Catalytic microtubular jet engines self-propelled by accumulated gas bubbles

A Bubble-Dragged Catalytic Polymer Microrocket

We report the bubble dragged microrocket consisting of functionalized multilayer polymer covered asymmetrically by platinum nanoparticles. The microrocket is pushed back during bubble growth over a small step and dragged forward over a big step during bubble explosion. Each bubble explosion induced a shock wave of gas which propagates in water at ultrafast speed. The bubble dragged microrocket can move along an approximate straight line instead of a fluctuating circle which is the trajectory of a bubble-pushed microrocket in most cases, which makes it a promising candidate for drug delivery and simulating rod-shaped bacteria.

Nonequilibrium atomistic molecular dynamics simulation of tubular nanomotor propelled by bubble propulsion

We develop a molecular nanoscaled model for tubular motors propelled by bubble propulsion. The motor is modeled by a carbon nanotube, and the bubble is represented by a particle interacting with water by a time-dependent potential. Effects of liquid viscosity, fuel concentration, geometry, and size of the tube on the performance of the motor are effectively encoded into two parameters: time scales of the bubble expansion and bubble formation. Our results are qualitatively consistent with experimental data of much larger motors. Simulations suggest that (i) the displacement of the tube is optimized if two time scales are as short as possible, (ii) the compromise between the performance and fuel consumption is achieved if the bubble formation time is shorter than the velocity correlation time of the tube, (iii) the motor efficiency is higher with slow expansion, short formation of the bubble than fast growth but long formation time, and (iv) the tube is propelled by strong forces on the order of mN, reaching high speeds up to ∼60 m/s. Our simulation may be useful for refining and encouraging future experimental work on nanomotors having the size of a few nanometers. The tiny size and high speed motors could have great potential applications in real life.

Microfluidic Fabrication of Bubble-Propelled Micromotors for Wastewater Treatment

Bubble-propelled micromotors with controllable shapes and sizes have been developed by a microfluidic method, which serves for effective wastewater treatment. Using the emulsion from microfluidics as the template, monodisperse micromotors can be fabricated in large quantities based on phase separation and UV-induced monomer polymerization. By adjusting the volume ratio of the two immiscible oils (ethoxylated trimethylolpropane triacrylate/paraffin oil) in the initial emulsion, the geometry of the resulting micromotor can be precisely controlled from nearly spherical, hemispherical to crescent-shaped. The size of the micromotor can be manipulated by varying the fluid flow parameters. In addition, by incorporating functional nanoparticles into the asymmetric structure, the micromotor can be functionalized flexibly for water remediation. In this research, Fe₃O₄ and MnO₂ nanoparticles were successfully loaded on Janus micromotors. Fe₃O₄ nanoparticles can act as catalysts for pollutant degradation and also control the movement direction of micromotors. MnO₂ nanoparticles on the concave of the micromotor catalyzed H₂O₂ to produce bubble propulsion motion in solution, which further enhanced the degradation of pollutants. Consequently, the obtained micromotor demonstrated effective degradation of methylene blue and can be easily recovered by magnets. Furthermore, this simple and flexible strategy offers a synthetic way for anisotropic Janus particles, which will broaden their potential application.

Engineering Micromotors with Droplet Microfluidics

Micromotors have promising potential in applications ranging from environmental remediation to targeted drug delivery and noninvasive microsurgery. However, there are inadequacies in the fabrication of artificial micromotors to improve the design of structure and composition for motion performance and multifunctionality. Here, we present a microfluidic fiber-confined approach to creating droplet-templated micromotors with precisely engineered anisotropies in 3D structures and material compositions. The shape anisotropy comes from controllable deformation in droplet templates, and material anisotropy originates from versatile emulsion templates. Containing Pt and magnetic nanoparticles (NPs), micromotors are endowed with both catalytic propulsion and magnetic guidance, which are capable of performing tasks of precise catching, skillful delivering, and on-demand releasing of cargos. Droplet microfluidics allows us to systematically and independently vary the shape and size of micromotors and the distribution and content of NPs for the study of their influences on motors' mobility and improve the design. Our results are useful for fabricating micromotors with well-controlled morphology and composition that is beneficial to designing sophisticated microrobotic systems for real-world applications.

