Rapid and continuous magnetic separation in droplet microfluidic devices

Abstract

We present a droplet microfluidic method to extract molecules of interest from a droplet in a rapid and continuous fashion. We accomplish this by first marginalizing functionalized super-paramagnetic beads within the droplet using a magnetic field, and then splitting the droplet into one droplet containing the majority of magnetic beads and one droplet containing the minority fraction. We quantitatively analysed the factors which affect the efficiency of marginalization and droplet splitting to optimize the enrichment of magnetic beads. We first characterized the interplay between the droplet velocity and the strength of the magnetic field and its effect on marginalization. We found that marginalization is optimal at the midline of the magnet and that marginalization is a good predictor of bead enrichment through splitting at low to moderate droplet velocities. Finally, we focused our efforts on manipulating the splitting profile to improve the enrichment provided by asymmetric splitting. We designed asymmetric splitting forks that employ capillary effects to preferentially extract the bead-rich regions of the droplets. Our strategy represents a framework to optimize magnetic bead enrichment methods tailored to the requirements of specific droplet-based applications. We anticipate that our separation technology is well suited for applications in single-cell genomics and proteomics. In particular, our method could be used to separate mRNA bound to poly-dT functionalized magnetic microparticles from single cell lysates to prepare single-cell cDNA libraries.

The Poisson distribution and beyond: methods for microfluidic droplet production and single cell encapsulation

“…7b) 101 and to control the ejection of particles in a single phase. 5c, 115 the concentration and separation of magnetic particles in droplets, 133,135,136 or the use of a centrifuge to eject and segment a cell-containing fluid from a glass capillary. 130 By pulsing the actuator, one or more droplets containing a number of cells could be ejected.…”

K-Channel: A Multifunctional Architecture for Dynamically Reconfigurable Sample Processing in Droplet Microfluidics

“…While some devices have reported larger droplet fractions removed (60–90%) or higher capture efficiencies (98–100%), K-channel magnetic separation proceeds at droplet frequencies more than an order of magnitude faster than these approaches (0.5–30 Hz). 27–29 We anticipate that K-channel bead retention would be similarly improved by velocity reduction to increase the residence time for beads in the magnetic field. Ongoing work seeks to concentrate magnetic beads in smaller droplet fractions per splitting event.…”

“…24–26 Coupled with antibody-functionalized magnetic beads, droplet splitting can even selectively concentrate samples and remove waste volume. 13,27–31 Overall, these well-characterized approaches provide a toolkit for in-droplet chemistry, and recent reviews highlight the broad range of droplet techniques and applications. 7,8,17,32,33 …”

Controlled Lateral Positioning of Microparticles Inside Droplets Using Acoustophoresis

“…8,10 Some researchers have addressed this and similar topics by controlling the position of microparticles inside droplets using magnetic forces, but this technique cannot easily be transferred into label-free cell-based assays since it requires magnetic properties of the microparticles. 11,12 A recent report also shows how hydrodynamics can be used to position microparticles inside droplets. 13 However, that technique relies upon particle sedimentation and requires slow flows to successfully handle biologically relevant microparticles, thus limiting its use for high-throughput screening applications.…”