Self-Growing and Serpentine Locomotion of Liquid Metal Induced by Copper Ions

Electrochemical Redox Synergism-Enhanced Liquid Metal Locomotion for Unrestricted Circuit Substrate Patterning

The critical challenge in advancing liquid metal circuits (LMCs) lies in achieving interfacial compatibility with diverse substrates while dynamically balancing fabrication efficiency and quality to ensure robust conductive stability. Here we introduce an electrochemical redox synergistic liquid metal (E-rsLM) that enables the controllable generation of diverse intermetallic bond transition layers (Cu, Au, or Fe-based) between liquid metal and unrestricted substrate surfaces, applicable in pH-universal electrolytes. It involves enhanced locomotion of the liquid metal, driven by synergistic electrochemical energy transduction from cyclic changes in gallium redox states. Characterized by expansion-contraction-expansion, it enables unique self-propelled bouncing, tuning spreading speed (up to ~26.8 mm/s) and elongation rate (up to 1192 %) with a volume of only 80 μL. Additionally, we demonstrate the adaptability of E-rsLM fabrication across 30 different substrates, highlighting its versatility. The patterning displays the superimposed efficiency and self-indicated quality, leading to superior conductivity (with time-cost savings of 30.7 % and 13.4 % in heating-cooling cycles, and a nearly 90 % reduction in output resistance). The practical viability of these circuits is further showcased by the assembly of integrated circuits, marking a significant step in expanding LMCs applications beyond laboratory-scale prototypes.

Production and recycling of the cutting edge material of gallium: A review

Gallium, an indispensable scattering element, is driving the development of an array of latest generation functional materials. Due to its exceptionally conductive, fluidic, thermal, flexible, and biocompatible attributes, gallium and its derivatives are increasingly introduced into diverse cutting edge industries. Meanwhile, the aggravated irreconcilable contradiction between the rapid growth of gallium consumption and the severe shortage of gallium resources also brings about big concern regarding its availability for the coming era. In this review, we conducted a comprehensive examination on the global distribution and reserves of gallium which indicates sporadic locations and low concentrations of gallium, highlighting the daunting challenge of extracting gallium. Following that, extensive assessments of gallium production and recovery treatments were presented, ranging from ore mining to high-purity gallium extraction, from major to minor production methods, and from primary gallium extraction to recycling gallium reclamation. Finally, based on evaluating ongoing trends over the field, a forecast of the future gallium production and recycling was given. Potential barriers and their corresponding mitigation strategies were interpreted.

Effect of Anions on Deformation of Gallium-Based Liquid Metal in Solution

In this study, deformation behaviors of gallium-based liquid metals in acidified cupric sulfate or cupric chloride solutions with varying concentrations of chloride anion or sulfate anion were investigated to explore their potential applications in soft machines and electronics. Gallium-based liquid metals are known for their unique deformability, making them promising materials for various fields. Previous research has shown that deformation of the liquid metal can be achieved in the presence of acidified cupric or ferric salts. However, the specific influence of different anions on the deformation process remains unclear. Our findings indicate that the deformation rate of the liquid metal increases with higher concentrations of chloride ions and decreases with higher concentrations of sulfate ions in the solution. UV-vis absorbance spectra of the solutions were analyzed to identify the formation of hydrated cupric cations. It was observed that increasing the concentration of Cl- ions promotes the formation of cupric-chloro complexes, thereby reducing the concentration of hydrated cupric ions in the solution. Furthermore, the addition of sulfate ions to the solution enhances the ionic strength of the medium, leading to the dissociation of cupric-chloro complexes. Additionally, sulfate ions can form insoluble layers with gallium ions, which impede the deformation of the liquid metal. The deformation rate of the liquid metal was found to be inversely correlated with the concentration of cupric ions in the solution. These results provide valuable insights into the deformable behavior of gallium-based liquid metals and their potential applications in liquid metal-based soft robots. This study highlights the importance of understanding the role of different anions in the deformation process of liquid metals, shedding light on the design and optimization of soft machines and electronics utilizing these materials.