Driving Forces of the Bubble-Driven Tubular Micromotor Based on the Full Life-Cycle of the Bubble

Micromotors show many advantages in practical applications, including small size, large push-to-weight ratio, and low power consumption. Micromotors have been widely used in a variety of applications, including cell manipulation, payload delivery, and removal of toxic components. Among them, bubble-driven micromotors have received great attention due to their large driving force and high speed. The driving force of the bubble-driven micromotor movement comes from the four stages of the life cycle of the bubble: nucleation, growth, slip, and ejection. At present, investigators are still unclear about the driving mechanism of the bubble-driven micromotors, the source of the driving force being still especially controversial. In response to this problem, this paper combines the mass transfer model, hydrodynamic theory, and numerical simulation to explain the driving force generated by the various stages of the life-cycle of the bubble. A mass transfer model was used to calculate the driving force of the motor contributed by the bubble nucleation and slip stage. Based on equilibrium of force and conservation of energy, a theoretical model of the driving force of the tubular micromotor in the growth and ejection stage of the bubble was established. The results show that the driving force contributed by the bubble in the nucleation and the slip stage is rather small. However, the stage of bubble growth and ejection provide most of the driving force. On further evaluating the effect of the bubble driving force on the motor speed, it was found that the growth stage plays a major role in the motion of the bubble-driven micromotor. The micromotor velocity based on the driving forces of the full life-cycle of bubbles agrees well with the experimental results.

Hydrogel micromotors with catalyst-containing liquid core and shell

Methacrylic anhydride-derived hydrogel microcapsules have unique properties, including reversibly tunable permeation, purification, and separation of dissolved molecular species. Endowing these dynamic encapsulant systems with autonomous motion will significantly enhance their efficiency and applicability. Here, hydrogel micromotors are realized using complex water-in-oil-in-water double emulsion drops and oil-in-water emulsion drops from glass capillary microfluidics and subsequent photopolymerization. Three hydrogel micromotor strategies are explored: microcapsules with thin shells and liquid cores with dispersed catalytic Pt nanoparticles, as well as water-cored microcapsules and homogeneous microparticles selectively coated with Ti/Pt catalytic layers. Autonomous motion of hydrogel particles and capsules is realized in hydrogen peroxide solutions, where generated oxygen microbubbles propel the dynamically responsive micromotors. The micromotors are balanced by weight, buoyancy, lateral capillary forces and show specific autonomous behaviours that significantly extend short range dynamic responses of hydrogels. Drop-based microfluidics represent a paradigm shift in the integration of multifunctional subsystems and high-throughput design of chemical micromachines in reasonable quantities towards their desired biomedical, environmental and flow/diffusion microreactor applications.

Design and Fabrication of Tubular Micro/Nanomotors via 3D Laser Lithography

Catalytic tubular micro/nanomachines convert chemical energy from a surrounding aqueous fuel solution into mechanical energy to generate autonomous movements, propelled by the oxygen bubbles decomposed by hydrogen peroxide and expelled from the microtubular cavity. With the development of nanotechnology, micro/nanomotors have attracted more and more interest due to their numerous potential for in vivo and in vitro applications. Here, highly efficient chemical catalytic microtubular motors were fabricated via 3D laser lithography and their motion behavior under the action of driving force in fluids was demonstrated. The frequency of catalytically-generated bubbles ejection was influenced by the geometrical shape of the micro/nanomotor and surrounding chemical fuel environment, resulting in the variation in motion speed. The micro/nanomotors generated with a rocket-like shape displayed a more active motion compared with that of a single tubular micro/nanomotor, providing a wider range of practical micro-/nanoscale applications in the future.

Simple-Structured Micromotors Based on Inherent Asymmetry in Crystalline Phases: Design, Large-Scale Preparation, and Environmental Application

The key principle of designing a micro/nanomotor is to introduce asymmetry to a micro/nanoparticle. However, micro/nanomotors designed based on external asymmetry and inherent chemical and geometrical asymmetry usually suffer from tedious small-scale preparation, high cost, and/or complexity of external power and control devices, making them face insurmountable hurdles in practical applications. Herein, considering the possible distinct properties of different polymorphs, we propose a novel design strategy of simple-structured micromotors by introducing inherent asymmetry in crystalline phases. The inherent phase asymmetry can be easily introduced in spherical TiO₂ particles by adjusting the calcination temperature to control the phase transition and growth of primary grains. The as-designed anatase/rutile TiO₂ micromotors not only show efficient autonomous motions controlled by light in liquid media stemming from the asymmetric surface photocatalytic redox reactions but also have a promising application in environmental remediation due to their high photocatalytic activity in "on-the-fly" degradation of organic pollutants, facile large-scale fabrication, and low cost. The proposed design strategy may pave the way for the large-scale productions and applications of micro/nanomotors.

