Low-Temperature Triggered Shape Transformation of Liquid Metal Microdroplets

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.

Review of Liquid Metal Fiber Based Biosensors and Bioelectronics

Liquid metal, as a novel material, has become ideal for the fabrication of flexible conductive fibers and has shown great potential in the field of biomedical sensing. This paper presents a comprehensive review of the unique properties of liquid metals such as gallium-based alloys, including their excellent electrical conductivity, mobility, and biocompatibility. These properties make liquid metals ideal for the fabrication of flexible and malleable biosensors. The article explores common preparation methods for liquid metal conductive fibers, such as internal liquid metal filling, surface printing with liquid metal, and liquid metal coating techniques, and their applications in health monitoring, neural interfaces, and wearable devices. By summarizing and analyzing the current research, this paper aims to reveal the current status and challenges of liquid metal conductive fibers in the field of biosensors and to look forward to their development in the future, which will provide valuable references and insights for researchers in the field of biomedical engineering.

Phase Transition Liquid Metal Enabled Emerging Biomedical Technologies and Applications

Phase change materials that can absorb or release large amounts of heat during phase transition, play a critical role in many important processes, including heat dissipation, thermal energy storage, and solar energy utilization. In general, phase change materials are usually encapsulated in passive modules to provide assurance for energy management. The shape and mechanical changes of these materials are greatly ignored. An emerging class of phase change materials, liquid metals (LMs) have attracted significant interest beyond thermal management, including in transformable robots, flexible electronics, soft actuators, and biomedicine. Interestingly, the melting point of LM is highly tunable around body temperature, allowing it to experience considerable stiffness change when interacting with human organisms during solid-liquid change, which brings about novel phenomena, applied technologies, and therapeutic methods, such as mechanical destruction of tumors, neural electrode implantation technique, and embolization therapy. This review focuses on the technology, regulation, and application of the phase change process along with diverse changes of LM to facilitate emerging biomedical applications based on the influences of mechanical stiffness change and versatile regulation strategies. Typical applications will also be categorized and summarized. Lastly, the advantages and challenges of using the unique and reversible process for biomedicine will be discussed.

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.

High internal phase emulsions gel ink for direct-ink-writing 3D printing of liquid metal

3D printing of liquid metal remains a big challenge due to its low viscosity and large surface tension. In this study, we use Carbopol hydrogel and liquid gallium-indium alloy to prepare a liquid metal high internal phase emulsion gel ink, which can be used for direct-ink-writing 3D printing. The high volume fraction (up to 82.5%) of the liquid metal dispersed phase gives the ink excellent elastic properties, while the Carbopol hydrogel, as the continuous phase, provides lubrication for the liquid metal droplets, ensuring smooth flow of the ink during shear extrusion. These enable high-resolution and shape-stable 3D printing of three-dimensional structures. Moreover, the liquid metal droplets exhibit an electrocapillary phenomenon in the Carbopol hydrogel, which allows for demulsification by an electric field and enables electrical connectivity between droplets. We have also achieved the printing of ink on flexible, non-planar structures, and demonstrated the potential for alternating printing with various materials.

Highly Elastic and Conductive Metallic Interconnect with Crystalline-Amorphous Nanolaminate

Nanolaminate with alternating layers of nanocrystalline Cu and amorphous CuZrTi is suggested as highly stretchable and conductive interconnect material in stretchable devices. 50 nm nanocrystalline Cu and 20 nm amorphous CuZrTi are the optimum thicknesses of the constituent layers, which result in an elastic deformation limit of 3.33% similar to that of the monolithic amorphous CuZrTi film and an electrical conductivity of 11.83 S/μm corresponding to 70% of that of the monolithic nanocrystalline Cu film. The enhanced elastic deformability and conductivity of the nanolaminates enable the maintenance of the interconnect performance for cyclic stretching with a tensile strain of 114% in the form of a free-standing serpentine structure and a tensile strain of 30% in the form of an ordinary circular coil on an elastomer substrate.

