Magnetic Beads inside Droplets for Agitation and Splitting Manipulation by Utilizing a Magnetically Actuated Platform

Abstract

We successfully developed a platform for the magnetic manipulation of droplets containing magnetic beads and examined the washing behaviors of the droplets, including droplet transportation, magnetic bead agitation inside droplets, and separation from parent droplets. Magnetic field gradients were produced with two layers of 6 × 1 planar coils fabricated by using printed circuit board technology. We performed theoretical analyses to understand the characteristics of the coils and successfully predicted the magnetic field and thermal temperature of a single coil. We then investigated experimentally the agitation and splitting kinetics of the magnetic beads inside droplets and experimentally observed the washing performance in different neck-shaped gaps. The performance of the washing process was evaluated by measuring both the particle loss ratio and the optical density. The findings of this work will be used to design a magnetic-actuated droplet platform, which will separate magnetic beads from their parent droplets and enhance washing performance. We hope that this study will provide digital microfluidics for application in point-of-care testing. The developed microchip will be of great benefit for genetic analysis and infectious disease detection in the future.

Keywords: agitation; digital microfluidics; magnetic manipulation; splitting; washing.

Figure 1

(a) Cartridge layout with seven compartments on a planar coil array and (b) composition of the liquid in each compartment along with the droplet movement sequence.

Figure 2

(a) Coil array chip and layout of droplet cartridge integrated with two-layer coils. (b) Top-side view and (c) lateral side view of the cartridge.

Figure 3

Experimental setup of the magnetically actuated platform. a: digital camera incorporating a CCD; b: Helmholtz coil; c: magnetically actuated chip; d: cooling system.

Figure 4

Experimental magnetic field (Bz,_Max) versus different applied currents at different locations. Here, z_A = 300 μm at location A and z_B = 400 μm at location B.

Figure 5

Experimental maximum temperature (T_Max) versus different applied currents. The surrounding temperature (T_∞) is 25 °C.

Figure 6

Illustrations depicting the (a) release, (b) attraction, and (c) repulsion of magnetic beads within a droplet. The red scalar represents 1.0 mm.

Figure 6

Illustrations depicting the (a) release, (b) attraction, and (c) repulsion of magnetic beads within a droplet. The red scalar represents 1.0 mm.

Figure 7

(a) Top-side and (b) lateral side views of photographic sequences showing mixing performance under agitation in the absence of magnetic beads at the different times of (i) t = 0 s; (ii) t = 25 s, and (iii) t = 50 s. In this case, the 2.88-μm magnetic beads have a mass of 100.0 μg, and no DC current is applied to the single coil. The red scalar represents 1.0 mm.

Figure 8

(a) Top-side view and (b) lateral sides views of photographic sequences showing mixing performance under magnetic bead agitation at the different times of (i) t = 0 s; (ii) t = 2.0 s, and (iii) t = 4.0 s. The conditions for this operation are as follows: a DC current of 1.5 A, a 2.88-μm magnetic beads’ mass of 100.0 μg, and an alternate frequency of 1.0 Hz. The red scalar represents 1.0 mm.

Figure 9

Photographic sequences showing droplet splitting assisted by a sieve structure. (a) Magnetic beads congregated at the droplet’s rim; (b) Aggregated magnetic beads traversing through the topological shape; (c) Clustered magnetic beads dispersing to create a satellite droplet. The established operational parameters are as follows: a DC current setting of 1.5 A, beads with a size of 2.8 μm and a collective mass of 100.0 μg within the parent droplet, and an alternating frequency of 1.0 Hz for the nearby coil. The red scalar represents 1.0 mm.

Figure 10

Satellite droplets continuously split from a parent droplet through a microfluidic orifice. (a) Splitting of a single satellite droplet; (b) Splitting of two satellite droplets; (c) Transportation and collection of two satellite droplets. The established operational parameters are as follows: a DC current setting of 1.0 A, beads with a size of 1.0 μm and a collective mass of 1000.0 μg within the parent droplet, and an alternating frequency of 1.0 Hz for the nearby coil. The red scalar represents 1.0 mm.

Figure 11

(a) Dye intensity in each chamber during the separation of magnetic beads from droplets. (b) Concentration of droplets normalized in accordance with different chambers. The operational conditions are set as follows: a DC current of 1.5 A, a beads’ mass of 100.0 μg in the droplet, and a frequency of 1.0 Hz alternation for the adjacent coil. The red scalar represents 1.0 mm.

Figure 12

Normalized concentration of blue dye after the fourth washing process in different neck-shaped channel gaps of (a) 500, (b) 750, and (c) 1000 μm. (d) Concentration of droplet normalized in accordance with different neck-shaped gaps. The operational conditions are set as follows: a DC current of 1.5 A, a bead mass of 100.0 μg in the droplet, and a frequency of 1.0 Hz alternation for the adjacent coil. The red scalar represents 1.0 mm.

Figure 13

The relationship of (a) particle loss ratio and (b) optical density (OD) with respect to different gap sizes of 500, 750, and 1000 μm. The operational conditions are set as follows: a DC current of 1.5 A, a beads’ mass of 100.0 μg in the droplet, and a frequency of 1.0 Hz alternation for the adjacent coil.