Reconfigurable Assembly of Active Liquid Metal Colloidal Cluster

Nanorobot Swarms Made with Laser-Induced Graphene@Fe3O4 Nanoparticles with Controllable Morphology for Targeted Drug Delivery

Magnetic nanorobot swarms can mimic group behaviors in nature and can be flexibly controlled by programmable magnetic fields, thereby having great potential in various applications. This paper presents a novel approach for the rapid and large-scale processing of laser-induced graphene (LIG) @Fe3O4-based-nanorobot swarms utilizing one-step UV laser processing technology. The swarm is capable of forming a variety of reversible morphologies under the magnetic field, including vortex-like and strip-like, as well as the interconversion of these, demonstrating high levels of controllability and flexibility. Moreover, the maximum forward motion speed of the nanorobot swarm is up to 2165 μm/s, and the drug loading and release ability of such a nanorobot swarm is enhanced about 50 times due to the presence of graphene, enabling the nanorobot swarm to show rapid and precise targeted drug delivery. Importantly, by controllable morphology transformation to conform to the complicated requirements for the magnetic field, the drug-loaded swarm can smoothly pass through a width-varying zigzag channel while maintaining 96% of the initial drug-loading, demonstrating that LIG @Fe3O4 NPs-based nanorobot swarm can provide effective and controllable targeted drug delivery in complex passages.

Bioinspired Liquid Metal Based Soft Humanoid Robots

The pursuit of constructing humanoid robots to replicate the anatomical structures and capabilities of human beings has been a long-standing significant undertaking and especially garnered tremendous attention in recent years. However, despite the progress made over recent decades, humanoid robots have predominantly been confined to those rigid metallic structures, which however starkly contrast with the inherent flexibility observed in biological systems. To better innovate this area, the present work systematically explores the value and potential of liquid metals and their derivatives in facilitating a crucial transition towards soft humanoid robots. Through a comprehensive interpretation of bionics, an overview of liquid metals' multifaceted roles as essential components in constructing advanced humanoid robots-functioning as soft actuators, sensors, power sources, logical devices, circuit systems, and even transformable skeletal structures-is presented. It is conceived that the integration of these components with flexible structures, facilitated by the unique properties of liquid metals, can create unexpected versatile functionalities and behaviors to better fulfill human needs. Finally, a revolution in humanoid robots is envisioned, transitioning from metallic frameworks to hybrid soft-rigid structures resembling that of biological tissues. This study is expected to provide fundamental guidance for the coming research, thereby advancing the area.

Multiple Magneto-Optical Microrobotic Collectives with Selective Control in Three Dimensions Under Water

Inspired by natural swarms, various methods are developed to create artificial magnetic microrobotic collectives. However, these magnetic collectives typically receive identical control inputs from a common external magnetic field, limiting their ability to operate independently. And they often rely on interfaces or boundaries for controlled movement, posing challenges for independent, three-dimensional(3D) navigation of multiple magnetic collectives. To address this challenge, self-assembled microrobotic collectives are proposed that can be selectively actuated in a combination of external magnetic and optical fields. By harnessing both actuation methods, the constraints of single actuation approaches are overcome. The magnetic field excites the self-assembly of colloids and maintains the self-assembled microrobotic collectives without disassembly, while the optical field drives selected microrobotic collectives to perform different tasks. The proposed magnetic-photo microrobotic collectives can achieve independent position and path control in the two-dimensional (2D) plane and 3D space. With this selective control strategy, the microrobotic collectives can cooperate in convection and mixing the dye in a confined space. The results present a systematic approach for realizing selective control of multiple microrobotic collectives, which can address multitasking requirements in complex environments.