Fish-Scale-Like Intercalated Metal Oxide-Based Micromotors as Efficient Water Remediation Agents

With compelling virtues of a large specific surface area, abundant active sites, and fast interfacial transport, nanomaterials have been demonstrated to be indispensable tools for water remediation applications. Accordingly, micro/nanomotors made by nanomaterials would also benefit from these properties. Though tuning the surface architecture on demand becomes a hot topic in the field of nanomaterials, there are still limited reports on the design of active surface architectures in chemically driven tubular micro/nanomachines. Here, a unique architecture composed of a fish-scale-like intercalated (FSI) surface structure and an active layer with 5 nm nanoparticles is constructed, which composes of Fe₂O₃ and ramsdellite MnO₂, Mn₂O₃, in the tubular micromotor using a versatile electrodeposition protocol. Tailoring the electrodeposition parameters enables us to modulate the active MnO₂ surface structure on demand, giving rise to a pronounced propulsion performance and catalytic activity. Upon exposure to the azo-dye waste solution, the degradation efficacy greatly raises by around 22.5% with FSI micromotor treatment when compared to the normal compact motors, owing to the synergistic effect between the Fe-related Fenton reaction and a large catalytic area offered by the hierarchically rough inner surface. Such unique micromachines with a large active surface area have great potential for environmental and biomedical applications.

Photochemically Excited, Pulsating Janus Colloidal Motors of Tunable Dynamics

Spontaneous periodicity is widely found in many biological and synthetic systems, and designing colloidal motors that mimic this feature may not only facilitate our understanding of how complexity emerges but also enable applications that benefit from a time-varying activity. However, there is so far no report on a colloidal motor system that shows controllable and spontaneous oscillation in speeds. Inspired by previous studies of oscillating silver microparticles, we report silver-poly(methyl methacrylate) microsphere Janus colloidal motors that moved, interacted with tracers, and exhibited negative gravitaxis all in an oscillatory fashion. Its dynamics, including pulsating speeds and magnitude, as well as whether moving forward in a pulsating or continuous mode, can be systematically modulated by varying chemical concentrations, light intensity, and the way light was applied. A qualitative mechanism is proposed to link the oscillation of Janus colloidal motors to ionic diffusiophoresis, while nonlinearity is suspected to arise from a sequence of autocatalytic decomposition of AgCl and its slow buildup in the presence of H₂O₂ and light. The generation of light-absorbing Ag nanoparticles is suspected to be the key. This study therefore establishes a robust model system of chemically driven, oscillatory colloidal motors with clear directionality, good tunability, and an improved mechanism, with which complex, emergent phenomena can be explored.

Rolled-Up Monolayer Graphene Tubular Micromotors: Enhanced Performance and Antibacterial Property

The motion of catalytic tubular micromotors are driven by the oxygen bubbles generated from chemical reaction and is influenced by the resistance from the liquid environment. Herein, we fabricated a rolled-up graphene tubular micromotor, in which the graphene layer was adopted as the outmost surface. Due to the hydrophobic property of the graphene layer, the fabricated micromotor performed a motion pattern that could escape from the attraction from the bubbles. In addition, Escherichia coli and Staphylococcus culture experiments proved that the graphene outer surface displays antibacterial property. Considering the bubble-avoiding and antibacterial properties, the rolled-up graphene tubular micromotor holds great potential for various applications such as in vivo drug delivery and biosensors.

