Reconfigurable Magnetic Liquid Metal Robot for High-Performance Droplet Manipulation

Light-Responsive Materials in Droplet Manipulation for Biochemical Applications

Miniaturized droplets, characterized by well-controlled microenvironments and capability for parallel processing, have significantly advanced the studies on enzymatic evolution, molecular diagnostics, and single-cell analysis. However, manipulation of small-sized droplets, including moving, merging, and trapping of the targeted droplets for complex biochemical assays and subsequent analysis, is not trivial and remains technically demanding. Among various techniques, light-driven methods stand out as a promising candidate for droplet manipulation in a facile and flexible manner, given the features of contactless interaction, high spatiotemporal resolution, and biocompatibility. This review therefore compiles an in-depth discussion of the governing mechanisms underpinning light-driven droplet manipulation. Besides, light-responsive materials, representing the core of light-matter interaction and the key character converting light into different forms of energy, are particularly assessed in this review. Recent advancements in light-responsive materials and the most notable applications are comprehensively archived and evaluated. Continuous innovations and rational engineering of light-responsive materials are expected to propel the development of light-driven droplet manipulation, equip droplets with enhanced functionality, and broaden the applications of droplets for biochemical studies and routine biochemical investigations.

Magnetointeractive Cr2Te3-Coated Liquid Metal Droplets for Flexible Memory Arrays and Wearable Sensors

Magnetic liquid metal droplets, featured by unique fluidity, metallic conductivity, and magnetic reactivity, are of growing significance for next-generation flexible electronics. Conventional fabrication routes, which typically incorporate magnetic nanoparticles into liquid metals, otherwise encounter the pitfall pertaining to surface adhesivity and corrosivity over device modules. Here, an innovative approach of synergizing liquid metals with 2D magnetic materials is presented, accordingly creating chromium(III)-telluride-coated liquid metal (CT-LM) droplets via a simple self-assembly process. The CT-LM droplets exhibit controllable deformation and locomotion under magnetic fields, demonstrate nonadherence to various surfaces, and enable cost-effective recycling of components. The functionality of CT-LM droplets is validated through their use in magnetointeractive memory devices to enable sensing/storing 64 magnetic paths and in wearable sensors as the flexible vibrator for dynamic gesture recognition with machine learning assistance. This work opens new avenues for the functional droplet design and broadens the horizons of flexible electronics.

Magnetically Responsive Gallium-Based Liquid Metal: Preparation, Property and Application

Magnetically responsive soft smart materials have garnered significant academic attention due to their flexibility, remote controllability, and reconfigurability. However, traditional soft materials used in the construction of these magnetically responsive systems typically exhibit low density and poor thermal and electrical conductivities. These limitations result in suboptimal performance in applications such as medical radiography, high-performance electronic devices, and thermal management. To address these challenges, magnetically responsive gallium-based liquid metals have emerged as promising alternatives. In this review, we summarize the methodologies for achieving magnetically responsive liquid metals, including the integration of magnetic agents into the liquid metal matrix and the utilization of induced Lorentz forces. We then provide a comprehensive discussion of the key physicochemical properties of these materials and the factors influencing them. Additionally, we explore the advanced and potential applications of magnetically responsive liquid metals. Finally, we discuss the current challenges in this field and present an outlook on future developments and research directions.

Electrically mediated self-assembly and manipulation of drops at an interface

The fluid-fluid interface is a complex environment for a floating object where the statics and dynamics may be governed by capillarity, gravity, inertia, and other external body forces. Yet, the alignment of these forces in intricate ways may result in beautiful pattern formation and self-assembly of these objects, as in the case of crystalline order observed with bubble rafts or colloidal particles. While interfacial self-assembly has been explored widely, controlled manipulation of floating objects, e.g. drops, at the fluid-fluid interface still remains a challenge largely unexplored. In this work, we reveal the self-assembly and manipulation of water drops floating at an oil-air interface. We show that the assembly occurs due to electrostatic interactions between the drops and their environment. We highlight the role of the boundary surrounding the system by showing that even drops with a net zero electric charge can self-assemble under certain conditions. Using experiments and theory, we show that the depth of the oil bath plays an important role in setting the distance between the self-assembled drops. Furthermore, we demonstrate ways to manipulate the drops actively and passively at the interface.

