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.

Preparation of eGaIn NDs/TPU Composites for X-ray Radiation Shielding Based on Electrostatic Spinning Technology.

Thermoplastic polyurethane (TPU) composites with eutectic gallium (Ga) and indium (In) (eGaIn) fillings of 0 wt%-75 wt% were prepared using the electrostatic spinning method. Field emission scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier-transform infrared (FTIR) spectroscopy were used to characterize the eGaIn NDs/TPU composites. To evaluate their X-ray shielding properties, Phy-X/PSD and WinXCom were employed to calculate the mass attenuation coefficients, linear attenuation coefficients, half-value layers, tenth value layers, mean free paths, and adequate atomic numbers of the eGaIn NDs/TPU composites. The SEM results indicated that the eGaIn nanodroplets were evenly distributed throughout the TPU fibers, and the flowable eGaIn was well-suited for interfacial compatibility with the TPU. A comparison of the eGaIn NDs/TPU composites with different content levels showed that the composite with 75 wt% eGaIn had the highest µm at all the evaluated energies, indicating a superior ability to attenuate X-rays. This non-toxic, lightweight, and flexible composite is a potential material for shielding against medical diagnostic X-rays.

Liquid-Metal Molecular Scissors.

Molecules are the smallest units of matter that can exist independently, relatively stable, maintaining their physical and chemical activities. The key factors that dominate the structures and properties of molecules include atomic species, alignment commands, and chemical bonds. Herein, we reported a generalized effect in which liquid metals can directly cut off oxygen-containing groups in molecular matter at room temperature, allowing the remaining groups to recombine to form functional materials. Thus, we propose basic liquid-metal scissors for molecular directional clipping and functional transformations. As a proof of concept, we demonstrate the capabilities of liquid-metal scissors and reveal that the gallium on the surface of liquid metals directly extracts oxygen atoms from H2O or CH3OH molecules to form oxides. After clipping, the remaining hydrogen atoms from the H2O molecules recombine to form H2, while the remaining fragments of CH3OH produce H2, carbon materials, and carboxylates. This finding refreshes our basic understanding of chemistry and should lead to the development of straightforward molecular weaving techniques, which can help to overcome the limitations of molecular substances with single purposes. It also opens a universal route for realizing future innovations in molecular chemical engineering, life sciences, energy and environment research, and biomedicine.

Liquid Metal-Based Flexible Bioelectrodes for Management of In-Stent-Restenosis: Potential Application.

Although vascular stents have been widely used in clinical practice, there is still a risk of in-stent restenosis after their implantation. Combining conventional vascular stents with liquid metal-based electrodes with impedance detection, irreversible electroporation, and blood pressure detection provides a new direction to completely solve the restenosis problem. Compared with conventional rigid electrodes, liquid metal-based electrodes combine high conductivity and stretchability, and are more compliant with the implantation process of vascular stents and remain in the vasculature for a long period of time. This perspective reviews the types and development of conventional vascular stents and proposes a novel stent that integrates liquid metal-based electrodes on conventional vascular stents. This vascular stent has three major functions of prediction, detection and treatment, and is expected to be a new generation of cardiovascular implant with intelligent sensing and real-time monitoring.

Amorphous TiO2 shells: an Essential Elastic Buffer Layer for High-Performance Self-Healing Eutectic GaSn Nano-Droplet Room-Temperature Liquid Metal Battery.

Gallium-based alloy liquid metal batteries currently face limitations such as volume expansion, unstable solid electrolyte interface (SEI) film and substantial capacity decay. In this study, amorphous titanium dioxide is used to coat eutectic GaSn nanodroplets (eGaSn NDs) to construct the core-shell structure of eGaSn@TiO2 nanodroplets (eGaSn@TiO2 NDs). The amorphous TiO2 shell (~6.5 nm) formed a stable SEI film, alleviated the volume expansion, and provided electron/ion transport channels to achieve excellent cycling performance and high specific capacity. The resulting eGaSn@TiO2 NDs exhibited high capacities of 580, 540, 515, 485, 456 and 426 mAh g-1 at 0.1, 0.2, 0.5, 1, 2 and 5 C, respectively. No significant decay was observed after more than 500 cycles with a capacity of 455 mAh g-1 at 1 C. In situ X-ray diffraction (in situ XRD) was used to explore the lithiation mechanism of the eGaSn negative electrode during discharge. This study elucidates the design of advanced liquid alloy-based negative electrode materials for high-performance liquid metal batteries (LMBs).