Metal-Organic Frameworks Based Nano/Micro/Millimeter-Sized Self-Propelled Autonomous Machines

Synthetic nano/micro/millimeter-sized machines that harvest energy from the surrounding environment and then convert it to motion have had a significant impact on many research areas such as biology (sensing, imaging, and therapy) and environmental applications. Autonomous motion is a key element of these devices. A high surface area is preferable as it leads to increased propellant or cargo-loading capability. Integrating highly ordered and porous metal-organic frameworks (MOFs) with self-propelled machines is demonstrated to have a significant impact on the field of nano/micro/millimeter-sized devices for a wide range of applications. MOFs have shown great potential in many research fields due to their tailorable pore size. These fields include energy storage and conversion; catalysis, biomedical application (e.g., drug delivery, imaging, and cancer therapy), and environmental remediation. The marriage of motors and MOFs may provide opportunities for many new applications for synthetic nano/micro/millimeter-sized machines. Herein, MOF-based micro- and nanomachines are reviewed with a focus on the specific properties of MOFs.

High-performance carbon/MnO2 micromotors and their applications for pollutant removal

The wide applications of particulate micromotors in practice, especially in the removal of environmental pollutants, have been limited by the low production yields and demand on high concentration of fuel such as H₂O₂. Carbon/MnO₂ micromotors were made hydrothermally using different carbon allotropes including graphite, carbon nanotube (CNT), and graphene for treatment of methylene blue and toxic Ag ions. The obtained micromotors showed high speed of self-propulsion. The highest speed of MnO₂-based micromotors to date was observed for CNT/MnO₂ (>2 mm/s, 5 wt% H₂O₂, 0.5 wt% surfactant). Moreover, different from previous studies, even with low H₂O₂ concentration (0.5 wt%) and without surfactant addition, the micromotors could also be well dispersed in water by the O₂ stream released from their reaction with H₂O₂. The carbon/MnO₂ micromotors removed both methylene blue (>80%) and Ag ions (100%) effectively within 15 min by catalytic decomposition and adsorption. Especially high adsorption capacity of Ag (600 mg/g) was measured on graphite/MnO₂ and graphene/MnO₂ micromotors.

Shape-directed rotation of homogeneous micromotors via catalytic self-electrophoresis

The pursuit of chemically-powered colloidal machines requires individual components that perform different motions within a common environment. Such motions can be tailored by controlling the shape and/or composition of catalytic microparticles; however, the ability to design particle motions remains limited by incomplete understanding of the relevant propulsion mechanism(s). Here, we demonstrate that platinum microparticles move spontaneously in solutions of hydrogen peroxide and that their motions can be rationally designed by controlling particle shape. Nanofabricated particles with n-fold rotational symmetry rotate steadily with speed and direction specified by the type and extent of shape asymmetry. The observed relationships between particle shape and motion provide evidence for a self-electrophoretic propulsion mechanism, whereby anodic oxidation and cathodic reduction occur at different rates at different locations on the particle surface. We develop a mathematical model that explains how particle shape impacts the relevant electrocatalytic reactions and the resulting electrokinetic flows that drive particle motion.

Self-propelled motors operating at the micro- or nanoscale can be powered by catalytic reactions and show appealing potential in robotic applications. Brooks et al. describe how the motions of platinum spinners in hydrogen peroxide solutions can be rationally designed by controlling particle shape.

Micromotors for drug delivery in vivo: The road ahead

Autonomously propelled/externally guided micromotors overcome current drug delivery challenges by providing (a) higher drug loading capacity, (b) localized delivery (less toxicity), (c) enhanced tissue penetration and (d) active maneuvering in vivo. These microscale drug delivery systems can exploit biological fluids, as well as exogenous stimuli, like light-NIR, ultrasound and magnetic fields (or a combination of these), towards propulsion/drug release. Ability of these wireless drug carriers towards localized targeting and controlled drug release, makes them a lucrative candidate for drug administration in complex microenvironments (like solid tumors or gastrointestinal tract). In this report, we discuss these microscale drug delivery systems for their therapeutic benefits under in vivo setting and provide a design-application rationale towards greater clinical significance. Also, a proof-of-concept depicting 'microbots-in-a-capsule' towards oral drug delivery has been discussed.

Experimental Study of the Motion of Patchy Particle Swimmers Near a Wall

In this work, we demonstrate our ability to precisely tailor the surface activity of self-propelled active colloids by varying the size of the active area. The quasi two-dimensional autonomous motion of spherical patchy particle swimmers is studied in a chemical environment in the vicinity of a solid boundary. Oxidative decomposition of hydrogen peroxide into oxygen and water occurs only on a well-defined Pt-coated section of the polystyrene particle surface. The asymmetric distribution of product molecules interacting with the particle leads to the autonomous motion, which is characterized as the patch size varies from 11 to 25 to 50% of the particle surface area. The phoretic motion of patchy particle swimmers is analytically predicted by a model developed by Popescu et al. and shows good agreement with the experimentally observed velocities when the influence of the wall on the preferential rotational motion of the particles near the solid boundary is considered. The study illustrates the potential to precisely engineer the motion of particles by controlling their properties rather than depending on changes in the environment.