Controllable Manipulation of Large-Volume Droplet on Non-Slippery Surfaces Based on Triboelectric Contactless Charge Injection

Controllable droplet manipulation is crucial in diverse scientific and engineering fields. Traditional electric-based methods usually rely on commercial high-voltage (HV) power sources, which are typically bulky, expensive, and potentially hazardous. The triboelectric nanogenerator (TENG) is a highly studied device that can generate HV output with limited current, showing great potential in droplet manipulation applications. However, current TENG-based approaches usually utilize traditional free-standing TENGs that produce short-pulsed alternating-current signals. This limitation hinders continuous electrostatic forces necessary for precise droplet control, leading to complex circuitry and suboptimal droplet motion control in terms of volume, distance, direction, and momentum. Here, a triboelectric contactless charge injection (TCCI) method employing a novel dual-functional triboelectric nanogenerator (DF-TENG), is proposed. The DF-TENG can produce both high voltage and constant current during unidirectional motion, enabling continuous corona discharges for contactless charge injection into the droplets. Using this method, a large-volume droplet (3000 µL) can be controlled with momentum up to 115.2 g mm s-1, quintupling the highest value recorded by the traditional methods. Moreover, the TCCI method is adaptable for a variety of non-slippery substrates and droplets of different compositions and viscosities, which makes it an ideal manipulation strategy for droplet transport, chemical reactions, and even driving solids.

Untethered soft actuators for soft standalone robotics

Soft actuators produce the mechanical force needed for the functional movements of soft robots, but they suffer from critical drawbacks since previously reported soft actuators often rely on electrical wires or pneumatic tubes for the power supply, which would limit the potential usage of soft robots in various practical applications. In this article, we review the new types of untethered soft actuators that represent breakthroughs and discuss the future perspective of soft actuators. We discuss the functional materials and innovative strategies that gave rise to untethered soft actuators and deliver our perspective on challenges and opportunities for future-generation soft actuators.

Droplet Manipulation on Bioinspired Slippery Surfaces: From Design Principle to Biomedical Applications

Droplet manipulation with high efficiency, high flexibility, and programmability, is essential for various applications in biomedical sciences and engineering. Bioinspired liquid-infused slippery surfaces (LIS), with exceptional interfacial properties, have led to expanding research for droplet manipulation. In this review, an overview of actuation principles is presented to illustrate how materials or systems can be designed for droplet manipulation on LIS. Recent progress on new manipulation methods on LIS is also summarized and their prospective applications in anti-biofouling and pathogen control, biosensing, and the development of digital microfluidics are presented. Finally, an outlook is made on the key challenges and opportunities for droplet manipulation on LIS.

Peptide Nucleic Acid Clamp-Assisted Photothermal Multiplexed Digital PCR for Identifying SARS-CoV-2 Variants of Concern

The unprecedented demand for variants diagnosis in response to the COVID-19 epidemic has brought the spotlight onto rapid and accurate detection assays for single nucleotide polymorphisms (SNPs) at multiple locations. However, it is still challenging to ensure simplicity, affordability, and compatibility with multiplexing. Here, a novel technique is presented that combines peptide nucleic acid (PNA) clamps and near-infrared (NIR)-driven digital polymerase chain reaction (dPCR) to identify the Omicron and Delta variants. This is achieved by simultaneously identifying highly conserved mutated signatures at codons 19, 614, and 655 of the spike protein gene. By microfluidically introducing graphene-oxide-nanocomposite into the assembled gelatin microcarriers, they achieved a rapid temperature ramping-up rate and switchable gel-to-sol phase transformation synchronized with PCR activation under NIR irradiation. Two sets of duplex PCR reactions, each classifying respective PNA probes, are emulsified in parallel and illuminated together using a homemade vacuum-based droplet generation device and a programmable NIR control module. This allowed for selective amplification of mutant sequences due to single-base-pair mismatch with PNA blockers. Sequence-recognized bioreactions and fluorescent-color scoring enabled quick identification of variants. This technique achieved a detection limit of 5,100 copies and a 5-fold quantitative resolution, which is promising to unfold minor differences and dynamic changes.

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.