Flexible Skin Patch Enabled Tumor Hybrid Thermophysical Therapy and Adaptive Antitumor Immune Response

Innovations on materials and technologies have greatly promoted the rapid development of wearable electronics from disease diagnosis to therapeutics. For superficial skin tumors, skin-attachable patches possess the advantages of minimally invasive property, alleviative side effects, and high efficiency. The development of noninvasive techniques and devices is still in urgent demands. Here, a flexible skin patch fabricated through a facile preparation method is reported for noninvasive hybrid thermophysical therapy and adaptative immune function enhancement. The liquid metal enabled skin patch is demonstrated with high conductivity, certain stability, biocompatibility, and an enhanced adhesive merit on skin surfaces for cryoablation therapy and magnetic hyperthermia therapy. The skin patch exhibits remarkably conformable heating and cooling performance toward the treatment of 4T1 breast tumors. The magnetic resonance images also indicate the significant tumor ablation effect. Interestingly, a relatively stable proportion of both CD8⁺ T and CD4⁺ T cells in the peripheral blood is identified after tumor therapy in comparison with the decreased trend in the untreated group, representing an efficient antitumor immune response induced by the skin patch. The developed skin patch would provide a promising noninvasive approach for tumor therapies by direct tumor destruction and maintenance of the antitumor immune response.

Liquid metal enabled conformal electronics

The application of three-dimensional common electronics that can be directly pasted on arbitrary surfaces in the fields of human health monitoring, intelligent robots and wearable electronic devices has aroused people's interest, especially in achieving stable adhesion of electronic devices on biological dynamic three-dimensional interfaces and high-quality signal acquisition. In recent years, liquid metal (LM) materials have been widely used in the manufacture of flexible sensors and wearable electronic devices because of their excellent tensile properties and electrical conductivity at room temperature. In addition, LM has good biocompatibility and can be used in a variety of biomedical applications. Here, the recent development of LM flexible electronic printing methods for the fabrication of three-dimensional conformal electronic devices on the surface of human tissue is discussed. These printing methods attach LM to the deformable substrate in the form of bulk or micro-nano particles, so that electronic devices can adapt to the deformation of human tissue and other three-dimensional surfaces, and maintain stable electrical properties. Representative examples of applications such as self-healing devices, degradable devices, flexible hybrid electronic devices, variable stiffness devices and multi-layer large area circuits are reviewed. The current challenges and prospects for further development are also discussed.

Toxicity and Biocompatibility of Liquid Metals

Recently, room-temperature liquid metals have attracted increasing attention from researchers owing to their excellent material properties. Systematic interpretation of the potential toxicity issues involved is essential for a wide range of applications, especially in the biomedical and healthcare fields. However, even with the exponential growth of related studies, investigation of the toxicological impact and possible hazards of liquid metals to organisms is still in its infancy. This review aims to provide a comprehensive summary of the current frontier of knowledge on liquid metal toxicology and biocompatibility in different environments. Based on recent studies, this review focuses on Ga and Bi-based in different states. It is necessary to evaluate their toxicity considering the rapid increase in research and utilization of such liquid metal composites. Finally, existing challenges are discussed and suggestions are provided for further investigation of liquid metal toxicology to clarify the toxicological mechanisms and strategies are provided to avoid adverse effects. In addition to resolving the doubts of public concern about the toxicity of liquid metals, this review is expected to promote the healthy and sustainable development of liquid metal-based materials and their use in diverse areas, especially those related to health care.

Protocol for designing and preparing gallium particles using cell membrane encapsulation for applications in melanoma cryoablation therapy

Liquid metals are increasingly applied in drug delivery, in vivo imaging, and biosensing. Herein, we describe a surface modification strategy, where cell membrane is introduced to encapsulate gallium (Ga) particles. We detail preparation steps of Ga microparticles by sonication, followed by Ga microparticles coating with purified tumor cell membranes and morphological assessment using TEM and cryo-TEM. We further describe cell uptake and establishment of tumor-bearing mouse models and steps to assess in vitro cytotoxicity and in vivo antitumor cryotherapy. For complete details on the use and execution of this protocol, please refer to Wang et al. (2022).

