Design criteria for developing low-resource magnetic bead assays using surface tension valves

Abstract

Many assays for biological sample processing and diagnostics are not suitable for use in settings that lack laboratory resources. We have recently described a simple, self-contained format based on magnetic beads for extracting infectious disease biomarkers from complex biological samples, which significantly reduces the time, expertise, and infrastructure required. This self-contained format has the potential to facilitate the application of other laboratory-based sample processing assays in lowresource settings. The technology is enabled by immiscible fluid barriers, or surface tension valves, which stably separate adjacent processing solutions within millimeter-diameter tubing and simultaneously permit the transit of magnetic beads across the interfaces. In this report, we identify the physical parameters of the materials that maximize fluid stability and bead transport and minimize solution carryover. We found that fluid stability is maximized with 0.8 mm i.d. tubing, valve fluids of similar density to the adjacent solutions, and tubing with 20 dyn/cm surface energy. Maximizing bead transport was achieved using !2.4 mm i.d. tubing, mineral oil valve fluid, and a mass of 1-3 mg beads. The amount of solution carryover across a surface tension valve was minimized using 0.2 mg of beads, tubing with 20 dyn/cm surface energy, and air separators. The most favorable parameter space for valve stability and bead transport was identified by combining our experimental results into a single plot using two dimensionless numbers. A strategy is presented for developing additional self-contained assays based on magnetic beads and surface tension valves for low-resource diagnostic applications.

A two-magnet strategy for improved mixing and capture from biofluids

“…While this illustrates that vortexing is an excellent means of dispersing beads throughout a fluid in a microcentrifuge tube, vortexing the pre-arrayed assay tube would result in hydrodynamic instabilities that would combine the otherwise separated fluid chambers. 27 This re-iterates the importance of mixing strategies to improve the rate of target biomarker binding, and hence shorten processing times, in pre-arrayed microfluidic cassettes designed for off-theshelf use.…”

“…27 The tube format has been demonstrated as a method for tuberculosis DNA extraction and amplification, protein isolation for malaria detection, cell counting for human immunodeficiency virus diagnosis, and clinical chemistry assays. 21,22,[27][28][29] In each of these assays, a single magnet was moved relative to the length of the tube to transfer the magnetic beads across the immiscible fluid interface and into the sequential processing chambers. We have recently described a twomagnet design for processing which worked particularly well for viscous sputum samples used for tuberculosis diagnosis.…”

Magnetically-enabled biomarker extraction and delivery system: towards integrated ASSURED diagnostic tools

“…The depositions tube’s millimeter-diameter tube aperture creates a stable surface tension valve that retained the sample and allowed beads to collect at the meniscus formed at the bottom of the tube during the 1 min bead collection. 32 The developed surface tension valve prevents premature transfer of magnetic beads to the LFA during magnetic bead gathering step. The cantilever-based tube holder was then depressed briefly by the user in order for the bead-filled meniscus to make contact with the conjugate pad, breaking the surface tension valve, and transferring the beads to the LFA.…”

A negative-pressure-driven microfluidic chip for the rapid detection of a bladder cancer biomarker in urine using bead-based enzyme-linked immunosorbent assay

“…Thus, the total measurement time with the chip is greatly reduced, totaling approximately 40 min, which is much shorter than the time needed to run a conventional plate-based ELISA (more than 4 h). In another bead-based microfluidic assay approach using surface tension valves for fluidic control, 33 the measurement of the concentration of the biomarker in that chip was performed off-chip, but in the proposed chip, it was performed in the same chip, which eliminates the need for manual operation and facilitates automatic control of the measurement process. 96-well plate.…”