Principles and methods of liquid metal actuators

As a promising material, liquid metals (LMs) have gained considerable interest in the field of soft robotics due to their ability to move as designed routines or change their shape dramatically under external stimuli. Inspired by the science fiction film Terminator, tremendous efforts have been devoted to liquid robots with high compliance and intelligence. How to manipulate LM droplets is crucial to achieving this goal. Accordingly, this review is dedicated to presenting the principles driving LMs and summarizing the potential methods to develop LM actuators of high maneuverability. Moreover, the recent progress of LM robots based on these methods is overviewed. The challenges and prospects of implementing autonomous robots have been proposed.

Room-Temperature Liquid Metals for Flexible Electronic Devices

Room-temperature gallium-based liquid metals (RT-GaLMs) have garnered significant interest recently owing to their extraordinary combination of fluidity, conductivity, stretchability, self-healing performance, and biocompatibility. They are ideal materials for the manufacture of flexible electronics. By changing the composition and oxidation of RT-GaLMs, physicochemical characteristics of the liquid metal can be adjusted, especially the regulation of rheological, wetting, and adhesion properties. This review highlights the advancements in the liquid metals used in flexible electronics. Meanwhile related characteristics of RT-GaLMs and underlying principles governing their processing and applications for flexible electronics are elucidated. Finally, the diverse applications of RT-GaLMs in self-healing circuits, flexible sensors, energy harvesting devices, and epidermal electronics, are explored. Additionally, the challenges hindering the progress of RT-GaLMs are discussed, while proposing future research directions and potential applications in this emerging field. By presenting a concise and critical analysis, this paper contributes to the advancement of RT-GaLMs as an advanced material applicable for the new generation of flexible electronics.

Magnetic-Actuated Jumping of Droplets on Superhydrophobic Grooved Surfaces: A Versatile Strategy for Three-Dimensional Droplet Transportation

On-demand droplet transportation is of great significance for numerous applications. Although various strategies have been developed for droplet transportation, out-of-surface three-dimensional (3D) transportation of droplets remains challenging. Here, a versatile droplet transportation strategy based on magnetic-actuated jumping (MAJ) of droplets on superhydrophobic grooved surfaces (SHGSs) is presented, which enables 3D, remote, and precise manipulation of droplets even in enclosed narrow spaces. To trigger MAJ, an electromagnetic field is utilized to deform the droplet on the SHGS with the aid of an attached magnetic particle, thereby the droplet acquires excess surface energy. When the electromagnetic field is quickly removed, the excess surface energy is partly converted into kinetic energy, allowing the droplet to jump atop the surface. Through high-speed imaging and numerical simulation, the working mechanism and size matching effect of MAJ are unveiled. It is found that the MAJ behavior can only be observed if the sizes of the droplets and the superhydrophobic grooves are matched, otherwise unwanted entrapment or pinch-off effects would lead to failure of MAJ. A regime diagram which serves as a guideline to design SHGSs for MAJ is proposed. The droplet transportation capacities of MAJ, including in-surface and out-of-surface directional transportation, climbing stairs, and crossing obstacles, are also demonstrated. With the ability to remotely manipulate droplets in enclosed narrow spaces without using any mechanical moving parts, MAJ can be used to design miniaturized fluidic platforms, which exhibit great potential for applications in bioassays, microfluidics, droplet-based switches, and microreactions.

Engineering Magnetic Soft and Reconfigurable Robots

Magnetic control has gained popularity recently due to its ability to enhance soft robots with reconfigurability and untethered maneuverability, among other capabilities. Several advancements in the fabrication and application of reconfigurable magnetic soft robots have been reported. This review summarizes novel fabrication techniques for designing magnetic soft robots, including chemical and physical methods. Mechanisms of reconfigurability and deformation properties are discussed in detail. The maneuverability of magnetic soft robots is then briefly discussed. Finally, the present challenges and possible future work in designing reconfigurable magnetic soft robots for biomedical applications are identified.

