All-soft, battery-free, and wireless chemical sensing platform based on liquid metal for liquid- and gas-phase VOC detection

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.

Development of a direct PMMA-PCB bonding method for low cost and rapid prototyping of microfluidic-based gas analysers

Rapid prototyping of microfluidic devices requires low cost materials and simple fabrication methods. PMMA and PCB have been used separately for the fabrication of microfluidic devices in a wide range of applications. Although the combined use of PMMA and PCB can have considerable merits, few works have been reported on the direct bonding of these materials. In this work we have investigated the fabrication of microfluidic devices using PMMA and PCB for the analysis of gaseous samples. In order to yield a reliable direct bonding method, four parameters including temperature, pressure, solvent and patterned interface material were experimentally investigated. Results of testing various prototypes showed that a patterned interface of concentric rectangular copper rings exposed to solvent at room temperature and under moderate pressure provided better adhesion strength, sealing and durability. After successful development of the PMMA-PCB direct bonding process, sample prototypes were designed and fabricated to practically assess the combined advantages of two materials. Presented concepts include implementation of heater on a PCB, array of gas sensors coupled with microchannels, serpentine microchannel and fast evaporation of liquid sample using an SMD resistor. It has been shown that advantages of utilizing PMMA such as fabricating the channel easily and with low cost, can be combined with benefits of a PCB including simple sensor installation and the use of copper tracks and electronic components for gas flow modulation. Moreover, it is possible to implement channel, circuit and other electronic components such as microprocessors on a single device.

Recent progress in multifunctional, reconfigurable, integrated liquid metal-based stretchable sensors and standalone systems

Possessing a unique combination of properties that are traditionally contradictory in other natural or synthetical materials, Ga-based liquid metals (LMs) exhibit low mechanical stiffness and flowability like a liquid, with good electrical and thermal conductivity like metal, as well as good biocompatibility and room-temperature phase transformation. These remarkable properties have paved the way for the development of novel reconfigurable or stretchable electronics and devices. Despite these outstanding properties, the easy oxidation, high surface tension, and low rheological viscosity of LMs have presented formidable challenges in high-resolution patterning. To address this challenge, various surface modifications or additives have been employed to tailor the oxidation state, viscosity, and patterning capability of LMs. One effective approach for LM patterning is breaking down LMs into microparticles known as liquid metal particles (LMPs). This facilitates LM patterning using conventional techniques such as stencil, screening, or inkjet printing. Judiciously formulated photo-curable LMP inks or the introduction of an adhesive seed layer combined with a modified lift-off process further provide the micrometer-level LM patterns. Incorporating porous and adhesive substrates in LM-based electronics allows direct interfacing with the skin for robust and long-term monitoring of physiological signals. Combined with self-healing polymers in the form of substrates or composites, LM-based electronics can provide mechanical-robust devices to heal after damage for working in harsh environments. This review provides the latest advances in LM-based composites, fabrication methods, and their novel and unique applications in stretchable or reconfigurable sensors and resulting integrated systems. It is believed that the advancements in LM-based material preparation and high-resolution techniques have opened up opportunities for customized designs of LM-based stretchable sensors, as well as multifunctional, reconfigurable, highly integrated, and even standalone systems.

Functional micro-/nanostructured gallium-based liquid metal for biochemical sensing and imaging applications

In recent years, liquid metals (LMs) have garnered increasing attention for their expanded applicability, and wide application potential in various research fields. Among them, gallium (Ga)-based LMs exhibit remarkable analytical performance in electrical and optical sensors, thanks to their excellent conductivity, large surface area, biocompatibility, small bandgap, and high elasticity. This review comprehensively summarizes the latest advancements in functional micro-/nanostructured Ga-based LMs for biochemical sensing and imaging applications. Firstly, the electrical, optical, and biocompatible features of Ga-based LM micro-/nanoparticles are briefly discussed, along with the manufacturing and functionalization processes. Subsequently, we demonstrate the utilization of Ga-based LMs in biochemical sensing techniques, encompassing electrochemistry, electrochemiluminescence, optical sensing techniques, and various biomedical imaging. Lastly, we present an insightful perspective on promising research directions and remaining challenges in LM-based biochemical sensing and imaging applications.

