Catalytically propelled micro-/nanomotors: how fast can they move?

Metal-Plastic Hybrid Additive Manufacturing to Realize Small-Scale Self-Propelled Catalytic Engines

Microengines driven by catalytic decomposition of a fuel have been an interesting research area recently due to their diverse applications, such as environmental monitoring and drug delivery. Literature reports a number of studies on this topic where researchers have made various attempts to manufacture such microengines. Some such methods are deposition of catalytic metal layers on sacrificial photoresists, electrochemical deposition of metal layers on polymeric structures, or 3D printing of structures followed by multi-step loading of structures with catalysts. These methods, even though proven to be effective, are tedious, time-consuming, and expensive. To address these issues, herein we report a 3D printing technique to realize microengines in a simple, rapid, and inexpensive single-step process. The printing of various shapes of microengines is achieved using digital light processing printing of a catalyst resin, where Pd(II) acts as a catalyst resin. The proposed integrated molding process can achieve cost-effective preparation of high-efficiency microengines. We demonstrate the locomotion of these microengines in 30% (w/w) H2O2 through the decomposition of H2O2 to generate oxygen to facilitate the self-propelled locomotion. The study characterizes the microengine based on several factors, such as the role of H2O2, Pd, shape, and design of the microengine, to get a full picture of the self-locomotion of microengines. The study shows that the developed method is feasible to manufacture microengines in a simple, rapid, and inexpensive manner to be suitable for numerous applications such as environmental monitoring, remediation, drug delivery, diagnosis, etc., through the modification of the catalyst resin and fuel, as desired.

In the View of Electrons Transfer and Energy Conversion: The Antimicrobial Activity and Cytotoxicity of Metal-Based Nanomaterials and Their Applications

The global pandemic and excessive use of antibiotics have raised concerns about environmental health, and efforts are being made to develop alternative bactericidal agents for disinfection. Metal-based nanomaterials and their derivatives have emerged as promising candidates for antibacterial agents due to their broad-spectrum antibacterial activity, environmental friendliness, and excellent biocompatibility. However, the reported antibacterial mechanisms of these materials are complex and lack a comprehensive understanding from a coherent perspective. To address this issue, a new perspective is proposed in this review to demonstrate the toxic mechanisms and antibacterial activities of metal-based nanomaterials in terms of energy conversion and electron transfer. First, the antimicrobial mechanisms of different metal-based nanomaterials are discussed, and advanced research progresses are summarized. Then, the biological intelligence applications of these materials, such as biomedical implants, stimuli-responsive electronic devices, and biological monitoring, are concluded based on trappable electrical signals from electron transfer. Finally, current improvement strategies, future challenges, and possible resolutions are outlined to provide new insights into understanding the antimicrobial behaviors of metal-based materials and offer valuable inspiration and instructional suggestions for building future intelligent environmental health.

Propulsion, deformation, and confinement response of hollow nanocellulose millimotors

HYPOTHESIS

Micromotor and nanomotor particles are typically made using dense solid particles that can sediment or be trapped in confined flow environments. Creation of much larger motors should be possible if a very low-density system is used with sufficient strength to carry liquid and still experience propulsive motion. Light, dense millimotors should also be able to deform more than dense solid ones in constrictions.

EXPERIMENTS

Millimotors are created from permeable capsules of bacterial cellulose that are coated with catalse-containing metal-organic frameworks, enabling reactive propulsion in aqueous hydrogen peroxide. The motion of the motors is quantified using particle tracking and the deformation is measured using microcapillary compression and flow through confined channels.

FINDINGS

Two different propulsion mechanisms are dominant depending on the motor surface chemistry: oxygen bubbles are expelled from hydrophilic millimotors, driving motion via recoil force and buoyancy. Hydrophobic millimotors remain attached to growing bubbles and move by buoyancy alone. Despite their large size, the low-density capsules compress to pass through contractions that would impede and be blocked by solid motors. The sparse structure but relatively large size of the motors enables them to transport significant volumes of liquid using minimal solid mass as a motor support structure.

Quantitative Analysis of Drag Force for Task-Specific Micromachine at Low Reynolds Numbers

Micromotors have spread widely in order to meet the needs of new applications, including cell operation, drug delivery, biosensing, precise surgery and environmental decontamination, due to their small size, low energy consumption and large propelling power, especially the newly designed multifunctional micromotors that combine many extra shape features in one device. Features such as rod-like receptors, dendritic biosensors and ball-like catalyzing enzymes are added to the outer surface of the tubular micromotor during fabrication to perform their special mission. However, the structural optimization of motion performance is still unclear. The main factor restricting the motion performance of the micromotors is the drag forces. The complex geometry of a micromotor makes its dynamic behavior more complicated in a fluid environment. This study aimed to design the optimum structure of tubular micromotors with minimum drag forces and obtain the magnitude of drag forces considering both the internal and external fluids of the micromotors. By using the computational fluid dynamics software Fluent 18.0 (ANSYS), the drag force and the drag coefficient of different conical micromotors were calculated. Moreover, the influence of the Reynolds numbers Re, the semi-cone angle δ and the ratios ξ and η on the drag coefficient was analyzed. The results show the drag force monotonically increased with Reynolds numbers Re and the ratio η. The extreme point of the drag curve is reached when the semi-cone angle δ is 8° and the ratio ξ is 3.846. This work provides theoretical support and guidance for optimizing the design and development of conical micromotors.