Effective Interactions between Chemically Active Colloids and Interfaces

Chemically active colloids can achieve force- and torque-free motility ("self-propulsion") via the promotion, on their surface, of catalytic chemical reactions involving the surrounding solution. Such systems are valuable both from a theoretical perspective, serving as paradigms for nonequilibrium processes, as well as from an application viewpoint, according to which active colloids are envisioned to play the role of carriers ("engines") in novel lab-on-a-chip devices. The motion of such colloids is intrinsically connected with a "chemical field", i.e., the distribution near the colloid of the number densities of the various chemical species present in the solution, and with the hydrodynamic flow of the solution around the particle. In most of the envisioned applications, and in virtually all reported experimental studies, the active colloids operate under spatial confinement (e.g., within a microfluidic channel, a drop, a free-standing liquid film, etc.). In such cases, the chemical field and the hydrodynamic flow associated with an active colloid are influenced by any nearby confining surfaces, and these disturbances couple back to the particle. Thus, an effective interaction with the spatial confinement arises. Consequently, the particle is endowed with means to perceive and to respond to its environment. Understanding these effective interactions, finding the key parameters which control them, and designing particles with desired, preconfigured responses to given environments, require interdisciplinary approaches which synergistically integrate methods and knowledge from physics, chemistry, engineering, and materials science. Here we review how, via simple models of chemical activity and self-phoretic motion, progress has recently been made in understanding the basic physical principles behind the complex behaviors exhibited by active particles near interfaces. First, we consider the occurrence of "interface-bounded" steady states of chemically active colloids near simple, nonresponsive interfaces. Examples include particles "sliding" along, or "hovering" above, a hard planar wall while inducing hydrodynamic flow of the solution. These states lay the foundations for concepts like the guidance of particles by the topography of the wall. We continue to discuss responsive interfaces: a suitable chemical patterning of a planar wall allows one to bring the particles into states of motion which are spatially localized (e.g., within chemical stripes or along chemical steps). These occur due to the wall responding to the activity-induced chemical gradients by generating osmotic flows, which encode the surface-chemistry of the wall. Finally, we discuss how, via activity-induced Marangoni stresses, long-ranged effective interactions emerge from the strong hydrodynamic response of fluid interfaces. These examples highlight how in this context a desired behavior can be potentially selected by tuning suitable parameters (e.g., the phoretic mobility of the particle, or the strength of the Marangoni stress at an interface). This can be accomplished via a judicious design of the surface chemistry of the particle and of the boundary, or by the choice of the chemical reaction in solution.

Effective squirmer models for self-phoretic chemically active spherical colloids

Various aspects of self-motility of chemically active colloids in Newtonian fluids can be captured by simple models for their chemical activity plus a phoretic-slip hydrodynamic boundary condition on their surface. For particles of simple shapes (e.g., spheres) --as employed in many experimental studies-- which move at very low Reynolds numbers in an unbounded fluid, such models of chemically active particles effectively map onto the well studied so-called hydrodynamic squirmers (S. Michelin and E. Lauga, J. Fluid Mech. 747, 572 (2014)). Accordingly, intuitively appealing analogies of "pusher/puller/neutral" squirmers arise naturally. Within the framework of self-diffusiophoresis we illustrate the above-mentioned mapping and the corresponding flows in an unbounded fluid for a number of choices of the activity function (i.e., the spatial distribution and the type of chemical reactions across the surface of the particle). We use the central collision of two active particles as a simple, paradigmatic case for demonstrating that in the presence of other particles or boundaries the behavior of chemically active colloids may be qualitatively different, even in the far field, from the one exhibited by the corresponding "effective squirmer", obtained from the mapping in an unbounded fluid. This emphasizes that understanding the collective behavior and the dynamics under geometrical confinement of chemically active particles necessarily requires to explicitly account for the dependence of the hydrodynamic interactions on the distribution of chemical species resulting from the activity of the particles.