Gallium-Based Liquid Metal Materials for Antimicrobial Applications

The hazards caused by drug-resistant bacteria are rocketing along with the indiscriminate use of antibiotics. The development of new non-antibiotic antibacterial drugs is urgent. The excellent biocompatibility and diverse multifunctionalities of liquid metal have stimulated the studies of antibacterial application. Several gallium-based antimicrobial agents have been developed based on the mechanism that gallium (a type of liquid metal) ions disorder the normal metabolism of iron ions. Other emerging strategies, such as physical sterilization by directly using LM microparticles to destroy the biofilm of bacteria or thermal destruction via infrared laser irradiation, are gaining increasing attention. Different from traditional antibacterial agents of gallium compounds, the pronounced property of gallium-based liquid metal materials would bring innovation to the antibacterial field. Here, LM-based antimicrobial mechanisms, including iron metabolism disorder, production of reactive oxygen species, thermal injury, and mechanical destruction, are highlighted. Antimicrobial applications of LM-based materials are summarized and divided into five categories, including liquid metal motors, antibacterial fabrics, magnetic field-responsive microparticles, liquid metal films, and liquid metal polymer composites. In addition, future opportunities and challenges towards the development and application of LM-based antimicrobial materials are presented.

A Liquid Metal-Enhanced Wearable Thermoelectric Generator

It is a key challenge to continuously power personal wearable health monitoring systems. This paper reports a novel liquid metal-enhanced wearable thermoelectric generator (LM-WTEG that directly converts body heat into electricity for powering the wearable sensor system. The gallium-based liquid metal alloys with room-temperature melting point (24~30 °C) and high latent heat density (about 500 MJ/m³) are used to design a new flexible finned heat sink, which not only absorbs the heat through the solid-liquid phase change of the LM and enhances the heat release to the ambient air due to its high thermal conduction. The LM finned is integrated with WTEG to present high biaxial flexibility, which could be tightly in contact with the skin. The LM-WTEG could achieve a super high output power density of 275 μW/cm² for the simulated heat source (37 °C) with the natural convective heat transfer condition. The energy management unit, the multi-parameter sensors (including temperature, humidity, and accelerometer), and Bluetooth module with a total energy consumption of about 65 μW are designed, which are fully powered from LM-WTEG through harvesting body heat.

Engineering Polymers via Understanding the Effect of Anchoring Groups for Highly Stable Liquid Metal Nanoparticles

Liquid metal nanoparticles (LMNPs) have recently attracted much attention as soft functional materials for various biorelated applications. Despite the fact that several reports demonstrate highly stable LMNPs in aqueous solutions or organic solvents, it is still challenging to stabilize LMNPs in biological media with complex ionic environments. LMNPs grafted with functional polymers (polymers/LMNPs) have been fabricated for maintaining their colloidal and chemical stability; however, to the best of our knowledge, no related work has been conducted to systematically investigate the effect of anchoring groups on the stability of LMNPs. Herein, various anchoring groups, including phosphonic acids, trithiolcarbonates, thiols, and carboxylic acids, are incorporated into brush polymers via reversible addition-fragmentation chain transfer (RAFT) polymerization to graft LMNPs. Both the colloidal and chemical stability of such polymer/LMNP systems are then investigated in various biological media. Moreover, the influence of multidentate ligands is also investigated by incorporating different numbers of carboxylic or phosphonic acid into the brush polymers. We discover that increasing the number of anchoring groups enhances the colloidal stability of LMNPs, while polymers bearing phosphonic acids provide the optimum chemical stability for LMNPs due to surface passivation. Thus, polymers bearing multidentate phosphonic acids are desirable to decorate LMNPs to meet complex environments for biological studies.

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.

Controlled Transformation of Liquid Metal Microspheres in Aqueous Solution Triggered by Growth of GaOOH

Liquid metals (LMs) are playing an increasingly important role in the fields of flexible devices, electronics, and thermal management due to their low melting point and excellent thermal and electrical conductivity, and the transformation of LMs in deionized water has recently received much attention. In this paper, we investigate the transformation process of EGaIn microspheres in deionized water and propose a two-step process of microspherical transformation, whereby the microspheres are first deformed into a spindle shape and then into lamellar nanorods. It is also shown that the growth of GaOOH crystals drives the transformation. Based on this result, EGaIn microspheres with controllable transformation could be prepared, such as spindle or lamellar rod shapes, extending the application area of LMs.

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.