Liquid Metal as Energy Conversion Sensitizers: Materials and Applications

Energy can exist in nature in a wide range of forms. Energy conversion refers to the process in which energy is converted from one form to another, and this process will be greatly enhanced by energy conversion sensitizers. Recently, an emerging class of new materials, namely liquid metals (LMs), shows excellent prospects as highly versatile materials. Notably, in terms of energy delivery and conversion, LMs functional materials are chemical responsive, heat-responsive, photo-responsive, magnetic-responsive, microwave-responsive, and medical imaging responsive. All these intrinsic virtues enabled promising applications in energy conversion, which means LMs can act as energy sensitizers for enhancing energy conversion and transport. Herein, first the unique properties of the light, heat, magnetic and microwave converting capacity of gallium-based LMs materials are summarized. Then platforms and applications of LM-based energy conversion sensitizers are highlighted. Finally, some of the potential applications and opportunities of LMs are prospected as energy conversion sensitizers in the future, as well as unresolved challenges. Collectively, it is believed that this review provides a clear perspective for LMs mediated energy conversion, and this topic will help deepen knowledge of the physical chemistry properties of LMs functional materials.

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.

Gallium-based liquid metals as smart responsive materials: Morphological forms and stimuli characterization

Gallium-based liquid metals (GaLMs) have garnered monumental attention from the scientific community due to their diverse actuation characteristics. These metals possess remarkable characteristics, including high surface tension, excellent electrical and thermal conductivity, phase transformation behaviour, minimal viscosity and vapour pressure, lack of toxicity, and biocompatibility. In addition, GaLMs have melting points that are either lower or near room temperature, making them incredibly beneficial when compared to solid metals since they can be easily deformed. Thus, there has been significant progress in developing multifunctional devices using GaLMs, including bio-devices, flexible and self-healing circuits, and actuators. Despite numerous reports on these liquid metals (LMs), there is an urgent need for consolidated and coherent literature regarding their actuation principles linked to the targeted application. This will ensure that the reader gets the flavour of physics behind the actuation mechanism and how it can be utilized in diverse fields. Moreover, the actuation mechanism has been scattered in the literature, and thus, the primary motive of this review is to provide a one-stop solution for the actuation mechanism and the associated dynamics while directing the readers to specialized literature. Thus, addressing this issue, we thoroughly examine and present a detailed account of the actuation mechanisms of GaLMs while highlighting the science behind them. We also discuss the various morphologies of GaLMs and their crucial physical characteristics which decide their targeted application. Furthermore, we also delve into commonly held beliefs about GaLMs in the literature, such as their toxicity and antibacterial properties, to offer readers a more accurate understanding. Finally, we have explored several key unanswered aspects of the LM that should be explored in future research. The core strength of this review lies in its simplistic approach in offering a starting point for researchers venturing this innovative field, while we make use of existing literature to develop a comprehensive understanding.

Bismuth-based liquid metals: advances, applications, and prospects

Bismuth-based liquid metals (LMs) are a large group of alloys with melting points slightly above room temperature. They are associated with fewer encapsulation constraints than room temperature LMs such as mercury, sodium-potassium alloys, and gallium-based alloys and are more likely to remain stable in the natural environment. In addition, their low melting point properties enable them to soften and melt via easy control. Bismuth-based alloys can also be modified with metal-based, carbon-based, and ceramic-based micro/nano particles as well as polymeric materials to create a series of novel composites owing to their outstanding functions. Based on these considerations, this review provides a comprehensive overview of bismuth-based LMs. The categories of bismuth and bismuth-based LMs are first briefly introduced to better systematize the physical and chemical properties of bismuth-based LMs. Based on these properties, bismuth-based LMs have been prepared using various methods, and this review briefly categorizes these preparation methods based on their finished forms (lumps, powders, and films). In addition, this review details the research progress of bismuth-based LMs in the fields of printed electronics, 3D printing, thermal management, biomedicine, chemical engineering, and deformable robotics. Finally, the challenges and future opportunities of bismuth-based LMs in the development process are discussed and visualized from different perspectives.

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.