Magnetoactive Millirobots with Ternary Phase Transition

Magnetoactive soft millirobots have made significant advances in programmable deformation, multimodal locomotion, and untethered manipulation in unreachable regions. However, the inherent limitations are manifested in the solid-phase millirobot as limited deformability and in the liquid-phase millirobot as low stiffness. Herein, we propose a ternary-state magnetoactive millirobot based on a phase transitional polymer embedded with magnetic nanoparticles. The millirobot can reversibly transit among the liquid, solid, and viscous-fluid phases through heating and cooling. The liquid-phase millirobot has elastic deformation and mobility for unimpeded navigation in a constrained space. The viscous-fluid phase millirobot shows irreversible deformation and large ductility. The solid-phase millirobot shows good shape stability and controllable locomotion. Moreover, the ternary-state magnetoactive millirobot can achieve prominent capabilities including stiffness change and shape reconfiguration through phase transition. The millirobot can perform potential functions of navigation in complex terrain, three-dimensional circuit connection, and simulated treatment in a stomach model. This magnetoactive millirobot may find new applications in flexible electronics and biomedicine.

Mechano-Driven Tribo-Electrophoresis Enabled Human-Droplet Interaction in 3D Space

Electronically controlled droplet manipulation has widespread applications in biochemistry, life sciences, and industry. However, current technologies such as electrowetting, electrostatics, and surface charge printing rely on complex electrode arrays and external power supplies, leading to inefficient manipulation. In light of these limitations, a novel method is proposed, which leverages tribo-electrophoresis (TEP) to pipette in an oil medium, thereby enabling human-droplet interactions to be constructed with greater efficiency. The approach involves the rational design of a triboelectric nanogenerator-electrostatic tweezer that generates an electric field to charge the droplet and improves the maneuverability of the charged droplet, including aligned/non-aligned pipetting and stable transport in the clamped state, which can be accomplished solely by hand motion. The TEP method not only provides droplets with freedom to move in three dimensions but also offers a feasibility case for chemical reactions in the liquid phase and non-invasive sample extraction. This breakthrough establishes a cornerstone for human-droplet interactions capitalized on triboelectric nanogenerators, opening new avenues for research in droplet manipulation.

Bidirectional Droplet Manipulation on Magnetically Actuated Superhydrophobic Ratchet Surfaces

Droplet manipulation has garnered significant attention in various fields due to its wide range of applications. Among many different methods, magnetic actuation has emerged as a promising approach for remote and instantaneous droplet manipulation. In this study, we present the bidirectional droplet manipulation on a magnetically actuated superhydrophobic ratchet surface. The surface consists of silicon strips anchored on elastomer ridges with superhydrophobic black silicon structures on the top side and magnetic layers on the bottom side. The soft magnetic properties of the strips enable their bidirectional tilting to form a ratchet surface and thus bidirectional droplet manipulation upon varying external magnetic field location and strength. Computational multiphysics models were developed to predict the tilting of the strips, demonstrating the concept of bidirectional tilting along with a tilting angle hysteresis theory. Experimental results confirmed the soft magnetic hysteresis and consequential bidirectional tilting of the strips. The superhydrophobic ratchet surface formed by the tilting strips induced the bidirectional self-propulsion of dispensed droplets through the Laplace pressure gradient, and the horizontal acceleration of the droplets was found to be positively correlated with the tilting angle of the strips. Additionally, a finite element analysis was conducted to identify the critical conditions for dispensed droplet penetration through the gaps between the strips, which hinder the droplet's self-propulsion. The models and findings here provide substantial insights into the design and optimization of magnetically actuated superhydrophobic ratchet surfaces to manipulate droplets in the context of digital microfluidic applications.

Reactive wetting enabled anchoring of non-wettable iron oxide in liquid metal for miniature soft robot

Magnetic liquid metal (LM) soft robots attract considerable attentions because of distinctive immiscibility, deformability and maneuverability. However, conventional LM composites relying on alloying between LM and metallic magnetic powders suffer from diminished magnetism over time and potential safety risk upon leakage of metallic components. Herein, we report a strategy to composite inert and biocompatible iron oxide (Fe3O4) magnetic nanoparticles into eutectic gallium indium LM via reactive wetting mechanism. To address the intrinsic interfacial non-wettability between Fe3O4 and LM, a silver intermediate layer was introduced to fuse with indium component into AgxIny intermetallic compounds, facilitating the anchoring of Fe3O4 nanoparticles inside LM with improved magnetic stability. Subsequently, a miniature soft robot was constructed to perform various controllable deformation and locomotion behaviors under actuation of external magnetic field. Finally, practical feasibility of applying LM soft robot in an ex vivo porcine stomach was validated under in-situ monitoring by endoscope and X-ray imaging.

