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

Abstract

By rapidly creating libraries of thousands of unique, miniaturized reactors, droplet microfluidics provides a powerful method for automating high-throughput chemical analysis. In order to engineer in-droplet assays, microfluidic devices must add reagents into droplets, remove fluid from droplets, and perform other necessary operations, each typically provided by a unique, specialized geometry. Unfortunately, modifying device performance or changing operations usually requires re-engineering the device among these specialized geometries, a time-consuming and costly process when optimizing in-droplet assays. To address this challenge in implementing droplet chemistry, we have developed the "K-channel," which couples a cross-channel flow to the segmented droplet flow to enable a range of operations on passing droplets. K-channels perform reagent injection (0-100% of droplet volume), fluid extraction (0-50% of droplet volume), and droplet splitting (1:1-1:5 daughter droplet ratio). Instead of modifying device dimensions or channel configuration, adjusting external conditions, such as applied pressure and electric field, selects the K-channel process and tunes its magnitude. Finally, interfacing a device-embedded magnet allows selective capture of 96% of droplet-encapsulated superparamagnetic beads during 1:1 droplet splitting events at ∼400 Hz. Addition of a second K-channel for injection (after the droplet splitting K-channel) enables integrated washing of magnetic beads within rapidly moving droplets. Ultimately, the K-channel provides an exciting opportunity to perform many useful droplet operations across a range of magnitudes without requiring architectural modifications. Therefore, we envision the K-channel as a versatile, easy to use microfluidic component enabling diverse, in-droplet (bio)chemical manipulations.

Figure 1

K-channel device operation. Arrows indicate flow directions. After forming at a T-junction (left), droplets (blue) flow through the main channel to the K-channel element (right). The K-channel fluid (orange), an aqueous or an oil phase, flows through the cross-channel in an anti-parallel direction relative to main channel flow. An electric field may be supplied via the electrode channels (gray) to destabilize passing droplets. Through the interaction at the K-channel, droplet size, number, composition, and spacing can be altered.

Figure 2

Multiple K-channel operations on a single device. Droplets flow left to right. The K-channel continuous phase flows right to left. (a) High K-channel pressure with electric field injects into droplets. (b) Low K-channel pressure with electric field extracts from droplets. (c) Moderate K-channel pressure without electric field maintains the droplet-K-channel interface. (d) Low K-channel pressure without electric field splits droplets under oil flow. (scale bars = 100 μm)

Figure 3

K-channel operation characterization. (a) Net droplet volume change becomes more positive with increasing applied K-channel pressure. In the highlighted region, a stable droplet-K-channel fluid interface occurs in the absence of an applied electric field. (b) K-channel volumetric flow rate into droplets is directly proportional to applied pressure (R² = 0.981). (c) Increasing the K-channel inlet hydraulic resistance (by decreasing the inlet channel width from 40 μm to 25 μm to 15 μm) decreases net droplet volume change. (d) Increasing applied K-channel pressure reduces the droplet fraction removed during oil flow-induced droplet splitting.

Figure 4

K-channel material exchange. During K-channel operations that merge droplets with the continuous aqueous phase, bi-directional exchange of material occurs. (a) The K-channel extracts fluorescein from droplets as it injects water. Continuous flow through the K-channel washes away extracted fluorescein to reduce the likelihood for droplet-to-droplet cross-contamination. The white arrow highlights the position of a single droplet across subsequent frames. (b) Monitoring the net change in droplet volume and fluorescein concentration at the K-channel enables (c) decoupling of the relative magnitudes of water injection, fluorescein extraction, and net volume change (the sum of injection and extraction) at each K-channel pressure. (scale bar = 100 μm)

Figure 5

Magnetic bead capture. (a) Schematic of bead capture device. Arrows indicate flow directions. Magnetic bead (brown) containing droplets (blue) form at a flow focusing geometry (left) followed by droplet splitting under oil flow at a K-channel (middle). During droplet splitting, a magnet (dark gray) pulls beads into only one of the two daughter droplets. Electrode channels (light gray) are not enabled during this operation. After droplet splitting, waste droplets without magnetic beads (upper) and sample droplets with magnetic beads (lower) flow to the detection channels (right). (b) The nearby magnet pulls beads (circled in red) into one of the two daughter droplets during droplet splitting at the K-channel. (c) The detection channels show high incidence (96%) of magnetic beads in the sample droplets (lower channel) and low incidence (4%) of magnetic bead loss (white arrow) into waste droplets (upper channel). (scale bar = 100 μm)

Figure 6

Magnetic Bead Washing. (a) Schematic of washing device. Light gray arrows indicate flow direction. After magnetic bead encapsulation in droplets (not shown) droplets (b) split at the leftmost K-channel under parallel oil flow, (c) respace at an oil channel, and (d) double in size upon injection at the rightmost K-channel using parallel water flow in an electric field (supplied by red electrode channels in schematic). Magnetic beads (highlighted by red arrows) are pulled toward the magnet (dark gray in schematic) and are retained during operations. (scale bars = 100 μm)