Activating Tumor-Selective Liquid Metal Nanomedicine through Galvanic Replacement

Advanced chemotherapeutic strategies including prodrug and nanocatalytic medicine have significantly advanced tumor-selective theranostics, but delicate prodrug screening, tedious synthesis, low degradability/biocompatibility of inorganic components, and unsatisfied reaction activity complicate treatment efficacies. Here, the intrinsic anticancer bioactivity of liquid metal nanodroplets (LMNDs) is explored through galvanic replacement. By utilizing a mechano-degradable ligand, the resultant size of the aqueous LMND is unexpectedly controlled as small as ≈20 nm (LMND20). It is demonstrated that LMND20 presents excellent tumor penetration and biocompatibility and activates tumor-selective carrier-to-drug conversion, synchronously depleting Cu2+ ions and producing Ga3+ ions through galvanic replacement. Together with abundant generation of reactive oxygen species, multiple anticancer pathways lead to selective apoptosis and anti-angiogenesis of breast cancer cells. Compared to the preclinical/clinical anticancer drugs of tetrathiomolybdate and Ga(NO3 )3 , LMND20 administration significantly improves the therapeutic efficacy and survival in a BCap-37 xenograft mouse model, yet without obvious side effects.

A liquid metal based, integrated parallel electroosmotic micropump cluster drive system

The application of liquid metal in a microfluidic system enables the fabrication of highly integrated on-chip electroosmotic micropumps (EOPs). In this work, a low-voltage driveable integrated parallel EOP cluster drive system is proposed. This system consists of two layers, a branch-channel layer and a trunk-channel layer. The lower branch-channel layer contains separate parallel pumping channels and a pair of comb liquid metal electrodes. The separated branch channels are connected together through the trunk channels in the upper layer. With this structural arrangement, the parallel micropumps form an integrated micropump cluster for larger pumping capacity. The distance between the pumping channel and the electrode next to it is controlled to 20 μm. To guide the pump design, parametric studies are performed and fully discussed. According to the experimental results, the micropump cluster can be driven at a low voltage of 0.5 V, and the flow rate reaches 274 nL min-1 at 5 V. In addition, the paper finally proposes an electrode protection strategy and an integrated pump-valve drive system which is expected to solve the shortcoming of electroosmotic pumps in terms of long-time storage and driving.

Ultrasensitive and ultrastretchable electrically self-healing conductors

Self-healing is a bioinspired strategy to repair damaged conductors under repetitive wear and tear, thereby largely extending the life span of electronic devices. The self-healing process often demands external triggering conditions as the practical challenges for the widespread applications. Here, a compliant conductor with electrically self-healing capability is introduced by combining ultrahigh sensitivity to minor damages and reliable recovery from ultrahigh tensile deformations. Conductive features are created in a scalable and low-cost fabrication process comprising a copper layer on top of liquid metal microcapsules. The efficient rupture of microcapsules is triggered by structural damages in the copper layer under stress conditions as a result of the strong interfacial interactions. The liquid metal is selectively filled into the damaged site for the instantaneous restoration of the metallic conductivity. The unique healing mechanism is responsive to various structural degradations including microcracks under bending conditions and severe fractures upon large stretching. The compliant conductor demonstrates high conductivity of ∼12,000 S/cm, ultrahigh stretchability of up to 1,200% strain, an ultralow threshold to activate the healing actions, instantaneous electrical recovery in microseconds, and exceptional electromechanical durability. Successful implementations in a light emitting diode (LED) matrix display and a multifunctional electronic patch demonstrate the practical suitability of the electrically self-healing conductor in flexible and stretchable electronics. The developments provide a promising approach to improving the self-healing capability of compliant conductors.

