Utilizing Iron's Attractive Chemical and Magnetic Properties in Microrocket Design, Extended Motion, and Unique Performance.

All-in-one material for microrocket propulsion featuring acid-based bubble generation and magnetic guidance is presented. Electrochemically deposited iron serves as both a propellant, toward highly efficient self-propulsion in acidic environments, and as a magnetic component enabling complete motion control. The new microrockets display longer lifetime and higher propulsion efficiency compared to previously reported active metal zinc-based microrockets due to the chemical properties of iron and the unique structure of the microrockets. These iron-based microrockets also demonstrate unique and attractive cargo towing and autonomous release capabilities. The latter is realized upon loss of the magnetic properties due to acid-driven iron dissolution. More interestingly, these bubble-propelled microrockets assemble via magnetic interactions into a variety of complex configurations and train structures, which enrich the behavior of micromachines. Modeling of the magnetic forces during the microrocket assembly and cargo capture confirms these unique experimentally observed assembly and cargo-towing behaviors. These findings provide a new concept of blending propellant and magnetic components into one, toward simplifying the design and fabrication of artificial micro/nanomachines, realizing new functions and capabilities for a variety of future applications.

Forecastable and Guidable Bubble-Propelled Microplate Motors for Cell Transport.

Cell transport is important to renew body functions and organs with stem cells, or to attack cancer cells with immune cells. The main hindrances of this method are the lack of understanding of cell motion as well as proper transport systems. In this publication, bubble-propelled polyelectrolyte microplates are used for controlled transport and guidance of HeLa cells. Cells survive attachment on the microplates and up to 22 min in 5% hydrogen peroxide solution. They can be guided by a magnetic field whereby increased friction of cells attached to microplates decreases the speed by 90% compared to pristine microplates. The motion direction of the cell-motor system is easier to predict due to the cell being opposite to the bubbles.

Wastewater Mediated Activation of Micromotors for Efficient Water Cleaning.

We present wastewater-mediated activation of catalytic micromotors for the degradation of nitroaromatic pollutants in water. These next-generation hybrid micromotors are fabricated by growing catalytically active Pd particles over thin-metal films (Ti/Fe/Cr), which are then rolled-up into self-propelled tubular microjets. Coupling of catalytically active Pd particles inside the micromotor surface in the presence of a 4-nitrophenol pollutant (with NaBH4 as reductant) results in autonomous motion via the bubble-recoil propulsion mechanism such that the target pollutant mixture (wastewater) is consumed as a fuel, thereby generating nontoxic byproducts. This study also offers several distinct advantages over its predecessors including no pH/temperature manipulation, limited stringent process control and complete destruction of the target pollutant mixture. The improved intermixing ability of the micromotors caused faster degradation ca. 10 times higher as compared to its nonmotile counterpart. The high catalytic efficiency obtained via a wet-lab approach has promising potential in creating hybrid micromotors comprising of multicatalytic systems assembled into one entity for sustainable environmental remediation and theranostics.

Magnetocatalytic Graphene Quantum Dots Janus Micromotors for Bacterial Endotoxin Detection.

Magnetocatalytic hybrid Janus micromotors encapsulating phenylboronic acid (PABA) modified graphene quantum dots (GQDs) are described herein as ultrafast sensors for the detection of deadly bacteria endotoxins. A bottom-up approach was adopted to synthesize an oil-in-water emulsion containing the GQDs along with a high loading of platinum and iron oxide nanoparticles on one side of the Janus micromotor body. The two different "active regions" enable highly efficient propulsion in the presence of hydrogen peroxide or magnetic actuation without the addition of a chemical fuel. Fluorescence quenching was observed upon the interaction of GQDs with the target endotoxin (LPS), whereby the PABA tags acted as highly specific recognition receptors of the LPS core polysaccharide region. Such adaptive hybrid operation and highly specific detection hold considerable promise for diverse clinical, agrofood, and biological applications and integration in future lab-on-chip technology.

Reactive Inkjet Printing of Biocompatible Enzyme Powered Silk Micro-Rockets.

Inkjet-printed enzyme-powered silk-based micro-rockets are able to undergo autonomous motion in a vast variety of fluidic environments including complex media such as human serum. By means of digital inkjet printing it is possible to alter the catalyst distribution simply and generate varying trajectory behavior of these micro-rockets. Made of silk scaffolds containing enzymes these micro-rockets are highly biocompatible and non-biofouling.

