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Guiding microswimmers by magnetic fields

​As part of an international collaboration, BIAM researchers have successfully produced 3D-printed microswimmers that can be guided by magnetic fields. Ultimately, these devices could be used within the human body for therapeutic purposes.

Published on 5 May 2021

The BIAM's Molecular and Environmental Microbiology team is currently working on microswimmers. These micron-sized devices, able to move within a fluid, are being considered for a multitude of biomedical applications.

Most of these microswimmers have helical shapes, since they are inspired both by the rotating flagella of bacteria and by man-made helices. Their movement can be controlled at the microscopic scale by a rotating magnetic field that acts on the propeller, which itself has a magnetic moment perpendicular to its axis. However, in these conditions, the propellers cannot be guided in every direction. This is why the group of BIAM researchers has focused on microswimmers with different shapes.

In their previous work on chemically synthesized microswimmers, the researchers observed that the movement of some of them, whose shape differed from that of conventional propellers, varied according to the rotation frequency of the magnetic field. At 20 rotations per second, they moved in one direction at a speed of about 2 µm/second, whereas 70 rotations per second propelled them in the opposite direction at a speed of about 3 µm/second.

In their latest work, the researchers thus selected these microswimmers, whose direction of movement changed according to the frequency. They then digitally reconstructed their shapes and reproduced them by 3D microprinting. This targeted selection revealed a variety of behaviors and speed-frequency dependencies indicating different magnetic properties for identical shapes.

This research resolves several technical difficulties including the selection and design of the most suitable shapes. At the same time, it uncovers other difficulties related to the precision of 3D printing, or the nickel layer coating of microswimmers. In particular, while this layer allows microswimmers to react to magnetic fields, it also modifies their shape and weight, and can also oxidize.

The applications of microswimmers will get multiplied in the near future. It will accordingly not only be necessary to produce complex shapes other than classical helices but also to understand the interaction between these shapes and their magnetic and hydrodynamic characteristics. Further developments should make it possible to control the magnetic properties of 3D-printed structures, for example by incorporating magnetic nanoparticles or even magnetic multilayer systems.

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