Millirobot Based on a Phase-Transformable Magnetorheological Liquid Metal

Droplet robots have attracted much attention in recent years due to their large-scale deformability and flexible mobility in confined spaces. However, droplet robots are always difficult to maintain rigid shapes, making them difficult to manipulate objects with large inertia. Moreover, their low conductivity makes them unable to complete tasks such as circuit repair. Herein, a millirobot made from magnetorheological liquid metal is proposed to address the problems. Specifically, the magnetorheological liquid metal (MLM) robot is made by engulfing iron particles into gallium-indium alloy, and the mass fraction of the MLM robot is determined by microscopic observation and rheological test. The MLM robot possesses both solid and liquid properties, enabling the robot with plasticity, large-scale deformability, good conductivity, motion flexibility, and good object manipulation. The MLM robot can achieve almost all of the functions of existing droplet robots, including splitting, merging, navigating in narrow channels, and pushing objects. In addition, it can also accomplish some other tasks that are difficult for existing droplet robots, such as pulling large objects, repairing damaged circuits selectively and reversibly, and repairing suspended circuits through plasticity. The demos show that MLM robots can traverse narrow spaces and repair circuit damage selectively and reversibly. It is believed that MLM robots can enrich diverse functionalities in the future.

Gallium Liquid Metal: Nanotoolbox for Antimicrobial Applications

The proliferation of drug resistance in microbial pathogens poses a significant threat to human health. Hence, treatment measures are essential to surmount this growing problem. In this context, liquid metal nanoparticles are promising. Gallium, a post-transition metal notable for being a liquid at physiological temperature, has drawn attention for its distinctive properties, high antimicrobial efficacy, and low toxicity. Moreover, gallium nanoparticles demonstrate anti-inflammatory properties in immune cells. Gallium can alloy with other metals and be prepared in various composites to modify and tailor its characteristics and functionality. More importantly, the bactericidal mechanism of gallium liquid metal could sidestep the threat of emerging drug resistance mechanisms. Building on this rationale, gallium-based liquid metal nanoparticles can enable impactful and innovative strategic pathways in the battle against antimicrobial resistance. This review outlines the characteristics of gallium-based liquid metals at the nanoscale and their corresponding antimicrobial mechanisms to provide a comprehensive yet succinct overview of their current antimicrobial applications. In addition, challenges and opportunities that require further research efforts have been identified and discussed.

Droplet-Based Microfluidics: Applications in Pharmaceuticals

Droplet-based microfluidics offer great opportunities for applications in various fields, such as diagnostics, food sciences, and drug discovery. A droplet provides an isolated environment for performing a single reaction within a microscale-volume sample, allowing for a fast reaction with a high sensitivity, high throughput, and low risk of cross-contamination. Owing to several remarkable features, droplet-based microfluidic techniques have been intensively studied. In this review, we discuss the impact of droplet microfluidics, particularly focusing on drug screening and development. In addition, we surveyed various methods of device fabrication and droplet generation/manipulation. We further highlight some promising studies covering drug synthesis and delivery that were updated within the last 5 years. This review provides researchers with a quick guide that includes the most up-to-date and relevant information on the latest scientific findings on the development of droplet-based microfluidics in the pharmaceutical field.

Responsive Magnetic Nanocomposites for Intelligent Shape-Morphing Microrobots

With the development of advanced biomedical theragnosis and bioengineering tools, smart and soft responsive microstructures and nanostructures have emerged. These structures can transform their body shape on demand and convert external power into mechanical actions. Here, we survey the key advances in the design of responsive polymer-particle nanocomposites that led to the development of smart shape-morphing microscale robotic devices. We overview the technological roadmap of the field and highlight the emerging opportunities in programming magnetically responsive nanomaterials in polymeric matrixes, as magnetic materials offer a rich spectrum of properties that can be encoded with various magnetization information. The use of magnetic fields as a tether-free control can easily penetrate biological tissues. With the advances in nanotechnology and manufacturing techniques, microrobotic devices can be realized with the desired magnetic reconfigurability. We emphasize that future fabrication techniques will be the key to bridging the gaps between integrating sophisticated functionalities of nanoscale materials and reducing the complexity and footprints of microscale intelligent robots.

