How to Make a Fast, Efficient Bubble-Driven Micromotor: A Mechanical View

Shape-controlled movement of Zn/SU-8 micromotors

Creating micromotors (MMs) that will have the highest possible velocities has become one of the main focuses in the field of autonomous microdevices research. The importance of velocity stems from various autonomous microdevices applications, ranging from faster drug delivery to the eradication of various bacterial biofilms using only mechanical movement. To investigate how different shapes affect the velocity of Zn/SU-8 micromotors in acid solution, we fabricated micromotors with various geometries (Zn/SU-8/Cylindrical, Zn/SU-8/Rectangular cuboid, Zn/SU-8/Triangular prism, Zn/SU-8/Pentagonal prism and Zn/SU-8/Pentagrammic prism MMs). This is the first comparative study where shape has been isolated as the critical factor influencing micromotor velocity under the same catalytic surface conditions. Our results demonstrate that Zn/SU-8/Rectangular cuboid and Zn/SU-8/Triangular prism MMs exhibit significantly higher average velocities compared to the other studied MMs. The shape-optimized Zn/SU-8 micromotors, characterized by their simple synthesis process and low cost, offer significant potential to enhance efficiency and navigation in both environmental and medical applications through precise movement control.

Molecular Memory Micromotors for Fast Snake Venom Toxin Dynamic Detection

The analysis and detection of snake venom toxins are a matter of great importance in clinical diagnosis for fast treatment and the discovery of new pharmaceutical products. Current detection methods have high associated costs and require the use of sophisticated bioreceptors, which in some cases are difficult to obtain. Herein, we report the synthesis of template-based molecularly imprinted micromotors for dynamic detection of α-bungarotoxin as a model toxin present in the venom of many-banded krait (Bungarus multicinctus). The specific recognition sites are built-in in the micromotors by incubation of the membrane template with the target toxin, followed by a controlled electrodeposition of a poly(3,4-ethylenedioxythiophene)/poly(sodium 4-styrenesulfonate) polymeric layer, a magnetic Ni layer to promote magnetic guidance and facilitate washing steps, and a Pt layer for autonomous propulsion in the presence of hydrogen peroxide. The enhanced fluid mixing and autonomous propulsion increase the likelihood of interactions with the target analyte as compared with static counterparts, retaining the tetramethylrhodamine-labeled α-bungarotoxin on the micromotor surface with extremely fast dynamic sensor response (after just 20 s navigation) in only 3 μL of water, urine, or serum samples. The sensitivity achieved meets the clinically relevant concentration postsnakebite (from 0.1 to 100 μg/mL), illustrating the feasibility of the approach for practical applications. The selectivity of the protocol is very high, as illustrated by the absence of fluorescence in the micromotor surface in the presence of α-cobratoxin as a representative toxin with a size and structure similar to those of α-bungarotoxin. Recoveries higher than 95% are obtained in the analysis of urine- and serum-fortified samples. The new strategy holds considerable promise for fast, inexpensive, and even onsite detection of several toxins using multiple molecularly imprinted micromotors with tailored recognition abilities.

Additively manufactured customized microhelix motors' bursting motion in mesoscopic tubes for vessel declogging

Magnetic microhelix motors are widely employed in various applications such as cargo transportation, drug delivery, toxic substance declogging, and cell manipulation, due to their unique adaptive magnetic manipulation capabilities. In this work, high-precision stereoscopic additive manufacturing techniques were used to produce customized microhelices with varying structural parameters, including different pitch numbers (2-4 pitches), sizes (0.1-0.25 mm), and taper angles (172°-180°). Their motion performance in mesoscopic tubes was systematically investigated. The magnetic microhelix motors' speed increases when circle numbers and taper angles decrease, while circle diameters increase. The magnetic microhelix motors' speed could achieve a 1500% enhancement reaching 0.16 mm s-1 in a 0.3 mm tube, with a pitch number of 3, diameter of 0.2 mm, and taper angle of 172°. Furthermore, their vessel declogging capability is confirmed in in vitro experiments.