Pressure Driven Rapid Reconfigurable Liquid Metal Patterning.

This paper proposes a method for pressure driven rapid reconfigurable liquid metal patterning. A sandwich structure of "pattern-film-cavity" is designed to complete this function. Both sides of the highly elastic polymer film are bonded with two PDMS slabs. One PDMS slab has microchannels patterned on the surface. The other PDMS slab has a large cavity on its surface for liquid metal storage. These two PDMS slabs are bonded together, face to face, with the polymer film in the middle. In order to control the distribution of the liquid metal in the microfluidic chip, the elastic film will deform under the high pressure of the working medium in the microchannels and then extrude the liquid metal into different patterns in the cavity. This paper studies the factors of liquid metal patterning in detail, including external control conditions, such as the type and pressure of the working medium and the critical dimensions of the chip structure. Moreover, both a single-pattern and a double-pattern chip are fabricated in this paper, which can form or reconfigure the liquid metal pattern within 800 ms. Based on the above methods, reconfigurable antennas of two frequencies are designed and fabricated. Meanwhile, their performance is simulated and tested by simulation and vector network tests. The operating frequencies of the two antennas are respectively significantly switching between 4.66 GHz and 9.97 GHz.

Liquid Metal-Based Electrode Array for Neural Signal Recording.

Neural electrodes are core devices for research in neuroscience, neurological diseases, and neural-machine interfacing. They build a bridge between the cerebral nervous system and electronic devices. Most of the neural electrodes in use are based on rigid materials that differ significantly from biological neural tissue in flexibility and tensile properties. In this study, a liquid-metal (LM) -based 20-channel neural electrode array with a platinum metal (Pt) encapsulation material was developed by microfabrication technology. The in vitro experiments demonstrated that the electrode has stable electrical properties and excellent mechanical properties such as flexibility and bending, which allows the electrode to form conformal contact with the skull. The in vivo experiments also recorded electroencephalographic signals using the LM-based electrode from a rat under low-flow or deep anesthesia, including the auditory-evoked potentials triggered by sound stimulation. The auditory-activated cortical area was analyzed using source localization technique. These results indicate that this 20-channel LM-based neural electrode array satisfies the demands of brain signal acquisition and provides high-quality-electroencephalogram (EEG) signals that support source localization analysis.

Phase change composites of octadecane and gallium with expanded graphite as a carrier.

Phase change materials (PCMs) have attracted more and more attention in the field of energy and thermal management due to the characteristic of exchanging heat with small temperature change. In order to obtain perfect PCMs, previous researchers usually prepared various phase change composites (PCCs), but there is still a gap toward the goal. Perhaps the development of PCMs with adjustable properties in a wide range to meet different needs is a feasible option. Given that the properties of organic PCMs and metal PCMs are highly complementary, using expanded graphite (EG) as a mediator, a stable PCC of octadecane and gallium that are difficult to directly mix, was successfully prepared. Octadecane and gallium are stored in the microstructures of EG, and the microstructures of EG play the role of storing nucleation embryos, and the suppression of supercooling can reach more than 86.82%. The test results showed that the properties of the PCC are indeed a balance between octadecane and gallium, and can be adjusted in a wide range. The PCC also has good structural and chemical stability, which can effectively avoid the corrosion risk caused by gallium leakage. The PCC samples containing equal amounts of gallium and paraffin were selected for thermal management performance tests. The results indicated that the PCC has application potential in related fields, and can provide a reference for the development of other PCCs.

A U-Shaped Dual-Frequency-Reconfigurable Monopole Antenna Based on Liquid Metal.