4D Printed Non-Euclidean-Plate Jellyfish Inspired Soft Robot in Diverse Organic Solvents.

Soft robots have the potential to assist and complement human exploration of extreme and harsh environments (i.e., organic solvents). However, soft robots with stable performance in diverse organic solvents are not developed yet. In the current research, a non-Euclidean-plate under-liquid soft robot inspired by jellyfish based on phototropic liquid crystal elastomers is fabricated via a 4D-programmable strategy. Specifically, the robot employs a 3D-printed non-Euclidean-plate, designed with Archimedean orientation, which undergoes autonomous deformation to release internal stress when immersed in organic solvents. With the assistance of near-infrared light illumination, the organic solvent inside the robot vaporizes and generates propulsion in the form of bubble streams. The developed NEP-Jelly-inspired soft robot can swim with a high degree of freedom in various organic solvents, for example, N, N-dimethylformamide, N, N-dimethylacetamide, tetrahydrofuran, dichloromethane, and trichloromethane, which is not reported before. Besides bionic jellyfish, various aquatic invertebrate-inspired soft robots can potentially be prepared via a similar 4D-programmable strategy.

Refillable Fuel-Loading Microshell Motors for Persistent Motion in a Fuel-Free Environment.

Artificial micro-/nanomotors that harvest environmental energy to move require energy surroundings; thus, their motion generally occurs in fuel solutions or under the real-time stimuli of external energy sources. Herein, inspired by vehicles, a refillable fuel-loading micromotor is proposed based on a 2 µm hemispherical multimetallic shell using catalase or platinum on its concave surface as the engine and the bowl structure as the fuel tank. H2O2 fuel is drawn into the microbowl by capillary action and restricted inside the bowl space through a self-generated O2 bubble cap on the microshell mouth. The periodic growth and burst of the O2 cap cause the enhanced diffusion motion of micromotors. This motion behavior can last for at least 30 min in a fuel-free environment with one H2O2 fueling. Additionally, the micromotor can be refilled repeatedly to achieve permanent motion. This demonstration of a refillable fuel-loading micromotor provides a model design of an energy built-in micromotor.

Biodegradable magnesium fuel-based Janus micromotors with surfactant induced motion direction reversal.

The speed and motion directionality of bubble-propelled micromotors is dependent on bubble lifetime, bubble formation frequency and bubble stabilization. Absence and presence of bubble stabilizing agents should significantly influence speed and propulsion pattern of a micromotor, especially for fast-diffusing molecules like hydrogen. This study demonstrates a fully biodegradable Janus structured micromotor, propelled by hydrogen bubbles generated by the chemical reaction between hydrochloric acid and magnesium. Six different concentrations of hydrochloric acid and five different concentrations of the surfactant Triton X-100 were tested, which also cover the critical micelle concentration at a pH corresponding to an empty stomach. The Janus micromotor reverses its propulsion direction depending on the availability and concentration of a surfactant. Upon surfactant-free condition, the Janus micromotor is propelled by bubble cavitation, causing the micromotor to be pulled at high speed for short time intervals into the direction of the imploding bubble and thus backwards. In case of available surfactant above the critical micelle concentration, the Janus micromotor is pushed forward by the generated bubbles, which emerge at high frequency and form a bubble trail. The finding of the propulsion direction reversal effect demonstrates the importance to investigate the motion properties of artificial micromotors in a variety of different environments prior to application, especially with surfactants, since biological media often contain large amounts of surface-active components.

Cost-Effective, High-Yield Production of Biotemplated Catalytic Tubular Micromotors as Self-Propelled Microcleaners for Water Treatment.

Micro/nano-motors (MNMs) that combine attributes of miniaturization and self-propelled swimming mobility have been explored for efficient environmental remediation in the past decades. However, their progresses in practical applications are now subject to several critical issues including a complicated fabrication process, low production yield, and high material cost. Herein, we propose a biotemplated catalytic tubular micromotor consisting of a kapok fiber (KF, abundant in nature) matrix and manganese dioxide nanoparticles (MnO2 NPs) deposited on the outer and inner walls of the KF and demonstrate its applications for rapid removal of methylene blue (MB) in real-world wastewater. The fabrication is straightforward via dipping the KF into a potassium permanganate (KMnO4) solution, featured with high yield and low cost. The distribution and amount of MnO2 can be easily controlled by varying the dipping time. The obtained motors are actuated and propelled by oxygen (O2) bubbles generated from MnO2-triggered catalytic decomposition of hydrogen peroxide (H2O2), with the highest speed at 615 µm/s (i.e., 6 body length per second). To enhance decontamination efficacy and also enable magnetic navigation/recycling, magnetite nanoparticles (Fe3O4 NPs) are adsorbed onto such motors via an electrostatic effect. Both the Fe3O4-induced Fenton reaction and hydroxyl radicals from MnO2-catalyzed H2O2 decomposition can account for the MB removal (or degradation). Results of this study, taken together, provide a cost-effective approach to achieve high-yield production of the MNMs, suggesting an automatous microcleaner able to perform practical wastewater treatment.

