Swimming with magnets: From biological organisms to synthetic devices
Résumé
The movement of microorganisms is not only an essential aspect of life, it also serves as a fruitful model for the development of synthetic microswimmers or microrobots with potential applications in environmental and medical fields. Here, we review the field of the magnetic microswimmers, which as indicated by the adjective, represents a dedicated branch of the general microswimmers where magnetism plays a role either for the orientation or for the locomotion of the swimmers. These swimmers, on the one hand, provide model system for studying the interplay of external physical forces with biological processes and are, on the other hand, attractive for various applications due to the possibility of remote steering by magnetic fields.
The review is structured in such a way that we first present the physical background associated with the microswimmers in general, with their locomotion at low Reynolds numbers and their orientation by magnetic fields and by other mechanisms. We then review illustrative examples of magnetic swimmers, starting with the biological ones, the magnetotactic bacteria, which are bacteria forming intracellular magnetic nanoparticles to orient in magnetic field. We continue with the hybrid and synthetic microswimmers, where magnetism again plays a role for the orientation / directionality, but can also provide propulsion mechanisms. We finally emphasize the control of multiple swimmers/devices along independent trajectories as well as their interactions and collective behavior. We will look forward to the continuous development of the field and hope that this review will stimulate further research in this vibrant research area.