Comprehensive Understanding of Self-Propelled Janus Pt/Fe2O3 Micromotor Dynamics: Impact of Size, Morphology, and Surface Structure

The increasing use of plastics has led to the accumulation of plastic waste in the oceans, resulting in significant global environmental challenges associated with microplastic pollution. Micromotors, capable of capturing and removing microplastics from aquatic systems, have emerged as a promising solution to addressing this problem. This research aims to analyze the factors affecting the speed of micromotors, including size, morphology, and surface structure, while elucidating the underlying mechanisms governing micromotor propulsion to develop efficient and ecofriendly micromotors. In this study, we systematically manipulate various parameters by modifying the synthesis method of hematite-based micromotors, subsequently comparing their propulsion speeds and uncovering the precise role of these parameters in determining the micromotor performance. Furthermore, we shed light on the intricate interplay between drag force and propulsive force, demonstrating how these forces vary under different H2O2 conditions. These findings provide valuable insights into the design of efficient micromotors tailored for dynamic aquatic environments.

Micro-/nanoscale robotics for chemical and biological sensing

The field of micro-/nanorobotics has attracted extensive interest from a variety of research communities and witnessed enormous progress in a broad array of applications ranging from basic research to global healthcare and to environmental remediation and protection. In particular, micro-/nanoscale robots provide an enabling platform for the development of next-generation chemical and biological sensing modalities, owing to their unique advantages as programmable, self-sustainable, and/or autonomous mobile carriers to accommodate and promote physical and chemical processes. In this review, we intend to provide an overview of the state-of-the-art development in this area and share our perspective in the future trend. This review starts with a general introduction of micro-/nanorobotics and the commonly used methods for propulsion of micro-/nanorobots in solution, along with the commonly used methods in their fabrication. Next, we comprehensively summarize the current status of the micro/nanorobotic research in relevance to chemical and biological sensing (e.g., motion-based sensing, optical sensing, and electrochemical sensing). Following that, we provide an overview of the primary challenges currently faced in the micro-/nanorobotic research. Finally, we conclude this review by providing our perspective detailing the future application of soft robotics in chemical and biological sensing.

Role of Bubble Evolution in the Bubble-Propelled Janus Micromotors

Bubble-propelled Janus micromotors have attracted extensive attention in recent years and have been regarded as powerful tools in the environmental and medical fields due to their excellent movement ability. The movement ability can mainly be attributed to the periodic growth, detachment, and/or collapse of the bubble. However, subjected to the experimental conditions, the mechanism of bubble evolution on the motion of the micromotor could not be elucidated clearly. In this work, a finite element method was employed for exploring the role of bubble evolution in bubble-propelled Janus micromotors, which emphasized the growth and collapse of bubbles. After the proposed model was verified by the scallop theorem, the influence of the growth and rapid collapse of bubbles on micromotors was investigated. Results show that the growth and collapse of a bubble can drive the micromotor to produce a displacement, but the displacement caused by a bubble collapse is significantly greater than that caused by bubble growth. The reasons for this phenomenon are analyzed and explained. In addition to the influence of bubble size, the collapse time of the bubble is also investigated.

Spatial Control over Catalyst Positioning for Increased Micromotor Efficiency

Motion is influenced by many different aspects of a micromotor's design, such as shape, roughness and the type of materials used. When designing a motor, asymmetry is the main requirement to take into account, either in shape or in catalyst distribution. It influences both speed and directionality since it dictates the location of propulsion force. Here, we combine asymmetry in shape and asymmetry in catalyst distribution to study the motion of soft micromotors. A microfluidic method is utilized to generate aqueous double emulsions, which upon UV-exposure form asymmetric microgels. Taking advantage of the flexibility of this method, we fabricated micromotors with homogeneous catalyst distribution throughout the microbead and micromotors with different degrees of catalyst localization within the active site. Spatial control over catalyst positioning is advantageous since less enzyme is needed for the same propulsion speed as the homogeneous system and it provides further confinement and compartmentalization of the catalyst. This proof-of-concept of our new design will make the use of enzymes as driving forces for motors more accessible, as well as providing a new route for compartmentalizing enzymes at interfaces without the need for catalyst-specific functionalization.

Fabrication, control, and modeling of robots inspired by flagella and cilia

Flagella and cilia are slender structures that serve important functionalities in the microscopic world through their locomotion induced by fluid and structure interaction. With recent developments in microscopy, fabrication, biology, and modeling capability, robots inspired by the locomotion of these organelles in low Reynolds number flow have been manufactured and tested on the micro-and macro-scale, ranging from medicalin vivomicrobots, microfluidics to macro prototypes. We present a collection of modeling theories, control principles, and fabrication methods for flagellated and ciliary robots.