Bubble-Propelled Jellyfish-like Micromotors for DNA Sensing.

A chemically powered jellyfish-like micromotor was proposed by using a multimetallic shell and a DNA assembly with catalase decorations modified on the concave surface to simulate the umbrella-shaped body and the muscle fibers on the inner umbrella of jellyfish. Relying on the catalytic generation of oxygen gas by catalase in H2O2 fuel, the jellyfish-like micromotor showed good bubble-propelled motion in different biomedia with speed exceeding 209 µm s-1 in 1.5% H2O2. The jellyfish-like micromotors could also be applied for motion detection of DNA based on a displacement hybridization-triggered catalase release. The proposed jellyfish-like micromotors showed advantages of easy fabrication, good motion ability, sensitive motion detection of DNA, and good stability and reproducibility, indicating considerable promise for biological application.

Motion of Enzyme-Powered Microshell Motors.

Microshells are attractive in constructing bubble-propelled micromotors due to the lower energy consumption for bubbles forming on a concave surface. In this work, enzyme-powered microshell motors were fabricated on multimetallic (Au/Ag/Au) microshells along with the modification of catalase on its concave surface. The catalase triggered the decomposition of hydrogen peroxide to oxygen gas, hence propelling the autonomous motion of microshell motors. A size-dependent motion behaviour was observed for the microshell motors in the form of slow tremble and fast translation motion for a size smaller and larger than 5 µm, respectively, according to the size, generation efficiency and ejection mechanism of bubbles and the intensity of Brownian motion. In addition, the effect of fuel concentration on the motion speed of microshells was dependent on whether the bubble generation was affected by the limited mass transfer in the microshell space. These findings play an important role for the design of microshell motors.

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.

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.

Janus-micromotor-based on-off luminescence sensor for active TNT detection.

An active TNT (2,4,6-trinitrotoluene) catalytic sensor based on Janus upconverting nanoparticle (UCNP)-functionalized micromotor capsules, displaying "on-off" luminescence with a low limit of detection has been developed. The Janus capsule motors were fabricated by layer-by-layer assembly of UCNP-functionalized polyelectrolyte microcapsules, followed by sputtering of a platinum layer onto one half of the capsule. By catalytic decomposition of hydrogen peroxide to oxygen bubbles, the Janus UCNP capsule motors are rapidly propelled with a speed of up to 110 µm s-1. Moreover, the Janus motors display efficient on-off luminescent detection of TNT. Owing to the unique motion of the Janus motor with bubble generation, the likelihood of collision with TNT molecules and the reaction rate between them are increased, resulting in a limit of detection as low as 2.4 ng mL-1 TNT within 1 minute. Such bubble-propelled Janus UCNP capsule motors have great potential for contaminated water analysis.

ZnO/ZnO2/Pt Janus Micromotors Propulsion Mode Changes with Size and Interface Structure: Enhanced Nitroaromatic Explosives Degradation under Visible Light.

Self-motile mesoporous ZnO/Pt-based Janus micromotors accelerated by bubble propulsion that provide efficient removal of explosives and dye pollutants via photodegradation under visible light are presented. Decomposition of H2O2 (the fuel) is triggered by a platinum catalytic layer asymmetrically deposited on the nanosheets of the hierarchical and mesoporous ZnO microparticles. The size-dependent motion behavior of the mesoporous micromotors is studied; the micromotors with average size ∼1.5 µm exhibit enhanced self-diffusiophoretic motion, whereas the fast bubble propulsion is detected for micromotors larger than 5 µm. The bubble-propelled mesoporous ZnO/Pt Janus micromotors show remarkable speeds of over 350 µm s-1 at H2O2 concentrations lower than 5 wt %, which is unusual for Janus micromotors based on dense materials such as ZnO. This high speed is related to efficient bubble nucleation, pinning, and growth due to the highly active and rough surface area of these micromotors, whereas the ZnO/Pt particles with a smooth surface and low surface area are motionless. We discovered new atomic interfaces of ZnO2 introduced into the ZnO/Pt micromotor system, as revealed by X-ray diffraction (XRD), which contribute to enhance their photocatalytic activity under visible light. Such coupling of the rapid movement with the high catalytic performance of ZnO/Pt Janus micromotors provides efficient removal of nitroaromatic explosives and dye pollutants from contaminated water under visible light without the need for UV irradiation. This paves the way for real-world environmental remediation efforts using microrobots.