Responsive Liquid Metal Droplets: From Bulk to Nano

Droplets exist widely in nature and play an extremely important role in a broad variety of industrial processes. Typical droplets, including water and oil droplets, have received extensive attention and research, however their single properties still cannot meet diverse needs. Fortunately, liquid metal droplets emerging in recent years possess outstanding properties, including large surface tension, excellent electrical and thermal conductivity, convenient chemical processing, easy transition between liquid and solid phase state, and large-scale deformability, etc. More interestingly, liquid metal droplets with unique features can respond to external factors, including the electronic field, magnetic field, acoustic field, chemical field, temperature, and light, exhibiting extraordinary intelligent response characteristics. Their development over the past decade has brought substantial breakthroughs and progress. To better promote the advancement of this field, the present article is devoted to systematically summarizing and analyzing the recent fundamental progress of responsive liquid metal droplets, not only involving droplet characteristics and preparation methods, but also focusing on their diverse response behaviors and mechanisms. On this basis, the challenges and prospects related to the following development of liquid metal droplets are also proposed. In the future, responsive liquid metal droplets with a rapid development trend are expected to play a key role in soft robots, biomedicine, smart matter, and a variety of other fields.

Liquid Metal Foaming via Decomposition Agents

As an emerging functional material, the liquid metal has demonstrated its encouraging potential in several areas with practical trials, while its global uniformity including high density and limited macroscopic interface might become a barrier for some tough application scenarios. Here, we proposed the concept of liquid metal foaming via decomposition agents, aiming to develop a generalized way to make porous foam metallic fluid, which would pave the way in achieving more structured features and adaptability of liquid metals. By introducing a greenness strategy with the help of an ecofriendly foaming agent, we realized a series of designed targeted liquid metal foams (LMFs). Compared with common liquid metals, LMFs possess many excellent properties, such as abundant interfaces, tunable conductivity, and adjustable stiffness, due to the controllable regulation of their porous structure. According to these unique characteristics, diversified values of LMFs were obtained. Benefiting from the naturally enriched interface in LMFs, the hydrogen evolution of LMFs in neutral deionized water was more efficient and more productive. Additionally, the compact LMF-air battery with high performance was originally manufactured. Moreover, the tunable LMF-enabled four-dimensional (4D) electromagnetic shielding materials possess excellent shielding performance. This material could open up broad vistas for the application of LMs.

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.

High performance liquid metal thermal interface materials

Conventional thermal interface materials (TIMs) as widely used in thermal management area is inherently limited by their relatively low thermal conductivity. From an alternative, the newly emerging liquid metal based thermal interface materials (LM-TIMs) open a rather promising way, which can pronouncedly improve the thermal contact resistance and offers tremendous opportunities for making powerful thermal management materials. The LM-TIMs thus prepared exhibits superior thermal conductivity over many conventional TIMs which guarantees its significant application prospect. And the nanoparticles mediated or tuned liquid metal further enable ever conductive LM-TIMs which suggests the ultimate goal of thermal management. In this review, a systematic interpretation on the basic features of LM-TIMs was presented. Representative exploration and progress on LM-TIMs were summarized. Typical approaches toward nanotechnology enhanced high performance LM-TIMs were illustrated. The perspect of this new generation thermal management material were outlined. Some involved challenges were raised. This work is expected to provide a guide line for future research in this field.

Liquid Metal Based Flexible and Implantable Biosensors

Biosensors are the core elements for obtaining significant physiological information from living organisms. To better sense life information, flexible biosensors and implantable sensors that are highly compatible with organisms are favored by researchers. Moreover, materials for preparing a new generation of flexible sensors have also received attention. Liquid metal is a liquid-state metallic material with a low melting point at or around room temperature. Owing to its high electrical conductivity, low toxicity, and superior fluidity, liquid metal is emerging as a highly desirable candidate in biosensors. This paper is dedicated to reviewing state-of-the-art applications in biosensors that are expounded from seven aspects, including pressure sensor, strain sensor, gas sensor, temperature sensor, electrical sensor, optical sensor, and multifunctional sensor, respectively. The fundamental scientific and technological challenges lying behind these recommendations are outlined. Finally, the perspective of liquid metal-based biosensors is present, which stimulates the upcoming design of biosensors.

