Many motile microorganisms swim and navigate in chemically and mechanically complex environments. These organisms can be functionalized and directly used for applications (biohybrid approach), but also inspire designs for fully synthetic microbots. The most promising designs of biohybrids and bioinspired microswimmers include one or several magnetic components, which lead to sustainable propulsion mechanisms and external controllability. This Review addresses such magnetic microswimmers, which are often studied in view of certain applications, mostly in the biomedical area, but also in the environmental field. First, propulsion systems at the microscale are reviewed and the magnetism of microswimmers is introduced. The review of the magnetic biohybrids and bioinspired microswimmers is structured gradually from mostly biological systems toward purely synthetic approaches. Finally, currently less explored parts of this field ranging from in situ imaging to swarm control are discussed.
Synthetic microswimmers mimicking biological movements at the microscale have been developed in recent years. Actuating helical magnetic materials with a homogeneous rotating magnetic field is one of the most widespread techniques for propulsion at the microscale, partly because the actuation strategy revolves around a simple linear relationship between the actuating field frequency and the propeller velocity. However, the full control of the swimmers' motion has remained a challenge. Increasing the controllability of micropropellers is crucial to achieve complex actuation schemes that in turn are directly relevant for numerous applications. The simplicity of the linear relationship though limits the possibilities and flexibilities of swarm control. Using a pool of randomly-shaped magnetic microswimmers, we show that the complexity of shape can advantageously be translated into enhanced control. In particular, directional reversal of sorted micropropellers is controlled by the frequency of the actuating field.2 This directionality change is linked to the balance between magnetic and hydrodynamic forces.We further show an example how this behavior can experimentally lead to simple and effective sorting of individual swimmers from a group. The ability of these propellers to reverse swimming direction solely by frequency increases the control possibilities and is an example for propeller designs, where the complexity needed for many applications is embedded directly in the propeller geometry rather than external factors such as actuation sequences. I. IntroductionMicroswimmers are envisioned for a multitude of applications ranging from solving environmental problems to being used for micro surgery [1][2][3]. Precise, versatile and noninvasive controllability is necessary to cover this broad scope of applications. These requirements are mostly matched by magnetic microswimmers. The fuel-free actuation by weak and homogeneous magnetic fields indeed allows remote controlling in many environments, the synthesis via nanofabrication makes them accessible even on a sub-micrometer scale [4][5][6]. In addition, the ability to functionalize their surface and the limited toxicity of the mostly iron-based propellers makes them appealing for medical applications [2,7]. Many of the current magnetic microswimmers use a helical shape with a fixed magnetic moment to rotate in an externally applied magnetic field, which enables stable propulsion. In this case, a simple linear relationship between the frequency of the actuating magnetic field and the velocity of micropropellers is used to precisely control the propeller [5,[8][9][10]. This leaves the sign of the swimming direction of the propeller to be determined by the rotation direction of the applied magnetic field, which limits the versatility of their actuation capability: when controlling two or more geometrically identical propellers, it is not possible to let them swim in a common propulsion mode respectively in the same direction and, if needed, in opposite directions, simpl...
19Bacteria propel and change direction by rotating long, helical filaments, called flagella. The 20 number of flagella, their arrangement on the cell body and their sense of rotation 21 hypothetically determine the locomotion characteristics of a species. The movement of the 22 most rapid microorganisms has in particular remained unexplored because of additional 23 experimental limitations. We show that magnetotactic cocci with two flagella bundles on 24 one pole swim faster than 500 µm·s -1 along a double helical path, making them one of the 25 fastest natural microswimmers. We additionally reveal that the cells reorient in less than 5 26 ms, an order of magnitude faster than reported so far for any other bacteria. Using 27 hydrodynamic modeling, we demonstrate that a mode where a pushing and a pulling bundle 28 cooperate is the only possibility to enable both helical tracks and fast reorientations. The 29 advantage of sheathed flagella bundles is the high rigidity, making high swimming speeds 30 possible. 31 32 Introduction 33The understanding of microswimmer motility has implications ranging from the 34 comprehension of phytoplankton migration to the autonomously acting microbots in 35 medical scenarios [1, 2]. The most present microswimmers in our daily lives are bacteria, 36 most of which use flagella for locomotion. Well-studied examples of swimming 37 microorganisms include the peritrichous (several flagella all over the body surface) 38Escherichia coli with an occasionally distorted hydrodynamic flagella bundling [3] and the 39 monotrichous (one polar flagellum) Vibrio alginolyticus, which are pushed or pulled by a 40 flagellum and exploit a mechanical buckling instability to change direction [4, 5]. The 41 3 swimming speeds of so far studied cells are in the range of several 10 µm s -1 and their 42 reorientation events occur on the time scale of 50-100 ms [5, 6]. 43Magnetococcus marinus (MC-1) is a magnetotactic, spherical bacterium that is capable of 44 swimming extremely fast [7][8][9][10]. MC-1 as well as the closely related strain are 45 equipped with two bundles of flagella on one hemisphere (bilophotrichous cells). The 46 bacterium also features a magnetosome chain, which imparts the cell with a magnetic 47 moment ('magnetotactic' cell). They are assumed to swim with the cell body in front of both 48 flagella, which synchronously push the cell forward [7]. This assumption leads to helical 49 motion in the presence of a strong magnetic field, which exerts a torque on the cell's 50 magnetic moment, as seen in hydrodynamic simulations [12]. In the absence of a magnetic 51 field, this model predicts rather straight trajectories. 52Our observations disagree with the above-mentioned model, indicating that an 53 understanding of the physics of their swimming is still missing, even though proof of concept 54 biomedical applications of these bacteria have already emerged [2]. Here we show that MC-55 1 not only reach speeds of over 400 µm s -1 but that this speed is recorded along an 56 unexplored double he...
Bacteria propel and change direction by rotating long, helical filaments, called flagella. The number of flagella, their arrangement on the cell body and their sense of rotation hypothetically determine the locomotion characteristics of a species. The movement of the most rapid microorganisms has in particular remained unexplored because of additional experimental limitations. We show that magnetotactic cocci with two flagella bundles on one pole swim faster than 500 µm·s−1 along a double helical path, making them one of the fastest natural microswimmers. We additionally reveal that the cells reorient in less than 5 ms, an order of magnitude faster than reported so far for any other bacteria. Using hydrodynamic modeling, we demonstrate that a mode where a pushing and a pulling bundle cooperate is the only possibility to enable both helical tracks and fast reorientations. The advantage of sheathed flagella bundles is the high rigidity, making high swimming speeds possible.
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