Self-propelled Micro Research Articles

Reconfigurable self-assembly of photocatalytic magnetic microrobots for water purification

The development of artificial small-scale robotic swarms with nature-mimicking collective behaviors represents the frontier of research in robotics. While microrobot swarming under magnetic manipulation has been extensively explored, light-induced self-organization of micro- and nanorobots is still challenging. This study demonstrates the interaction-controlled, reconfigurable, reversible, and active self-assembly of TiO2/α-Fe2O3 microrobots, consisting of peanut-shaped α-Fe2O3 (hematite) microparticles synthesized by a hydrothermal method and covered with a thin layer of TiO2 by atomic layer deposition (ALD). Due to their photocatalytic and ferromagnetic properties, microrobots autonomously move in water under light irradiation, while a magnetic field precisely controls their direction. In the presence of H2O2 fuel, concentration gradients around the illuminated microrobots result in mutual attraction by phoretic interactions, inducing their spontaneous organization into self-propelled clusters. In the dark, clusters reversibly reconfigure into microchains where microrobots are aligned due to magnetic dipole-dipole interactions. Microrobots’ active motion and photocatalytic properties were investigated for water remediation from pesticides, obtaining the rapid degradation of the extensively used, persistent, and hazardous herbicide 2,4-Dichlorophenoxyacetic acid (2,4D). This study potentially impacts the realization of future intelligent adaptive metamachines and the application of light-powered self-propelled micro- and nanomotors toward the degradation of persistent organic pollutants (POPs) or micro- and nanoplastics.

Self-Propelled Micro-/Nanomotors of ZnO Nanoshuttles Induced by Surface Defects

To accomplish various complex tasks in the microworld, micro-/nanomotors (MNMs) with diverse swimming characteristics have attracted more and more attention. In this work, ZnO nanoshuttles were synthesized by a simple hydrothermal method. In addition, MNMs based on ZnO nanoshuttles have been designed, exhibiting self-propelled swimming characteristics with three motion modes of shake, rotation, and translation in water. It is revealed that the asymmetrically distributed defects on the surface of ZnO nanoshuttles are responsible for the swimming modes. The unique shuttle-like structure helps to reduce the drag in a low Reynolds number environment and endows the self-propelled ZnO nanoshuttle motor with a relatively high swimming velocity (5.16 μm/s). In addition, to demonstrate the versatility and openness of ZnO nanoshuttle MNMs, ZnO/Co magnetic MNMs and ZnO/TiO2 Janus MNMs were prepared by simple Co doping and TiO2 coating, respectively. This study reveals the possible role of surface defects in regulating the motion behavior of ZnO-based MNMs and also provides a feasible low-cost method for the large-scale fabrication of self-asymmetric fuel-free micro-/nanomotors.

Biomedical Micro-/Nanomotors: From Overcoming Biological Barriers to In Vivo Imaging.

Self-propelled micro- and nanomotors (MNMs) have shown great potential for applications in the biomedical field, such as active targeted delivery, detoxification, minimally invasive diagnostics, and nanosurgery, owing to their tiny size, autonomous motion, and navigation capacities. To enter the clinic, biomedical MNMs request the biodegradability of their manufacturing materials, the biocompatibility of chemical fuels or externally physical fields, the capability of overcoming various biological barriers (e.g., biofouling, blood flow, blood-brain barrier, cell membrane), and the in vivo visual positioning for autonomous navigation. Herein, the recent advances of synthetic MNMs in overcoming biological barriers and in vivo motion-tracking imaging techniques are highlighted. The challenges and future research priorities are also addressed. With continued attention and innovation, it is believed that, in the future, biomedical MNMs will pave the way to improve the targeted drug delivery efficiency.

Application of self-propelled micro-/nanomotors in active targeted drug delivery

As a new type of micro-/nanomachines, self-propelled micro-/nanomotors (MNMs) can convert chemical or external energies from the surrounding environment into mechanical forces to produce autonomous motion. The ability of autonomous movement allows these MNMs to move actively to the targeted locations, and thus confers great potentials on the MNMs for applications in biomedicine, especially in drug delivery. MNMs have been shown to effectively load therapeutic payloads for active delivery to the disease site, which greatly improves the therapeutic efficacy and reduces side effects compared with the traditional nanodrugs. In this review, we provide an overview of different propulsion mechanisms of MNMs, including chemical propulsion based on redox reaction and external field propulsion driven by external energy such as light, magnetic field, electric field and ultrasound, followed by a review of the recent progress in active drug delivery based on MNMs in the past decade. We also discuss the current challenges and future perspectives of the application of the MNMs.