AcousticRobots: Smart acoustically powered micro-/nanoswimmers for precise biomedical applications

Although nanotechnology has evolutionarily progressed in biomedical field over the past decades, achieving satisfactory therapeutic effects remains difficult with limited delivery efficiency. Ultrasound could provide a deep penetration and maneuverable actuation to efficiently power micro-/nanoswimmers with little harm, offering an emerging and fascinating alternative to the active delivery platform. Recent advances in novel fabrication, controllable concepts like intelligent swarm and the integration of hybrid propulsions have promoted its function and potential for medical applications. In this review, we will summarize the mechanisms and types of ultrasonically propelled micro/nanorobots (termed here as "AcousticRobots"), including the interactions between AcousticRobots and acoustic field, practical design considerations (e.g., component, size, shape), the synthetic methods, surface modification, controllable behaviors, and the advantages when combined with other propulsion approaches. The representative biomedical applications of functional AcousticRobots are also highlighted, including drug delivery, invasive surgery, eradication on the surrounding bio-environment, cell manipulation, detection, and imaging, etc. We conclude by discussing the challenges and outlook of AcousticRobots in biomedical applications.

Light-Induced Dynamic Manipulation of Liquid Metal Droplets in the Ambient Atmosphere

Dynamic manipulation of liquid metal (LM) droplets, a material combining metallicity and fluidity, has recently revealed tremendous potential in developing unconstrained microrobots. LM manipulating techniques based on magnetic fields, electric fields, chemical reactions, and ion concentration gradients in liquid environments have advanced considerably, but dynamic manipulation in air remains a challenge. Herein, a photoresponsive pyroelectric superhydrophobic (PPS) platform is proposed for noncontact, flexible, and controllable manipulation in the ambient atmosphere. The PPS can generate additional free charges when illuminated by light, thus generating the driving force to manipulate liquid metal droplets. By using the synergistic effect of dielectrophoretic and electrostatic forces generated under light navigation, liquid metal droplets can achieve a series of complex motion behaviors, such as climbing slopes, going over steps, avoiding obstacles, crossing mazes, etc. We further extend the light control of liquid metal droplets to robots applied in electronic circuits, including circuit switching robots and circuit welding robots. This light strategy for manipulating liquid metal droplets provides insights into the development of intelligent, responsive interfaces and simultaneously provides possibilities for the application of liquid metals.

Enzymatic micro/nanomotors in biomedicine: from single motors to swarms

Micro/nanomotors (MNMs) have evolved from single self-propelled entities to versatile systems capable of performing one or multiple biomedical tasks. When single MNMs self-assemble into coordinated swarms, either under external control or triggered by chemical reactions, they offer advantages that individual MNMs cannot achieve. These benefits include intelligent multitasking and adaptability to changes in the surrounding environment. Here, we provide our perspective on the evolution of MNMs, beginning with the development of enzymatic MNMs since the first theoretical model was proposed in 2005. These enzymatic MNMs hold immense promise in biomedicine due to their advantages in biocompatibility and fuel availability. Subsequently, we introduce the design and application of single motors in biomedicine, followed by the control of MNM swarms and their biomedical applications. In the end, we propose viable solutions for advancing the development of MNM swarms and anticipate valuable insights into the creation of more intelligent and controllable MNM swarms for biomedical applications.

Nanoarchitectonics with a Membrane-Embedded Electron Shuttle Mimics the Bioenergy Anabolism of Mitochondria

Enhanced bioenergy anabolism through transmembrane redox reactions in artificial systems remains a great challenge. Here, we explore synthetic electron shuttle to activate transmembrane chemo-enzymatic cascade reactions in a mitochondria-like nanoarchitecture for augmenting bioenergy anabolism. In this nanoarchitecture, a dendritic mesoporous silica microparticle as inner compartment possesses higher load capacity of NADH as proton source and allows faster mass transfer. In addition, the outer compartment ATP synthase-reconstituted proteoliposomes. Like natural enzymes in the mitochondrion respiratory chain, a small synthetic electron shuttle embedded in the lipid bilayer facilely mediates transmembrane redox reactions to convert NADH into NAD+ and a proton. These facilitate an enhanced outward proton gradient to drive ATP synthase to rotate for catalytic ATP synthesis with improved performance in a sustainable manner. This work opens a new avenue to achieve enhanced bioenergy anabolism by utilizing a synthetic electron shuttle and tuning inner nanostructures, holding great promise in wide-range ATP-powered bioapplications.