Erythromycin Formulations—A Journey to Advanced Drug Delivery

Erythromycin (ERY) is a macrolide compound with a broad antimicrobial spectrum which is currently being used to treat a large number of bacterial infections affecting the skin, respiratory tract, intestines, bones and other systems, proving great value from a clinical point of view. It became popular immediately after its discovery in 1952, due to its therapeutic effect against pathogens resistant to other drugs. Despite this major advantage, ERY exhibits several drawbacks, raising serious clinical challenges. Among them, the very low solubility in water and instability under acidic conditions cause a limited efficacy and bioavailability. Apart from this, higher doses promote drug resistance and undesirable effects. In order to overcome these disadvantages, during the past decades, a large variety of ERY formulations, including nanoparticles, have emerged. Despite the interest in ERY-(nano)formulations, a review on them is lacking. Therefore, this work was aimed at reviewing all efforts made to encapsulate ERY in formulations of various chemical compositions, sizes and morphologies. In addition, their preparation/synthesis, physico-chemical properties and performances were carefully analysed. Limitations of these studies, particularly the quantification of ERY, are discussed as well.

Hybrid Inorganic-Organic Visible-Light-Driven Microrobots Based on Donor-Acceptor Organic Polymer for Degradation of Toxic Psychoactive Substances

Light-driven microrobots based on organic semiconductors have received tremendous attention in the past few years due to their unique properties, such as ease of reactivity tunability, band-gap modulation, and low cost. However, their fabrication with defined morphologies is a very challenging task that results in amorphous microrobots with poor motion efficiencies. Herein, we present hybrid inorganic-organic photoactive microrobots with a tubular shape and based on the combination of a mesoporous silica template with an active polymer containing thiophene and triazine units (named as Tz-Th microrobots). Owing to their well-defined tubular structure, such Tz-Th microrobots showed efficient directional motion under fuel-free conditions. Depending on the accumulation of the polymer coating, these microdevices also exhibited stand-up and rotation motion. As a proof-of-concept, we use these hybrid microrobots for the capture and degradation of toxic psychoactive drugs commonly found in wastewater effluents such as methamphetamine derivatives. We found that the microrobots were able to decompose the drug into small organic fragments after 20 min of visible light irradiation, reaching total intermediates removal after 2 h. Therefore, this approach represents a versatile and low-cost strategy to fabricate structured organic microrobots with efficient directional motion by using inorganic materials as the robot chassis, thereby maintaining the superior photocatalytic performance usually associated with such organic polymers.

Autonomously Propelled Colloids for Penetration and Payload Delivery in Complex Extracellular Matrices

For effective treatment of diseases such as cancer or fibrosis, it is essential to deliver therapeutic agents such as drugs to the diseased tissue, but these diseased sites are surrounded by a dense network of fibers, cells, and proteins known as the extracellular matrix (ECM). The ECM forms a barrier between the diseased cells and blood circulation, the main route of administration of most drug delivery nanoparticles. Hence, a stiff ECM impedes drug delivery by limiting the transport of drugs to the diseased tissue. The use of self-propelled particles (SPPs) that can move in a directional manner with the application of physical or chemical forces can help in increasing the drug delivery efficiency. Here, we provide a comprehensive look at the current ECM models in use to mimic the in vivo diseased states, the different types of SPPs that have been experimentally tested in these models, and suggest directions for future research toward clinical translation of SPPs in diverse biomedical settings.

Peroxidase driven micromotors for dynamic bioremediation

Phenolics are size products present in tons concentrations in industrial wastewater that can cause adverse health effects when released in the environment. As such, there is a growing interest in the development of efficient strategies for the removal of phenolic compounds from polluted water. Herein we describe the use of poly(3,4-ethylenedioxythiophene) (PEDOT)-Au/peroxidase micromotors as dynamic biocatalytic platforms for the removal of model phenolics (phenol, bisphenol A, guaiacol, pyrogallol and catechol). Micromotors are synthetized by using a simplified template electrodeposition protocol followed by covalent enzyme immobilization in the inner Au layer. Kinetic parameters revealed that enzyme immobilization in the inner micromotor layer increased over 2-fold the enzymatic activity, along with increasing operational pH and thermal stabilities. The micromotors can propel at speed of up to 60 µm/s, generating an enhanced fluid mixing that results in removal efficiencies of up to 60% as compared with the 27% removal when using free peroxidase under the same conditions. In addition, excellent activities of almost 100% were obtained within ten cycles of removal using the micromotors. This newly developed bioremediation strategy holds considerable promise in for its application in large scale water treatment systems and many relevant environmental processes.

