Magneto-Acoustic Field-Induced Unstable Interface of Magnetic Microswarm.

Research on the interfacial instability of two-phase systems can help in gaining a better understanding of various hydrodynamic instabilities in nature. However, owing to the nonlinear and complex spatiotemporal dynamics of the unstable interface, the instability is challenging to control and suppress. This paper presents a novel interfacial instability of the magnetic microswarm induced by the competition between the destabilizing effect of magnetic field and the stabilizing effect of acoustic field. The physics underlying this novel phenomenon is discussed by analyzing the contributions of the external fields. Unlike previous studies, this study demonstrates that the instability is independent of the interfacial force or diffusion effect and can persist without dissipation over time. The manipulation of the unstable interface is further achieved by adjusting the configuration of the magneto-acoustic system. This approach can be used in thermal encoding metamaterials and has great potential applications in systems where the instability is detrimental.

Mimicking Motor Proteins: Wall-Guided Self-Navigation of Microwheels.

Untethered micro/nanorobots (MNRs) show great promise in biomedicine. However, high-precision targeted in vivo navigation of MNRs into both deep and tiny microtube networks comes with big challenges because the present medical imaging cannot simultaneously meet the requirements of high resolution, high penetration depth, and high real-time performance. Inspired by intracellular motor proteins that transport cargo along cytoskeletal tracks, this study proposed a microtube inwall-guided targeted self-navigation strategy of magnetic microwheels (µ-wheels) that relies only on interactions with a microtube inwall, compared to conventional techniques that rely on real-time imaging and tracking of MNRs. By presetting the direction of the rotating magnetic field, the µ-wheel realized targeted navigation along the inwall. The propulsion principles behind it are elaborated. The targeted self-navigation of the µ-wheels in three-dimensional microtube networks, a spiral microtube, and an intrahepatic bile duct of a pig was conducted. Lastly, based on the strategy, a practical tumor early detection method was proposed and verified by means of magnetic resonance imaging. The microtube inwall-guided targeted self-navigation strategy reduces the dependence of in vivo targeted navigation of MNRs on the real-time performance of medical imaging technology and greatly contributes to the development of MNRs in biomedical applications.

Three-Dimensional Rock Core-Like Microstructure Fabricated by Additive Manufacturing for Petroleum Engineering.

To improve the recovery rate of oil in the formation, oil recovery technology has been continuously studied. Considering the experimental cost and data measurement in oil recovery research, laboratory oil recovery is the most effective method. The rock core model used in the simulation directly affects whether the research results are credible. However, the current three-dimensional rock core model manufacturing methods and corresponding models lack of reproducible, customizable, and visualized characteristics. In this study, a reproducible rock core model of microsphere accumulation based on the structure of natural rock core was designed and manufactured by microstereolithography. Oil recovery experiments and simulation studies show that the rock core model has similar flow characteristics to natural rock cores. In addition, resin rock core models with different structures and hydrogel rock core models with deformability are also manufactured by microstereolithography and used for simulation analysis. This research provides an effective and reproducible rock core structure model for the experiment of oil recovery research.

A magnetic field-driven multi-functional "medical ship" for intestinal tissue collection in vivo.

The incidence of intestinal cancer has risen significantly. Because of the many challenges posed by the complex environment of the intestine, it is difficult to diagnose accurately and painlessly using conventional methods, which requires the development of new body-friendly diagnostic methods. Micro- and nanomotors show great potential for biomedical applications in restricted environments. However, the difficulty of recycling has been a constraint in the collection of biological tissues for diagnostic purposes. Here, we propose a multi-functional "medical ship" (MFMS) that can be rapidly driven by a magnetic field and can reversibly "open" and "close" its internal storage space under NIR laser irradiation. It provides a transportation and recovery platform for micro- and nanomotors and cargoes. In addition, fast selection of the MFMS and magnetic nanoparticles (MNPs) can be realized through adjusting the strength and frequency of the external magnetic field. Rapid encapsulation of intestinal tissues by MNPs was achieved using a low-frequency rotating magnetic field. In addition, we demonstrated the controlled release of MNPs using the MFMS and the collection of intestinal tissues. The proposed MFMS is an intelligent and controllable transportation platform with a simple structure, which is expected to be a new tool for performing medical tasks within the digestive system.

