Controllable propulsion of microscale and nanoscale devices enhanced with additional functionality would enable the realization of miniaturized robotic swimmers applicable to transport and assembly, actuators, and drug delivery systems. Following biological examples, existing magnetically actuated microswimmers have been designed to use flexibility or chirality, presenting fabrication challenges. Here we show that, contrary to biomimetic expectations, magnetically actuated geometries with neither flexibility nor chirality can produce propulsion, through both experimental demonstration and a theoretical analysis, which elucidates the fundamental constraints on micropropulsion via magnetetic rotation. Our results advance existing paradigms of low-Reynolds-number propulsion, possibly enabling simpler fabrication and design of microswimmers and nanoswimmers.
Untethered robotic microswimmers are very promising to significantly improve various types of minimally invasive surgeries by offering high accuracy at extremely small scales. A prime example is drug delivery, for which a large number of microswimmers is required to deliver sufficient dosages to target sites. For this reason, the controllability of groups of microswimmers is essential. In this paper, we demonstrate simultaneous control of multiple geometrically similar but magnetically different microswimmers using a single global rotating magnetic field. By exploiting the differences in their magnetic properties, we triggered different swimming behaviors from the microswimmers by controlling the frequency and the strength of the global field, for example, one swim and the other does not while exposed to the same control input. Our results show that the balance between the applied magnetic torque and the hydrodynamic torque can be exploited for simultaneous control of two microswimmers to swim in opposite directions, with different velocities, and with similar velocities. This work will serve to establish important concepts for future developments of control systems to manipulate multiple magnetically actuated microswimmers and a step towards using swarms of microswimmers as viable workforces for complex operations.
We induce artificial magnetotaxis in Tetrahymena pyriformis, a eukaryotic ciliate, using ferro-magnetic nanoparticles and an external time-varying magnetic field. Magnetizing internalized iron oxide particles (magnetite), allows control of the swimming direction of an individual cell using two sets of electromagnets. Real-time feedback control was performed with a vision tracking system, which demonstrated controllability of a single cell. Since the endogenous motility of the cell is combined in one system with artificial magnetotaxis, the motion of artificially magnetotactic T. pyriformis is finely controllable. Thus, artificially magnetotactic T. pyriformis is a promising candidate microrobot for microassembly and transport in microfluidic environments.
Wirelessly controlled nanoscale robots have the potential to be used for both in vitro and in vivo biomedical applications. So far, the vast majority of reported micro- and nanoscale swimmers have taken the approach of mimicking the rotary motion of helical bacterial flagella for propulsion, and are often composed of monolithic inorganic materials or photoactive polymers. However, currently no man-made soft nanohelix has the ability to rapidly reconfigure its geometry in response to multiple forms of environmental stimuli, which has the potential to enhance motility in tortuous heterogeneous biological environments. Here, we report magnetic actuation of self-assembled bacterial flagellar nanorobotic swimmers. Bacterial flagella change their helical form in response to environmental stimuli, leading to a difference in propulsion before and after the change in flagellar form. We experimentally and numerically characterize this response by studying the swimming of three flagellar forms. Also, we demonstrate the ability to steer these devices and induce flagellar bundling in multi-flagellated nanoswimmers.
A biomimetic, microscale system using the mechanics of swimming bacteria has been fabricated and controlled in a low Reynolds number fluidic environment. The microswimmer consists of a polystyrene microbead conjugated to a magnetic nanoparticle via a flagellar filament using avidin-biotin linkages. The flagellar filaments were isolated from the bacterium, Salmonella typhimurium. Propulsion energy was supplied by an external rotating magnetic field designed in an approximate Helmholtz configuration. Further, the finite element analysis software, COMSOL MULTIPHYSICS, was used to develop a simulation of the robotic devices within the magnetic controller. The robotic microswimmers exhibited flagellar propulsion in two-dimensional magnetic fields, which demonstrate controllability of the biomimetically designed devices for future biomedical applications.
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