Micro/nano-machines for spilled-oil cleanup and recovery: A review

High-efficiency, safe and economically viable nano-engineered platforms for oil spill cleanup and recovery are of great importance. This review takes account of the concept of nanomotors and micromotors and their most advancements in use for oil spill treatment. The fundamental facets of artificial micro- and nano-machines/nanobots/nanomotors (MNMs) are first documented, followed by the most recent influencing developments in chemical engineering approaches toward their specific utilizations. The surface chemistry of these MNMs, their behaviors in different water matrices and their roles in the removal of oil are examined, revealing great rooms for improvement. The strategies for surface and structural modification of these tiny machines toward enhancing their reactivity in the removal of oil and coupled tasking are discussed in details, highlighting the significance of fit-for-duty design and tailored fabrication. The engineering limitations and practical implementation barriers of this emerging technology and how it can be overcome are also considered. Finally, some engineering boundaries and perspectives of this fast-evolving field are proposed at the end.

Switching Propulsion Mechanisms of Tubular Catalytic Micromotors

Different propulsion mechanisms have been suggested for describing the motion of a variety of chemical micromotors, which have attracted great attention in the last decades due to their high efficiency and thrust force, enabling several applications in the fields of environmental remediation and biomedicine. Bubble-recoil based motion, in particular, has been modeled by three different phenomena: capillary forces, bubble growth, and bubble expulsion. However, these models have been suggested independently based on a single influencing factor (i.e., viscosity), limiting the understanding of the overall micromotor performance. Therefore, the combined effect of medium viscosity, surface tension, and fuel concentration is analyzed on the micromotor swimming ability, and the dominant propulsion mechanisms that describe its motion more accurately are identified. Using statistically relevant experimental data, a holistic theoretical model is proposed for bubble-propelled tubular catalytic micromotors that includes all three above-mentioned phenomena and provides deeper insights into their propulsion physics toward optimized geometries and experimental conditions.

Discretized Motion of Surface Walker under a Nonuniform AC Magnetic Field

The motion of peanut-shaped magnetic microrods (PSMRs) with different magnetic moment (M_s) orientations φ_M under a nonuniform AC magnetic field has been investigated systematically. When gradually changing φ_M from 90° (perpendicular to the long axis of the PSMR) to 0°, the motion of the PSMR evolves from rolling to precession, then to tumbling. Systematic investigations on the translational velocity v_p versus the magnitude of the applied magnetic field B and the angular velocity ω_B show that the overall motion of the PSMRs can be divided into four different zones: Brownian motion zone, synchronized zone, asynchronized zone, and oscillation zone. The v_p-ω_B relationship can be rescaled by a critical frequency ω_c, which is determined by M_s, B, and a hydrodynamic term. An intrinsic quality factor q_m for the translational motion of a magnetically driven micro-/nanomotor is defined and is found to range from 0.73 to 13.65 T^-1 in the literature, while the Fe PSMRs in the current work give the highest q_m (= 25.48 T^-1). High speed movies reveal that both the tumbling and precession motions of the PSMRs have a discretized nature. At the instances when the magnetic field changes direction, the PSMR performs an instantaneous rotation and the strong hydrodynamic wall effect would impose a driving force to move the PSMR translationally, and about more than 60% of the time, the PSMR neither rotates nor moves translationally. Based on this discretized motion nature, an analytic expression for q_m is found to be determined by the shape of the surface walker, the hydrodynamics near a wall, and the magnetic properties of the surface walker. This work can help us to better understand the motion of magnetic surface walkers and gain insight into designing better micro-/nanomotors.

Light-Responsive Inorganic Biomaterials for Biomedical Applications

Light-responsive inorganic biomaterials are an emerging class of materials used for developing noninvasive, noncontact, precise, and controllable medical devices in a wide range of biomedical applications, including photothermal therapy, photodynamic therapy, drug delivery, and regenerative medicine. Herein, a range of biomaterials is discussed, including carbon-based nanomaterials, gold nanoparticles, graphite carbon nitride, transition metal dichalcogenides, and up-conversion nanoparticles that are used in the design of light-responsive medical devices. The importance of these light-responsive biomaterials is explored to design light-guided nanovehicle, modulate cellular behavior, as well as regulate extracellular microenvironments. Additionally, future perspectives on the clinical use of light-responsive biomaterials are highlighted.