Controlling the Collective Behaviors of Ultrasound-Driven Nanomotors via Frequency Regulation

Controlling the collective behavior of micro/nanomotors with ultrasound may enable new functionality in robotics, medicine, and other engineering disciplines. Currently, various collective behaviors of nanomotors, such as assembly, reconfiguration, and disassembly, have been explored by using acoustic fields with a fixed frequency, while regulating their collective behaviors by varying the ultrasound frequency still remains challenging. In this work, we designed an ultrasound manipulation methodology that allows nanomotors to exhibit different collective behaviors by regulating the applied ultrasound frequency. The experimental results and FEM simulations demonstrate that the secondary ultrasonic waves produced from the edge of the sample cell lead to the formation of complex acoustic pressure fields and microfluidic patterns, which causes these collective behaviors. This work has important implications for the design of artificial actuated nanomotors and optimize their performances.

Ultrasound-Induced Enrichment of Ultra-Trace miRNA Biosensing in Nanoliter Samples

The rapid and accurate analysis of micro-samples is a crucial foundation for precision medicine, particularly for early screening and monitoring of cancer, where it holds significant importance. Ultrasound-based multifunctional biocompatible manipulation techniques have been extensively applied in a variety of biomedical fields, providing insights for the development of rapid, cost-effective, and accurate biomarker detection strategies. In this chapter, we combine ultrasound-based gradient pressure fields with functionalized microsphere enrichment to develop a biosensing method for ultra-trace miRNA enrichment in nanoliter samples without PCR. This system relies on inexpensive capillaries, enabling simultaneous visual imaging and trace sample detection.

Liquid Metal Combinatorics toward Materials Discovery

Liquid metals and their derivatives provide several opportunities for fundamental and practical exploration worldwide. However, the increasing number of studies and shortage of desirable materials to fulfill different needs also pose serious challenges. Herein, to address this issue, a generalized theoretical frame that is termed as "Liquid Metal Combinatorics" (LMC) is systematically presented, and summarizes promising candidate technical routes toward new generation material discovery. The major categories of LMC are defined, and eight representative methods for manufacturing advanced materials are outlined. It is illustrated that abundant targeted materials can be efficiently designed and fabricated via LMC through deep physical combinations, chemical reactions, or both among the main bodies of liquid metals, surface chemicals, precipitated ions, and other materials. This represents a large class of powerful, reliable, and modular methods for innovating general materials. The achieved combinatorial materials not only maintained the typical characteristics of liquid metals but also displayed distinct tenability. Furthermore, the fabrication strategies, wide extensibility, and pivotal applications of LMC are classified. Finally, by interpreting the developmental trends in the area, a perspective on the LMC is provided, which warrants its promising future for society.

Bioinspired self-assembled colloidal collectives drifting in three dimensions underwater

Active matter systems feature a series of unique behaviors, including the emergence of collective self-assembly structures and collective migration. However, realizing collective entities formed by synthetic active matter in spaces without wall-bounded support makes it challenging to perform three-dimensional (3D) locomotion without dispersion. Inspired by the migration mechanism of plankton, we propose a bimodal actuation strategy in the artificial colloidal systems, i.e., combining magnetic and optical fields. The magnetic field triggers the self-assembly of magnetic colloidal particles to form a colloidal collective, maintaining numerous colloids as a dynamically stable entity. The optical field allows the colloidal collectives to generate convective flow through the photothermal effect, enabling them to use fluidic currents for 3D drifting. The collectives can perform 3D locomotion underwater, transit between the water-air interface, and have a controlled motion on the water surface. Our study provides insights into designing smart devices and materials, offering strategies for developing synthetic active matter capable of controllable collective movement in 3D space.

