Helical Swimming Research Articles

Tail length influences swimming speed of helical swimmers in granular media

Tail length influences swimming speed of helical swimmers in granular media

Concentration- and time-dependent behavioural effects of ethanol on Lumbriculus variegatus.

Ethanol is one of the most widely used drugs in the world. Ethanol induces profound physiological and behavioural responses in invertebrate model organisms, such as Caenorhabditis elegans and Drosophila melanogaster. Lumbriculus variegatus (Annelida, Oligochaete) is an aquatic worm which shows behavioural responses to common drugs and thus is potentially useful in pharmacological research. The effects of ethanol are unknown in this organism. In this study, we examine the effects of acute exposure to ethanol (0-500 mM) on the stereotypical movements and locomotor activity of L. variegatus and examine the concentration- (0-500 mM) and time-dependent (0-210 min) effects of ethanol in L. variegatus. We show that ≥250 mM ethanol reversibly reduced the ability of tactile stimulation to elicit stereotypical movements, namely body reversal and helical swimming and locomotor activity (p < 0.05, N = 8). We also found that 2 min of exposure to ≥250 mM ethanol rapidly induces steady-state hypokinesis (p < 0.05, N = 11) and confirm ethanol absorption into L. variegatus tissues. Additionally, we also observed acute ethanol tolerance after 150 min of exposure to 500 mM ethanol (p < 0.05, N = 24). This study is the first to report the behavioural effects of ethanol in L. variegatus. Our results show that this is a model organism for use in ethanol studies, providing further evidence for its utility in pharmacological research.

Leeches Predate on Fast-Escaping and Entangling Blackworms by Spiral Entombment.

We investigate how the Helobdella sp. freshwater leeches capture and consume Lumbriculus variegatus blackworms despite the blackworm's ultrafast helical swimming escape reflex and ability to form large tangled "blobs". We describe a spiral "entombment" predation strategy, where Helobdellid leeches latch onto blackworms with their anterior sucker and envelop them in a spiral cocoon. Quantitative analysis shows that larger leeches succeed more often in entombing prey, while longer worms tend to escape. The rate of spiral contraction correlates with entombment outcomes, with slower rates associated with success. These insights highlight the complex interactions between predator and prey in freshwater ecosystems, providing new perspectives on ecological adaptability and predator-prey dynamics.

Propulsion of a three-sphere microrobot in a porous medium.

Microorganisms and synthetic microswimmers often encounter complex environments consisting of networks of obstacles embedded into viscous fluids. Such settings include biological media, such as mucus with filamentous networks, as well as environmental scenarios, including wet soil and aquifers. A fundamental question in studying their locomotion is how the impermeability of these porous media impacts their propulsion performance compared with the case of that in a purely viscous fluid. Previous studies showed that the additional resistance due to the embedded obstacles leads to an enhanced propulsion of different types of swimmers, including undulatory swimmers, helical swimmers, and squirmers. In this paper, we employ a canonical three-sphere swimmer model to probe the impact of propulsion in porous media. The Brinkman equationis utilized to model a sparse network of stationary obstacles embedded into an incompressible Newtonian liquid. We present both a far-field theory and numerical simulations to characterize the propulsion performance of the swimmer in such porous media. In contrast to enhanced propulsion observed in other swimmer models, our results reveal that both the propulsion speed and efficiency of the three-sphere swimmer are largely reduced by the impermeability of the porous medium. We attribute the substantial reduction in propulsion performance to the screened hydrodynamic interactions among the spheres due to the more rapid spatial decays of flows in Brinkman media. These results highlight how enhanced or hindered propulsion in porous media is largely dependent on individual propulsion mechanisms. The specific example and physical insights provided here may guide the design of synthetic microswimmers for effective locomotion in porous media in their potential biological and environmental applications.

Spinning and corkscrewing of oceanic macroplankton revealed through in situ imaging.

Helical motion is prevalent in nature and has been shown to confer stability and efficiency in microorganisms. However, the mechanics of helical locomotion in larger organisms (>1 centimeter) remain unknown. In the open ocean, we observed the chain forming salp, Iasis cylindrica, swimming in helices. Three-dimensional imaging showed that helicity derives from torque production by zooids oriented at an oblique orientation relative to the chain axis. Colonies can spin both clockwise and counterclockwise and longer chains (>10 zooids) transition from spinning around a linear axis to a helical swimming path. Propulsive jets are non-interacting and directed at a small angle relative to the axis of motion, thus maximizing thrust while minimizing destructive interactions. Our integrated approach reveals the biomechanical advantages of distributed propulsion and macroscale helical movement.