Ballistic supercavitating nanoparticles driven by single Gaussian beam optical pushing and pulling forces

Directed high-speed motion of nanoscale objects in fluids can have a wide range of applications like molecular machinery, nano robotics, and material assembly. Here, we report ballistic plasmonic Au nanoparticle (NP) swimmers with unprecedented speeds (~336,000 μm s⁻¹) realized by not only optical pushing but also pulling forces from a single Gaussian laser beam. Both the optical pulling and high speeds are made possible by a unique NP-laser interaction. The Au NP excited by the laser at the surface plasmon resonance peak can generate a nanoscale bubble, which can encapsulate the NP (i.e., supercavitation) to create a virtually frictionless environment for it to move, like the Leidenfrost effect. Certain NP-in-bubble configurations can lead to the optical pulling of NP against the photon stream. The demonstrated ultra-fast, light-driven NP movement may benefit a wide range of nano- and bio-applications and provide new insights to the field of optical pulling force.

Control of small particles in fluid can have a range of applications. The authors explore a phenomenon that allows an extremely low friction environment around a nanoparticle, demonstrating high-speed nanoparticles driven by optical forces in both directions of an optical beam.

Active matter therapeutics

Nanotherapies based on micelles, liposomes, polymersomes, nanocapsules, magnetic nanoparticles, and noble metal nanoparticles have been at the forefront of drug delivery in the past few decades. Some of these nanopharmaceuticals have been commercially applied to treat a wide range of diseases, from dry eye syndrome to cancer. However, the majority involve particles that are passive, meaning that they do not change shape, and they lack motility; the static features can limit their therapeutic efficacy. In this review, we take a critical look at an emerging field that seeks to utilize active matter for therapeutics. In this context, active matter can be broadly referred to as micro or nanosized constructs that energetically react with their environment or external fields and translate, rotate, vibrate or change shape. Essentially, the recent literature suggests that such particles could significantly augment present-day drug delivery, by enhancing transport and increasing permeability across anatomical barriers by transporting drugs within solid tumor microenvironments or disrupting cardiovascular plaque. We discuss examples of such particles and link the transport and permeability properties of active matter to potential therapeutic applications in the context of two major diseases, namely cancer and heart disease. We also discuss potential challenges, opportunities, and translational hurdles.

The effect of size-scale on the kinematics of elastic energy release

Elastically-driven motion has been used as a strategy to achieve high speeds in small organisms and engineered micro-robotic devices. We examine the size-scaling relations determining the limit of elastic energy release from elastomer bands that efficiently cycle mechanical energy with minimal loss. The maximum center-of-mass velocity of the elastomer bands was found to be size-scale independent, while smaller bands demonstrated larger accelerations and shorter durations of elastic energy release. Scaling relationships determined from these measurements are consistent with the performance of small organisms and engineered devices which utilize elastic elements to power motion.

Radioactive Uranium Preconcentration via Self-Propelled Autonomous Microrobots Based on Metal-Organic Frameworks

Self-propelled micromachines have recently attracted attention for environmental remediation, yet their use for radioactive waste management has not been addressed. Engineered micromotors that are able to combine highly adsorptive capabilities together with fast autonomous motion in liquid media are promising tools for the removal of nuclear waste, which is one of the most difficult types to manage. Herein, we fabricate self-propelled micromotors based on metal-organic frameworks (MOFs) via template-based interfacial synthesis and show their potential for efficient removal of radioactive uranium. A crucial challenge of the MOF-based motors is their stability in the presence of fuel (hydrogen peroxide) and acidic media. We have ensured their structural stability by Fe doping of zeolitic imidazolate framework-8 (ZIF-8). The implementation of magnetic ferroferric oxide nanoparticles (Fe₃O₄ NPs) and catalytic platinum nanoparticles (Pt NPs) results in the magnetically responsive and bubble-propelled micromotors. In the presence of 5 wt % H₂O₂, these micromotors are propelled at a high speed of ca. 860 ± 230 μm·s^-1 (i.e., >60 body lengths per second), which is significantly faster than that of other microrod-based motors in the literature. These micromotors demonstrate a highly efficient removal of uranium (96%) from aqueous solution within 1 h, with the subsequent recovery under magnetic control, as well as stable recycling ability and high selectivity. Such self-propelled magnetically recoverable micromotors could find a role in the management and remediation of radioactive waste.

Biocompatibility of artificial micro/nanomotors for use in biomedicine

The advent of micro/nanomotors (MNMs) has shed light on the innovation of active biomedical systems or devices that might bring revolutionary solutions to traditional biomedical strategies. In spite of development beyond expectation over the last decade with a fair number of proof-of-concept demonstrations, the in vivo practical application of MNMs for clinical use is still in its infancy. The biocompatibility of MNMs is the first consideration before realizing practicality, taking into account the complicated interactions between the self-propelled MNMs and biological systems. Therefore, in this review, we focused on the biocompatibility of MNMs with regard to the fabrication materials and propulsion mechanisms by means of in-depth discussions on the advantages and limitations of MNMs for operating under physiological conditions. The future prospective and suggestions on the development of MNMs toward practical biomedical applications will also be proposed.