Active Anion Delivery by Self-Propelled Microswimmers.

Self-propelled micro- and nanomachines are at the forefront of materials research, branching into applications in biomedical science and environmental remediation. Cationic frameworks enabling the collection and delivery of anionic species (A-) are highly required, due to the large variety of life-threatening pollutants, such as radioactive technetium and carcinogenic chromium, and medicines, such as dexamethasone derivatives with negative charges. However, such autonomous moving carriers for active transport of the anions have been barely discussed. A polymeric viologen (PV++)-consisting of electroactive bicationic subunits-is utilized in a tubular autonomous microswimmer to selectively deliver A- of different sizes and charge densities. The cargo loading is based on a facile anion exchange mechanism. The packed crystal structure of PV++ allows removal of an exceptionally high quantity of anions per one microswimmer (2.55 × 10-13 mol anions per microswimmer), a critical factor often neglected regarding the real-world application of microswimmers. Notably, there was virtually no leakage of anions during the delivery process or upon keeping the loaded microswimmers under ambient conditions for at least 4 months. Multiple release mechanisms, compatible with different environments, including electrochemical, photochemical, and a metathesis reaction, with high efficiencies up to 98% are introduced. Such functional autonomous micromachines provide great promise for the next generation of functional materials for biomedical and environmental applications.

One-Step Preparation of Fuel-Containing Anisotropic Nanocapsules with Stimuli-Regulated Propulsion.

One of the dreams of nanotechnology is to create tiny objects, nanobots, that are able to perform difficult tasks in dimensions and locations that are not directly accessible. One basic function of these nanobots is motility. Movements created by self-propelled micro- and nanovehicles are usually dependent on the production of propellants from catalytic reactions of fuels present in the environment. Developing self-powered nanovehicles with internally stored fuels that display motion regulated by external stimuli represents an intriguing and challenging alternative. Herein, a one-step preparation of fuel-containing nanovehicles that feature a motion that can be regulated by external stimuli is reported. Nanovehicles are prepared via a sol-gel process confined at the oil/water interface of miniemulsions. The nanovehicles display shapes ranging from mushroom-like to truncated cones and a core-shell structure so that the silica shell acts as a hull for the nanovehicles while the core is used to store the fuel. Azo-based initiators are loaded in the nanovehicles, which are activated to release nitrogen gas upon increase of temperature or exposure to UV light. Enhanced diffusion of nanovehicles is achieved upon decomposition of the fuel.

Enzyme-Powered Gated Mesoporous Silica Nanomotors for On-Command Intracellular Payload Delivery

The introduction of stimuli-responsive cargo release capabilities on self-propelled micro- and nanomotors holds enormous potential in a number of applications in the biomedical field. Herein, we report the preparation of mesoporous silica nanoparticles gated with pH-responsive supramolecular nanovalves and equipped with urease enzymes which act as chemical engines to power the nanomotors. The nanoparticles are loaded with different cargo molecules ([Ru(bpy)3]Cl2 (bpy = 2,2'-bipyridine) or doxorubicin), grafted with benzimidazole groups on the outer surface, and capped by the formation of inclusion complexes between benzimidazole and cyclodextrin-modified urease. The nanomotor exhibits enhanced Brownian motion in the presence of urea. Moreover, no cargo is released at neutral pH, even in the presence of the biofuel urea, due to the blockage of the pores by the bulky benzimidazole:cyclodextrin-urease caps. Cargo delivery is only triggered on-command at acidic pH due to the protonation of benzimidazole groups, the dethreading of the supramolecular nanovalves, and the subsequent uncapping of the nanoparticles. Studies with HeLa cells indicate that the presence of biofuel urea enhances nanoparticle internalization and both [Ru(bpy)3]Cl2 or doxorubicin intracellular release due to the acidity of lysosomal compartments. Gated enzyme-powered nanomotors shown here display some of the requirements for ideal drug delivery carriers such as the capacity to self-propel and the ability to "sense" the environment and deliver the payload on demand in response to predefined stimuli.

Frontispiece: Fabrication of Self‐Propelled Micro‐ and Nanomotors Based on Janus Structures

Frontispiece: Fabrication of Self‐Propelled Micro‐ and Nanomotors Based on Janus Structures

Motion Control of Polymeric Nanomotors Based on Host-Guest Interactions.

