Rheotaxis of Bimetallic Micromotors Driven by Chemical-Acoustic Hybrid Power

Capillary-based, multifunctional manipulation of particles and fluids via focused surface acoustic waves

Surface acoustic wave (SAW)-enabled acoustofluidic technologies have recently atttracted increasing attention for applications in biology, chemistry, biophysics, and medicine. Most SAW acoustofluidic devices generate acoustic energy which is then transmitted into custom microfabricated polymer-based channels. There are limited studies on delivering this acoustic energy into convenient commercially-available glass tubes for manipulating particles and fluids. Herein, we have constructed a capillary-based SAW acoustofluidic device for multifunctional fluidic and particle manipulation. This device integrates a converging interdigitated transducer to generate focused SAWs on a piezoelectric chip, as well as a glass capillary that transports particles and fluids. To understand the actuation mechanisms underlying this device, we performed finite element simulations by considering piezoelectric, solid mechanic, and pressure acoustic physics. This experimental study shows that the capillary-based SAW acoustofluidic device can perform multiple functions including enriching particles, patterning particles, transporting particles and fluids, as well as generating droplets with controlled sizes. Given the usefulness of these functions, we expect that this acoustofluidic device can be useful in applications such as pharmaceutical manufacturing, biofabrication, and bioanalysis.

Pair Interactions of Self-Propelled SiO2-Pt Janus Colloids: Chemically Mediated Encounters

Driven by the necessity to achieve a thorough comprehension of the bottom-up fabrication process of functional materials, this experimental study investigates the pairwise interactions or collisions between chemically active SiO2-Pt Janus colloids. These collisions are categorized based on the Janus colloids' orientations before and after they make physical contact. In addition to the hydrodynamic interactions, the Janus colloids are also known to affect each other's chemical field, resulting in chemophoretic interactions, which depend on the degree of surface anisotropy in reactivity of Janus colloid and the solute-surface interaction at play. Our study reveals that these interactions lead to a noticeable decrease in particle speed and changes in orientation that correlate with the contact duration and yield different collision types. Distinct configurations of contact during collisions were found, whose mechanisms and likelihood are found to be dependent primarily on the chemical interactions. Such estimates of collision and their characterization in dilute suspensions shall have a key impact in determining the arrangement and time scales of dynamical structures and assemblies of denser suspensions and potentially the functional materials of the future.

AcousticRobots: Smart acoustically powered micro-/nanoswimmers for precise biomedical applications

Although nanotechnology has evolutionarily progressed in biomedical field over the past decades, achieving satisfactory therapeutic effects remains difficult with limited delivery efficiency. Ultrasound could provide a deep penetration and maneuverable actuation to efficiently power micro-/nanoswimmers with little harm, offering an emerging and fascinating alternative to the active delivery platform. Recent advances in novel fabrication, controllable concepts like intelligent swarm and the integration of hybrid propulsions have promoted its function and potential for medical applications. In this review, we will summarize the mechanisms and types of ultrasonically propelled micro/nanorobots (termed here as "AcousticRobots"), including the interactions between AcousticRobots and acoustic field, practical design considerations (e.g., component, size, shape), the synthetic methods, surface modification, controllable behaviors, and the advantages when combined with other propulsion approaches. The representative biomedical applications of functional AcousticRobots are also highlighted, including drug delivery, invasive surgery, eradication on the surrounding bio-environment, cell manipulation, detection, and imaging, etc. We conclude by discussing the challenges and outlook of AcousticRobots in biomedical applications.

Acoustically Powered Nano- and Microswimmers: From Individual to Collective Behavior

Micro- and nanoscopic particles that swim autonomously and self-assemble under the influence of chemical fuels and external fields show promise for realizing systems capable of carrying out large-scale, predetermined tasks. Different behaviors can be realized by tuning swimmer interactions at the individual level in a manner analogous to the emergent collective behavior of bacteria and mammalian cells. However, the limited toolbox of weak forces with which to drive these systems has made it difficult to achieve useful collective functions. Here, we review recent research on driving swimming and particle self-organization using acoustic fields, which offers capabilities complementary to those of the other methods used to power microswimmers. With either chemical or acoustic propulsion (or a combination of the two), understanding individual swimming mechanisms and the forces that arise between individual particles is a prerequisite to harnessing their interactions to realize collective phenomena and macroscopic functionality. We discuss here the ingredients necessary to drive the motion of microscopic particles using ultrasound, the theory that describes that behavior, and the gaps in our understanding. We then cover the combination of acoustically powered systems with other cross-compatible driving forces and the use of ultrasound in generating collective behavior. Finally, we highlight the demonstrated applications of acoustically powered microswimmers, and we offer a perspective on the state of the field, open questions, and opportunities. We hope that this review will serve as a guide to students beginning their work in this area and motivate others to consider research in microswimmers and acoustic fields.

Open Questions of Chemically Powered Nano- and Micromotors

Chemically powered nano- and micromotors are microscopic devices that convert chemical energy into motion. Interest in these motors has grown over the past 20 years because they exhibit interesting collective behaviors and have found potential uses in biomedical and environmental applications. Understanding how these motors operate both individually and collectively and how environments affect their operation is of both fundamental and applied significance. However, there are still significant gaps in our knowledge. This Perspective highlights several open questions regarding the propulsion mechanisms of, interactions among, and impact of confinements on nano- and micromotors driven by self-generated chemical gradients. These questions are based on my own experience as an experimentalist. For each open question, I describe the problem and its significance, analyze the status-quo, identify the bottleneck problem, and propose potential solutions. An underlying theme for these questions is the interplay among reaction kinetics, physicochemical distributions, and fluid flows. Unraveling this interplay requires careful measurements as well as a close collaboration between experimentalists and theoreticians/numerical experts. The interdisciplinary nature of these challenges suggests that their solutions could bring new revelations and opportunities across disciplines such as colloidal sciences, material sciences, soft matter physics, robotics, and beyond.

