Breaking the Interface: Efficient Extraction of Magnetic Beads from Nanoliter Droplets for Automated Sequential Immunoassays

Abstract

Droplet-based microfluidic systems offer a high potential for miniaturization and automation. Therefore, they are becoming an increasingly important tool in analytical chemistry, biosciences, and medicine. Heterogeneous assays commonly utilize magnetic beads as a solid phase. However, the sensitivity of state of the art microfluidic systems is limited by the high bead concentrations required for efficient extraction across the water–oil interface. Furthermore, current systems suffer from a lack of technical solutions for sequential measurements of multiple samples, limiting their throughput and capacity for automation. Taking advantage of the different wetting properties of hydrophilic and hydrophobic areas in the channels, we improve the extraction efficiency of magnetic beads from aqueous nanoliter-sized droplets by 2 orders of magnitude to the low μg/mL range. Furthermore, the introduction of a switchable magnetic trap enables repetitive capture and release of magnetic particles for sequential analysis of multiple samples, enhancing the throughput. In comparison to conventional ELISA-based sandwich immunoassays on microtiter plates, our microfluidic setup offers a 25–50-fold reduction of sample and reagent consumption with up to 50 technical replicates per sample. The enhanced sensitivity and throughput of this system open avenues for the development of automated detection of biomolecules at the nanoliter scale.

Particle ID: A Multiplexed Hydrogel Bead Platform for Biomedical Applications

“…The use of cross-linking technology based on such preformed polymers has an advantage over the often-described polymerization reactions in liquid droplet based microfluidic environments in that no monomer is involved, which allows performance of the reactions outside a chemical laboratory. 6,17,29 Shape-Based and Optical Barcoding of Hydrogel Beads. Barcodes provide information about an object in a machine-readable layout.…”

“…Incorporation of Superparamagnetic and ID Particles into the Beads. Magnetic particles are used in many heterogeneous assays 29,32 as they allow simplified collection and washing of the beads by applying an external magnetic field. 33 Also for the incorporation of magnetic particles, the photo-cross-linking process provides an optimal platform.…”

Digital droplet immunoassay based on a microfluidic chip with magnetic beads for the detection of prostate‐specific antigen

“…Additionally, the microfluidic droplet technology we used allows for homogeneous and stable droplets to be generated within reaction chambers at high throughputs. Lastly, microfluidic immunoassay technology also provides high sensitivity, low sample size, and high throughput over flow cytometry and microplate readers, making it an increasingly popular choice for analysis research [28, 38–43].…”

Recent progress of microfluidic sample preparation techniques

“…These on-chip sample transfers can be executed by droplet microfluidics. Metzler et al [86] reported an automated microfluidic immunoassay enables target capture, labeling, magnetic preconcentration, and fluorescence detection sequentially processed in microdroplet flow. In their microchip device, two auxiliary channels were connected to the main analysis channel via T-junctions to merge droplets containing antibody-coated magnetic beads with the sample as well as fluorophorelabeled detection antibodies sequentially, and two external magnets were placed downstream of the main analysis channel to accumulate magnetic sandwiched immunocomplexes for fluorescence detection.…”