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.

Active, Yet Little Mobility: Asymmetric Decomposition of H2O2 Is Not Sufficient in Propelling Catalytic Micromotors

A popular principle in designing chemical micromachines is to take advantage of asymmetric chemical reactions such as the catalytic decomposition of H₂O₂. Contrary to intuition, we use Janus micromotors half-coated with platinum (Pt) or catalase as an example to show that this ingredient is not sufficient in powering a micromotor into self-propulsion. In particular, by annealing a thin Pt film on a SiO₂ microsphere, the resulting microsphere half-decorated with discrete Pt nanoparticles swims ∼80% more slowly than its unannealed counterpart in H₂O₂, even though they both catalytically produce comparable amounts of oxygen. Similarly, SiO₂ microspheres half-functionalized with the enzyme catalase show negligible self-propulsion despite high catalytic activity toward decomposing H₂O₂. In addition to highlighting how surface morphology of a catalytic cap enables/disables a chemical micromotor, this study offers a refreshed perspective in understanding how chemistry powers nano- and microscopic objects (or not): our results are consistent with a self-electrophoresis mechanism that emphasizes the electrochemical decomposition of H₂O₂ over nonelectrochemical pathways. More broadly, our finding is a critical piece of the puzzle in understanding and designing nano- and micromachines, in developing capable model systems of active colloids, and in relating enzymes to active matter.

Flying Squirrel-Inspired Motion Control of a Light-Deformed Pt-PAzoMA Micromotor through Drag Force Manipulation

Micromotors require stable and precise motion control for complex tasks such as microsurgery, drug delivery in vivo, or environmental monitoring ex vivo. However, a continuous control signal is needed for micromotors to achieve motion control during their whole journey, which hinders their application in areas where external control stimuli are limited or unavailable. Fortunately, nature suggests an excellent solution that flying squirrel exhibits motion tuning capability by deforming itself when jumping off a tall tree. Inspired by this, we propose a Pt-PAzoMA Janus micromotor that precisely changes its shape (from a spherical shape to an elliptical shape) under a brief light signal (450 nm) and maintains this deformation until next signal reception. The deformed elliptical micromotor performs relatively low-speed motion compared to the spherical one, which is further confirmed by massive simulation results. In addition, by investigating motion behavior experimentally and theoretically, it is proved that the motion modulation is caused by the drag force changing brought from the deformation. This method represents a different route to regulate the motion of micromotors without a continuous signal, which is useful in application scenarios where the environmental control signal is inaccessible/limited or long-time operation with minimum energy input is required to maintain motion manipulation. With further function modification, this kind of shape-changing micromotor has potential in optimizing drug diffusion efficiency by speed altering and long-term monitoring at the diseased area by confining the active range of the micromotor in the targeted area through deformation.

Chemokinesis-driven accumulation of active colloids in low-mobility regions of fuel gradients

Many motile cells exhibit migratory behaviors, such as chemotaxis (motion up or down a chemical gradient) or chemokinesis (dependence of speed on chemical concentration), which enable them to carry out vital functions including immune response, egg fertilization, and predator evasion. These have inspired researchers to develop self-propelled colloidal analogues to biological microswimmers, known as active colloids, that perform similar feats. Here, we study the behavior of half-platinum half-gold (Pt/Au) self-propelled rods in antiparallel gradients of hydrogen peroxide fuel and salt, which tend to increase and decrease the rods' speed, respectively. Brownian Dynamics simulations, a Fokker-Planck theoretical model, and experiments demonstrate that, at steady state, the rods accumulate in low-speed (salt-rich, peroxide-poor) regions not because of chemotaxis, but because of chemokinesis. Chemokinesis is distinct from chemotaxis in that no directional sensing or reorientation capabilities are required. The agreement between simulations, model, and experiments bolsters the role of chemokinesis in this system. This work suggests a novel strategy of exploiting chemokinesis to effect accumulation of motile colloids in desired areas.