Biomimetic submicromotor with NIR light triggered motion and cargo release inspired by cuttlefish

Biomimetic design is very helpful and significant for the smart construction of micro/nanomotors with artificial intelligence. In this work, inspired by cuttlefish, who can rapidly eject poisonous ink and are also capable of fast movement to escape, we designed and fabricated a biomimetic submicromotor with the ability of simultaneous quick movement and a temperature threshold caused explosive cargo release triggered by near infra-red (NIR) light irradiation, which was approximately equivalent to the action of cuttlefish when encountering a predator. The yolk@shell structured polydopamine@mesoporous silica (PDA@MS60) with immovable and asymmetric yolk distribution was employed as a platform, and this was followed by the simultaneous encapsulation of phase change materials (PCM) and cargo molecules. The NIR light irradiation could not only propel the direct motion of the submicromotor, but also caused the explosive release of the cargo loaded in the submicromotor when the temperature exceeded the melting point of the PCM.

Horizontal and Vertical Coalescent Microrobotic Collectives Using Ferrofluid Droplets

Many artificial miniature robotic collectives have been developed to overcome the inherent limitations of inadequate individual capabilities. However, the basic building blocks of the reported collectives are mainly in the solid state, where the morphological boundaries of internal individuals are clear and cannot genuinely merge. Miniature robotic collectives based on liquid units still need to be explored; such on-demand mergeable swarm systems are advantageous for adapting to the changing external environment. Here, a strategy to achieve a coalescent collective system we presented that exploits the ferrofluid droplets' splitting and coalescence properties to trigger the formation of horizontal multimodal and vertical gravity-resistant collectives and unveil pattern-enabled robotic functionalities. When subjected to a time-varying magnetic field, the droplet swarm exhibits a variety of morphologies ranging from horizontal collectives, including vortex-like, chain-like, and crystal-like patterns to vertical layer-upon-layer patterns. Using experiments and simulations, the formation and transformation of different morphological collectives are shown and their robust environmental adaptability are demonstrated. Potential applications of the multimodal droplet collectives are presented, including exploring an unknown environment, targeted object delivery, and fluid flow filtration in a lab-on-a-chip. This work may facilitate the design of microrobotic swarm systems and expand the range of materials for miniature robots.

High-Thermal-Conductivity and High-Fluidity Heat Transfer Emulsion with 89 wt % Suspended Liquid Metal Microdroplets

Colloidal suspensions of thermally conductive particles in a carrier fluid are considered promising heat transfer fluids for various thermal energy transfer applications, such as transportation, plants, electronics, and renewable energy systems. The thermal conductivity (k) of the particle-suspended fluids can be improved substantially by increasing the concentration of conductive particles above a "thermal percolation threshold," which is limited because of the vitrification of the resulting fluid at the high particle loadings. In this study, eutectic Ga-In liquid metal (LM) was employed as a soft high-k filler dispersed as microdroplets at high loadings in paraffin oil (as a carrier fluid) to produce an emulsion-type heat transfer fluid with the combined advantages of high thermal conductivity and high fluidity. Two types of the LM-in-oil emulsions, which were produced via the probe-sonication and rotor-stator homogenization (RSH) methods, demonstrated significant improvements in k, i.e., Δk ∼409 and ∼261%, respectively, at the maximum investigated LM loading of 50 vol % (∼89 wt %), attributed to the enhanced heat transport via high-k LM fillers above the percolation threshold. Despite the high filler loading, the RSH-produced emulsion retained remarkably high fluidity, with a relatively low viscosity increase and no yield stress, demonstrating its potential as a circulatable heat transfer fluid.

Small-Scale Robotics with Tailored Wettability

Small-scale robots (SSRs) have emerged as promising and versatile tools in various biomedical, sensing, decontamination, and manipulation applications, as they are uniquely capable of performing tasks at small length scales. With the miniaturization of robots from the macroscale to millimeter-, micrometer-, and nanometer-scales, the viscous and surface forces, namely adhesive forces and surface tension have become dominant. These forces significantly impact motion efficiency. Surface engineering of robots with both hydrophilic and hydrophobic functionalization presents a brand-new pathway to overcome motion resistance and enhance the ability to target and regulate robots for various tasks. This review focuses on the current progress and future perspectives of SSRs with hydrophilic and hydrophobic modifications (including both tethered and untethered robots). The study emphasizes the distinct advantages of SSRs, such as improved maneuverability and reduced drag forces, and outlines their potential applications. With continued innovation, rational surface engineering is expected to endow SSRs with exceptional mobility and functionality, which can broaden their applications, enhance their penetration depth, reduce surface fouling, and inhibit bacterial adhesion.