Portable Electrochemical Sensor for Micromotor Speed Monitoring

Autonomous movement promotes practical applications of micromotors. Understanding the moving speeds is a crucial step in micromotor studies. The current analysis method relies on an expensive optical microscope, which is limited to laboratory settings. Herein, we have developed a lightweight (0.15 g), portable (2.0 × 3.5 cm2), and low-cost (approximately $0.26) micromotor sensor (μ-Motor sensor), composed of water-sensitive materials for micromotor speed monitoring. Moving micromotors induce fluid flow, enhancing the evaporation rate of the liquid medium. Consequently, a high correlation between motor speed and water molecule concentration above the moving medium has been established. The μ-Motor sensor enables a real-time readout of the moving speed in various settings, with high accuracy (≥95% in the lab and ≥90% in field studies at a local beach). The μ-Motor sensor opens up a new way for detecting micro/nanomachine movements, illuminating future applications of micro/nanorobotics for diverse scenarios.

Strategies in design of self-propelling hybrid micro/nanobots for bioengineering applications

Micro/nanobots are integrated devices developed from engineered nanomaterials that have evolved significantly over the past decades. They can potentially be pre-programmed to operate robustly at numerous hard-to-reach organ/tissues/cellular sites for multiple bioengineering applications such as early disease diagnosis, precision surgeries, targeted drug delivery, cancer therapeutics, bio-imaging, biomolecules isolation, detoxification, bio-sensing, and clearing up clogged arteries with high soaring effectiveness and minimal exhaustion of power. Several techniques have been introduced in recent years to develop programmable, biocompatible, and energy-efficient micro/nanobots. Therefore, the primary focus of most of these techniques is to develop hybrid micro/nanobots that are an optimized combination of purely synthetic or biodegradable bots suitable for the execution of user-defined tasks more precisely and efficiently. Recent progress has been illustrated here as an overview of a few of the achievable construction principles to be used to make biomedical micro/nanobots and explores the pivotal ventures of nanotechnology-moderated development of catalytic autonomous bots. Furthermore, it is also foregrounding their advancement offering an insight into the recent trends and subsequent prospects, opportunities, and challenges involved in the accomplishments of the effective multifarious bioengineering applications.

Ultrasound Manipulation and Extrusion of Active Nanorods

Synthetic self-propelled nano and microparticles have a growing appeal for targeted drug delivery, collective functionality, and manipulation at the nanoscale. However, it is challenging to control their positions and orientations under confinement, e.g., in microchannels, nozzles, and microcapillaries. This study reports on the synergistic effect of acoustic and flow-induced focusing in microfluidic nozzles. In a microchannel with a nozzle, the balance between the acoustophoretic forces and the fluid drag due to streaming flows generated by the acoustic field controls the microparticle's dynamics. This study manipulates the positions and orientations of dispersed particles and dense clusters inside the channel at a fixed frequency by tuning the acoustic intensity. The main findings are: first, this study successfully manipulates the positions and orientations of individual particles and dense clusters inside the channel at a fixed frequency by tuning the acoustic intensity. Second, when an external flow is applied, the acoustic field separates and selectively extrudes shape-anisotropic passive particles and self-propelled active nanorods. Finally, the observed phenomena are explained by multiphysics finite-element modeling. The results shed light on the control and extrusion of active particles in confined geometries and enable applications for acoustic cargo (e.g., drug) delivery, particle injection, and additive manufacturing via printed self-propelled active particles.

Dependence of the acoustic propulsion of nano- and microcones on their orientation and aspect ratio

Recent research revealed the orientation-dependent propulsion of a cone-shaped colloidal particle that is exposed to a planar traveling ultrasound wave. Here, we extend the previous research by considering nano- and microcones with different aspect ratios and studying how the propulsion of a particle depends on its orientation and aspect ratio. We also study how the orientation-averaged propulsion of a cone-shaped particle, which corresponds to an isotropic ultrasound field, depends on its aspect ratio and identify an aspect ratio of 1/2 where the orientation-averaged propulsion is particularly strong. To make our simulation results easier reusable for follow-up research, we provide a corresponding simple analytic representation.

Fully Microfabricated Surface Acoustic Wave Tweezer for Collection of Submicron Particles and Human Blood Cells

Precise manipulation of (sub)micron particles is key for the preparation, enrichment, and quality control in many biomedical applications. Surface acoustic waves (SAW) hold tremendous promise for manipulation of (bio)particles at the micron to nanoscale ranges. In commonly used SAW tweezers, particle manipulation relies on the direct acoustic radiation effect whose superior performance fades rapidly when progressing from micron to nanoscale particles due to the increasing dominance of a second order mechanism, termed acoustic streaming. Through reproducible and high-precision realization of stiff microchannels to reliably actuate the microchannel cross-section, here we introduce an approach that allows the otherwise competing acoustic streaming to complement the acoustic radiation effect. The synergetic effect of both mechanisms markedly enhances the manipulation of nanoparticles, down to 200 nm particles, even at relatively large wavelength (300 μm). Besides spherical particles ranging from 0.1 to 3 μm, we show collections of cells mixed with different sizes and shapes inherently existing in blood including erythrocytes, leukocytes, and thrombocytes.

Ultrasonic Steering Wheels: Turning Micromotors by Localized Acoustic Microstreaming

The ability to steer micromotors in specific directions and at precise speeds is highly desired for their use in complex environments. However, a generic steering strategy that can be applied to micromotors of all types and surface coatings is yet to be developed. Here, we report that ultrasound of ∼100 kHz can spin a spherical micromotor so that it turns left or right when moving forward, or that it moves in full circles. The direction and angular speeds of their spinning and the radii of circular trajectories are precisely tunable by varying ultrasound voltages and frequencies, as well as particle properties such as its radius, materials, and coating thickness. Such spinning is hypothesized to originate from the circular microstreaming flows localized around a solid microsphere vibrating in ultrasound. In addition to causing a micromotor to spin, such streaming flows also helped release cargos from a micromotor during a capture-transport-release mission. Localized microstreaming does not depend on or interference with a specific propulsion mechanism and can steer a wide variety of micromotors. This work suggests that ultrasound can be used to steer microrobots in complex, biologically relevant environments as well as to steer microorganisms and cells.