The Energy Conversion behind Micro-and Nanomotors

Inspired by the autonomously moving organisms in nature, artificially synthesized micro-nano-scale power devices, also called micro-and nanomotors, are proposed. These micro-and nanomotors that can self-propel have been used for biological sensing, environmental remediation, and targeted drug transportation. In this article, we will systematically overview the conversion of chemical energy or other forms of energy in the external environment (such as electrical energy, light energy, magnetic energy, and ultrasound) into kinetic mechanical energy by micro-and nanomotors. The development and progress of these energy conversion mechanisms in the past ten years are reviewed, and the broad application prospects of micro-and nanomotors in energy conversion are provided.

Parameters Optimization of Catalytic Tubular Nanomembrane-Based Oxygen Microbubble Generator

A controllable generation of oxygen gas during the decomposition of hydrogen peroxide by the microreactors made of tubular catalytic nanomembranes has recently attracted considerable attention. Catalytic microtubes play simultaneous roles of the oxygen bubble producing microreactors and oxygen bubble-driven micropumps. An autonomous pumping of peroxide fuel takes place through the microtubes by the recoiling microbubbles. Due to optimal reaction–diffusion processes, gas supersaturation, leading to favorable bubble nucleation conditions, strain-engineered catalytic microtubes with longer length produce oxygen microbubbles at concentrations of hydrogen peroxide in approximately ×1000 lower in comparison to shorter tubes. Dynamic regimes of tubular nanomembrane-based oxygen microbubble generators reveal that this depends on microtubes’ aspect ratio, hydrogen peroxide fuel concentration and fuel compositions. Different dynamic regimes exist, which produce specific bubble frequencies, bubble size and various amounts of oxygen. In this study, the rolled-up Ti/Cr/Pd microtubes integrated on silicon substrate are used to study oxygen evolution in different concentrations of hydrogen peroxide and surfactants. Addition of Sodium dodecyl sulfate (SDS) surfactants leads to a decrease of bubble diameter and an increase of frequencies of bubble recoil. Moreover, an increase of temperature (from 10 to 35 °C) leads to higher frequencies of oxygen bubbles and larger total volumes of produced oxygen.

Mg-Based Micromotors with Motion Responsive to Dual Stimuli

Mg-based micromotors have emerged as an extremely attractive artificial micro/nanodevice, but suffered from uncontrollable propulsion and limited motion lifetime, restricting the fulfillment of complex tasks. Here, we have demonstrated Mg-based micromotors composed of Mg microspheres asymmetrically coated with Pt and temperature-sensitive poly(N-isopropylacrylamide) (PNIPAM) hydrogel layers in sequence. They can implement different motion behaviors stemming from the driving mechanism transformation when encountering catalyzed substrates such as H₂O₂ and respond to both H₂O₂ concentration and temperature in aqueous environment. The as-constructed Mg-based micromotors are self-propelled by Pt-catalyzed H₂O₂ decomposition following the self-consuming Mg-H₂O reaction. In this case, they could further generate bilateral bubbles and thus demonstrate unique self-limitation motion like hovering when the phase transformation of PNIPAM is triggered by decreasing temperature or when the H₂O₂ concentration after permeating across the PNIPAM hydrogel layer is high enough to facilitate bubble nucleation. Our work for the first time provides a stimuli-induced "hovering" strategy for self-propelled micromotors, which endows Mg-based micromotors with an intelligent response to the surroundings besides the significant extension of their motion lifetime.

Visible-Light-Driven Single-Component BiVO4 Micromotors with the Autonomous Ability for Capturing Microorganisms

Light-driven micro/nanomotors represent the next generation of automotive devices that can be easily actuated and controlled by using an external light source. As the field evolves, there is a need for developing more sophisticated micromachines that can fulfill diverse tasks in complex environments. Herein, we introduce single-component BiVO₄ micromotors with well-defined micro/nanostructures that can swim both individually and as collectively assembled entities under visible-light irradiation. These devices can perform cargo loading and transport of passive particles as well as living microorganisms without any surface functionalization. Interestingly, after photoactivation, the BiVO₄ micromotors exhibited an ability to seek and adhere to yeast cell walls, with the possibility to control their attachment/release by switching the light on/off, respectively. Taking advantage of the selective motor/fungal cells attachment, the fungicidal activity of BiVO₄ micromotors under visible illumination was also demonstrated. The presented star-shaped BiVO₄ micromotors, obtained by a hydrothermal synthesis, contribute to the potential large-scale fabrication of light-powered micromotors. Moreover, these multifunctional single-component micromachines with controlled self-propulsion, collective behavior, cargo transportation, and photocatalytic activity capabilities hold promising applications in sensing, biohybrids assembly, cargo delivery, and microbiological water pollution remediation.