Current Progress in Gallium-based Liquid Metals for Combinatory Phototherapeutic Anticancer Applications

A variety of therapeutic approaches using liquid metal (LM) have been intensively investigated, due to its unique physico-chemical properties that include high surface tension, fluidity, shape deformability, thermal conductivity, and electrical conductivity. Among a series of LMs, the relatively lower toxicity and minimal volatility of gallium (Ga)-based LMs (GaLMs) enables their usage in a series of potential biomedical applications, especially implantable platforms, to treat multiple diseases. In addition, the highly efficient conversion of light energy into thermal or chemical energy via GaLMs has led to recent developments in photothermal and photodynamic applications for anticancer treatments. As attractive photothermal agents or photosensitizers, a systematic interpretation of the structural characteristics and photo-responsive behaviors of GaLMs is necessary to develop effective anticancer engineering applications. Therefore, the aim of this review is to provide a comprehensive summary of currently suggested GaLM-mediated photo-therapeutic cancer treatments. In particular, the review summarizes (1) surface coating techniques to form stable and multifunctional GaLM particulates, (2) currently investigated GaLM-mediated photothermal and photodynamic anticancer therapies, (3) synergistic efficacies with the aid of additional interventions, and (4) 3D composite gels embedded with GaLMs particles, to convey the potential technological advances of LM in this field.

Active control of equilibrium, near-equilibrium, and far-from-equilibrium colloidal systems

The development of top-down active control over bottom-up colloidal assembly processes has the potential to produce materials, surfaces, and objects with applications in a wide range of fields spanning from computing to materials science to biomedical engineering. In this review, we summarize recent progress in the field using a taxonomy based on how active control is used to guide assembly. We find there are three distinct scenarios: (1) navigating kinetic pathways to reach a desirable equilibrium state, (2) the creation of a desirable metastable, kinetically trapped, or kinetically arrested state, and (3) the creation of a desirable far-from-equilibrium state through continuous energy input. We review seminal works within this framework, provide a summary of important application areas, and present a brief introduction to the fundamental concepts of control theory that are necessary for the soft materials community to understand this literature. In addition, we outline current and potential future applications of actively-controlled colloidal systems, and we highlight important open questions and future directions.

Sequentially Selective Coalescence of Binary Self-Propelled Droplets upon Collective Motion

Subsequent synthesis and detection using droplets as microreactors have shown promise in the development of novel materials and drugs because microreactors enable small-scale synthesis and detection of covalent/non-covalent intermolecular interactions. Self-organization exhibited by autonomous droplets under non-equilibrium conditions is beneficial for manipulating the sequentiality and selectivity of droplet coalescence because expensive equipment or elaborate techniques are not required with the autonomy of droplets. However, to our knowledge, selective coalescence caused by the collective motion of self-propelled droplets has not been demonstrated in inanimate systems. Here, we report sequentially selective coalescence based on the dynamic collective pattern of self-propelled droplets composed of ethyl salicylate (ES) or butyl salicylate (BS). When ES and BS droplets were placed on an aqueous sodium dodecyl sulfate (SDS) solution, the collective motion of droplets resulted in three stages of selective coalescence on the time development. Initially, coalescence was observed only between different types of self-propelled droplets. Subsequently, the formed droplets selectively coalesced with ES droplets. Finally, mature droplets merged with BS droplets. The sequentially selective coalescence was discussed from the dynamic pattern formation of swarming droplets and the collapse of the SDS monolayer at the o/w interface caused by the difference in Laplace pressure and the interfacial instability at the contact point between droplets. Thus, this study formulates a strategy of sequentially selective coalescence of droplets via the collective motion of non-identical self-propelled droplets, promoting a new type of powerful and efficient automation technology based on an autonomous inanimate manner of spatiotemporal pattern formation under non-equilibrium conditions for the droplet manipulation.

