEF) core. The device produced about 20–30 % core-less solid beads, which needs to be removed from the population of liquid beads. We first examined the dielectrophoretic (DEP) responses for liquid beads and solid beads. Using a relatively simple setup with a steel needle and indium tin oxide (ITO)-coated glass electrodes, we demonstrated that beads exhibit both positive and negative DEP under alternate-current (AC) and direct-current (DC) electric fields. Subsequently, we designed and tested a microfluidic sorting device to leverage the differential DEP responses for continuous sorting of liquid beads and solid beads. In a DC field, solid beads experience positive DEP, whereas liquid beads show negative DEP, enabling their separation with an efficiency of approximately 80 %. Our sorting method provides a precise, controllable, label-free, and scalable solution for obtaining consistent liquid beads for further applications.