Colloidal Active Matter Mimics the Behavior of Biological Microorganisms-An Overview

This article provides a review of the recent development of biomimicking behaviors in active colloids. While the behavior of biological microswimmers is undoubtedly influenced by physics, it is frequently guided and manipulated by active sensing processes. Understanding the respective influences of the surrounding environment can help to engineering the desired response also in artificial swimmers. More often than not, the achievement of biomimicking behavior requires the understanding of both biological and artificial microswimmers swimming mechanisms and the parameters inducing mechanosensory responses. The comparison of both classes of microswimmers provides with analogies in their dependence on fuels, interaction with boundaries and stimuli induced motion, or taxis.

Collective Behaviors of Active Matter Learning from Natural Taxes Across Scales

Taxis orientation is common in microorganisms, and it provides feasible strategies to operate active colloids as small-scale robots. Collective taxes involve numerous units that collectively perform taxis motion, whereby the collective cooperation between individuals enables the group to perform efficiently, adaptively, and robustly. Hence, analyzing and designing collectives is crucial for developing and advancing microswarm toward practical or clinical applications. In this review, natural taxis behaviors are categorized and synthetic microrobotic collectives are discussed as bio-inspired realizations, aiming at closing the gap between taxis strategies of living creatures and those of functional active microswarms. As collective behaviors emerge within a group, the global taxis to external stimuli guides the group to conduct overall tasks, whereas the local taxis between individuals induces synchronization and global patterns. By encoding the local orientations and programming the global stimuli, various paradigms can be introduced for coordinating and controlling such collective microrobots, from the viewpoints of fundamental science and practical applications. Therefore, by discussing the key points and difficulties associated with collective taxes of different paradigms, this review potentially offers insights into mimicking natural collective behaviors and constructing intelligent microrobotic systems for on-demand control and preassigned tasks.

Controlled propulsion of micro/nanomotors: operational mechanisms, motion manipulation and potential biomedical applications

Inspired by natural mobile microorganisms, researchers have developed micro/nanomotors (MNMs) that can autonomously move by transducing different kinds of energies into kinetic energy. The rapid development of MNMs has created tremendous opportunities for biomedical fields including diagnostics, therapeutics, and theranostics. Although the great progress has been made in MNM research, at a fundamental level, the accepted propulsion mechanisms are still a controversial matter. In practical applications such as precision nanomedicine, the precise control of the motion, including the speed and directionality, of MNMs is also important, which makes advanced motion manipulation desirable. Very recently, diverse MNMs with different propulsion strategies, morphologies, sizes, porosities and chemical structures have been fabricated and applied for various uses. Herein, we thoroughly summarize the physical principles behind propulsion strategies, as well as the recent advances in motion manipulation methods and relevant biomedical applications of these MNMs. The current challenges in MNM research are also discussed. We hope this review can provide a bird's eye overview of the MNM research and inspire researchers to create novel and more powerful MNMs.

Microswimmers in vortices: dynamics and trapping

Biological and artificial microswimmers often self-propel in external flows of vortical nature; relevant examples include algae in small-scale ocean eddies, spermatozoa in uterine peristaltic flows and bacteria in microfluidic devices. A recent experiment has shown that swimming bacteria in model vortices are expelled from the vortex all the way to a well-defined depletion zone (A. Sokolov and I. S. Aranson, Rapid expulsion of microswimmers by a vortical flow. Nat. Commun., 2016, 7, 11114). In this paper, we propose a theoretical model to investigate the dynamics of elongated microswimmers in elementary vortices, namely active particles in two- and three-dimensional rotlets. A deterministic model first reveals the existence of bounded orbits near the centre of the vortex and unbounded orbits elsewhere. We further discover a conserved quantity of motion that allows us to map the phase space according to the type of the orbit (bounded vs unbounded). We next introduce translational and rotational noise into the system. Using a Fokker-Planck formalism, we quantify the quality of trapping near the centre of the vortex by examining the probability of escape and the mean time of escape from the region of deterministically bounded orbits. We finally show how to use these findings to formulate a prediction for the radius of the depletion zone, which compares favourably with the experiments (A. Sokolov and I. S. Aranson, Rapid expulsion of microswimmers by a vortical flow. Nat. Commun., 2016, 7, 11114).

Multistimuli-responsive microrobots: A comprehensive review

Untethered robots of the size of a few microns have attracted increasing attention for the potential to transform many aspects of manufacturing, medicine, health care, and bioengineering. Previously impenetrable environments have become available for high-resolution in situ and in vivo manipulations as the size of the untethered robots goes down to the microscale. Nevertheless, the independent navigation of several robots at the microscale is challenging as they cannot have onboard transducers, batteries, and control like other multi-agent systems, due to the size limitations. Therefore, various unconventional propulsion mechanisms have been explored to power motion at the nanoscale. Moreover, a variety of combinations of actuation methods has also been extensively studied to tackle different issues. In this survey, we present a thorough review of the recent developments of various dedicated ways to actuate and control multistimuli-enabled microrobots. We have also discussed existing challenges and evolving concepts associated with each technique.