Self-propelled predator-prey of swarming Janus micromotors

It is a long-standing challenge to accomplish bionic microrobot that acts in a similar way of white blood cell, chasing bacteria in complex environment. Without an effective external control field, most swarming microrobots systems are usually unable to perform directional movement and redirect their motion to capture the target. Here we report the predatory-prey dynamics of self-propelled clusters of Janus micromotors. The active cluster generates an oxygen bubbles cloud around itself by decomposing H₂O₂, which levitated it above the substrate, enhancing its mobility in solution to wander around to devour other clusters. The fast decomposition of H₂O₂ also induced a tubular low-concentration zone that bridges two clusters far separated from each other, resulting in a diffusio-osmotic pressure that drives the two clusters to meet. This predatory-prey phenomena mimic white blood cells chasing bacteria and swarming flocks in nature, shedding light on emergent collective intelligence in biology.

Collective Behaviors of Active Matter Learning from Natural Taxes Across Scales

Taxis orientation is common in microorganisms, and it provides feasible strategies to operate active colloids as small-scale robots. Collective taxes involve numerous units that collectively perform taxis motion, whereby the collective cooperation between individuals enables the group to perform efficiently, adaptively, and robustly. Hence, analyzing and designing collectives is crucial for developing and advancing microswarm toward practical or clinical applications. In this review, natural taxis behaviors are categorized and synthetic microrobotic collectives are discussed as bio-inspired realizations, aiming at closing the gap between taxis strategies of living creatures and those of functional active microswarms. As collective behaviors emerge within a group, the global taxis to external stimuli guides the group to conduct overall tasks, whereas the local taxis between individuals induces synchronization and global patterns. By encoding the local orientations and programming the global stimuli, various paradigms can be introduced for coordinating and controlling such collective microrobots, from the viewpoints of fundamental science and practical applications. Therefore, by discussing the key points and difficulties associated with collective taxes of different paradigms, this review potentially offers insights into mimicking natural collective behaviors and constructing intelligent microrobotic systems for on-demand control and preassigned tasks.

Smart Eutectic Gallium-Indium: From Properties to Applications

Eutectic gallium-indium (EGaIn), a liquid metal with a melting point close to or below room temperature, has attracted extensive attention in recent years due to its excellent properties such as fluidity, high conductivity, thermal conductivity, stretchability, self-healing capability, biocompatibility, and recyclability. These features of EGaIn can be adjusted by changing the experimental condition, and various composite materials with extended properties can be further obtained by mixing EGaIn with other materials. In this review, not only the are unique properties of EGaIn introduced, but also the working principles for the EGaIn-based devices are illustrated and the developments of EGaIn-related techniques are summarized. The applications of EGaIn in various fields, such as flexible electronics (sensors, antennas, electronic circuits), molecular electronics (molecular memory, opto-electronic switches, or reconfigurable junctions), energy catalysis (heat management, motors, generators, batteries), biomedical science (drug delivery, tumor therapy, bioimaging and neural interfaces) are reviewed. Finally, a critical discussion of the main challenges for the development of EGaIn-based techniques are discussed, and the potential applications in new fields are prospected.

Using H2O2 as a green oxidant to produce fluorescent GaOOH nanomaterials from a liquid metal

We report a simple and rapid method for the synthesis of fluorescent gallium oxyhydroxide (GaOOH) nanoparticles from liquid Ga by a probe sonication method in the presence of H₂O₂ as an oxidant. The aspect ratio of the GaOOH nanoparticles is determined by the concentration of H₂O₂ and solution pH, as well as the probe energy and sonication time. Further surface modification with cyclodextrin to achieve biocompatibility for potential biomedical applications is reported where an example of cell uptake and fluorescence imaging is shown.