Programmable Digital Liquid Metal Droplets in Reconfigurable Magnetic Fields

Gallium-based liquid metals exhibit excellent locomotion and deformation capabilities under external stimuli and has potential in developing intelligent robots. Programing the locomotion and morphology of the Liquid metal (LM) to endow it with functionalities and intelligence as robots is charming but remains challenging. In this study, we develop a programmable digital LM (PDLM) control platform that can realize versatile locomotion and morphological manipulation of magnetic LM (MLM) droplets using arrays of electromagnets. We demonstrate on-demand transportation, deformation, breakup, and merging of multiple MLM droplets simultaneously and precisely. We find that the intriguing behaviors of MLM under a magnetic field are due to the interplay of surface tension and magnetic forces. Furthermore, we present a functional cooperative droplet robot by equipping the MLM droplets with three-dimensionally printed microtool modules. We show that both the position and orientation of a rod-shaped object can be precisely manipulated by the cooperation of the MLM droplet robots. More interestingly, we explore the capability of the MLM droplet robots for cooperatively handling a copper wire to connect and disconnect electronic circuits. Finally, we demonstrate that the PDLM control platform is capable of programing a group of MLM droplets to accomplish a digital display task. We believe that the PDLM control system presents a promising potential in developing LM-based reconfigurable circuits, digital display systems, and biomimetic soft robotic systems with high controllability, multifunctionalities, and intelligence.

Galvanic replacement of liquid metal Galinstan with copper for the formation of photocatalytically active nanomaterials

Galvanic replacement of liquid metal Galinstan under mechanical agitation with copper creates a multi-elemental system that is photocatalytically active for the degradation of organic dyes where reuseability is achieved via immobilisation on a solid support.

Galvanic replacement of liquid metal galinstan with Pt for the synthesis of electrocatalytically active nanomaterials

The galvanic replacement reaction is a verstile method for the fabrication of bimetallic nanomaterials which is usually limited to solid precursors. Here we report on the galvanic replacement of liquid metal galinstan with Pt which predominantly results in the formation of a Pt5Ga1 material. During the galvanic replacement process an interesting phenomenon was observed whereby a plume of nanomaterial is ejected upwards from the centre of the liquid metal droplet into solution which is due to surface tension gradients on the liquid metal surface that induces surface convection. It was also found that hydrogen gas was liberated during the process facilitated by the formation of the Pt rich nanomaterial which is a highly effective catalyst for the hydrogen evolution reaction (HER). The material was characterised by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction and dynamic light scattering measurements. It was found that Pt5Ga1 was highly effective for the electrochemical oxidation of methanol and ethanol and outperformed a commercial Pt/C catalyst. Density functional theory calculations confirmed that the increased activity is due to the anti poisoning properties of the surface towards CO upon the incorporation of Ga atoms into a Pt catalyst. The use of liquid metals and galvanic replacement offers a simple approach to fabricating Ga based alloy nanomaterials that may have use in many other types of applications.

Spontaneous Dispersion and Large-Scale Deformation of Gallium-Based Liquid Metal Induced by Ferric Ions

A gallium-based liquid metal (LM) exhibits the largest interfacial tension among all the room-temperature liquids, which gives it strong deformability and promises its role in the field of soft machines. Paradoxically, such a material always remains nearly spherical in solution because of large interfacial tension, which in turn hinders the construction of LM-based soft machines. Consequently, it is of significant theoretical and practical value to regulate the interfacial tension of a LM in order to carry out richer deformation. In this study, spontaneous dispersion and large-scale deformation of a bulk LM were disclosed to be induced by ferric ions. It was found that the bulk LM immersed in the FeCl₃ solution can spontaneously disperse into a large amount of droplets. In addition, the dispersed LM droplets could move and deform by increasing the concentration of the solution or adding acids. The mechanisms behind the untraditional phenomena lie in the nonuniform interfacial tension over the entire surface of the LM, which is associated with the space-time distribution of the FeCl₃ solution. Further, directional locomotion and periodic oscillation occur because of the nonuniform interfacial tension, which leads to the autonomous dispersion and deformation of the LM. Overall, the unique redox reactions between the LM and the FeCl₃ solution play an essential role in ensuring the continuity of deformation. The present spontaneous dispersion and deformation capability of the LM signify a paradigm shift and open up new possibilities for the development of chemistry-enabled soft machines in the future.