Nature-inspired miniaturized magnetic soft robotic swimmers

State-of-the-art biomedical applications such as targeted drug delivery and laparoscopic surgery are extremely challenging because of the small length scales, the requirements of wireless manipulation, operational accuracy, and precise localization. In this regard, miniaturized magnetic soft robotic swimmers (MSRS) are attractive candidates since they offer a contactless mode of operation for precise path maneuvering. Inspired by nature, researchers have designed these small-scale intelligent machines to demonstrate enhanced swimming performance through viscous fluidic media using different modes of propulsion. In this review paper, we identify and classify nature-inspired basic swimming modes that have been optimized over large evolutionary timescales. For example, ciliary swimmers like Paramecium and Coleps are covered with tiny hairlike filaments (cilia) that beat rhythmically using coordinated wave movements for propulsion and to gather food. Undulatory swimmers such as spermatozoa and midge larvae use traveling body waves to push the surrounding fluid for effective propulsion through highly viscous environments. Helical swimmers like bacteria rotate their slender whiskers (flagella) for locomotion through stagnant viscid fluids. Essentially, all the three modes of swimming employ nonreciprocal motion to achieve spatial asymmetry. We provide a mechanistic understanding of magnetic-field-induced spatiotemporal symmetry-breaking principles adopted by MSRS for the effective propulsion at such small length scales. Furthermore, theoretical and computational tools that can precisely predict the magnetically driven large deformation fluid–structure interaction of these MSRS are discussed. Here, we present a holistic descriptive review of the recent developments in these smart material systems covering the wide spectrum of their fabrication techniques, nature-inspired design, biomedical applications, swimming strategies, magnetic actuation, and modeling approaches. Finally, we present the future prospects of these promising material systems. Specifically, synchronous tracking and noninvasive imaging of these external agents during in vivo clinical applications still remains a daunting task. Furthermore, their experimental demonstrations have mostly been limited to in vitro and ex vivo phantom models where the dynamics of the testing conditions are quite different compared the in vivo conditions. Additionally, multi-shape morphing and multi-stimuli-responsive modalities of these active structures demand further advancements in 4D printing avenues. Their multi-state configuration as an active solid-fluid continuum would require the development of multi-scale models. Eventually, adding multiple levels of intelligence would enhance their adaptivity, functionalities, and reliability during critical biomedical applications.

Model Predictive Control of Magnetic Helical Swimmers in Two-Dimensional Plane

Magnetic micro/mini-swimmers have a great potential application in biomedical research and have gained broad attention. Recent research studies focus on the automatic control methods of magnetic micro/mini-swimmers, such as artificial intelligence methods. However, their autonomous manipulation remains a challenge since they are subject to various disturbances from the external environment and model uncertainties. The current methods employ a status observer to estimate these disturbances and uncertainties. In this paper, we apply a data-driven technique that utilizes the nonlinear approximation ability of neural networks (NNs). To be specific, a flexible structure of NNs, Broad Learning System (BLS), is employed to model the input-output mapping relationship between the direction of the rotating magnetic field and the swimming direction of the helical miniature swimmer. Then, according to the dual-variable decoupling control, a levitation controller is formulated and a path following controller is proposed based on a planar three-degree-of-freedom model of magnetic helical swimmers, which is inspired by the model of wheeled mobile robots. Simulations and experiments are conducted to quantitatively validate the proposed control method using different planar paths. The experiment results show that the mean absolute error of the path following control is about 3 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\%$</tex-math> </inline-formula> of the body length which is less than 0.5 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$mm$</tex-math> </inline-formula> . Our proposed control method provides a preliminary study to alleviate the impact of disturbances and uncertainties on the control performance of magnetic micro/mini-swimmers. <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Note to Practitioners</i> —This paper is mainly motivated by the potential application of artificial intelligence methods in magnetic micro/mini-robot community, especially for the applications that reject disturbances and uncertainties. In practice, a path planner is employed to compute a pre-defined reference path that connects the start and the targeted locations. Then our control method is applied to the magnetic helical swimmer and it is guided to follow the reference path. Tackling external disturbances and model uncertainties is still challenging during the control progress. Neural network-based technique is an intuitive approach since their nonlinear modeling ability and generalization are suitable for estimating these disturbances and uncertainties. As for training the neural-network model, the training data about the angle parameters should be recorded by manual control of the helical swimmer. These angle parameters define the rotating axis of the uniform rotating magnetic field and the self-rotating axis of the magnetic helical swimmer. According to the planar motion model, the optimal controller is formulated using the feedback distance error and angle error, and the sum of the control signal and the prediction of the compensating model is used as the final control input. Our electromagnetic coil system features easy operation and configuration of cameras or other sensors. Simulations and experiments validate the performance of the neural network-based compensating method and the proposed optimal control method using magnetic micro/mini-swimmers.