Controlling the motion of artificial self-propelled micro- and nanomotors independent of the fuel concentration is still a great challenge. Here we describe the first report of speed manipulation of supramolecular nanomotors via blue light-responsive valves, which can regulate the access of hydrogen peroxide fuel into the motors. Light-sensitive polymeric nanomotors are built up via the self-assembly of functional block copolymers, followed by bowl-shaped stomatocyte formation and incorporation of platinum nanoparticles. Subsequent addition of β-cyclodextrin (β-CD) leads to the formation of inclusion complexes with the trans-isomers of the azobenzene derivatives grafted from the surfaces of the stomatocytes. β-CDs attachment decreases the diffusion rate of hydrogen peroxide into the cavities of the motors because of partly blocking of the openings of the stomatocyte. This results in a lowering of the speed of the nanomotors. Upon blue light irradiation, the trans-azobenzene moieties isomerize to the cis-form, which lead to the detachment of the β-CDs due to their inability to form complexes with the cis-isomer. As a result, the speed of the nanomotors increases accordingly. Such a conformational change provides us with the unique possibility to control the speed of the supramolecular nanomotor via light-responsive host-guest complexation. We envision that such artificial responsive nano-systems with controlled motion could have potential applications in drug delivery.

Continuously Variable Regulation of the Speed of Bubble-Propelled Janus Microcapsule Motors Based on Salt-Responsive Polyelectrolyte Brushes.

The engineering of self-propelled micro-/nanomotors (MNMs) with continuously variable speeds, akin to macroscopic automobiles equipped with a continuously variable transmission, is still a huge challenge. Herein, after grafting with salt-responsive poly[2-(methacryloyloxy)ethyltrimethylammonium chloride] (PMETAC) brushes, bubble-propelled Janus microcapsule motors with polyelectrolyte multilayers exhibited adjustable speeds when the type and concentration of the counterion was changed. Reversible switching between low- and high-speed states was achieved by modulating the PMETAC brushes between hydrophobic and hydrophilic configurations by ion exchange with ClO4 - and polyphosphate anions. This continuously variable regulation enabled control of the speed in an accurate and predictable manner and an autonomous response to the local chemical environment. This study suggests that the integration of polymer brushes with precisely adjustable responsiveness offers a promising route for motion control of smart MNMs that act like their counterparts in living systems.

Self-Propelled Micro Swimmer with Red-Blood-Cell Size

Self-Propelled Micro Swimmer with Red-Blood-Cell Size

Polymer platforms for micro- and nanomotor fabrication.

Artificial, self-propelled micro- and nanomotors are small devices capable of autonomous movement, which are a powerful scientific innovation for solving various medical and environmental issues. Their design is frequently inspired by complex biological structures which are composed of biopolymers and their composites. The choice of materials for nano- and micromachines is crucial for their shape, mechanism and efficiency of propulsion. In this review, we discuss the utilization and fabrication of polymers as soft components of micro- and nanomotors.

Stretching and Heating Single DNA Molecules with Optically Trapped Gold–Silica Janus Particles

Self-propelled micro- and nanoscale motors are capable of autonomous motion typically by inducing local concentration gradients or thermal gradients in their surrounding medium. This is a result of the heterogeneous surface of the self-propelled structures that consist of materials with different chemical or physical properties. Here we present a self-thermophoretically driven Au–silica Janus particle that can simultaneously stretch and partially melt a single double-stranded DNA molecule. We show that the effective force acting on the DNA molecule is in the ∼pN range, well suited to probe the entropic stretching regime of DNA, and we demonstrate that the local temperature enhancement around the gold side of the particle produces partial DNA dehybridization.

Motion Manipulation of Micro- and Nanomotors.

Inspired by the self-migration of microorganisms in nature, artificial micro- and nanomotors can mimic this fantastic behavior by converting chemical fuel or external energy into mechanical motion. These self-propelled micro- and nanomotors, designed either by top-down or bottom-up approaches, are able to achieve different applications, such as environmental remediation, sensing, cargo/sperm transportation, drug delivery, and even precision micro-/nanosurgery. For these various applications, especially biomedical applications, regulating on-demand the motion of micro- and nanomotors is quite essential. However, it remains a continuing challenge to increase the controllability over motors themselves. Here, we will discuss the recent advancements regarding the motion manipulation of micro- and nanomotors by different approaches.