Active Droplet Transport Induced by Moving Meniscus on a Slippery Magnetic Responsive Micropillar Array

Intelligent droplet manipulation plays a crucial role in both scientific research and industrial technology. Inspired by nature, meniscus driving is an ingenious way to spontaneously transport droplets. However, the shortages of short-range transport and droplet coalescence limit its application. Here, an active droplet manipulation strategy based on the slippery magnetic responsive micropillar array (SMRMA) is reported. With the aid of a magnetic field, the micropillar array bends and induces the infusing oil to form a moving meniscus, which can attract nearby droplets and transport them for a long range. Significantly, clustered droplets on SMRMA can be isolated by micropillars, avoiding droplet coalescence. Moreover, through adjusting the arrangement of the micropillars of SMRMA, multi-functional droplet manipulation such as unidirectional droplet transport, multi-droplet transport, droplet mixing, and droplet screening can be achieved. This work provides a promising approach for intelligent droplet manipulation and unfolds broad application prospects in microfluidics, microchemical reaction, biomedical engineering, and other fields.

Ultrasonic tweezer for multifunctional droplet manipulation

Spatiotemporally controllable droplet manipulation is essential in diverse applications, ranging from thermal management to microfluidics and water harvesting. Despite considerable advances, droplet manipulation without surface or droplet pretreatment is still challenging in terms of response and functional adaptability. Here, a droplet ultrasonic tweezer (DUT) based on phased array is proposed for versatile droplet manipulation. The DUT can generate a twin trap ultrasonic field at the focal point for trapping and maneuvering the droplet by changing the position of the focal point, which enables a highly flexible and precise programmable control. By leveraging the acoustic radiation force resulting from the twin trap, the droplet can pass through a confined slit 2.5 times smaller than its own size, cross a slope with an inclination up to 80°, and even reciprocate in the vertical direction. These findings provide a satisfactory paradigm for robust contactless droplet manipulation in various practical settings including droplet ballistic ejection, droplet dispensing, and surface cleaning.

Water-Immiscible Coacervate as a Liquid Magnetic Robot for Intravascular Navigation

Developing magnetic ultrasoft robots to navigate through extraordinarily narrow and confined spaces like capillaries in vivo requires synthesizing materials with excessive deformability, responsive actuation, and rapid adaptability, which are difficult to achieve with the current soft polymeric materials, such as elastomers and hydrogels. We report a magnetically actuatable and water-immiscible (MAWI) coacervate based on the assembled magnetic core-shell nanoparticles to function as a liquid robot. The degradable and biocompatible millimeter-sized MAWI coacervate liquid robot can remain stable under changing pH and salt concentrations, release loaded cargoes on demand, squeeze through an artificial capillary network within seconds, and realize intravascular targeting in vivo guided by an external magnetic field. We believe the proposed "coacervate-based liquid robot" can implement demanding tasks beyond the capability of conventional elastomer or hydrogel-based soft robots in the field of biomedicine and represents a distinct design strategy for high-performance ultrasoft robots.

Liquid Metal Patterning and Unique Properties for Next-Generation Soft Electronics

Room-temperature liquid metal (LM)-based electronics is expected to bring advancements in future soft electronics owing to its conductivity, conformability, stretchability, and biocompatibility. However, various difficulties arise when patterning LM because of its rheological features such as fluidity and surface tension. Numerous attempts are made to overcome these difficulties, resulting in various LM-patterning methods. An appropriate choice of patterning method based on comprehensive understanding is necessary to fully utilize the unique properties. Therefore, the authors aim to provide thorough knowledge about patterning methods and unique properties for LM-based future soft electronics. First, essential considerations for LM-patterning are investigated. Then, LM-patterning methods-serial-patterning, parallel-patterning, intermetallic bond-assisted patterning, and molding/microfluidic injection-are categorized and investigated. Finally, perspectives on LM-based soft electronics with unique properties are provided. They include outstanding features of LM such as conformability, biocompatibility, permeability, restorability, and recyclability. Also, they include perspectives on future LM-based soft electronics in various areas such as radio frequency electronics, soft robots, and heterogeneous catalyst. LM-based soft devices are expected to permeate the daily lives if patterning methods and the aforementioned features are analyzed and utilized.