This study presents a U-shaped dual-frequency-reconfigurable liquid-metal monopole antenna. Eutectic Gallium-Indium (EGaIn) was used as a conductive fluid and filled in the two branches of the U-shaped glass tube. A precision syringe pump was connected to one of the branches of the U-shaped tube by a silicone tube to drive EGaIn, forming a height difference between the two liquid levels. When the height of liquid metal in the two branches met the initial condition of L1 = L2 = 10 mm, and L1 increased from 10 mm to 18 mm, the two branches obtained two working bandwidths of 2.27-4.98 GHz and 2.71-8.58 GHz, respectively. The maximum peak gain was 4.00 dBi. The initial amount of EGaIn also affected the available operating bandwidth. When the liquid metal was perfused according to the initial condition: L1 = L2 = 12 mm, and L1 was adjusted within the range of 12-20 mm, the two branches had the corresponding working bandwidths of 2.18-4.32 GHz and 2.57-9.09 GHz, and the measured maximum peak gain was 3.72 dBi. The simulation and measurement data corresponded well. A series of dual-frequency-reconfigurable antennas can be obtained by changing the initial amount of EGaIn. This series of antennas may have broad application prospects in fields such as base stations and navigation.

Self-Assembled Copper Film-Enabled Liquid Metal Core-Shell Composite.

Liquid metal (LM) droplets covered with functional materials, especially metallic, often make breakthroughs in performance and functionality. In this study, self-assembly was used to synthesize copper films on the surface of LM. Herein, using CuO nanoparticles as the monomers, driven by the electrostatic interaction between CuO and eutectic gallium-indium (EGaIn) in the alkaline environment, EGaIn@Cu is realized by taking advantage of the reducing property of the EGaIn-alkaline interface. The copper film is smooth and dense, and under its protection, a layer of gallium oxide remains on the reaction interface between copper and LM, which enabled EGaIn@Cu to possess the volt-ampere curves similar to the Schottky mode, showing that the proposed mechanism has the potential to be used in the bottom-up synthesis of the semiconductor junction. Owing to the support of the copper film, the stiffness coefficient of the LM droplet can be increased by 56.9%. Coupled with the melting latent heat of 55.46 J/g and the natural high density of metal, EGaIn@Cu is also a potential phase change capsule. In addition, a method based on stream jetting and self-breaking up mechanisms of LM to batch-produce sub-millimeter capsules was also introduced. The above structural and functional characteristics demonstrate the value of this work in related fields.

A handy reversible bonding technology and its application on fabrication of an on-chip liquid metal micro-thermocouple.

We report a novel reversible bonding technique for liquid metal (LM) microelectrode fabrication in this study. This technique greatly simplifies the process of LM micro-electrode fabrication and can be used to achieve the rapid fabrication of LM blind-end electrodes. Three kinds of treatments, including heat treatment, plasma treatment and heat/plasma treatment, were tested for bonding strength. The experimental results showed that the heat/plasma treatment has the strongest bonding strength. All the three treatments can be completely released by simple water treatment. This handy fabrication method can help to integrate micro-liquid metal electrodes vertically in a microchannel. At the end of this work, this fabrication method was used to integrate liquid metal thermocouples in a microchannel, which greatly shortened the fabrication time and lowered the cost compared with traditional deposition or sputtering methods.

A Gravity-Triggered Liquid Metal Patch Antenna with Reconfigurable Frequency.

In this paper, a gravity-triggered liquid metal microstrip patch antenna with reconfigurable frequency is proposed with experimental verification. In this work, the substrate of the antenna is quickly obtained through three-dimensional (3D) printing technology. Non-toxic EGaIn alloy is filled into the resin substrate as a radiation patch, and the NaOH solution is used to remove the oxide film of EGaIn. In this configuration, the liquid metal inside the antenna can be flexibly flowed and deformed with different rotation angles due to the gravity to realize different working states. To validate the conception, the reflection coefficients and radiation patterns of the prototyped antenna are then measured, from which it can be observed that the measured results closely follow the simulations. The antenna can obtain a wide operating bandwidth of 3.69-4.95 GHz, which coverage over a range of frequencies suitable for various channels of the 5th generation (5G) mobile networks. The principle of gravitational driving can be applied to the design of reconfigurable antennas for other types of liquid metals.