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.

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.

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.

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.

Bilayer Tubular Micromotors for Simultaneous Environmental Monitoring and Remediation

There are two main aspects of environmental governance including monitoring and remediation, both of which are essential for environmental protection. Self-propelled micro/nanomotors (MNM) have shown promising potential for achieving on-demand tasks in environmental field, including environmental sensing and pollutant removal or degradation. However, most of the current MNM used in environmental protection can hardly accomplish the two major tasks of both monitoring and pollutant degradation. Hereby, we present a bubble-propelled mesoporous silica-coated titania (TiO₂@mSiO₂) bilayer tubular micromotor with platinum (Pt) and magnetic Fe₃O₄ nanoparticles modified on their inner walls. The outer mesoporous silica (mSiO₂) layer can effectively adsorb and collect the pollutants, and the adsorption capacity of the TiO₂@mSiO₂ tube is about 3 times higher than that of the TiO₂ tube due to the presence of mSiO₂ shell. By magnetic manipulation, the micromotors can be recovered to release the collected pollutant for precise analysis of the composition of the pollutants, such us pollutant molecule identification by surface-enhanced Raman scattering. The active motion and photocatalytic TiO₂ inner layer of the micromotors can greatly enhance the degradation rate of the model pollutant rhodamine 6G (R6G). Our results show that within 30 min, up to 98% of R6G can be degraded by the motors. The successful demonstration of the TiO₂@mSiO₂ bilayer tubular motors for simultaneous environmental monitoring and pollutant degradation paves the way for future development of active and intelligent micro/nanorobots for advanced environmental governance.

A Dynamic Model of Drag Force for Catalytic Micromotors Based on Navier⁻Stokes Equations

In past decades, considerable advances have been achieved in micro and nanomotors. Particular attention has been given to self-propelled catalytic micromotors, which have been widely used in cell separation, drug delivery, microsurgery, lithography and environmental remediation. Fast moving, long life micromotors appear regularly, however it seems there are no solutions yet that thoroughly clarify the hydrodynamic behavior of catalytic micromotors moving in fluid. Dynamic behavior of this kind of micromotors is mainly determined by the driving force and drag force acting on the micromotors. Based on the hydromechanics theory, a hydrodynamic model is established to predict the drag force for a conical micromotor immersed in the flow field. By using the computational fluid dynamics software Fluent 18.0 (ANSYS), the drag force and the drag coefficient of different conical micromotors are calculated. A mathematical model was proposed to describe the relationship among Reynolds numbers Re, the ratio λ, the semi-cone angle δ and the drag coefficient Cd of the micromotors. This work provides theoretical support and reference for optimizing the design and development of conical micromotors.

Examination of the Statistical Effects Associated with Tracking Propulsive Particles

Particle tracking of active colloidal particles can be used to compute mean-squared displacements that are fit to extract properties of the particles including the propulsive speed. Statistical errors in the mean-squared displacement leads to errors in the extracted properties especially for more weakly propelling particles. Brownian dynamics simulations in which the particle parameters are prescribed were used to examine the statistics of tracking self-propelling objects. It was found that the manner in which tracking data is analyzed has a profound impact on the precision and accuracy of measurements. To properly extract particle parameters, it was necessary to apply a nonlinear fit of the mean-squared displacement over a time region that includes transition behavior from ballistic to diffusive. The dependence of the statistics on the number of particles tracked and the length of movies was examined, showing how and why weakly propelling particles are difficult to analyze.

Micro/nanomotors towards in vivo application: cell, tissue and biofluid

Inspired by highly efficient natural motors, synthetic micro/nanomotors are self-propelled machines capable of converting the supplied fuel into mechanical motion. A significant advance has been made in the construction of diverse motors over the last decade. These synthetic motor systems, with rapid transporting and efficient cargo towing abilities, are expected to open up new horizons for various applications. Utilizing emergent motor platforms for in vivo applications is one important aspect receiving growing interest as conventional therapeutic methodology still remains limited for cancer, heart, or vasculature diseases. In this review we will highlight the recent efforts towards realistic in vivo application of various motor systems. With ever booming research enthusiasm in this field and increasing multidisciplinary cooperation, micro/nanomotors with integrated multifunctionality and selectivity are on their way to revolutionize clinical practice.