Lab-on-a-micromotor: catalytic Janus particles as mobile microreactors for tailored synthesis of nanoparticles

Catalytic Janus micromotors encapsulating Cd2+ or citrate are used here as mobile microreactors for "on the fly" CdS quantum dot and gold nanoparticle synthesis. Micromotor navigation in microliter "reagent solutions" results in the generation of the corresponding nanoparticles inside the micromotor body with high yield and negligible waste generation. Nanoparticle generation can be attributed to convective diffusion of reagents into the moving reactor body. "On-demand" modulation of nanoparticle size and catalytic activities can be achieved by judicious control of the motion behavior of the microreactor. The use of confined reagents in connection with such enhanced movement allows for efficient operation in very low (less than 800 μL) volumes. The new microreactors developed here hold considerable promise for reactions in aqueous environments for novel synthetic schemes in different sites along with multiplexed capabilities for a myriad of catalytic reactions.

High-Motility Visible Light-Driven Ag/AgCl Janus Micromotors

Visible light-driven nano/micromotors are promising candidates for biomedical and environmental applications. This study demonstrates blue light-driven Ag/AgCl-based spherical Janus micromotors, which couple plasmonic light absorption with the photochemical decomposition of AgCl. These micromotors reveal high motility in pure water, i.e., mean squared displacements (MSD) reaching 800 µm² within 8 s, which is 100× higher compared to previous visible light-driven Janus micromotors and 7× higher than reported ultraviolet (UV) light-driven AgCl micromotors. In addition to providing design rules to realize efficient Janus micromotors, the complex dynamics revealed by individual and assemblies of Janus motors is investigated experimentally and in simulations. The effect of suppressed rotational diffusion is focused on, compared to UV light-driven AgCl micromotors, as a reason for this remarkable increase of the MSD. Moreover, this study demonstrates the potential of using visible light-driven plasmonic Ag/AgCl-based Janus micromotors in human saliva, phosphate-buffered saline solution, the most common isotonic buffer that mimics the environment of human body fluids, and Rhodamine B solution, which is a typical polluted dye for demonstrations of photocatalytic environmental remediation. This new knowledge is useful for designing visible light driven nano/micromotors based on the surface plasmon resonance effect and their applications in assays relevant for biomedical and ecological sciences.

A Supramolecular Approach to Nanoscale Motion: Polymersome-Based Self-Propelled Nanomotors

Autonomous micro- and nanoscale systems have revolutionized the way scientists look into the future, opening up new frontiers to approach and solve problems via a more bioinspired route. However, to achieve systems with higher complexity, superior output control, and multifunctionality, an in-depth study of the different factors that affect micro- and nanomotor behavior is crucial. From a fundamental perspective, the mechanical response of micro- and nanomotors still requires further study in order to have a better understanding of how exactly these systems operate and the different mechanisms of motion that can be combined into one system to achieve an optimal response. From a design engineering point of view, compatibility, degradability, specificity, sensitivity, responsiveness, and efficiency of the active systems fabricated to this point have to be addressed, with respect to the potential of these devices for biomedical applications. Nonetheless, optimizing the system with regards to all these areas is a challenging task with the micro- and nanomotors studied to date, as most of them consist of materials or designs that are unfavorable for further chemical or physical manipulation. As this new field of self-powered systems moves forward, the need for motor prototypes with different sizes, shapes, chemical functionalities, and architectures becomes increasingly important and will define not only the way active systems are powered, but also the methods for motor fabrication. Bottom-up supramolecular approaches have recently emerged as great candidates for the development of active structures that allow for chemical or physical functionalization, shape transformation, and compartmentalization, in a structure that provides a soft interface to improve molecular recognition and cell uptake. Our group pioneers the use of supramolecular structures as catalytically propelled systems via the fabrication of stomatocyte or tubular-shaped motors capable of displaying active motion in a substrate concentration-dependent fashion. This behavior demonstrates the potential of bottom-up assemblies for powering motion at the micro- or nanoscale, with a system that can be readily tuned and controlled at the molecular level. In this Account, we highlight the steps we have taken in order to understand and optimize the design of catalytically powered polymersome-based motors. Our research has been focused on addressing the importance of motor architecture, motion activation, direction control, and biological integration. While our work supports the feasibility of supramolecular structures for the design of active systems, we strongly believe that we are still in the initial stages of unveiling the full potential of supramolecular chemistry in the micro- and nanomotor field. We look forward to using this approach for the development of multifunctional and stimuli-responsive systems in the near future.