The Design and Manufacturing Process of an Electrolyte-Free Liquid Metal Frequency-Reconfigurable Antenna.

This communication provides an integrated process route of smelting gallium-based liquid metal (GBLM) in a high vacuum, and injecting GBLM into the antenna channel in high-pressure protective gas, which avoids the oxidation of GBLM during smelting and filling. Then, a frequency-reconfigurable antenna, utilizing the thermal expansion characteristic of GBLM, is proposed. To drive GBLM into an air-proof space, the thermal expansion characteristics of GBLM are required. The dimensions of the radiating element of the liquid metal antenna can be adjusted at different temperatures, resulting in the reconfigurability of the operating frequency. To validate the proposed concept, an L-band antenna prototype was fabricated and measured. Experimental results demonstrate that the GBLM in the antenna was well filled, and the GBLM was not oxidized. Due to the GBLM being in an air-proof channel, the designed liquid metal antenna without electrolytes could be used in an air environment for a long time. The antenna is able to achieve an effective bandwidth of over 1.25-2.00 GHz between 25 °C and 100 °C. The maximum radiation efficiency and gain in the tunable range are 94% and 2.9 dBi, respectively. The designed antenna also provides a new approach to the fabrication of a temperature sensor that detects temperature in some situations that are challenging for conventional temperature sensing technology.

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.

A Liquid-Metal-Based Dielectrophoretic Microdroplet Generator.

This paper proposes a novel microdroplet generator based on the dielectrophoretic (DEP) force. Unlike the conventional continuous microfluidic droplet generator, this droplet generator is more like "invisible electric scissors". It can cut the droplet off from the fluid matrix and modify droplets' length precisely by controlling the electrodes' length and position. These electrodes are made of liquid metal by injection. By applying a certain voltage on the liquid-metal electrodes, the electrodes generate an uneven electric field inside the main microfluidic channel. Then, the uneven electric field generates DEP force inside the fluid. The DEP force shears off part from the main matrix, in order to generate droplets. To reveal the mechanism, numerical simulations were performed to analyze the DEP force. A detailed experimental parametric study was also performed. Unlike the traditional droplet generators, the main separating force of this work is DEP force only, which can produce one droplet at a time in a more precise way.

Stretchable Pressure Sensor with Leakage-Free Liquid-Metal Electrodes.

Nowadays, with the development of wearable devices, stretchable pressure sensors have been widely adopted in all kinds of areas. Most of the sensors aim to detect small pressure, such as fingertip tactile sensing, but only a few are focused on high-pressure sensing, such as foot pressure sensing during men's walking. In this work, a liquid metal-based stretchable sensor for large-pressure measurement is investigated. This sensor is fully stretchable because it is made of soft materials. However, when the soft sensor is subjected to high pressure, the liquid metal easily leaks from microchannels because it maintains the liquid state at room temperature. We therefore propose to fabricate liquid metal-based leakage-free electrodes to handle the liquid-metal leak. Parametric studies are conducted to compare this sensor with liquid-metal-only electrodes and leakage-free electrodes. The leakage-free electrodes increase the measurement ranges from 0.18, 0.18, and 0.15 MPa to 0.44 MPa, with higher linearity and precision. The improvement in the liquid-metal electrode enables the sensors to work stably within 0.44 MPa pressure and 20% strain. In addition, we integrate two capacitors, namely, a working capacitor and a reference capacitor, into one sensor to reduce the influence of parasitic capacitance brought about by external interference. This stretchable capacitive sensor capable of working under a wide range of pressure with good repeatability, sensitivity, and linearity, exhibits great potential use for wearable electronics. Finally, the method for fabricating leakage-free electrodes shows great value for hyperelastic electronics manufacturing and micromachine technology.

Development of a High Flow Rate 3-D Electroosmotic Flow Pump.