Isolation of HL-60 cancer cells from the human serum sample using MnO2-PEI/Ni/Au/aptamer as a novel nanomotor and electrochemical determination of thereof by aptamer/gold nanoparticles-poly(3,4-ethylene dioxythiophene) modified GC electrode

Herein, aptamer-modified self-propelled nanomotors were used for transportation of human promyelocytic leukemia cells (HL-60) from a human serum sample. For this purpose, the fabricated manganese oxide nanosheets-polyethyleneimine decorated with nickel/gold nanoparticles (MnO₂-PEI/Ni/Au) as nanomotors were added to a vial containing thiolated aptamer KH1C12 solution as a capture aptamer to attach to the gold nanoparticles on the surface of nanomotors covalently. The aptamer-modified self-propelled nanomotors (aptamer_KH1C12/nanomotors) were then separated by placing the vial in a magnetic stand. The aptamer-modified self-propelled nanomotors were rinsed three times with water to remove the non-attached aptamers. Then, the resulting aptamer_KH1C12/nanomotors were applied for the on-the-fly" transporting of HL-60 cancer cell from a human serum sample. To release of the captured HL-60 cancer cells, the complementary nucleotide sequences of KH1C12 aptamer solution (releasing aptamer) that has a with capture aptamer was added to phosphate buffer solution (1 M, pH 7.4) containing HL-60/aptamer_KH1C12/nanomotors. Because of the high affinity of capture aptamer to complementary nucleotide sequences of aptamer_KH1C12, the HL-60 cancer cells released on the surface of aptamer_KH1C12/nanomotors into the solution. The second goal of the present work was determining the concentration of HL-60 cancer cell in the human serum samples. The electrochemical impedance spectroscopy technique (EIS) was used for the determination of HL-60 cancer cell. The concentration of separated cancer cell was determined by aptamer/gold nanoparticles-poly(3,4-ethylene dioxythiophene) modified GC electrode (GC/PEDOT-Au_nano/aptamer _KH1C12). The proposed aptasensor exhibited a good response to the concentration of HL-60 cancer cells in the range of 2.5 × 10¹ to 5 × 10⁵ cells mL^-1 with a low limit of detection of 250 cells mL^-1.

Light-driven micro/nanomotors: from fundamentals to applications

Light, as an external stimulus, is capable of driving the motion of micro/nanomotors (MNMs) with the advantages of reversible, wireless and remote manoeuvre on demand with excellent spatial and temporal resolution. This review focuses on the state-of-the-art light-driven MNMs, which are able to move in liquids or on a substrate surface by converting light energy into mechanical work. The general design strategies for constructing asymmetric fields around light-driven MNMs to propel themselves are introduced as well as the photoactive materials for light-driven MNMs, including photocatalytic materials, photothermal materials and photochromic materials. Then, the propulsion mechanisms and motion behaviors of the so far developed light-driven MNMs are illustrated in detail involving light-induced phoretic propulsion, bubble recoil and interfacial tension gradient, followed by recent progress in the light-driven movement of liquid crystalline elastomers based on light-induced deformation. An outlook is further presented on the future development of light-driven MNMs towards overcoming key challenges after summarizing the potential applications in biomedical, environmental and micro/nanoengineering fields.

ZnO-based microrockets with light-enhanced propulsion

Improving the propulsion of artificial micro-nanomotors represents an exciting nanotechnology challenge, especially considering their cargo delivery ability and fuel efficiency. In light of the excellent photocatalytic performance of zinc oxide (ZnO) and chemical catalytic properties of platinum (Pt), ZnO-Pt microrockets with light-enhanced propulsion have been developed by atomic layer deposition (ALD) technology. The velocity of such microrockets is dramatically doubled upon irradiation by 77 mW cm^-2 ultraviolet (UV) light in 10% H₂O₂ and is almost 3 times higher than the classic poly(3,4-ethylenedioxythiophene)-Pt microrockets (PEDOT-Pt microrockets) even in 6% H₂O₂ under the same UV light. In addition, such micromotors not only retain the standard approach to improve propulsion by varying the fuel concentration, but also demonstrate a simple way to enhance the movement velocity by adjusting the UV light intensity. High reversibility and controllable "weak/strong" propulsion can be easily achieved by switching the UV irradiation on or off. Finally, light-enhanced propulsion has been investigated by electrochemical measurements which further confirm the enhanced photocatalytic properties of ZnO and Pt. The successful demonstration of ZnO-based microrockets with excellent light-enhanced propulsion is significant for developing highly efficient synthetic micro-nanomotors which have strong delivery ability and economic fuel requirements for future practical applications in the micro-nanoscale world.

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

Micromotors, which can be moved at a micron scale, have special functions and can perform microscopic tasks. They have a wide range of applications in various fields with the advantages of small size and high efficiency. Both high speed and efficiency for micromotors are required in various conditions. However, the dynamical mechanism of bubble-driven micromotors movement is not clear, owing to various factors affecting the movement of micromotors. This paper reviews various factors acting on micromotor movement, and summarizes appropriate methods to improve the velocity and efficiency of bubble-driven micromotors, from a mechanical view. The dynamical factors that have significant influence on the hydrodynamic performance of micromotors could be divided into two categories: environment and geometry. Improving environment temperature and decreasing viscosity of fluid accelerate the velocity of motors. Under certain conditions, raising the concentration of hydrogen peroxide is applied. However, a high concentration of hydrogen peroxide is not applicable. In the environment of low concentration, changing the geometry of micromotors is an effective mean to improve the velocity of micromotors. Increasing semi-cone angle and reducing the ratio of length to radius for tubular and rod micromotors are propitious to increase the speed of micromotors. For Janus micromotors, reducing the mass by changing the shape into capsule and shell, and increasing the surface roughness, is applied. This review could provide references for improving the velocity and efficiency of micromotors.