Electric current-assisted manipulation of liquid metals using a stylus at micro-and nano-scales

A novel methodology, based on wetting and electromigration, for transporting liquid metal, over long distances, at micro-and nano-scale using a stylus is reported. The mechanism is analogous to a dropper that uses 'suction and release' actions to 'collect and dispense' liquid. In our methodology, a stylus coated with a thin metal film acts like the dropper that collects liquid metal from a reservoir upon application of an electric current, holds the liquid metal via wetting while carrying the liquid metal over large distances away from the reservoir and drops it on the target location by reversing the direction of electric current. Essentially, the working principle of the technique relies on the directionality of electromigration force and adhesive force due to wetting. The working of the technique is demonstrated by using an Au-coated Si micropillar as the stylus, liquid Ga as the liquid metal to be transported, and a Kleindiek-based position micro-manipulator to traverse the stylus from the liquid reservoir to the target location. For demonstrating the potential applications, the technique is utilized for closing a micro-gap by dispensing a minuscule amount of liquid Ga and conformally coating the desired segment of the patterned thin films with liquid Ga. This study confirms the promising potential of the developed technique for reversible, controlled manipulation of liquid metal at small length scales.

Meniscus-Induced Directional Self-Transport of Submerged Bubbles on a Slippery Oil-Infused Pillar Array with Height-Gradient

Directional manipulation of submerged bubbles is fundamental for both theoretical research and industrial production. However, most current strategies are limited to the upward motion direction, complex surface topography, and additional apparatuses. Here, we report a meniscus-induced self-transport platform, namely, a slippery oil-infused pillar array with height-gradient (SOPAH) by combining femtosecond laser drilling and replica mold technology. Owing to the unbalanced capillary force and Laplace pressure difference, bubbles on SOPAH tend to spontaneously transport along the meniscus gradient toward a higher elevation. The self-transport performances of bubbles near the pillars depend on the complex meniscus shape. Significantly, to understand the underlying transport mechanism, the 3D meniscus profile is simulated by solving the Young-Laplace equation. It is found that the concave valleys formed between the adjacent pillars can change the gradient direction of the meniscus and lead to the varied transport performances. Finally, by taking advantage of a water electrolysis system, the assembled SOPAH serving as a bubble-collecting device is successfully deployed. This work should not only bring new insights into the meniscus-induced self-transport dynamics but also benefit potential applications in the field of intelligent bubble manipulation.

Total Liquid Transfer with Enhanced Contact Line Slippage

A new surface treatment method is developed to achieve total liquid transfer. The transfer process of a liquid droplet is recorded through high-speed photography and analyzed via image analysis to investigate the hydrodynamic interactions. For a pristine PMMA surface, a viscous and viscoelastic liquid facilitates transfer by increased viscous and inertial forces and delayed liquid bridge breakage but is limited by slow contact line slippage. Hydrophobic surface treatments can increase contact line slippage and the receding angle to achieve transfer ratios up to 98%. However, pinning and contact angle hysteresis from surface roughness features limit liquid transfer, especially for smaller droplets and higher separation velocities. A lubricant-infused surface treatment with PDMS and a thin layer of less viscous silicone oil provides a smooth, homogeneous surface with fast slippage, low contact angle hysteresis, and only a slight oil wetting ridge. Liquid could then transfer at high ratios (∼99.9%), regardless of droplet size and separation velocity. Finally, complete transfer liquid from indented cells is demonstrated to show the potential of this surface modification method for gravure printing.

New Strategy and Excellent Fluorescence Property of Unique Core-Shell Structure Based on Liquid Metals/Metal Halides

The further applications of liquid metals (LMs) are limited by their common shortcoming of silver-white physical appearance, which deviates from the impose stringent requirements for color and aesthetics. Herein, a concept is proposed for constructing fluorescent core-shell structures based on the components and properties of LMs, and metal halides. The metal halides endow LMs with polychromatic and stable fluorescence characteristics. As a proof-of-concept, LMs-Al obtained by mixing of LMs with aluminum (Al) is reported. The surface of LMs-Al is transformed directly from Al to a multi-phase metal halide of K₃ AlCl₆ with double perovskites structure, via redox reactions with KCl + HCl solution in a natural environment. The formation of core-shell structure from the K₃ AlCl₆ and LMs is achieved, and the shell with different phases can emit a cyan light by the superimposition of the polychromatic spectrum. Furthermore, the LMs can be directly converted into a fluorescent shell without affecting their original features. In particular, the luminescence properties of shells can be regulated by the components in LMs. This study provides a new direction for research in spontaneous interfacial modification and fluorescent functionalization of LMs and promises potential applications, such as lighting and displays, anti-counterfeiting measures, sensing, and chameleon robots.