Lighting up Micro-/Nanorobots with Fluorescence

Micro-/nanorobots (MNRs) can be autonomously propelled on demand in complex biological environments and thus may bring revolutionary changes to biomedicines. Fluorescence has been widely used in real-time imaging, chemo-/biosensing, and photo-(chemo-) therapy. The integration of MNRs with fluorescence generates fluorescent MNRs with unique advantages of optical trackability, on-the-fly environmental sensitivity, and targeting chemo-/photon-induced cytotoxicity. This review provides an up-to-date overview of fluorescent MNRs. After the highlighted elucidation about MNRs of various propulsion mechanisms and the introductory information on fluorescence with emphasis on the fluorescent mechanisms and materials, we systematically illustrate the design and preparation strategies to integrate MNRs with fluorescent substances and their biomedical applications in imaging-guided drug delivery, intelligent on-the-fly sensing and photo-(chemo-) therapy. In the end, we summarize the main challenges and provide an outlook on the future directions of fluorescent MNRs. This work is expected to attract and inspire researchers from different communities to advance the creation and practical application of fluorescent MNRs on a broad horizon.

Acoustic Propulsion of Nano- and Microcones: Dependence on the Viscosity of the Surrounding Fluid

This article investigates how the acoustic propulsion of cone-shaped colloidal particles that are exposed to a traveling ultrasound wave depends on the viscosity of the fluid surrounding the particles. Using acoustofluidic computer simulations, we found that the propulsion of such nano- and microcones decreases strongly and even changes sign for increasing shear viscosity. In contrast, we found only a weak dependence of the propulsion on the bulk viscosity. The obtained results are in line with the findings of previous theoretical and experimental studies.

Recent Process in Microrobots: From Propulsion to Swarming for Biomedical Applications

Recently, robots have assisted and contributed to the biomedical field. Scaling down the size of robots to micro/nanoscale can increase the accuracy of targeted medications and decrease the danger of invasive operations in human surgery. Inspired by the motion pattern and collective behaviors of the tiny biological motors in nature, various kinds of sophisticated and programmable microrobots are fabricated with the ability for cargo delivery, bio-imaging, precise operation, etc. In this review, four types of propulsion-magnetically, acoustically, chemically/optically and hybrid driven-and their corresponding features have been outlined and categorized. In particular, the locomotion of these micro/nanorobots, as well as the requirement of biocompatibility, transportation efficiency, and controllable motion for applications in the complex human body environment should be considered. We discuss applications of different propulsion mechanisms in the biomedical field, list their individual benefits, and suggest their potential growth paths.

Enhanced and Robust Directional Propulsion of Light-Activated Janus Micromotors by Magnetic Spinning and the Magnus Effect

Advances in the versatile design and synthesis of nanomaterials have imparted diverse functionalities to Janus micromotors as autonomous vehicles. However, a significant challenge remains in maneuvering Janus micromotors by following desired trajectories for on-demand motility and intelligent control due to the inherent rotational Brownian motion. Here, we present the enhanced and robust directional propulsion of light-activated Fe₃O₄@TiO₂/Pt Janus micromotors by magnetic spinning and the Magnus effect. Once exposed to a low-intensity rotating magnetic field, the micromotors become physically actuated, and their rotational Brownian diffusion is quenched by the magnetic rotation. Photocatalytic propulsion can be triggered by unidirectional irradiation based on a self-electrophoretic mechanism. Thus, a transverse Magnus force can be generated due to the rotational motion and ballistic motion (photocatalytic propulsion) of the micromotors. Both the self-electrophoretic propulsion and the Magnus force are periodically changed due to the magnetic rotation, which results in an overall directed motion moving toward a trajectory with a deflection angle from the direction of incident light with enhanced speed, maneuverability, and steering robustness. Our study illustrates the admirable directional motion capabilities of light-driven Janus micromotors based on magnetic spinning and the Magnus effect, which unfolds a new paradigm for addressing the limitations of directionality control in the current asymmetric micromotors.

Twin-Engine Janus Supramolecular Nanomotors with Counterbalanced Motion

Supramolecular nanomotors were created with two types of propelling forces that were able to counterbalance each other. The particles were based on bowl-shaped polymer vesicles, or stomatocytes, assembled from the amphiphilic block copolymer poly(ethylene glycol)-block-polystyrene. The first method of propulsion was installed by loading the nanocavity of the stomatocytes with the enzyme catalase, which enabled the decomposition of hydrogen peroxide into water and oxygen, leading to a chemically induced motion. The second method of propulsion was attained by applying a hemispherical gold coating on the stomatocytes, on the opposite side of the opening, making the particles susceptible to near-infrared laser light. By exposing these Janus-type twin engine nanomotors to both hydrogen peroxide (H₂O₂) and near-infrared light, two competing driving forces were synchronously generated, resulting in a counterbalanced, "seesaw effect" motion. By precisely manipulating the incident laser power and concentration of H₂O₂, the supramolecular nanomotors could be halted in a standby mode. Furthermore, the fact that these Janus stomatocytes were equipped with opposing motile forces also provided a proof of the direction of motion of the enzyme-activated stomatocytes. Finally, the modulation of the "seesaw effect", by tuning the net outcome of the two coexisting driving forces, was used to attain switchable control of the motile behavior of the twin-engine nanomotors. Supramolecular nanomotors that can be steered by two orthogonal propulsion mechanisms hold considerable potential for being used in complex tasks, including active transportation and environmental remediation.

Rational Design of Polymer Conical Nanoswimmers with Upstream Motility

Utilizing bottom-up controllable molecular assembly, the bio-inspired polyelectrolyte multilayer conical nanoswimmers with gold-nanoshell functionalization on different segments are presented to achieve the optimal upstream propulsion performance. The experimental investigation reveals that the presence of the gold nanoshells on the big openings of the nanoswimmers could not only bestow efficient directional propulsion but could also minimize the impact from the external flow. The gold nanoshells at the big openings of nanoswimmers facilitate the acoustically powered propulsion against a flow velocity of up to 2.00 mm s^-1, which is higher than the blood velocity in capillaries and thus provides a proof-of-concept design for upstream nanoswimmers.