Breaking through Barriers: Ultrafast Microbullet Based on Cavitation Bubble.

Micromotors hold great promise for extensive practical applications such as those in biomedical domains and reservoir exploration. However, insufficient propulsion of the micromotor limits its application in crossing biological barriers and breaking reservoir boundaries. In this study, an ultrafast microbullet based on laser cavitation that can utilize the energy of a cavitation bubble and realize its own hurtling motion is reported. The experiments are performed using high-speed photography. A boundary integral method is adopted to reveal the motion mechanism of a polystyrene (PS)/magnetic nanoparticle (MNP) microbullet under the action of laser cavitation. Furthermore, the influence of certain factors (including laser intensity, microbullet size, and ambient temperature) on the motion of the microbullet was explored. For the PS/MNP microbullet driven by laser cavitation, the instantaneous velocity obtained can reach 5.23 m s-1 . This strategy of driving the PS/MNP microbullet provides strong penetration ability and targeted motion. It is believed that the reported propulsion mechanism opens up new possibilities for micromotors in a wide range of engineering applications.

Wheel-like Magnetic-Driven Microswarm with a Band-Aid Imitation for Patching Up Microscale Intestinal Perforation.

Microscale intestinal perforation can cause considerable mortality and is very difficult to treat using conventional methods owing to the numerous challenges associated with microscale operations, which require the development of new body-friendly and effective treatment methods. Swarming micro- and nanomotors have shown great potential in biomedical applications in complex and hard-to-reach environments. Herein, we present a wheel-like magnetic-driven microswarm (WLM) with a band-aid imitation to patch microscale intestinal perforations by pasting on the perforation point in mucus-filled environments. A method called "packing under rolling" was applied to make the formed microswarms denser and rounder. Microswarms with variable aspect ratios can be fabricated by tuning the frequency and strength of the external magnetic field. Actuation and navigation in a confined complex environment, locomotion on three-dimensional surfaces, and multiple switchable motion modes have been realized by combining AC and DC magnetic fields. Moreover, we demonstrated WLM controllable navigation, movement, and microscale perforation patching in the chicken intestines ex vivo. The proposed strategy will contribute to the treatment of microscale intestinal perforation and may be applicable to novel, precise topical medication and microsurgery.

Nature-inspired micro/nanomotors.

Living things in nature have evolved with unique morphologies, structures, materials, behaviors, and functions to survive in complex natural environments. Nature has inspired the design ideas, preparation methods, and applications of versatile micro/nanomotors. This review summarizes diverse nature-inspired micro/nanomotors, which can be divided into five groups: (i) natural morphology-inspired micro/nanomotors, whose shapes are designed to imitate the morphologies of plants, animals, and objects in nature. (ii) Natural structure-inspired micro/nanomotors, which use structures from plants, red blood cells, and platelet cells as components of micro/nanomotors, or directly use sperm cells and microorganisms as the engines of micro/nanomotors. (iii) Natural behavior-inspired micro/nanomotors, which are proposed to mimic natural behaviors such as motion behavior, swarm behavior, and communication behavior between individuals. (iv) Micro/nanomotors inspired by both natural morphology and behavior. Nature makes it possible for synthetic micro/nanomotors to possess interesting morphologies, novel preparation methods, new propulsion modes, innovative functions, and broad applications. The nature-inspired micro/nanomotors could provide a promising platform for various practical fields.

Enhanced Light-Harvesting Efficiency and Adaptation: A Review on Visible-Light-Driven Micro/Nanomotors.