Autonomous Collision-Free Navigation of Microvehicles in Complex and Dynamically Changing Environments.

Self-propelled micro- and nanoscale robots represent a rapidly emerging and fascinating robotics research area. However, designing autonomous and adaptive control systems for operating micro/nanorobotics in complex and dynamically changing environments, which is a highly demanding feature, is still an unmet challenge. Here we describe a smart microvehicle for precise autonomous navigation in complicated environments and traffic scenarios. The fully autonomous navigation system of the smart microvehicle is composed of a microscope-coupled CCD camera, an artificial intelligence planner, and a magnetic field generator. The microscope-coupled CCD camera provides real-time localization of the chemically powered Janus microsphere vehicle and environmental detection for path planning to generate optimal collision-free routes, while the moving direction of the microrobot toward a reference position is determined by the external electromagnetic torque. Real-time object detection offers adaptive path planning in response to dynamically changing environments. We demonstrate that the autonomous navigation system can guide the vehicle movement in complex patterns, in the presence of dynamically changing obstacles, and in complex biological environments. Such a navigation system for micro/nanoscale vehicles, relying on vision-based close-loop control and path planning, is highly promising for their autonomous operation in complex dynamic settings and unpredictable scenarios expected in a variety of realistic nanoscale scenarios.

Hybrid colloidal microswimmers through sequential capillary assembly.

Active colloids, also known as artificial microswimmers, are self-propelled micro- and nanoparticles that convert uniform sources of fuel (e.g. chemical) or uniform external driving fields (e.g. magnetic or electric) into directed motion by virtue of asymmetry in their shape or composition. These materials are currently attracting enormous scientific attention as models for out-of-equilibrium systems and with the promise to be used as micro- and nanoscale devices. However, current fabrication of active colloids is limited in the choice of available materials, geometries, and modes of motion. Here, we use sequential capillarity-assisted particle assembly (sCAPA) to link microspheres of different materials into hybrid clusters of prescribed shapes ("colloidal molecules") that can actively translate, circulate and rotate powered by asymmetric electro-hydrodynamic flows. We characterize the active motion of the clusters and highlight the range of parameters (composition and shape) that can be used to tune their trajectories. Further engineering provides active colloids that switch motion under external triggers or perform simple pick-up and transport tasks. By linking their design, realization and characterization, our findings enable and inspire both physicists and engineers to create customized active colloids to explore novel fundamental phenomena in active matter and to investigate materials and propulsion schemes that are compatible with future applications.

Research progress about the applications of self-propellant Janus particles in water environment

Today, environmental sustainability is one of the important challenges that we are facing, especially in the field of water environment. Self-propelled micro-/nano- motors have two distinct physical or chemical properties, so that the huge gradient fields can be established across micro-/nano- motors under different external stimulation sources, which induce the self-propulsion of motors. This feature of self-propulsion holds the considerable promise for water environmental application. In this paper, we reviewed the recent research progress of self-propelled micro-/nano- motors in water environmental monitoring and water remediation. Furthermore, the relationship between the active transport, the membrane filtration processes and micro-/nano- motors’ motion mechanisms was also explored. In addition, the reclaiming methods of self-propelled Janus particles were discussed in order to seek a better way to improve their recycling rate and reduce the negative impact on the environment. Finally, we put forward some potential studies in this field.

Design strategies and structure simplification methods of self-propelled micro-/nanomotors