Bacterial active matter

Bacteria are among the oldest and most abundant species on Earth. Bacteria successfully colonize diverse habitats and play a significant role in the oxygen, carbon, and nitrogen cycles. They also form human and animal microbiota and may become sources of pathogens and a cause of many infectious diseases. Suspensions of motile bacteria constitute one of the most studied examples of active matter: a broad class of non-equilibrium systems converting energy from the environment (e.g., chemical energy of the nutrient) into mechanical motion. Concentrated bacterial suspensions, often termed active fluids, exhibit complex collective behavior, such as large-scale turbulent-like motion (so-called bacterial turbulence) and swarming. The activity of bacteria also affects the effective viscosity and diffusivity of the suspension. This work reports on the progress in bacterial active matter from the physics viewpoint. It covers the key experimental results, provides a critical assessment of major theoretical approaches, and addresses the effects of visco-elasticity, liquid crystallinity, and external confinement on collective behavior in bacterial suspensions.

Oscillatory rheotaxis of artificial swimmers in microchannels

Biological microswimmers navigate upstream of an external flow with trajectories ranging from linear to spiralling and oscillatory. Such a rheotactic response primarily stems from the hydrodynamic interactions triggered by the complex shapes of the microswimmers, such as flagellar chirality. We show here that a self-propelling droplet exhibits oscillatory rheotaxis in a microchannel, despite its simple spherical geometry. Such behaviour has been previously unobserved in artificial swimmers. Comparing our experiments to a purely hydrodynamic theory model, we demonstrate that the oscillatory rheotaxis of the droplet is primarily governed by both the shear flow characteristics and the interaction of the finite-sized microswimmer with all four microchannel walls. The dynamics can be controlled by varying the external flow strength, even leading to the rheotactic trapping of the oscillating droplet. Our results provide a realistic understanding of the behaviour of active particles navigating in confined microflows relevant in many biotechnology applications.

Micro-Nano Motors with Taxis Behavior: Principles, Designs, and Biomedical Applications

As a novel mobile nanodevice, micro-nano motors (MNMs) can convert the energy of the surrounding environment into mechanical motion. With this unique ability, they promise revolutionary potential in bio-applications including precise drug delivery, bio-sensing, and noninvasive surgery. Yet for practically reaching the target and fulfilling these tasks in dynamically changing bio-environment, environment adaptivity beyond propulsion is important yet challenging. MNMs with taxis behavior/autonomous target-seeking ability offer a desirable solution. These motors can adaptively move to the target location and complete the task. Thanks to the persistent efforts of researchers, tactic MNMs have shown automatic navigation to target under various energy fields, not only in static environments, but also in shear rheological conditions that simulate blood flow. Therefore, tactic motors with self-targeting capability lay a concrete foundation for targeted drug delivery, cell transplantation, and thrombus ablation. This review systematically presents the moving principle, design, and biological applications of tactic MNMs under different energy fields. Through in-depth analysis of state-of-art progress, the obstacles of the field and possible solutions are discussed. With the continuous innovation and breakthroughs of multi-disciplinary researchers, MNMs with taxis behavior are expected to provide a revolutionary solution for cancer and other major diseases in the biomedical field.

Orientation-Dependent Propulsion of Triangular Nano- and Microparticles by a Traveling Ultrasound Wave

Previous studies on ultrasound-propelled nano- and microparticles have considered only systems in which the particle orientation is perpendicular to the direction of propagation of the ultrasound. However, in future applications of these particles, they will typically be able to attain other orientations. Therefore, using direct acoustofluidic simulations, here we study how the propulsion of triangular nano- and microparticles, which are known to have a particularly efficient acoustic propulsion and are therefore promising candidates for future applications, depends on their orientation relative to the propagation direction of a traveling ultrasound wave. Our results reveal that the propulsion of the particles depends strongly on their orientation relative to the direction of wave propagation and that the particles tend to orient perpendicularly to the wave direction. We also address the orientation-averaged translational and angular velocities of the particles, which correspond to the particles' effective propulsion for an isotropic exposure to ultrasound. Our results allow assessment of how free ultrasound-propelled colloidal particles move in three spatial dimensions and thus constitute an important step toward the realization of envisaged future applications of such particles.

Upstream Rheotaxis of Catalytic Janus Spheres

Fluid flow is ubiquitous in many environments that form habitats for microorganisms. Therefore, it is not surprising that both biological and artificial microswimmers show responses to flows that are determined by the interplay of chemical and physical factors. In particular, to deepen the understanding of how different systems respond to flows, it is crucial to comprehend the influence played by swimming pattern. The tendency of organisms to navigate up or down the flow is termed rheotaxis. Early theoretical studies predicted a positive rheotactic response for puller-type spherical Janus micromotors. However, recent experimental studies have focused on pusher-type Janus particles, finding that they exhibit cross-stream migration in externally applied flows. To study the response to the flow of swimmers with a qualitatively different flow pattern, we introduce Cu@SiO₂ micromotors that swim toward their catalytic cap. On the basis of experimental observations, and supported by flow field calculations using a model for self-electrophoresis, we hypothesize that they behave effectively as a puller-type system. We investigate the effect of externally imposed flow on these spherically symmetrical Cu@SiO₂ active Janus colloids, and we indeed observe a steady upstream directional response. Through a simple squirmer model for a puller, we recover the major experimental observations. Additionally, the model predicts a "jumping" behavior for puller-type micromotors at high flow speeds. Performing additional experiments at high flow speeds, we capture this phenomenon, in which the particles "roll" with their swimming axes aligned to the shear plane, in addition to being dragged downstream by the fluid flow.