A Viscosity-Based Model for Bubble-Propelled Catalytic Micromotors

Micromotors have shown significant potential for diverse future applications. However, a poor understanding of the propelling mechanism hampers its further applications. In this study, an accurate mechanical model of the micromotor has been proposed by considering the geometric asymmetry and fluid viscosity based on hydrodynamic principles. The results obtained from the proposed model are in a good agreement with the experimental results. The effects of the semi-cone angle on the micromotor are re-analyzed. Furthermore, other geometric parameters, like the length-radius aspect ratio, exert great impact on the velocity. It is also observed that micromotors travel much slower in highly viscous solutions and, hence, viscosity plays an important role.

Dye-Enhanced Self-Electrophoretic Propulsion of Light-Driven TiO2-Au Janus Micromotors

Light-driven synthetic micro-/nanomotors have attracted considerable attention in recent years due to their unique performances and potential applications. We herein demonstrate the dye-enhanced self-electrophoretic propulsion of light-driven TiO₂-Au Janus micromotors in aqueous dye solutions. Compared to the velocities of these micromotors in pure water, 1.7, 1.5, and 1.4 times accelerated motions were observed for them in aqueous solutions of methyl blue (10^-5 g L^-1), cresol red (10^-4 g L^-1), and methyl orange (10^-4 g L^-1), respectively. We determined that the micromotor speed changes depending on the type of dyes, due to variations in their photodegradation rates. In addition, following the deposition of a paramagnetic Ni layer between the Au and TiO₂ layers, the micromotor can be precisely navigated under an external magnetic field. Such magnetic micromotors not only facilitate the recycling of micromotors, but also allow reusability in the context of dye detection and degradation. In general, such photocatalytic micro-/nanomotors provide considerable potential for the rapid detection and "on-the-fly" degradation of dye pollutants in aqueous environments.

Influence of Asymmetry and Driving Forces on the Propulsion of Bubble-Propelled Catalytic Micromotors

Bubble-propelled catalytic micromotors have recently been attracting much attention. A bubble-propulsion mechanism has the advantage of producing a stronger force and higher speed than other mechanisms for catalytic micromotors, but the nature of the fluctuated bubble generation process affects the motions of the micromotors, making it difficult to control their motions. Thus, understanding of the influence of fluctuating bubble propulsion on the motions of catalytic micromotors is important in exploiting the advantages of bubble-propelled micromotors. Here, we report experimental demonstrations of the bubble-propelled motions of propeller-shaped micromotors and numerical analyses of the influence of fluctuating bubble propulsion on the motions of propeller-shaped micromotors. We found that motions such as trochoid-like motion and circular motion emerged depending on the magnitude or symmetricity of fluctuations in the bubble-propulsion process. We hope that those results will help in the construction and application of sophisticated bubble-propelled micromotors in the future.

Programmed Transport and Release of Cells by Self-Propelled Micromotors

Autonomous transport and release of bacterial cells by self-propelled micromotors were achieved. The motors consisted of zinc and platinum hemispheres formed on polystyrene beads and moved as a result of simultaneous redox reactions occurring on both metal ends. The highly negative redox potential of zinc enabled the selection of a wide variety of organic redox compounds as fuels, such as methanol and p-benzoquinone. The movement of motors was observed in solutions of fuels. To realize autonomous capture, transport, and release of cargo, a self-assembled monolayer (SAM) was formed on the platinum part of the motor. This SAM could be desorbed by coupling the reaction with the dissolution of zinc, which could also be controlled by adjusting the concentration of Zn(2+) ions. Escherichia coli (E. coli) cells were captured by the motor (due to hydrophobic interactions), transported, and released following SAM desorption at the mixed potential.

Rocket Science at the Nanoscale

Autonomous propulsion at the nanoscale represents one of the most challenging and demanding goals in nanotechnology. Over the past decade, numerous important advances in nanotechnology and material science have contributed to the creation of powerful self-propelled micro/nanomotors. In particular, micro- and nanoscale rockets (MNRs) offer impressive capabilities, including remarkable speeds, large cargo-towing forces, precise motion controls, and dynamic self-assembly, which have paved the way for designing multifunctional and intelligent nanoscale machines. These multipurpose nanoscale shuttles can propel and function in complex real-life media, actively transporting and releasing therapeutic payloads and remediation agents for diverse biomedical and environmental applications. This review discusses the challenges of designing efficient MNRs and presents an overview of their propulsion behavior, fabrication methods, potential rocket fuels, navigation strategies, practical applications, and the future prospects of rocket science and technology at the nanoscale.