Bubble-Pair Propelled Colloidal Kayaker

We report a hollow dumbbell-shaped manganese dioxide (MnO₂) colloidal kayaker capable of converting a pair of breathing oxygen bubbles into self-propelled movement. The bubble pair generated by catalytic decomposition of hydrogen peroxide fuel grew either synchronously or asynchronously, driving the colloidal kayaker to move along a fluctuating circle. The synchronous or asynchronous breathing mode of bubble pair is governed by the asymmetric catalytic sites of the colloidal kayakers. This imbalanced distribution of bubble propulsion force generates the driving force and the centripetal force on the colloidal kayaker. The dynamics of colloidal kayakers is well-described by the overdamped Langevin equation and fluid field simulation. Such bubble-pair propelled colloidal kayakers could advance applications of catalytic nanomotors, offering effective implementation for diverse tasks for a wide range of practical applications.

Efficient Propulsion and Hovering of Bubble-Driven Hollow Micromotors underneath an Air-Liquid Interface

Bubble-driven micromotors have attracted substantial interest due to their remarkable self-motile and cargo-delivering abilities in biomedical or environmental applications. Here, we developed a hollow micromotor that experiences fast self-propulsion underneath an air-liquid interface by periodic bubble growth and collapse. The collapsing of a single microbubble induces a ∼1 m·s^-1 impulsive jetting flow that instantaneously pushes the micromotor forward. Unlike previously reported micromotors propelled by the recoiling of bubbles, cavitation-induced jetting further utilizes the energy stored in the bubble to propel the micromotor and thus enhances the energy conversion efficiency by 3 orders of magnitude. Four different modes of propulsion are, for the first time, identified by quantifying the dependence of propulsion strength on microbubble size. Meanwhile, the vertical component of the jetting flow counteracts the buoyancy of the micromotor-bubble dimer and facilitates counterintuitive hovering underneath the air-liquid interface. This work not only enriches the understanding of the propulsion mechanism of bubble-driven micromotors but also gives insight into the physical aspects of cavitation bubble dynamics near the air-liquid interface on the microscale.

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Flexible Guidance of Microengines by Dynamic Topographical Pathways in Ferrofluids

In this work, we demonstrate a simple, versatile, and real-time motion guidance strategy for artificial microengines and motile microorganisms in a ferrofluid by dynamic topographical pathways (DTPs), which are assembled from superparamagnetic nanoparticles in response to external magnetic field ( H). In this general strategy, the DTPs can exert anisotropic resistance forces on autonomously moving microengines and thus regulate their orientation. As the DTPs with different directions and lengths can be reversibly and swiftly assembled in response to the applied H, the microengines in the ferrofluid can be guided on demand with controlled motion directions and trajectories, including circular, elliptical, straight-line, semi-sine, and sinusoidal trajectories. The as-demonstrated control strategy obviates reliance on the customized responses of micromotors and applies to autonomously propelling agents swimming both in bulk and near substrate walls. Furthermore, the microengines (or motile microorganisms) in a ferrofluid can be considered as an integrated system, and it may inspire the development of intelligent systems with cooperative functions for biomedical and environmental applications.

Poly(ionic liquid)s Based Brush Type Nanomotor

A brush type nanomotor was fabricated via assembly assistant polymerization of poly(ionic liquid) and surface grafting polymerization. The method for large-scale fabrication of brush nanomotors with soft surfaces is described. These soft locomotive particles are based on core-shell brush nanoparticles assembled from poly(ionic liquid) as core and thermoresponsive PNIPAM as brush shells on which platinum nanoparticle (PtNP) were grown in situ. The particles show non-Brownian motion in H₂O₂ solution.