Untethered Microrobots for Active Drug Delivery: From Rational Design to Clinical Settings

Recent advances of untethered microrobots, which navigate the complex regions in vivo for therapeutics, have presented promising multiple applications on future healthcare. Microrobots used for active drug delivery system (DDS) have been demonstrated for advanced targeting distribution, improved delivery efficiency, and reduced systemic side effects. In this review, the therapeutic benefits of active DDS are presented compared to the traditional passive DDS, which illustrate the historical reasons for choosing active DDS. An integrated 5D radar chart analysis model containing the core capabilities of the active DDS is innovatively proposed. It would be a practical tool for measurement and mapping of the field of active delivery, followed by the evolutions and bottlenecks of each technical module. The comprehensive consideration of microrobots before clinical application is also discussed from the aspects of robot ethics, dosage, quality control and stability control in actual production. Gastrointestinal and blood administration, as two major clinical scenes of drug delivery, are discussed in detail as examples of the potential bedside applications of active DDS. Finally, combined with the reported analysis model, the current status and future outlook from the translation prospect to the clinical scenes of microrobots are provided.

Acoustically propelled nano- and microcones: fast forward and backward motion

We focus on cone-shaped nano- and microparticles, which have recently been found to show particularly strong propulsion when they are exposed to a traveling ultrasound wave, and study based on direct acoustofluidic computer simulations how their propulsion depends on the cones' aspect ratio. The simulations reveal that the propulsion velocity and even its sign are very sensitive to the aspect ratio, where short particles move forward whereas elongated particles move backward. Furthermore, we identify a cone shape that allows for a particularly large propulsion speed. Our results contribute to the understanding of the propulsion of ultrasound-propelled colloidal particles, suggest a method for separation and sorting of nano- and microcones concerning their aspect ratio, and provide useful guidance for future experiments and applications.

Interactions in active colloids

The past two decades have seen a remarkable progress in the development of synthetic colloidal agents which are capable of creating directed motion in an unbiased environment at the microscale. These self-propelling particles are often praised for their enormous potential to self-organize into dynamic nonequilibrium structures such as living clusters, synchronized super-rotor structures or self-propelling molecules featuring a complexity which is rarely found outside of the living world. However, the precise mechanisms underlying the formation and dynamics of many of these structures are still barely understood, which is likely to hinge on the gaps in our understanding of how active colloids interact. In particular, besides showing comparatively short-ranged interactions which are well known from passive colloids (Van der Waals, electrostatic etc), active colloids show novel hydrodynamic interactions as well as phoretic and substrate-mediated 'osmotic' cross-interactions which hinge on the action of the phoretic field gradients which are induced by the colloids on other colloids in the system. The present article discusses the complexity and the intriguing properties of these interactions which in general are long-ranged, non-instantaneous, non-pairwise and non-reciprocal and which may serve as key ingredients for the design of future nonequilibrium colloidal materials. Besides providing a brief overview on the state of the art of our understanding of these interactions a key aim of this review is to emphasize open key questions and corresponding open challenges.

Acoustics at the nanoscale (nanoacoustics): A comprehensive literature review.: Part I: Materials, devices and selected applications

In the past decade, acoustics at the nanoscale (i.e., nanoacoustics) has evolved rapidly with continuous and substantial expansion of capabilities and refinement of techniques. Motivated by research innovations in the last decade, for the first time, recent advancements of acoustics-associated nanomaterials/nanostructures and nanodevices for different applications are outlined in this comprehensive review, which is written in two parts. As part I of this two part review, firstly, active and passive nanomaterials and nanostructures for acoustics are presented. Following that, representative applications of nanoacoustics including material property characterization, nanomaterial/nanostructure manipulation, and sensing, are discussed in detail. Finally, a summary is presented with point of views on the current challenges and potential solutions in this burgeoning field.

Light-Activated Fuel-Free Janus Metal Organic Framework Colloidal Motors for the Removal of Heavy Metal Ions

Light-powered fuel-free colloidal motors possess significant potential for practical applications ranging from nanomedicine to environmental remediation. However, current light-powered colloidal motors often require the incorporation of expensive metals or high concentrations of toxic chemical fuels, which is a severe limitation for their practical applications. Integrating highly ordered and porous materials with a large surface area into colloidal motors is a promising strategy for upsurging their self-propelled speed and adsorption, which will benefit many applications. Here, highly efficient, fuel-free, and light-activated metal organic framework (MOF)-3-trimethoxysilyl propyl methacrylate Janus colloidal motors with a hierarchical morphology are reported. These colloidal motors can be driven by UV or visible light, with a self-propelled speed tuned by the light intensity. The speed can be further enhanced by morphology optimization or by the addition of H₂O₂ as a fuel. The colloidal motors display a superior efficiency in removing heavy metal ions of Hg, which is up to ∼90% within 40 min from the contaminated water, attributed to their high surface area, hierarchical morphology, large number of active sites, and high mobility. This work not only offers a facile approach to incorporate a versatile MOF family into the design of fuel-free and light-powered Janus colloidal motors, but also demonstrates their potential for real-life applications such as environmental remediation.

Multifunctional micro/nanomotors as an emerging platform for smart healthcare applications

Self-propelling micro- and nano-motors (MNMs) are emerging as a multifunctional platform for smart healthcare applications such as biosensing, bioimaging, and targeted drug delivery with high tissue penetration, stirring effect, and rapid drug transport. MNMs can be propelled and/or guided by chemical substances or external stimuli including ultrasound, magnetic field, and light. In addition, enzymatically powered MNMs and biohybrid micromotors have been developed using the biological components in the body. In this review, we describe emerging MNMs focusing on their smart propulsion systems, and diagnostic and therapeutic applications. Finally, we highlight several MNMs for in vivo applications and discuss the future perspectives of MNMs on their current limitations and possibilities toward further clinical applications.

A Practical Guide to Analyzing and Reporting the Movement of Nanoscale Swimmers

The recent invention of nanoswimmers-synthetic, powered objects with characteristic lengths in the range of 10-500 nm-has sparked widespread interest among scientists and the general public. As more researchers from different backgrounds enter the field, the study of nanoswimmers offers new opportunities but also significant experimental and theoretical challenges. In particular, the accurate characterization of nanoswimmers is often hindered by strong Brownian motion, convective effects, and the lack of a clear way to visualize them. When coupled with improper experimental designs and imprecise practices in data analysis, these issues can translate to results and conclusions that are inconsistent and poorly reproducible. This Perspective follows the course of a typical nanoswimmer investigation from synthesis through to applications and offers suggestions for best practices in reporting experimental details, recording videos, plotting trajectories, calculating and analyzing mobility, eliminating drift, and performing control experiments, in order to improve the reliability of the reported results.