Recent Progress on Bioinspired Self-Propelled Micro/Nanomotors via Controlled Molecular Self-Assembly

The combination of bottom-up controllable self-assembly technique with bioinspired design has opened new horizons in the development of self-propelled synthetic micro/nanomotors. Over the past five years, a significant advances toward the construction of bioinspired self-propelled micro/nanomotors has been witnessed based on the controlled self-assembly technique. Such a strategy permits the realization of autonomously synthetic motors with engineering features, such as sizes, shapes, composition, propulsion mechanism, and function. The construction, propulsion mechanism, and movement control of synthetic micro/nanomotors in connection with controlled self-assembly in recent research activities are summarized. These assembled nanomotors are expected to have a tremendous impact on current artificial nanomachines in future and hold potential promise for biomedical applications including drug targeted delivery, photothermal cancer therapy, biodetoxification, treatment of atherosclerosis, artificial insemination, crushing kidney stones, cleaning wounds, and removing blood clots and parasites.

Photochromic Spatiotemporal Control of Bubble-Propelled Micromotors by a Spiropyran Molecular Switch

Controlling the environment in which bubble-propelled micromotors operate represents an attractive strategy to influence their motion, especially when the trigger is as simple as light. We demonstrate that spiropyrans, which isomerize to amphiphilic merocyanines under UV irradiation, can act as molecular switches that drastically affect the locomotion of the micrometer-sized engines. The phototrigger could be either a point or a field source, thus allowing different modes of control to be executed. A whole ensemble of micromotors was repeatedly activated and deactivated by just altering the spiropyran-merocyanine ratio with light. Moreover, the velocity of individual micromotors was altered using a point irradiation source that caused only localized changes in the environment. Such selective manipulation, achieved here with an optical microscope and a photochromic additive in the medium, reveals the ease of the methodology, which can allow micro- and nanomotors to reach their full potential of not just stochastic, but directional controlled motion.

Recent Progress on Man-Made Inorganic Nanomachines

The successful development of nanoscale machinery, which can operate with high controllability, high precision, long lifetimes, and tunable driving powers, is pivotal for the realization of future intelligent nanorobots, nanofactories, and advanced biomedical devices. However, the development of nanomachines remains one of the most difficult research areas, largely due to the grand challenges in fabrication of devices with complex components and actuation with desired efficiency, precision, lifetime, and/or environmental friendliness. In this work, the cutting-edge efforts toward fabricating and actuating various types of nanomachines and their applications are reviewed, with a special focus on nanomotors made from inorganic nanoscale building blocks, which are introduced according to the employed actuation mechanism. The unique characteristics and obstacles for each type of nanomachine are discussed, and perspectives and challenges of this exciting field are presented.

Bifurcation phenomena of two self-propelled camphor disks on an annular field depending on system length

Mode selection and bifurcation of a synchronized motion involving two symmetric self-propelled objects in a periodic one-dimensional domain were investigated numerically and experimentally by using camphor disks placed on an annular water channel. Newton's equation of motion for each camphor disk, whose driving force was the difference in surface tension, and a reaction-diffusion equation for camphor molecules on water were used in the numerical calculations. Among various dynamical behaviors found numerically, four kinds of synchronized motions (reversal oscillation, stop-and-move rotation, equally spaced rotation, and clustered rotation) were also observed in experiments by changing the diameter of the water channel. The mode bifurcation of these motions, including their coexistence, were clarified numerically and analytically in terms of the number density of the disk. These results suggest that the present mathematical model and the analysis of the equations can be worthwhile in understanding the characteristic features of motion, e.g., synchronization, collective motion, and their mode bifurcation.

Synthetic Nano- and Micromachines in Analytical Chemistry: Sensing, Migration, Capture, Delivery, and Separation

Synthetic nano- and microscale machines move autonomously in solution or drive fluid flows by converting sources of energy into mechanical work. Their sizes are comparable to analytes (sub-nano- to microscale), and they respond to signals from each other and their surroundings, leading to emergent collective behavior. These machines can potentially enable hitherto difficult analytical applications. In this article, we review the development of different classes of synthetic nano- and micromotors and pumps and indicate their possible applications in real-time in situ chemical sensing, on-demand directional transport, cargo capture and delivery, as well as analyte isolation and separation.

An efficient polymeric micromotor doped with Pt nanoparticle@carbon nanotubes for complex bio-media

A highly efficient polymeric tubular micromotor doped with Pt nanoparticle@carbon nanotubes is fabricated by template-assisted electrochemical growth.

Synthetic micro/nanomotors in drug delivery

Nanomachines offer considerable promise for the treatment of diseases. The ability of man-made nanomotors to rapidly deliver therapeutic payloads to their target destination represents a novel nanomedicine approach. Synthetic nanomotors, based on a multitude of propulsion mechanisms, have been developed over the past decade toward diverse biomedical applications. In this review article, we journey from the use of chemically powered drug-delivery nanovehicles to externally actuated (fuel-free) drug-delivery nanomachine platforms, and conclude with future prospects and challenges for such practical propelling drug-delivery systems. As future micro/nanomachines become more powerful and functional, these tiny devices are expected to perform more demanding biomedical tasks and benefit different drug delivery applications.