Motion-Based Immunological Detection of Zika Virus Using Pt-Nanomotors and a Cellphone

Zika virus (ZIKV) infection is an emerging pandemic threat to humans that can be fatal in newborns. Advances in digital health systems and nanoparticles can facilitate the development of sensitive and portable detection technologies for timely management of emerging viral infections. Here we report a nanomotor-based bead-motion cellphone (NBC) system for the immunological detection of ZIKV. The presence of virus in a testing sample results in the accumulation of platinum (Pt)-nanomotors on the surface of beads, causing their motion in H₂O₂ solution. Then the virus concentration is detected in correlation with the change in beads motion. The developed NBC system was capable of detecting ZIKV in samples with virus concentrations as low as 1 particle/μL. The NBC system allowed a highly specific detection of ZIKV in the presence of the closely related dengue virus and other neurotropic viruses, such as herpes simplex virus type 1 and human cytomegalovirus. The NBC platform technology has the potential to be used in the development of point-of-care diagnostics for pathogen detection and disease management in developed and developing countries.

Nanoscale Biosensors Based on Self-Propelled Objects

This review provides a comprehensive overview of the latest developments (2016⁻2018 period) in the nano and micromotors field for biosensing applications. Nano and micromotor designs, functionalization, propulsion modes and transduction mechanism are described. A second important part of the review is devoted to novel in vitro and in vivo biosensing schemes. The potential and future prospect of such moving nanoscale biosensors are given in the conclusions.

Progress toward Catalytic Micro- and Nanomotors for Biomedical and Environmental Applications

Synthetic micro- and nanomotors (MNMs) are tiny objects that can autonomously move under the influence of an appropriate source of energy, such as a chemical fuel, magnetic field, ultrasound, or light. Chemically driven MNMs are composed of or contain certain reactive material(s) that convert chemical energy of a fuel into kinetic energy (motion) of the particles. Several different materials have been explored over the last decade for the preparation of a wide variety of MNMs. Here, the discovery of materials and approaches to enhance the efficiency of chemically driven MNMs are reviewed. Several prominent applications of the MNMs, especially in the fields of biomedicine and environmental science, are also discussed, as well as the limitations of existing materials and future research directions.

Biohybrid and Bioinspired Magnetic Microswimmers

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.

Reconfigurable Vanadium Dioxide Nanomembranes and Microtubes with Controllable Phase Transition Temperatures

Two additional structural forms, free-standing nanomembranes and microtubes, are reported and added to the vanadium dioxide (VO₂) material family. Free-standing VO₂ nanomembranes were fabricated by precisely thinning as-grown VO₂ thin films and etching away the sacrificial layer underneath. VO₂ microtubes with a range of controllable diameters were rolled-up from the VO₂ nanomembranes. When a VO₂ nanomembrane is rolled-up into a microtubular structure, a significant compressive strain is generated and accommodated therein, which decreases the phase transition temperature of the VO₂ material. The magnitude of the compressive strain is determined by the curvature of the VO₂ microtube, which can be rationally and accurately designed by controlling the tube diameter during the rolling-up fabrication process. The VO₂ microtube rolling-up process presents a novel way to controllably tune the phase transition temperature of VO₂ materials over a wide range toward practical applications. Furthermore, the rolling-up process is reversible. A VO₂ microtube can be transformed back into a nanomembrane by introducing an external strain. Because of its tunable phase transition temperature and reversible shape transformation, the VO₂ nanomembrane-microtube structure is promising for device applications. As an example application, a tubular microactuator device with low driving energy but large displacement is demonstrated at various triggering temperatures.

Maneuverability of Magnetic Nanomotors Inside Living Cells

Spatiotemporally controlled active manipulation of external micro-/nanoprobes inside living cells can lead to development of innovative biomedical technologies and inspire fundamental studies of various biophysical phenomena. Examples include gene silencing applications, real-time mechanical mapping of the intracellular environment, studying cellular response to local stress, and many more. Here, for the first time, cellular internalization and subsequent intracellular manipulation of a system of helical nanomotors driven by small rotating magnetic fields with no adverse effect on the cellular viability are demonstrated. This remote method of fuelling and guidance limits the effect of mechanical transduction to cells containing external probes, in contrast to ultrasonically or chemically powered techniques that perturb the entire experimental volume. The investigation comprises three cell types, containing both cancerous and noncancerous types, and is aimed toward analyzing and engineering the motion of helical propellers through the crowded intracellular space. The studies provide evidence for the strong anisotropy, heterogeneity, and spatiotemporal variability of the cellular interior, and confirm the suitability of helical magnetic nanoprobes as a promising tool for future cellular investigations and applications.