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.

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.

A Review of Fast Bubble-Driven Micromotors Powered by Biocompatible Fuel: Low-Concentration Fuel, Bioactive Fluid and Enzyme

Micromotors are extensively applied in various fields, including cell separation, drug delivery and environmental protection. Micromotors with high speed and good biocompatibility are highly desirable. Bubble-driven micromotors, propelled by the recoil effect of bubbles ejection, show good performance of motility. The toxicity of concentrated hydrogen peroxide hampers their practical applications in many fields, especially biomedical ones. In this paper, the latest progress was reviewed in terms of constructing fast, bubble-driven micromotors which use biocompatible fuels, including low-concentration fuels, bioactive fluids, and enzymes. The geometry of spherical and tubular micromotors could be optimized to acquire good motility using a low-concentration fuel. Moreover, magnesium- and aluminum-incorporated micromotors move rapidly in water if the passivation layer is cleared in the reaction process. Metal micromotors demonstrate perfect motility in native acid without any external chemical fuel. Several kinds of enzymes, including catalase, glucose oxidase, and ureases were investigated to serve as an alternative to conventional catalysts. They can propel micromotors in dilute peroxide or in the absence of peroxide.

Buoyant force-induced continuous floating and sinking of Janus micromotors

A novel bubble-induced ultrafast floating and sinking of micromotors based on the difference between buoyant force and gravity is proposed. Asymmetric micromotors were prepared by modification with Au and Pt layers for the two faces of glassy carbon beads (GCBs) by the bipolar electrodeposition technique. After the accumulation of enough oxygen bubbles by the decomposition of H2O2 at the Pt layer, the upward net force acting on the micromotor drove its movement to the air/solution interface. In order to reverse the direction of net force for the sinking of the micromotors, sodium dodecyl sulfate (SDS) was added into the fuel solution, which could facilitate the release of bubbles and decrease the diameter of the bubbles. However, the lifetime of the bubbles was increased significantly. After the addition of a small amount of salt, the lifetime of the bubbles was obviously reduced. As a consequence, the breakup of bubbles on the micromotor changed the direction of the net force from up to down which pulled the micromotor down to the bottom of the solution. The velocity of the micromotor was dependent on the net force exerted on the micromotor, leading to an ultrafast motion of the micromotor. It still reached 1.2 cm s-1 after 3 h. Moreover, the simple asymmetric deposition technique showed great promise for the further application of the micromotors in bioanalysis and environmental remediation.

Multifunctional and self-propelled spherical Janus nano/micromotors: recent advances

Recent progress in autonomous self-propelled multifunctional Janus nano/micromotors, which are able to convert chemical or light energy into mechanical motion, is presented. This technology of moving micro- and nanodevices is at the forefront of materials research and is a promising and growing technology with the possibility of using these motors in both biomedical and environmental applications. The development of novel multifunctional Janus motors together with their motion mechanisms is discussed. Different preparation and synthesis routes are compared. The effects of the size, interfacial structures and porosity on the directional motion and the speed of Janus micromotors are discussed. For light-derived Janus micromotors, newly developed techniques that are able to observe directly the interfaces' charge distribution on a nanometer scale are presented in order to clarify the underlying electrophoresis motion mechanism. This review aims to encourage further research in the field of micromotors using new and facile methodologies for obtaining novel Janus motors with enhanced motion and activity.

Geometry Design, Principles and Assembly of Micromotors

Discovery of bio-inspired, self-propelled and externally-powered nano-/micro-motors, rotors and engines (micromachines) is considered a potentially revolutionary paradigm in nanoscience. Nature knows how to combine different elements together in a fluidic state for intelligent design of nano-/micro-machines, which operate by pumping, stirring, and diffusion of their internal components. Taking inspirations from nature, scientists endeavor to develop the best materials, geometries, and conditions for self-propelled motion, and to better understand their mechanisms of motion and interactions. Today, microfluidic technology offers considerable advantages for the next generation of biomimetic particles, droplets and capsules. This review summarizes recent achievements in the field of nano-/micromotors, and methods of their external control and collective behaviors, which may stimulate new ideas for a broad range of applications.

Carbon dioxide bubble-propelled microengines in carbonated water and beverages

We demonstrate a new type of gaseous fuel for rolled-up tubular Ti/Cr microengine powered by carbon dioxide microbubbles in carbonated water and brewed beverages.