As visible light accounts for a larger proportion of solar energy and is harmless to living organisms, it has the potential to be the energy source of micro/nanomotors, which transform visible-light energy into mechanical motion, for different applications, especially in environmental remediation. However, how to precisely control the motion of visible-light-driven micro/nanomotors (VLD-MNMs) and efficiently utilize the weak visible-light photon energy to acquire rapid motion are significant challenges. This review summarizes the most critical aspects, involving photoactive materials, propulsion mechanisms, control methods, and applications of VLD-MNMs, and discusses strategies to systematically enhance the energy-harvesting efficiency and adaptation. At first, the photoactive materials have been divided into inorganic and organic photoactive materials and comprehensively discussed. Then, different propulsion mechanisms of the current VLD-MNMs are presented to explain the improvement in the actuation force, speed, and environmental adaptability. In addition, considering the characteristics of easy control of VLD-MNMs, we summarized the direction, speed, and cluster control methods of VLD-MNMs for different application requirements. Subsequently, the potential applications of VLD-MNMs, e.g., in environmental remediation, micropumps, cargo delivery, and sensing in microscale, are presented. Finally, discussions and suggestions for future directions to enhance the energy-harvesting efficiency and adaptation of VLD-MNMs are provided.

Self-Propelled Janus Microdimer Swimmers under a Rotating Magnetic Field.

Recent strides in micro- and nanofabrication technology have enabled researchers to design and develop new micro- and nanorobots for biomedicine and environmental monitoring. Due to its non-invasive remote actuation and convenient navigation abilities, magnetic propulsion has been widely used in micro- and nanoscale robotic systems. In this article, a highly efficient Janus microdimer swimmer propelled by a rotating uniform magnetic field was investigated experimentally and numerically. The velocity of the Janus microdimer swimmer can be modulated by adjusting the magnetic field frequency with a maximum speed of 133 µm·s-1 (≈13.3 body length s-1) at the frequency of 32 Hz. Fast and accurate navigation of these Janus microdimer swimmers in complex environments and near obstacles was also demonstrated. This efficient propulsion behavior of the new Janus microdimer swimmer holds considerable promise for diverse future practical applications ranging from nanoscale manipulation and assembly to nanomedicine.

Motile Micropump Based on Synthetic Micromotors for Dynamic Micropatterning.

Micropump systems show great potential on the micropatterning process as a result of remarkable performance and functionality. However, existing micropumps cannot be employed as direct writing tools to perform the complex micropatterning process because of their lacking motility and controllability. Here, we propose a motile micropump system based on the combination of a water-driven ZnO/Ni/polystyrene Janus micromotor with a traditional immobilized micropump. This novel motile micropump system can translate the trajectory of Janus micromotors into predefined micropatterns by pumping away passive silica particles around the micromotor under the effect of diffusiophoresis. The resolution and efficiency of the micropatterning process can be regulated by controlling the diameters of Janus micromotors. Diverse surface micropatterns can be fabricated though remote magnetic control of the motile micropump system. Such ability to transform the versatile motile micropump into predetermined surface micropatterns creates new opportunities for mask-free micropatterning.

Coexisting Cooperative Cognitive Micro-/Nanorobots.

Interaction behaviors are a crucial, distinctive feature of organisms in ecological systems. Inspired by nature, micro-/nanorobots also show interaction behaviors with a significant number of novel and advanced functionalities. The micro-/nanoscale size and self-driven abilities of micro-/nanorobots have attracted considerable attention for potential applications. The mission of micro-/nanorobots has evolved from basic mechanical transfer or assistance to a diverse range of complex tasks, such as targeted drug delivery, biosensing, and environmental remediation. Herein, we focus on the interaction behavior of coexisting cooperative cognitive (Tri-Co) micro-/nanorobots, summarize recent research, and classify the interactions into three broad types: with the environment, with robot peers, and with human external control.

Micro-/Nanorobots Propelled by Oscillating Magnetic Fields.