Design and analysis of a novel magnetic helical swimmer

Magnetic helical swimmers are one type of robots that swim at low Reynolds number environments by rotating around the helix axis. Considering the importance and dramatic increase in the use of robots and microrobots in the near future, optimizing and increasing their efficiency is very important and noteworthy. Propulsion force and translational velocity are among the most important features of the magnetic helical swimmer, which improves the function of the swimmer as each of them increases. In this paper, a new design has been proposed for the magnetic helical swimmer by changing the geometry of the tail region, which has increased the propulsion force and improved its translational velocity. A suitable experimental setup has been designed and built in accordance with the required experiments to evaluate the translational velocity of the proposed swimmer. Using the experimental results, two models have been presented to express the translational velocity and propulsion force of the swimmer in terms of its angular velocity. The results of the experiments show that the propulsion force of the built swimmer is 698.89 % higher than that of the common magnetic helical swimmer with similar dimensions and the same environmental conditions in Newtonian fluid. At the end of the experiments, the motion of the proposed swimmer is simulated in the COMSOL software, and the results of the experiments are used to validate the simulation results. Finally, the effect of parameters such as the helix pitch and the number of turns of the helix on the translational velocity of the swimmer is investigated using the computer simulations.

Selective-collective scalable locomotion control of helical magnetic swimmers within confined channels using global magnetic fields

Achieving selective-individual and collective control over multiple magnetic microrobots remains a formidable challenge. Existing techniques for controlling identical magnetic swimmers are often limited to two-dimensional, small working spaces, while heterogeneous swimmers can only be controlled in small numbers without the option for collective control. In this study, we present a purely magnetic control methodology for the selective-scalable locomotion of helical magnetic swimmers within confined channels. By employing a unique gradient field distribution, referred to as the "selection field," along with space-uniform rotating fields, we establish a "selection volume" that allows swimmers within it to fully rotate, while those outside remain stationary. We explore various conditions of the selection volume to analyze the locomotion dynamics of the swimmers in relation to the position of the selection volume. Experimental results demonstrate the selective-scalable locomotion of identical robots, ranging from one to an arbitrary number (N) of robots, as well as their selective-individual manipulation for multitasking applications. Furthermore, compatibility of the proposed method with different magnetic torque-driven robots is exemplified through the selective control of rolling robots. Moreover, three-dimensional selective-scalable control is also demonstrated for four swimmers in a quasi-three-dimensional channel, as well as for two swimmers within a true three-dimensional channel.

Helical swimming motion driven by coordinated rotation of flagellar apparatus in marine bacterial cells

Helical swimming motion driven by coordinated rotation of flagellar apparatus in marine bacterial cells

Reversible Microwheel Translation Induced by Polymer Depletion.

For in vivo applications, microbots (μbots) must move, which is a need that has led to designs, such as helical swimmers, that translate through the bulk fluid. We have previously demonstrated that, upon application of a rotating magnetic field, colloidal particles in aqueous systems can be reversibly assembled from superparamagnetic particles into μbots that translate along surfaces using wet friction. Here, we show that high-molecular-weight polymers of a size that approaches the length scale of the gap between the μbot and surface can be excluded, impacting μbot transport. Using xanthan gum as a convenient high-molecular-weight model, we determine that polymer depletion imparts only a weak effect on colloid-surface interactions but has a significant influence on local viscosity, which is an effect great enough to induce a reversal in the μbot translation direction.