Photoactive rolled-up TiO2 microtubes: fabrication, characterization and applications†Electronic supplementary information (ESI) available. See DOI: 10.1039/c4tc00796dClick here for additional data file

Because of its unique properties, titania (TiO2) represents a promising candidate in a wide variety of research fields. In this paper, some of the properties and potential applications of titania within rolled-up nanotechnology are explored. It is shown how the structural and optical properties of rolled titania microtubes can be controlled by properly tuning the microfabrication parameters. The rolling up of titania films on different sacrificial layers and containing different shapes, achieving a control on the diameter of the fabricated titania microtubes, is presented. In order to obtain the more photoactive crystalline form of titania, one during-fabrication and two post-fabrication methods are demonstrated. Interesting applications in the fields of photocatalysis and photonics are suggested: the use of titania rolled-up microtubes as micromotors and optical microresonators is presented.

Catalytic nanomotors for environmental monitoring and water remediation

Self-propelled nanomotors hold considerable promise for developing innovative environmental applications. This review highlights the recent progress in the use of self-propelled nanomotors for water remediation and environmental monitoring applications, as well as the effect of the environmental conditions on the dynamics of nanomotors. Artificial nanomotors can sense different analytes-and therefore pollutants, or "chemical threats"-can be used for testing the quality of water, selective removal of oil, and alteration of their speeds, depending on the presence of some substances in the solution in which they swim. Newly introduced micromotors with double functionality to mix liquids at the microscale and enhance chemical reactions for the degradation of organic pollutants greatly broadens the range of applications to that of environmental. These "self-powered remediation systems" could be seen as a new generation of "smart devices" for cleaning water in small pipes or cavities difficult to reach with traditional methods. With constant improvement and considering the key challenges, we expect that artificial nanomachines could play an important role in environmental applications in the near future.

Motor-based autonomous microsensor for motion and counting immunoassay of cancer biomarker

A motor-based autonomous microsensor is proposed for in situ visualization immunoassay of cancer biomarkers through motion readout or tag counting. The microsensor is prepared by functionalizing a newly designed gold-nanoparticle-modified self-propelled polyaniline/Pt (AuNP/PANI/Pt) micromotor with capture antibody. The autonomous movement of the microsensor in the fuel-enhanced sample mixture results in the fast and selective recognition of the protein target and subsequent loading of the secondary-antibody-modified glycidyl methacrylate microspheres (GMA), which slows down the movement of the sensing microengine. The velocity of the microsensor and the number of GMA conjugated on the microsensor can be conveniently visualized using optical microscopy. They are negatively and positively correlated with the target concentration, respectively. Therefore, the microsensor can conveniently distinguish the concentration of carcinoembryonic antigen in a range of 1-1000 ng/mL. The motor-based microsensor can be easily prepared in batch using AuNP/PANI/Pt. The whole detection procedure for protein target can be completed in 5 min without any washing and separation step. This method shows considerable promise for diverse clinical and diagnostic applications.

Biomimetic artificial inorganic enzyme-free self-propelled microfish robot for selective detection of Pb(2+) in water

The availability of drinking water is of utmost importance for the world population. Anthropogenic pollutants of water, such as heavy-metal ions, are major problems in water contamination. The toxicity assays used range from cell assays to animal tests. Herein, we replace biological toxicity assays, which use higher organisms, with artificial inorganic self-propelled microtubular robots. The viability and activity of these robots are negatively influenced by heavy metals, such as Pb(2+) , in a similar manner to that of live fish models. This allows the establishment of a lethal dose (LD50 ) of heavy metal for artificial inorganic microfish robots. The self-propelled microfish robots show specific response to Pb(2+) compared to other heavy metals, such as Cd(2+) , and can be used for selective determination of Pb(2+) in water. It is a first step towards replacing the biological toxicity assays with biomimetic inorganic autonomous robotic systems.

Propulsion Mechanism of Catalytic Microjet Engines

We describe the propulsion mechanism of the catalytic microjet engines that are fabricated using rolled-up nanotech. Microjets have recently shown numerous potential applications in nanorobotics but currently there is a lack of an accurate theoretical model that describes the origin of the motion as well as the mechanism of self-propulsion. The geometric asymmetry of a tubular microjet leads to the development of a capillary force, which tends to propel a bubble toward the larger opening of the tube. Because of this motion in an asymmetric tube, there emerges a momentum transfer to the fluid. In order to compensate this momentum transfer, a jet force acting on the tube occurs. This force, which is counterbalanced by the linear drag force, enables tube velocities of the order of 100 μm/s. This mechanism provides a fundamental explanation for the development of driving forces that are acting on bubbles in tubular microjets.