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.

Internally/Externally Bubble-Propelled Photocatalytic Tubular Nanomotors for Efficient Water Cleaning.

We describe a highly effective bubble-propelled nanomotor for the photocatalytic decomposition of organic pollutants in water. Two different tubular TiO2 nanomotor systems are presented: one with Pt nanoparticles decorated on the inner surface and the other with Pt nanoparticles decorated on the outer surface. This is the first time that we have observed the autonomous movement of a tubular nanomotor without the aid of any surfactant, as well as a tubular nanomotor externally decorated with Pt propelled by oxygen bubbles. The synergy between the Pt nanoparticles and the superhydrophilic wetting behavior of the TiO2 nanotubes endows the two nanomotor systems with high speed at very low H2O2 fuel concentrations without the addition of any surfactant. The efficient photodecomposition of rhodamine B demonstrates the intermixing and photocatalytic ability of the two nanomotor systems, which opens new avenues for the development of multifunctional bubble-propelled micro/nanomotors with myriad practical applications.

Converting Chemical Energy to Electricity through a Three-Jaw Mini-Generator Driven by the Decomposition of Hydrogen Peroxide.

Energy conversion from a mechanical form to electricity is one of the most important research advancements to come from the horizontal locomotion of small objects. Until now, the Marangoni effect has been the only propulsion method to produce the horizontal locomotion to induce an electromotive force, which is limited to a short duration because of the specific property of surfactants. To solve this issue, in this article we utilized the decomposition of hydrogen peroxide to provide the propulsion for a sustainable energy conversion from a mechanical form to electricity. We fabricated a mini-generator consisting of three parts: a superhydrophobic rotator with three jaws, three motors to produce a jet of oxygen bubbles to propel the rotation of the rotator, and three magnets integrated into the upper surface of the rotator to produce the magnet flux. Once the mini-generator was placed on the solution surface, the motor catalyzed the decomposition of hydrogen peroxide. This generated a large amount of oxygen bubbles that caused the generator and integrated magnets to rotate at the air/water interface. Thus, the magnets passed under the coil area and induced a change in the magnet flux, thus generating electromotive forces. We also investigated experimental factors, that is, the concentration of hydrogen peroxide and the turns of the solenoid coil, and found that the mini-generator gave the highest output in a hydrogen peroxide solution with a concentration of 10 wt % and under a coil with 9000 turns. Through combining the stable superhydrophobicity and catalyst, we realized electricity generation for a long duration, which could last for 26 000 s after adding H2O2 only once. We believe this work provides a simple process for the development of horizontal motion and provides a new path for energy reutilization.

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.

Self-propelled particles that transport cargo through flowing blood and halt hemorrhage.

Delivering therapeutics deep into damaged tissue during bleeding is challenging because of the outward flow of blood. When coagulants cannot reach and clot blood at its source, uncontrolled bleeding can occur and increase surgical complications and fatalities. Self-propelling particles have been proposed as a strategy for transporting agents upstream through blood. Many nanoparticle and microparticle systems exhibiting autonomous or collective movement have been developed, but propulsion has not been used successfully in blood or used in vivo to transport therapeutics. We show that simple gas-generating microparticles consisting of carbonate and tranexamic acid traveled through aqueous solutions at velocities of up to 1.5 cm/s and delivered therapeutics millimeters into the vasculature of wounds. The particles transported themselves through a combination of lateral propulsion, buoyant rise, and convection. When loaded with active thrombin, these particles worked effectively as a hemostatic agent and halted severe hemorrhage in multiple animal models of intraoperative and traumatic bleeding. Many medical applications have been suggested for self-propelling particles, and the findings of this study show that the active self-fueled transport of particles can function in vivo to enhance drug delivery.