Robust, Healable, Self-Locomotive Integrated Robots Enabled by Noncovalent Assembled Gradient Nanostructure

Integration, being lightweight, and intelligence are important orientations for the future advancement of soft robots. However, existing soft robots are generally hydrogels or silicone rubber, which are inherently mechanically inferior and easily damaged and difficult to integrate functions. Here, inspired by nacre, an elastomer actuator with sulfonated graphene-based gradient nanostructures is constructed via supramolecular multiscale assembly. The resulting nanocomposite possesses an ultrahigh toughness of 141.19 MJ/m³ and high room-temperature self-healing efficiency (89%). The proof-of-concept robot is demonstrated to emphasize its maximum swimming speed of 2.67 body length per second, whose speed is comparable to that of plankton, representing the outperformance of most artificial soft robots. Furthermore, the robot can stably absorb pollutants and recover its robustness and functionality even when damaged. This study breaks the mutual exclusivity of functional execution and fast locomotions, and we anticipate that our nanostructural design will offer an effective extended path to other integrated robots that required multifunction integration.

Solid-Liquid State Transformable Magnetorheological Millirobot

Magnetically actuated soft millirobots (magneto-robot) capable of accomplishing on-demand tasks in a remote-control manner using noninvasive magnetic fields are of great interest in biomedical settings. However, the solid magneto-robots are usually restricted by the limited deformability due to the predesigned shape, while the liquid magneto-robots are capable of in situ shape reconfiguration but limited by the low stiffness and geometric instability due to the fluidity. Herein, we propose a magneto-active solid-liquid state transformable millirobot (named MRF-Robot) made from a magnetorheological fluid (MRF). The MRF-Robot can transform freely and rapidly between the Newtonian fluid in the liquid state upon a weak magnetic field (∼0 mT) and the Bingham plasticity in the solid state upon a strong magnetic field (∼100 mT). The MRF-Robot in the liquid state can realize diverse behaviors of large deformation, smooth navigation, in situ splitting, merging, and gradient pulling actuated by a weak magnetic field with a high gradient. The MRF-Robot in the solid state is distinguished for the controllable locomotion with reconfigured shapes and versatile object manipulations (including pull, push, and rotate the objects) driven by a strong magnetic field with a high gradient. Moreover, the MRF-Robot could continuously maneuver to accomplish diverse tasks in the comprehensive scenes and achieve liquid-drug delivery, thrombus clearance, and fluid-flow blockage in the phantom vascular model under magnetic actuation.

Self-Healable and Recyclable Dual-Shape Memory Liquid Metal–Elastomer Composites

Liquid metal (LM)–polymer composites that combine the thermal and electrical conductivity of LMs with the shape-morphing capability of polymers are attracting a great deal of attention in the fields of reconfigurable electronics and soft robotics. However, investigation of the synergetic effect between the shape-changing properties of LMs and polymer matrices is lacking. Herein, a self-healable and recyclable dual-shape memory composite, comprising an LM (gallium) and a Diels–Alder (DA) crosslinked crystalline polyurethane (PU) elastomer, is reported. The composite exhibits a bilayer structure and achieves excellent shape programming abilities, due to the phase transitions of the LM and the crystalline PU elastomers. To demonstrate these shape-morphing abilities, a heat-triggered soft gripper, which can grasp and release objects according to the environmental temperature, is designed and built. Similarly, combining the electrical conductivity and the dual-shape memory effect of the composite, a light-controlled reconfigurable switch for a circuit is produced. In addition, due to the reversible nature of DA bonds, the composite is self-healable and recyclable. Both the LM and PU elastomer are recyclable, demonstrating the extremely high recycling efficiency (up to 96.7%) of the LM, as well as similar mechanical properties between the reprocessed elastomers and the pristine ones.