Materials and Schemes of Multimodal Reconfigurable Micro/Nanomachines and Robots: Review and Perspective

Mechanically programmable, reconfigurable micro/nanoscale materials that can dynamically change their mechanical properties or behaviors, or morph into distinct assemblies or swarms in response to stimuli have greatly piqued the interest of the science community due to their unprecedented potentials in both fundamental research and technological applications. To date, a variety of designs of hard and soft materials, as well as actuation schemes based on mechanisms including chemical reactions and magnetic, acoustic, optical, and electric stimuli, have been reported. Herein, state-of-the-art micro/nanostructures and operation schemes for multimodal reconfigurable micro/nanomachines and swarms, as well as potential new materials and working principles, challenges, and future perspectives are discussed.

Construction of Nanomotors with Replaceable Engines by Supramolecular Machine-Based Host-Guest Assembly and Disassembly

Micro/nanomotors (MNMs) are miniaturized devices capable of performing self-propelled motion and on-demand tasks, which have brought revolutionary renovations in nanomedicine, environmental remediation, biochemical sensing, etc. Numerous methods of either chemical synthesis or physical fabrications have been extensively investigated to prepare MNMs of various shapes and functions. However, MNMs with replaceable engines that can be flexibly assembled and disassembled, resembling that of a macroscopic machine, have not been achieved. Here, for the first time, we report a demonstration of control over the engine replacement of self-propelled nanomotors based on hollow mesoporous silica nanoparticles (HMSNPs) via supramolecular machine-based host-guest assembly and disassembly between azobenzene (Azo) and β-cyclodextrin (β-CD). Nanomotors with different driving mechanisms can be rapidly constructed by selecting corresponding β-CD-modified nanoengines of urease, Pt, or Fe₃O₄, to assemble with the azobenzene-modified HMSNPs (HMSNPs-Azo). In virtue of photoresponsive cis/trans isomer conversion of azobenzene molecules, engine switching can be accomplished by remote light triggered host-guest assembly or disassembly between HMSNPs-Azo and β-CD-modified engines. Moreover, this method can quickly include multiple engines on the surface of the HMSNPs-Azo to prepare a hybrid MNM with enhanced motion capability. This strategy not only is cost-effective for the rapid and convenient preparation of nanomotors with different propulsion mechanism but also paves a new path to future multiple functionalization of MNMs for on-demand task assignment.

Reconfigurable multi-component micromachines driven by optoelectronic tweezers

There is great interest in the development of micromotors which can convert energy to motion in sub-millimeter dimensions. Micromachines take the micromotor concept a step further, comprising complex systems in which multiple components work in concert to effectively realize complex mechanical tasks. Here we introduce light-driven micromotors and micromachines that rely on optoelectronic tweezers (OET). Using a circular micro-gear as a unit component, we demonstrate a range of new functionalities, including a touchless micro-feed-roller that allows the programming of precise three-dimensional particle trajectories, multi-component micro-gear trains that serve as torque- or velocity-amplifiers, and micro-rack-and-pinion systems that serve as microfluidic valves. These sophisticated systems suggest great potential for complex micromachines in the future, for application in microrobotics, micromanipulation, microfluidics, and beyond.

Light-driven micromotors can convert energy to motion in sub-millimeter dimensions. Here, the authors extend this concept and introduce reconfigurable micromachines with multiple components, driven by optoelectronic tweezers, and demonstrate new functionalities.

Cargo Transportation and Methylene Blue Degradation by Using Fuel-Powered Micromotors

In the past two decades, micromotors have experienced rapid development, especially in environmental remediation, the biomedical field, and in cargo delivery. In this study micromotors have been synthesized from a variety of materials. Different functional layers and catalytic layers are formed through template electrodeposition (the bottom-up method). At the same time, the article analyzes the influence of hydrogen peroxide concentration, surfactant type and concentration on the speed of the micromotors. Cargo transportation through tubular micromotors has always been a problem that people are eager to solve. In this article, we electrodeposit a layer of Ni in the microtubes, which effectively guides the microtubular motors to complete the cargo transportation. The potential applications of micromotors are also being explored. We added the prepared micromotors to the methylene blue solution to effectively enhance the degradation.

Dipole-Moment Induced Phototaxis and Fuel-Free Propulsion of ZnO/Pt Janus Micromotors

Light-driven micromotors have stimulated considerate interests due to their potentials in biomedicine, environmental remediation, or serving as the model system for non-equilibrium physics of active matter. Simultaneous control over the motion direction and speed of micro/nanomotors is crucial for their functionality but still difficult since Brownian motion always randomizes the orientations. Here, a highly efficient light-driven ZnO/Pt Janus micromotor capable of aligning itself to illumination direction and exhibiting negative phototaxis at high speeds (up to 32 µm s^-1 ) without the addition of any chemical fuels is developed. A light-triggered self-built electric field parallel to the light illumination exists due to asymmetrical surface chemical reactions induced by the limited penetration depth of light along the illumination. The phototactic motion of the motor is achieved through electrophoretic rotation induced by the asymmetrical distribution of zeta potential on the two hemispheres of the Janus micromotor, into alignment with the electric field. Notably, similar phototactic propulsion is also achieved on TiO₂ /Pt and CdS/Pt micromotors, which presents explicit proof of extending the mechanism of dipole-moment induced phototactic propulsion in other light-driven Janus micromotors. Finally, active transportation of yeast cells are achieved by the motor, proving its capability in performing complex tasks.