A low voltage 3D parallel electroosmotic flow (EOF) pump composed of two electrode layers and a fluid layer is proposed in this work. The fluid layer contains twenty parallel fluid channels and is set at the middle of the two electrode layers. The distance between fluid and electrode channels was controlled to be under 45 µm, to reduce the driving voltage. Room temperature liquid metal was directly injected into the electrode channels by syringe to form non-contact electrodes. Deionized (DI) water with fluorescent particles was used to test the pumping performance of this EOF pump. According to the experimental results, a flow rate of 5.69 nL/min was reached at a driving voltage of 2 V. The size of this pump is small, and it shows a great potential for implanted applications. This structure could be easily expanded for more parallel fluid channels and larger flow rate.

A liquid metal based capacitive soft pressure microsensor.

A liquid-metal based capacitive soft pressure microsensor is proposed in this work for measuring pressure in microchannels. To measure the pressure of the target microchannel, a short detection channel is fabricated and connected to the target microchannel. Because the detection channel has only one outlet at the end which is connected to the target microchannel, the fluid in the detection channel will stay still during the measurement and the pressure remains constant inside the detection channel. A segment of reference fluid which is immiscible with the working fluid is sealed inside the detection channel. Because the chip material is soft, the pressure change will lead to the movement of the interface between the reference fluid and working fluid inside the detection channel. A pair of liquid metal electrodes are fabricated on both sides of the detection channel. By measuring the capacitance between these two liquid metal electrodes, the movement of the interface can be detected, and thus the pressure change can be detected as well. To minimize the influence from the environment, two liquid metal shield layers are placed on the top and the bottom of the microchannel layer separately. The microsensor was first tested in a microfluidic system and then utilized to measure the blood pressure of rabbit carotid artery in vivo. The experimental results showed excellent stability and linear correlation between capacitance and the value of fluid pressure. The pressure sensor can achieve a resolution of 7.5 mmHg within a pressure range of 20-300 mmHg. This work provides a promising approach to develop an implantable blood or intraocular pressure-monitoring device for clinical use.

A Handy Flexible Micro-Thermocouple Using Low-Melting-Point Metal Alloys.

A handy, flexible micro-thermocouple using low-melting-point metal alloys is proposed in this paper. The thermocouple has the advantages of simple fabrication and convenient integration. Bismuth/gallium-based mixed alloys are used as thermocouple materials. To precisely inject the metal alloys to the location of the sensing area, a micro-polydimethylsiloxane post is designed within the sensing area to prevent outflow of the metal alloy to another thermocouple pole during the metal-alloy injection. Experimental results showed that the Seebeck coefficient of this thermocouple reached -10.54 µV/K, which was much higher than the previously reported 0.1 µV/K. The thermocouple was also be bent at 90° more than 200 times without any damage when the mass ratio of the bismuth-based alloy was <60% in the metal-alloy mixture. This technology mitigated the difficulty of depositing traditional thin⁻film thermocouples on soft substrates. Therefore, the thermocouple demonstrated its potential for use in microfluidic chips, which are usually flexible devices.

A Handy Liquid Metal Based Non-Invasive Electrophoretic Particle Microtrap.

A handy liquid metal based non-invasive particle microtrap was proposed and demonstrated in this work. This kind of microtrap can be easily designed and fabricated at any location of a microfluidic chip to perform precise particle trapping and releasing without disturbing the microchannel itself. The microsystem demonstrated in this work utilized silicon oil as the continuous phase and fluorescent particles (PE-Cy5, SPHEROTM Fluorescent Particles, BioLegend, San Diego, CA, USA, 10.5 µm) as the target particles. To perform the particle trapping, the micro system utilized liquid-metal-filled microchannels as noncontact electrodes to generate different patterns of electric field inside the fluid channel. According to the experimental results, the target particle can be selectively trapped and released by switching the electric field patterns. For a better understanding the control mechanism, a numerical simulation of the electric field was performed to explain the trapping mechanism. In order to verify the model, additional experiments were performed and are discussed.