Robust acoustic trapping and perturbation of single-cell microswimmers illuminate three-dimensional swimming and ciliary coordination

We report a label-free acoustic microfluidic method to confine single, cilia-driven swimming cells in space without limiting their rotational degrees of freedom. Our platform integrates a surface acoustic wave (SAW) actuator and bulk acoustic wave (BAW) trapping array to enable multiplexed analysis with high spatial resolution and trapping forces that are strong enough to hold individual microswimmers. The hybrid BAW/SAW acoustic tweezers employ high-efficiency mode conversion to achieve submicron image resolution while compensating for parasitic system losses to immersion oil in contact with the microfluidic chip. We use the platform to quantify cilia and cell body motion for wildtype biciliate cells, investigating effects of environmental variables like temperature and viscosity on ciliary beating, synchronization, and three-dimensional helical swimming. We confirm and expand upon the existing understanding of these phenomena, for example determining that increasing viscosity promotes asynchronous beating. Motile cilia are subcellular organelles that propel microorganisms or direct fluid and particulate flow. Thus, cilia are critical to cell survival and human health. The unicellular alga Chlamydomonas reinhardtii is widely used to investigate the mechanisms underlying ciliary beating and coordination. However, freely swimming cells are difficult to image with sufficient resolution to capture cilia motion, necessitating that the cell body be held during experiments. Acoustic confinement is a compelling alternative to use of a micropipette, or to magnetic, electrical, and optical trapping that may modify the cells and affect their behavior. Beyond establishing our approach to studying microswimmers, we demonstrate a unique ability to mechanically perturb cells via rapid acoustic positioning.

Helical Locomotion in Yield Stress Fluids.

We report three stages for locomotion of a helical swimmer in yield stress fluids. In the first stage, the swimmer must overcome the material's yield strain to generate rotational motion. However, exceeding the first threshold is not sufficient for locomotion. Only when the viscous forces are sufficiently strong to plastically deform the material to a finite distance away from the swimmer will net locomotion occur. Once locomotion is underway in the third stage, the yield stress retards swimming at small pitch angles. Conversely, at large pitch angles, yield stress dominates the flow by enhancing swimming speed. Flow visualizations reveal a highly localized flow near the swimmer in yield stress fluids.

Magnetically Actuated Continuum Medical Robots: A Review

The magnetic field has unique advantages in manipulating miniature robots working inside the human body, such as high transparency to biological tissue and good controllability for field generation. Generally, the actuated magnetic robot can be classified into two categories: tethered devices like intravascular microcatheters and untethered devices like helical swimmers. Among these, the tethered devices have a long history and good clinical application prospects, considering their high‐dose delivery and easy removal after the procedure. As an evolution of traditional continuum medical devices, the integration with magnetic actuation provides them with better scalability and improved dexterity. Although rapidly developed in the last two decades, the field of tethered magnetic robots requires further advancements in terms of design, fabrication, modeling, and control, especially for clinical applications. Herein, the recent progress of magnetically actuated continuum medical robots is focused on, intending to offer readers a comprehensive survey of the state‐of‐the‐art technologies and an information collection for future system design.

Hydrodynamic Synchronization of Chiral Microswimmers.

We study synchronization in bulk suspensions of spherical microswimmers with chiral trajectories using large scale numerics. The model is generic. It corresponds to the lowest order solution of a general model for self-propulsion at low Reynolds numbers, consisting of a nonaxisymmetric rotating source dipole. We show that both purely circular and helical swimmers can spontaneously synchronize their rotation. The synchronized state corresponds to velocity alignment with high orientational order in both the polar and azimuthal directions. Finally, we consider a racemic mixture of helical swimmers where intraspecies synchronization is observed while the system remains as a spatially uniform fluid. Our results demonstrate hydrodynamic synchronization as a natural collective phenomenon for microswimmers with chiral trajectories.

Analysing the motion of scallop-like swimmers in a noisy environment.

A scallop-like swimmer going back-and-forth (reciprocal motion) does not produce any net motility. We discuss a similar artificial microswimmer that is powered by magnetic fields. In the presence of thermal noise, the helical swimmer exhibits enhanced diffusivity during reciprocal actuation. The external magnetic drive can be further modified to break the reciprocity. Equipped with only the information on swimmer trajectories and orientations, we discuss quantitative methods to estimate the degree of reciprocity and non-reciprocity in such scenarios. The paper proposes a quantitative measure and validates the same with numerical simulations, further supported by experiments.

Reconstitution of a minimal motility system based on Spiroplasma swimming by two bacterial actins in a synthetic minimal bacterium.