Self-propelled micro-/nanomotors (MNMs), which are defined as micro-/nanodevices capable of converting various energy into autonomous motion, can be used to pick up, transport, and release various cargoes within a liquid medium. They have important potential applications, for example, in drug delivery, biosensors, protein and cell separation, microsurgeries and environment remediation. This review comprehensively introduces the design strategies and structures of self-propelled MNMs along with an outlook for their future development. It starts with the summary of the propulsion mechanisms of self-propelled MNMs of bubble recoiling and self-phoresis induced by the asymmetric release of products or heat. For bubble recoiling propulsion, the continuous momentum change is caused by a jet of bubbles, while for self-phoresis propulsion, the MNMs move in a local electric field, concentration gradient, surface tension gradient, or temperature gradient, etc. After systematically and in-depth understanding these propulsion mechanisms, it has been pointed out that the key to design self-propelled MNMs is to construct an asymmetric field across micro-/ nanoparticles. Following this clue, the structures evolution and simplification methods of self-propelled MNMs are reviewed. Janus structures and multilayer-tubular structures, which are prepared through asymmetric modification process, electrochemical synthesis, template-assisted method, rolled-up nanotech, etc., have been firstly proposed to construct asymmetric fields across micro-/nanoparticles for their propulsion. However, the complicated structure and preparation process hinder the application of MNMs. Anisotropic single-component irregular particles, tubes and bowl-like MNMs, which are obtained by dry spinning method, “growing-bubble”-templated self-assembly, etc., have been subsequently achieved by utilizing their anomalous morphology and the nucleation preference of bubble molecules on different surfaces. This kind of MNMs show somewhat simple structure and can be easily fabricated, but the motion direction is still difficult to control because of the Brownian motion. Isotropic semiconducting MNMs have been recently developed by taking advantage of the limited light penetration depth in the isotropic photoresponsive particles, of which the motion is independent of the rotational Brownian motion. This suggests a remarkable breakthrough in design strategy of MNMs due to the simple isotropic structure of the motor and the controllability in both motion direction and speed by light. Besides the evolution of self-propelled MNMs from the complicated structure to the simplified one, some remarkable progresses have also been made on the motion control, functionalization, etc. For example, the speed and state of MNMs can so far be easily adjusted by the concentration of fuels, the intensity of external fields, etc. The direction can be controlled accurately by magnetic field, electric field, light, etc. Numerous complex tasks can also be performed effectively, such as protein separation, drug delivery, environmental detection and remediation, etc. Lastly, an outlook is also provided on the future development and main challenges of self-propelled MNMs. The future development of MNMs should be focused on improving energy conversion efficiency through optimization of structures, exploring new propulsion mechanisms and endowing MNMs with environmental responses for self-navigation, detection, and specific operations. In this way, MNMs will approach to the practical applications in biomedicine, environment treatment, microengineering, etc.

Fe(0) Nanomotors in Ton Quantities (10(20) Units) for Environmental Remediation.

Despite demonstrating potential for environmental remediation and biomedical applications, the practical environmental applications of autonomous self-propelled micro-/nanorobots have been limited by the inability to fabricate these devices in large (kilograms/tons) quantities. In view of the demand for large-scale environmental remediation by micro-/nanomotors, which are easily synthesized and powered by nontoxic fuel, we have developed bubble-propelled Fe(0) Janus nanomotors by a facile thermally induced solid-state procedure and investigated their potential as decontamination agents of pollutants. These Fe(0) Janus nanomotors, stabilized by an ultrathin iron oxide shell, were fuelled by their decomposition in citric acid, leading to the asymmetric bubble propulsion. The degradation of azo-dyes was dramatically increased in the presence of moving self-propelled Fe(0) nanomotors, which acted as reducing agents. Such enhanced pollutant decomposition triggered by biocompatible Fe(0) (nanoscale zero-valent iron motors), which can be handled in the air and fabricated in ton quantities for low cost, will revolutionize the way that environmental remediation is carried out.

Force Analysis of IPMC Actuated Fin and Wing Assembly of a Micro Scanning Device through Two-Way Fluid Structure Interaction Approach

In this paper, a methodology is presented to perform force analysis of wing and fin assembly of a micro fish like device through strongly coupled two-way fluid structure interaction approach. The scanning device operates underwater and is towed by a surface vessel through a tow cable. Device fins are actuated by ionic polymer metal composite (IPMC) actuators, an EAP actuator. Fins act as riser, depressor and stabiliser against roll motion of the device. During tow, wing and fin assembly of the device come under hydrodynamic forces. These forces are influenced by fin displacement under IPMC actuation and wing's angle of attack for same towing conditions. To fully investigate wing and fin assembly performance, we must consider the interaction between their structure and fluid (water) and model the coupling mechanism accurately for fluid structure interaction (FSI) analysis. To obtain an accurate prediction to the hydrodynamic forces on wing and fin assembly of the device, it is necessary to conduct dynamic analysis of the surrounding fluid by computational fluid dynamics (CFD). A numerical simulation of three dimensional model of the assembly is performed in ANSYS WORKBENCH by coupling transient structural and Fluid Flow (CFX) analysis systems. The objectives of this study are as follows: 1) To build an accurate three-dimensional CFD model of the wing and IPMC actuated fin 2) To quantify the lift and drag forces acting on the wing and their corresponding coefficients 3) To demonstrate the influence of wing's angle of attack and fin displacement on generation of lift and drag forces. The presented methodology is also applicable to self-propelled micro robots.