Imposing Local Magnetic Fields to Control Magnetotactic Bacteria Through Combining Microfabrication and Magnetism

The ability to impose localized magnetic fields on magnetic bacteria has a variety of applications including in biophysics, nature inspired materials, and non-destructive sensors. Herein we demonstrate our ability to locally direct swimming magnetotactic bacteria, Magnetospirillum magneticum strain AMB-1, through microfabricated magnetic field concentrators. To accomplish this, a pair of Helmholtz coils was built to magnetize microfabricated permalloy structures of shapes including circles, squares, and triangles, which may affect the local magnetic fields near the structures. The permalloy structures were fabricated through photolithography and depositing a thin film of NiFe on a master mold. The substrate was a #1 glass coverslip that permitted us to observe the interactions of the magnetotactic bacteria with the permalloy structures. These microfabricated structures were positioned at the center and along the axes of the Helmholtz coils. The response of the bacteria around the structure was quantified through analyzing the images of the bacteria in terms of orientation and velocity. The results indicate that the AMB-1 appeared to cross or stop at the regions on the structures possessing higher magnetic field intensity relative to the surroundings. Furthermore, the bacteria appeared to alter their orientation as they approached sharp corners of the structures. We also developed a model using Finite Element Method Magnetics (FEMM) to investigate the regions of local magnetic field gradients, which indicated higher magnetic fields at the protrusions and the sharp points of the magnetic structures. This work supports the idea of using these magnetotactic bacteria as an aqueous non-destructive sensor and also has implications in fields including physics, biology, material science, and engineering.