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 low-resource 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.

Figure 1

Illustration of the self-contained format for extraction of RNA biomarkers. Surface tension valves separate unique processing solutions arrayed within a single length of 1.6 mm i.d. tubing. Functionalized magnetic beads used to capture the biomarker of interest are drawn through the surface tension valves into each processing solution using an externally applied magnetic field (i.e., a permanent cube magnet).

Figure 2

Selected video images of magnetic beads under the influence of the magnetic field of a permanent magnet moving from one solution to the next through an air surface tension valve (a) or a mineral oil surface tension valve (b) (enhanced online).

Figure 3

The properties of the materials tested in these studies span a wide range of values. (a) The surface free energy of the various materials is related to the advancing contact angle with water. The surface free energies of the materials used in these studies (solid squares) span the range of available materials (open squares). (b) Tubing, solutions, and valve types tested span a wide range of interfacial energies. (c) Images showing the curvature of the menisci for tubing materials, solutions, and valve fluids evaluated in these studies. From left to right: (i) water and an air valve in tubing of decreasing surface energies. (ii) Tygon tubing and an air valve with solutions of decreasing interfacial tensions. (iii) Tygon tubing and a mineral oil valve with solutions of decreasing interfacial tensions.

Figure 4

The effect of material properties on the stability of the surface tension valve. (a) Surface tension valves within tubing with smaller inner diameter are much more stable than those within tubing with larger diameters. (b) Surface tension valves within tubing with low surface energy are more stable than those within tubing with high surface energy. (c) The stability of mineral oil valves decreases with increasing interfacial tension (solid circles), whereas the stability of air valves increases linearly with increasing interfacial tension (open circles). (d) Surface tension valves interfaced with solutions of similar density are much more stable than those interfaced with solutions with a greater difference in density. (e) Valve stability increases sharply with valve lengths smaller than 0.3 cm and remains consistent with longer valve lengths. (f) The volume of water flanking the valve has little effect on the stability of the valve. The symbol * indicates that the valve did not fail at maximum RCF tested. (n = 3, mean ± s.d.; if not visible, error bars are obscured by the symbols)

Figure 5

The effect of material properties on force required to pull beads through the solution/valve interface. (a) The force required decreases when using tubing of a larger diameter. (b) With the exception of Tygon tubing, the force required to pull beads across a surface tension valve increases with the surface energy of the tubing. (c) The force required to pull beads through mineral oil valves (solid circles) is significantly less than the force required to pull beads across air valves (open circles). Force required increases with interfacial tension with both types of valves. (d) The magnetic field gradient along the x axis that is required to pull the beads through the valve (squares) increases with the amount of beads, whereas the magnetic field required (circles) decreases. (e) The mass susceptibility of the bead used has little influence over the force required to pull beads across a surface tension valve. (f) Scanning electron microscopy images of the three commercially available silica-coated magnetic beads (scale bars = 5 μm). (n = 3, mean ± s.d.; if not visible, error bars are obscured by the symbols)

Figure 6

The effect of the material properties on the amount of solution carryover between solutions. (a) Illustration of magnetic beads under the influence of a magnetic field moving from a solution through a surface tension valve within small-diameter tubing. As beads traverse the solution/valve interface, a small amount of solution is retained amidst the beads and is carried across the valve and into the next solution. (b) The carryover volume increases linearly with an increased number of beads. (c) The surface energy of the tubing has little effect on the carryover volume. (d) In all solutions except 80% ethanol, there is more carryover when using mineral oil valves (solid bars) compared to air valves (open bars). (n = 3, mean ± s.d.; if not visible, error bars are obscured by the symbols)

Figure 7

Comparison of the valve fluid stability and penetrability for various material configurations. Solutions interfaced with mineral oil valves (solid squares) are more stable and easier to penetrate than solutions interfaced with air valves (open squares). There are tradeoffs between stability and penetrability with the range of tubing diameters tested (open circles). Tubing surface energy (solid circles) and bead mass (open triangles) influence valve penetrability but not valve stability. The minimum mass of beads that can be pulled through a valve (solid triangles) can be optimized for valve stability and penetrability. The relative size of the symbols corresponds to the relative material property values (e.g., large solid squares have higher interfacial energy than smaller solid squares). The positions of the zone boundaries indicated by the dotted lines are approximations.