Microswimmers have the potential for wide-ranging medical applications, such as drug delivery. As a step toward the creation of fast, reliable and versatile robotic microswimmers, Rogowski et al. attached bacterial flagella to the surface of magnetic microparticles and monitored their propulsive behavior and ability to follow desired trajectories.

“A multitude of bacterial organisms propel by rotating and bundling their individual flagellar filaments together to produce propulsive behavior,” said author Min Jun Kim. “However, while bacteria have flagella that are each controlled with their own independent biomolecular motors, the microswimmers we developed have flagella that all fall fixed along the microparticles’ surface and rotate in tandem with the rotation of the microparticle.”

Due to the natural left-handedness of the flagella, the microswimmers – which typically have between 12 and 24 flagella about 8 micrometers in length – were found to exhibit different behaviors depending on the direction of their rotation under an oscillating magnetic field. They propelled linearly with frequency while rotating counterclockwise, but exhibited a surprising reversal in their swimming direction beyond a critical frequency. Under clockwise rotation, the behavior of the microswimmers was less uniform.

By applying these findings, the authors were able to guide the microswimmers along a desired navigation trajectory, reaching within 5% of their intended position more than half of the time.

“While the microswimmers were able to perform the trajectories we provided easily using the simple controller we utilized, we plan on developing nonlinear controllers to better optimize their trajectory planning and accomplish more complex navigation tasks in the future,” Kim said. Such an improvement can open the door to potential targeted drug delivery and fluid sensing applications.

Source: “Heterogeneously flagellated microswimmer behavior in viscous fluids,” by Louis William Rogowski, Micah Oxner, Jiannan Tang, and Min Jun Kim, Biomicrofluidics (2020). The article can be accessed at https://doi.org/10.1063/1.5137743.