Magnetic Microswarm and Fluoroscopy-Guided Platform for Biofilm Eradication in Biliary Stents

Biofilm eradication from medical implants is of fundamental importance, and the treatment of biofilm-associated pathogen infections on inaccessible biliary stents remains challenging. Magnetically driven microrobots with controlled motility, accessibility to the tiny lumen, and swarm enhancement effects can physically disrupt the deleterious biostructures while not developing drug resistance. Magnetic urchin-like capsule robots (MUCRs) loaded with magnetic liquid metal droplets (MLMDs, antibacterial agents) are designed using natural sunflower pollen, and the therapeutic effect of swarming MUCR@MLMDs is explored for eradicating complex mixtures of bacterial biofilm within biliary stents collected from patients. The external magnetic field triggers the emergence of the microswarm and induces MLMDs to transform their shape into spheroids and rods with sharp edges. The inherent natural microspikes of MUCRs and the obtained sharp edges of MLMDs actively rupture the dense biological matrix and multiple species of embedded bacterial cells by exerting mechanical force, finally achieving synergistic biofilm eradication. The microswarm is precisely and rapidly deployed into the biliary stent via endoscopy in 10 min. Notably, fluoroscopy imaging is used to track and navigate the locomotion of microswarm in biliary stents in real-time. The microswarm has great potential for treating bacterial biofilm infections associated with medical implants.

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.

Reconfigurable assembly of colloidal motors towards interactive soft materials and systems

Due to the highly flexible reconfiguration of swarms, collective behaviors have provided various natural organisms with a powerful adaptivity to the complex environment. To mimic these natural systems and construct artificial intelligent soft materials, self-propelled colloidal motors that can convert diverse forms of energy into swimming-like movement in fluids afford an ideal model system at the micro-/nanoscales. Through the coupling of local gradient fields, colloidal motors driven by chemical reactions or externally physical fields can assembly into swarms with adaptivity. Here, we summarize the progress on reconfigurable assembly of colloidal motors which is driven and modulated by chemical reactions and external fields (e.g., light, ultrasonic, electric, and magnetic fields). The adaptive reconfiguration behaviors and the corresponding mechanisms are discussed in detail. The future directions and challenges are also addressed for developing colloidal motor-based interactive soft matter materials and systems with adaptation and interactive functions comparable to that of natural systems.

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.

Electrostatic Stabilization of Nano Liquid Metals in Doped Nonpolar Liquids

Liquid metals and alloys are attracting renewed attention owing to their potential for application in various advanced technologies. Eutectic gallium-indium (EGaIn) has been focused on in particular because of its integrated advantages of high conductivity, low melting point, and low toxicity. In this study, the colloidal behavior of nano-dispersed EGaIn in nonpolar oils is investigated. Although the nonpolar oil continuous phase is commonly considered to be free of electric charges, electrostatic repulsion appears to be crucial in the colloidal stabilization of the nano-dispersed EGaIn phases, the modulation of which is possible by doping the oil phases with different types of oil-soluble surfactants. The qualitative correlation between the observed colloidal stabilities and the "zero field" particle mobilities inferred from the field-dependent electrophoretic mobilities indicates that the electric charging of EGaIn particles in surfactant-doped nonpolar oils is a static phenomenon that is maintained in equilibrium, rather than a solely field-induced process. A systematic investigation of the charging properties of these unique biphasic particles, consisting of the liquid Ga-In bulk and the solid Ga₂ O₃ surface that formed spontaneously, reveals the complicated system-dependent nature of the charging mechanisms mediated by ionic and nonionic surfactants in nonpolar media.