Motility is one of the most important features of life, but its evolutionary origin remains unknown. In this study, we focused on Spiroplasma, commensal, or parasitic bacteria. They swim by switching the helicity of a ribbon-like cytoskeleton that comprises six proteins, each of which evolved from a nucleosidase and bacterial actin called MreB. We expressed these proteins in a synthetic, nonmotile minimal bacterium, JCVI-syn3B, whose reduced genome was computer-designed and chemically synthesized. The synthetic bacterium exhibited swimming motility with features characteristic of Spiroplasma swimming. Moreover, combinations of Spiroplasma MreB4-MreB5 and MreB1-MreB5 produced a helical cell shape and swimming. These results suggest that the swimming originated from the differentiation and coupling of bacterial actins, and we obtained a minimal system for motility of the synthetic bacterium.

Smart helical swimmer: Nested and uncoiled designs

Micro- and nanobots capable of controlled propulsion at low Reynolds number are expected to change many aspects of medicine by enabling targeted diagnosis and therapy and allowing minimally invasive surgery. Several types of helical swimmers with different actuation mechanisms have been proposed. As materials become smarter and more advanced, the boundaries between robots and materials have become less apparent. The goal of the present work is to develop a smart nested design or uncoiled design of temperature-sensitive helical swimmers adapted to low Reynolds number. The regulation of swimming velocity should provide numerous options for designing various small-sized, high-speed, motion-controllable robots for environmental and biomedical applications. To investigate the thermomechanical properties of swimmers, we present systematically theoretical modeling, experiments, and numerical calculations of temperature-sensitive shape-memory helical structures. Moreover, this work demonstrates differences in movement attributed to single or nested, folded or unfolded, and coiled or uncoiled helical structures with diverse configurations, which provide a reliable design strategy. The swimming capability can be regulated by the configurations, especially the swimmer radius. We also provide an intuitive Ashby selection map and explain the mechanical mechanism by which structure affects locomotion capability. The proposed smart helix-based swimmers should find thermomechanical coupling in applications involving active matter, biomedical sensing, and targeted drug delivery.

Fabrication of magnetic helical microribbons made of nickel thin films sandwiched between silicon nitride layers for microswimming applications

Helical swimming is adopted by microswimming robots since it is an efficient mechanism and commonly observed among microorganisms swimming at low Reynolds numbers. However, manufacturing of micro-helices made of sub-micron magnetic thin layers is neither straightforward nor well-established, advanced materials and methods are necessary to obtain such structures as reported in the literature. In this paper, a topological patterning method utilizing basic microfabrication methods is presented for the self-assembly of magnetic micro-helices made of a sandwiched nickel thin film (50–150 nm) between two silicon nitride layers. Strain mismatch between the thin films and the geometric anisotropy introduced by the slanted patterns on the top nitride layer result in self-rolled-up helical microribbons. Moreover, inspired by the actual release process during the wet-etching of the microribbon from the substrate, moving boundary conditions are incorporated in a numerical model to simulate the self-rolling of trilayer ribbons. The simulation results are compared and validated by experimental data within 7% error for all cases, including the geometries that do not result in a helical shape. The swimming performance of the magnetized micro-helix is demonstrated inside a capillary glass tube experimentally and cross-validated with a numerical model.

Smart Magnetic Microrobots Learn to Swim with Deep Reinforcement Learning.

Swimming microrobots are increasingly developed with complex materials and dynamic shapes and are expected to operate in complex environments in which the system dynamics are difficult to model and positional control of the microrobot is not straightforward to achieve. Deep reinforcement learning is a promising method of autonomously developing robust controllers for creating smart microrobots, which can adapt their behavior to operate in uncharacterized environments without the need to model the system dynamics. This article reports the development of a smart helical magnetic hydrogel microrobot that uses the soft actor critic reinforcement learning algorithm to autonomously derive a control policy which allows the microrobot to swim through an uncharacterized biomimetic fluidic environment under control of a time varying magnetic field generated from a three-axis array of electromagnets. The reinforcement learning agent learned successful control policies from both state vector input and raw images, and the control policies learned by the agent recapitulated the behavior of rationally designed controllers based on physical models of helical swimming microrobots. Deep reinforcement learning applied to microrobot control is likely to significantly expand the capabilities of the next generation of microrobots.