Integrated Microfluidic-Electromagnetic System to Probe Single-Cell Magnetotaxis in Microconfinement

Abstract

Magnetotactic bacteria have great potential for use in biomedical and environmental applications due to the ability to direct their navigation with a magnetic field. Applying and accurately controlling a magnetic field within a microscopic region during bacterial magnetotaxis studies at the single-cell level is challenging due to bulky microscope components and the inherent curvilinear field lines produced by commonly used bar magnets. In this paper, a system that integrates microfluidics and electromagnetic coils is presented for generating a linear magnetic field within a microenvironment compatible with microfluidics, enabling magnetotaxis analysis of groups or single microorganisms on-chip. The platform, designed and optimised via finite element analysis, is integrated into an inverted fluorescent microscope, enabling visualisation of bacteria at the single-cell level in microfluidic devices. The electromagnetic coils produce a linear magnetic field throughout a central volume where the microfluidic device containing the magnetotactic bacteria is located. The magnetic field, at this central position, can be accurately controlled from 1 to 10 mT, which is suitable for directing the navigation of magnetotactic bacteria. Potential heating of the microfluidic device from the operating coils was evaluated up to 2.5 A, corresponding to a magnetic field of 7.8 mT, for 10 min. The maximum measured heating was 8.4 °C, which enables analysis without altering the magnetotaxis behaviour or the average swimming speed of the bacteria. Altogether, this work provides a design, characterisation and experimental test of an integrated platform that enables the study of individual bacteria confined in microfluidics, under linear and predictable magnetic fields that can be easily and accurately applied and controlled.

Keywords: bacterial taxis; magnetotactic bacteria; magnetotaxis; microfluidics; microswimmer; single-cell analysis.

Figure 1

(a) A schematic diagram of the microfluidic–electromagnetic coils platform, and (b) an actual picture of the device. A microfluidic device can be placed between the two coils in the 3D-printed slide holder, and MTB can be visualised in the microscope during experiments. The platform enables the integration of macroscopic scale electromagnetic coils and holders, a millimeter-scale microfluidic device with microscopic features and the visualisation of micrometer-scale MTB. The two wooden blocks at the ends can be screwed into the microscope stage to ensure stability of the setup.

Figure 2

Simulated magnetic flux density of (a) a pair of electromagnetic coils with a current of 1.3 A and (b) a neodymium bar magnet. White arrows represent the magnitude and direction of the magnetic flux.

Figure 3

(a) Simulated magnetic flux density along the centre axis of the electromagnetic coils for a coil separation from 2R to 5R with a current of 1.3 A. As the separation increases, the intensity of the magnetic flux density decreases at the centre between the coils. Inset: A schematic representation of the microfluidic–electromagnetic coils platform demonstrating the coil separations increasing, represented by the red arrows. (b) The simulated magnetic flux densities for coil separations from 2R to 5R on an x–y-plane at the z = 0 position, which intersects the coils symmetrically.

Figure 4

(a) Simulated magnetic flux density along the center axis of the electromagnetic coils at different currents with the fabricated coil separation. (b) The magnetic flux density at a scale relevant to MTB experiments for 1.2 A and 1.4 A. (c) Simulated and experimental magnetic flux density at the centre of the coils for a varied current.

Figure 5

Temperature increase on the surface of a microfluidic device placed between the coils at different currents applied for a period of 10 min. The plot has been normalised to account for a difference in room temperature. For 1.5 A, 2.0 A and 2.5 A, the temperature was extrapolated by fitting the data to an exponential model. Error bars indicate standard deviation (n = 3).

Figure 6

(a) A frame-by-frame sequence of an MTB cell turning to align with a 4.0 mT field. (b) Tracking overlay of 20 MTB swimming in a 4.0 mT horizontal magnetic field. Scale bars = 20 μm.

Figure 7

MTB velocities under the influence of applied electromagnetic fields at 0.5 and 2.5 A. (a) Box plot of the average velocity of the bacteria at 0, 5 and 10 min for the two different currents, showcasing the median and limits based on the quartiles (n = 10). The diamond represents a single data point for average speed at 2.5 A. (b) Radial plots of the average angles of the MTB swimming trajectories (initial and final points).