Recent progress in actuation technologies of micro/nanorobots

As a research field of robotics, micro/nanorobots have been extensively studied in recent years because of their important application prospects in biomedical fields, such as medical diagnosis, nanoscale surgery, and targeted therapy. In this article, recent progress on micro/nanorobots is reviewed regarding actuation technologies. First, the different actuation mechanisms are divided into two types, external field actuation and self-actuation. Then, a few latest achievements on actuation methods are presented. On this basis, the principles of various actuation methods and their limitations are also analyzed. Finally, some key challenges in the development of micro/nanorobots are summarized and the next development direction of the field is explored.

Metallic microswimmers driven up the wall by gravity

Experiments on autophoretic bimetallic nanorods propelling within a fuel of hydrogen peroxide show that tail-heavy swimmers preferentially orient upwards and ascend along inclined planes. We show that such gravitaxis is strongly facilitated by interactions with solid boundaries, allowing even ultraheavy microswimmers to climb nearly vertical surfaces. Theory and simulations show that the buoyancy or gravitational torque that tends to align the rods is reinforced by a fore-aft drag asymmetry induced by hydrodynamic interactions with the wall.

Enzymatic/Magnetic Hybrid Micromotors for Synergistic Anticancer Therapy

Micro/nanomotors (MNMs), which propel by transforming various forms of energy into kinetic energy, have emerged as promising therapeutic nanosystems in biomedical applications. However, most MNMs used for anticancer treatment are only powered by one engine or provide a single therapeutic strategy. Although double-engined micromotors for synergistic anticancer therapy can achieve more flexible movement and efficient treatment efficacy, their design remains challenging. In this study, we used a facile preparation method to develop enzymatic/magnetic micromotors for synergetic cancer treatment via chemotherapy and starvation therapy (ST), and the size of micromotors can be easily regulated during the synthetic process. The enzymatic reaction of glucose oxidase, which served as the chemical engine, led to self-propulsion using glucose as a fuel and ST via a reduction in the energy available to cancer cells. Moreover, the incorporation of Fe₃O₄ nanoparticles as a magnetic engine enhanced the kinetic power and provided control over the direction of movement. Inherent pH-responsive drug release behavior was observed owing to the acidic decomposition of drug carriers in the intracellular microenvironment of cancer cells. This system displayed enhanced anticancer efficacy owing to the synergetic therapeutic strategies and increased cellular uptake in a targeted area because of the improved motion behavior provided by the double engines. Therefore, the demonstrated micromotors are promising candidates for anticancer biomedical microsystems.

'Sweeping rods': cargo transport by self-propelled bimetallic microrods moving perpendicular to their long axis

A possible application of self-propelling particles is the transport of microscopic cargo. Maximizing the collection and transport efficiency of particulate matter requires the area swept by the moving particle to be as large as possible. One such particle geometry are rods propelled perpendicular to their long axis, that act as "sweepers" for collecting particles. Here we report on the required Janus coating to achieve such motion, and on the dynamics of the collection and transport of microscopic cargo by sideways propelled Janus rods.

Programmable topotaxis of magnetic rollers in time-varying fields

We describe how spatially uniform, time-periodic magnetic fields can be designed to power and direct the migration of ferromagnetic spheres up (or down) local gradients in the topography of a solid substrate. Our results are based on a dynamical model that considers the time-varying magnetic torques on the particle and its motion through the fluid at low Reynolds number. We use both analytical theory and numerical simulation to design magnetic fields that maximize the migration velocity up (or down) an inclined plane. We show how "topotaxis" of spherical particles relies on differences in the hydrodynamic resistance to rotation about axes parallel and perpendicular to the plane. Importantly, the designed fields can drive multiple independent particles to move simultaneously in different directions as determined by gradients in their respective environments. Experiments on ferromagnetic spheres provide evidence for topotactic motions up inclined substrates. The ability to program the autonomous navigation of driven particles within anisotropic environments is relevant to the design of colloidal robots.

Chemically Propelled Nano and Micromotors in the Body: Quo Vadis?

The active delivery of drugs to disease sites in response to specific biomarkers is a holy grail in theranostics. If successful, it would greatly diminish the therapeutic dosage and reduce collateral cytotoxicity. In this context, the development of nano and micromotors that are able to harvest local energy to move directionally is an important breakthrough. However, serious hurdles remain before such active systems can be employed in vivo in therapeutic applications. Such motors and their energy sources must be safe and biocompatible, they should be able to move through complex body fluids, and have the ability to reach specific cellular targets. Given the complexity in the design and deployment of nano and micromotors, it is also critically important to show that they are significantly superior to inactive "smart" nanoparticles in theranostics. Furthermore, receiving regulatory approval requires the ability to scale-up the production of nano and micromotors with uniformity in structure, function, and activity. In this essay, the limitations of the current nano and micromotors and the issues that need to be resolved before such motors are likely to find theranostic applications are discussed.

Bio-inspired Acousto-magnetic Microswarm Robots with Upstream Motility

The ability to propel against flows, i.e., to perform positive rheotaxis, can provide exciting opportunities for applications in targeted therapeutics and non-invasive surgery. To date, no biocompatible technologies exist for navigating microparticles upstream when they are in a background fluid flow. Inspired by many naturally- occurring microswimmers such as bacteria, spermatozoa, and plankton that utilize the non-slip boundary conditions of the wall to exhibit upstream propulsion, here, we report on the design and characterization of self-assembled microswarms that can execute upstream motility in a combination of external acoustic and magnetic fields. Both acoustic and magnetic fields are safe to humans, non-invasive, can penetrate deeply into the human body, and are well-developed in clinical settings. The combination of both fields can overcome the limitations encountered by single actuation methods. The design criteria of the acoustically-induced reaction force of the microswarms, which is needed to perform rolling-type motion, are discussed. We show quantitative agreement between experimental data and our model that captures the rolling behaviour. The upstream capability provides a design strategy for delivering small drug molecules to hard-to-reach sites and represents a fundamental step toward the realization of micro- and nanosystem-navigation against the blood flow.