Universal Control for Micromotor Swarms with a Hybrid Sonoelectrode

Enabled by active motion of microrobots, conventional biological detection and chemical analyses limited by passive diffusion can be significantly enhanced with fast testing speed and unique sensitiveness. However, controlled release and precise enrichment of microrobot swarms are still difficult to accomplish and thus prohibit them away from practical applications. Here, an efficient and versatile strategy utilizing a needle-shaped hybrid sonoelectrode to disperse and aggregate distinct micromotors is presented, remarkably accelerating mass transfer and enhancing the signal intensity. Hydrogen bubbles generated at the tip of charged electrode can oscillate as actuated by the acoustic field, creating intensified vortexes to disperse micromotors spontaneously. Via removing the attached bubble, the sonoelectrode serving as solid needle isolator is capable of collecting micromotors in a large scale with acoustic streaming in the working reservoir at higher ultrasound frequency. Numerical calculation reveals the streaming profiles with/without microbubbles, and manipulations on classic spherical and tubular micromotor models confirm that the acoustic-powered prototype device is effective for controlling different swarming behaviors in microfluidic channels. Overall, the proposed hybrid sonoelectrode offers a universal and rapid strategy to tailor micromotor swarm behaviors, advancing intelligent microrobots to be featured with active enrichment and compatible for next-generation sensitive portable detection microsystems.

Enzymatic/Magnetic Hybrid Micromotors for Synergistic Anticancer Therapy

Micro/nanomotors (MNMs), which propel by transforming various forms of energy into kinetic energy, have emerged as promising therapeutic nanosystems in biomedical applications. However, most MNMs used for anticancer treatment are only powered by one engine or provide a single therapeutic strategy. Although double-engined micromotors for synergistic anticancer therapy can achieve more flexible movement and efficient treatment efficacy, their design remains challenging. In this study, we used a facile preparation method to develop enzymatic/magnetic micromotors for synergetic cancer treatment via chemotherapy and starvation therapy (ST), and the size of micromotors can be easily regulated during the synthetic process. The enzymatic reaction of glucose oxidase, which served as the chemical engine, led to self-propulsion using glucose as a fuel and ST via a reduction in the energy available to cancer cells. Moreover, the incorporation of Fe₃O₄ nanoparticles as a magnetic engine enhanced the kinetic power and provided control over the direction of movement. Inherent pH-responsive drug release behavior was observed owing to the acidic decomposition of drug carriers in the intracellular microenvironment of cancer cells. This system displayed enhanced anticancer efficacy owing to the synergetic therapeutic strategies and increased cellular uptake in a targeted area because of the improved motion behavior provided by the double engines. Therefore, the demonstrated micromotors are promising candidates for anticancer biomedical microsystems.

Generation of Nonspherical Liquid Metal Microparticles with Tunable Shapes Exhibiting an Electrostatic-Responsive Performance

Nonspherical liquid metal microparticles (NLMs) show extraordinary potential in various applications due to their multifunctional and structural advantages. To one-step-produce shaped NLMs with high efficiency, high controllability, and free of template, a facile microfluidic strategy named rotary flow shearing (RFS) is reported. A high-speed viscous shearing flow is provided by two counter-rotating rotors in the carrier fluid, inducing continuous pinch-off of liquid metal flowing from a capillary tube positioned in face of the slit between two rotors. The real-time oxidation realizes the rapid solidification of the pinching neck and the liquid metal surface during the RFS process, resulting in massive NLMs. Different from other microfluidic methods, the RFS enables tunable shapes of NLMs, especially for working materials at high viscosities. The collected NLMs exhibit special electrostatic-responsive performances including translation, rotation, reciprocation, and lining up under the manipulation of an external electric field. Such NLMs can be promisingly used for the construction of novel micromotors and soft electronics.

Mini/Micro/Nano Scale Liquid Metal Motors

Swimming motors navigating in complex fluidic environments have received tremendous attention over the last decade. In particular, liquid metal (LM) as a new emerging material has shown considerable potential in furthering the development of swimming motors, due to their unique features such as fluidity, softness, reconfigurability, stimuli responsiveness, and good biocompatibility. LM motors can not only achieve directional motion but also deformation due to their liquid nature, thus providing new and unique capabilities to the field of swimming motors. This review aims to provide an overview of the recent advances of LM motors and compare the difference in LM macro and micromotors from fabrication, propulsion, and application. Here, LM motors below 1 cm, named mini/micro/nano scale liquid metal motors (MLMTs) will be discussed. This work will present physicochemical characteristics of LMs and summarize the state-of-the-art progress in MLMTs. Finally, future outlooks including both opportunities and challenges of mini/micro/nano scale liquid metal motors are also provided.