Recent strides in micro- and nanomanufacturing technologies have sparked the development of micro-/nanorobots with enhanced power and functionality. Due to the advantages of on-demand motion control, long lifetime, and great biocompatibility, magnetic propelled micro-/nanorobots have exhibited considerable promise in the fields of drug delivery, biosensing, bioimaging, and environmental remediation. The magnetic fields which provide energy for propulsion can be categorized into rotating and oscillating magnetic fields. In this review, recent developments in oscillating magnetic propelled micro-/nanorobot fabrication techniques (such as electrodeposition, self-assembly, electron beam evaporation, and three-dimensional (3D) direct laser writing) are summarized. The motion mechanism of oscillating magnetic propelled micro-/nanorobots are also discussed, including wagging propulsion, surface walker propulsion, and scallop propulsion. With continuous innovation, micro-/nanorobots can become a promising candidate for future applications in the biomedical field. As a step toward designing and building such micro-/nanorobots, several types of common fabrication techniques are briefly introduced. Then, we focus on three propulsion mechanisms of micro-/nanorobots in oscillation magnetic fields: (1) wagging propulsion; (2) surface walker; and (3) scallop propulsion. Finally, a summary table is provided to compare the abilities of different micro-/nanorobots driven by oscillating magnetic fields.

Biomimetic Platelet-Camouflaged Nanorobots for Binding and Isolation of Biological Threats.

One emerging and exciting topic in robotics research is the design of micro-/nanoscale robots for biomedical operations. Unlike industrial robots that are developed primarily to automate routine and dangerous tasks, biomedical nanorobots are designed for complex, physiologically relevant environments, and tasks that involve unanticipated biological events. Here, a biologically interfaced nanorobot is reported, made of magnetic helical nanomotors cloaked with the plasma membrane of human platelets. The resulting biomimetic nanorobots possess a biological membrane coating consisting of diverse functional proteins associated with human platelets. Compared to uncoated nanomotors which experience severe biofouling effects and hence hindered propulsion in whole blood, the platelet-membrane-cloaked nanomotors disguise as human platelets and display efficient propulsion in blood over long time periods. The biointerfaced nanorobots display platelet-mimicking properties, including adhesion and binding to toxins and platelet-adhering pathogens, such as Shiga toxin and Staphylococcus aureus bacteria. The locomotion capacity and platelet-mimicking biological function of the biomimetic nanomotors offer efficient binding and isolation of these biological threats. The dynamic biointerfacing platform enabled by platelet-membrane cloaked nanorobots thus holds considerable promise for diverse biomedical and biodefense applications.

Bioinspired Chemical Communication between Synthetic Nanomotors.

While chemical communication plays a key role in diverse natural processes, the intelligent chemical communication between synthetic nanomotors remains unexplored. The design and operation of bioinspired synthetic nanomotors is presented. Chemical communication between nanomotors is possible and has an influence on propulsion behavior. A chemical "message" is sent from a moving activator motor to a nearby activated (receiver) motor by release of Ag+ ions from a Janus polystyrene/Ni/Au/Ag activator motor to the activated Janus SiO2 /Pt nanomotor. The transmitted silver signal is translated rapidly into a dramatic speed change associated with the enhanced catalytic activity of activated motors. Selective and successive activation of multiple nanomotors is achieved by sequential localized chemical communications. The concept of establishing chemical communication between different synthetic nanomotors paves the way to intelligent nanoscale robotic systems that are capable of cooperating with each other.

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.

Magnetically Propelled Fish-Like Nanoswimmers.

The swimming locomotion of fish involves a complex interplay between a deformable body and induced flow in the surrounding fluid. While innovative robotic devices, inspired by physicomechanical designs evolved in fish, have been created for underwater propulsion of large swimmers, scaling such powerful locomotion into micro-/nanoscale propulsion remains challenging. Here, a magnetically propelled fish-like artificial nanoswimmer is demonstrated that emulates the body and caudal fin propulsion swimming mechanism displayed by fish. To mimic the deformable fish body for periodic shape changes, template-electrosynthesized multisegment nanowire swimmers are used to construct the artificial nanofishes (diameter 200 nm; length 4.8 µm). The resulting nanofish consists a gold segment as the head, two nickel segments as the body, and one gold segment as the caudal fin, with three flexible porous silver hinges linking each segment. Under an oscillating magnetic field, the propulsive nickel elements bend the body and caudal fin periodically to generate travelling-wave motions with speeds exceeding 30 µm s-1 . The propulsion dynamics is studied theoretically using the immersed boundary method. Such body-deformable nanofishes exhibit a high swimming efficiency and can serve as promising biomimetic nanorobotic devices for nanoscale biomedical applications.