Smart Eutectic Gallium-Indium: From Properties to Applications

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

Tuning the sensing responses towards room-temperature hypersensitive methanol gas sensor using exfoliated graphene-enhanced ZnO quantum dot nanostructures

A suitable and non-invasive methanol sensor workable in ambient temperature conditions with a high response has gained wide interest to prevent detrimental consequences for industrial workers from its low-level intoxication. In this work, we present a tunable and highly responsive ppb-level methanol gas sensor device working at room temperature via a bottom-up synthetic approach using exfoliated graphene sheet (EGs) and ZnO quantum dots (QDs) on an aluminum anodic oxide (AAO) template. It is verified that EGs-supported AAO with a vertical electrode configuration enabled high and fast-responsive methanol sensing. Moreover, the hydroxyl and carboxyl groups of the high surface area EGs and ZnO QDs with a 3.37 eV bandgap efficiently absorbing UV light led to 56 times high response due to the enhanced polarization on the sensor surface compared to non-UV-radiated EGs/AAO at 800 ppb of methanol. The optimal resonance frequency of methanol is determined to be 100 kHz, which could detect methanol with high response of 2.65% at 100 ppm. The limit of detection (LOD) concentration is obtained at 2 ppb level. This study demonstrates the potential of UV-assisted ZnO, EGs, and AAO-based capacitance sensor material for rapidly detecting hazardous gaseous light organic molecules at ambient conditions, and the overall approach can be easily expanded to a novel non-invasive monitoring strategy for light and hazardous volatile organic exposures.

Emerging Liquid Metal Biomaterials: From Design to Application

Liquid metals (LMs) as emerging biomaterials possess unique advantages including their favorable biosafety, high fluidity, and excellent electrical and thermal conductivities, thus providing a unique platform for a wide range of biomedical applications ranging from drug delivery, tumor therapy, and bioimaging to biosensors. The structural design and functionalization of LMs endow them with enhanced functions such as enhanced targeting ability and stimuli responsiveness, enabling them to achieve better and even multifunctional synergistic therapeutic effects. Herein, the advantages of LMs in biomedicine are presented. The design of LM-based biomaterials with different scales ranging from micro-/nanoscale to macroscale and various components is explored in-depth to promote the understanding of structure-property relationships, guiding their performance optimization and applications. Furthermore, the related advanced progress in the development of LM-based biomaterials in biomedicine is summarized. Current challenges and prospects of LMs in the biomedical field are also discussed.

Emerging Role of Liquid Metals in Sensing

Low melting point metals and alloys are the group of materials that combine metallic and liquid properties, simultaneously. The fascinating characteristics of liquid metals (LMs) including softness and high electrical and thermal conductivity, as well as their unique interfacial chemistry, have started to dominate various research disciplines. Utilization of LMs as responsive interfaces, enabling sensing in a flexible and versatile manner, is one of the most promising traits demonstrated for LMs. In the context of LMs-enabled sensors, gallium (Ga) and its alloys have emerged as multipurpose functional materials with many compelling physical and chemical properties. Responsiveness to different stimuli and easy-to-functionalize interfaces of Ga-based LMs make them ideal candidates for a variety of sensing applications. However, despite the vast capabilities of Ga-based LMs in sensing, applications of these materials for developing different sensors have not been fully explored. In the present review, we provide a comprehensive overview regarding the applications of Ga-based LMs in a wide range of sensing approaches that cover different physical and chemical sensors. The unique features of Ga-based LMs, which make them promising materials for sensing, are discussed in subsections followed by relevant case studies. Finally, challenges as well as the prospected future and developing motifs are highlighted for each type of LM-based sensors.

Fluid pumping by liquid metal droplet utilizing ac electric field

We report a unique phenomenon in which liquid metal droplets (LMDs) under a pure ac electric field pump fluid. Unlike the directional pumping that occurs upon reversing the electric field polarity under a dc signal, this phenomenon allows the direction of fluid motion to be switched by simply shifting the position of the LMD within the cylindrical chamber. The physical mechanism behind this phenomenon has been termed Marangoni flow, caused by nonlinear electrocapillary stress. Under the influence of a localized, asymmetric ac electric field, the polarizable surface of the position-offset LMD produces a net time-averaged interfacial tension gradient that scales with twice the field strength, and thus pumps fluid unidirectionally. However, the traditional linear RC circuit polarization model of the LMD/electrolyte interface fails to capture the correct pump-flow direction when the thickness of the LMD oxide skin is non-negligible compared to the Debye length. Therefore, we developed a physical description by treating the oxide layer as a distributed capacitance with variable thickness and connected with the electric double layer. The flow profile is visualized via microparticle imaging velocimetry, and excellent consistency is found with simulation results obtained from the proposed nonlinear model. Furthermore, we investigate the effects of relevant parameters on fluid pumping and discuss a special phenomenon that does not exist in dc control systems. To our knowledge, no previous work addresses LMDs in this manner and uses a zero-mean ac electric field to achieve stable, adjustable directional pumping of a low-conductivity solution.

Liquid Metal-Based Soft Electronics for Wearable Healthcare

Wearable healthcare devices have garnered substantial interest for the realization of personal health management by monitoring the physiological parameters of individuals. Attaining the integrity between the devices and the biological interfaces is one of the greatest challenges to achieving high-quality body information in dynamic conditions. Liquid metals, which exist in the liquid phase at room temperatures, are advanced intensively as conductors for deformable devices because of their excellent stretchability and self-healing ability. The unique surface chemistry of liquid metals allows the development of various sensors and devices in wearable form. Also, the biocompatibility of liquid metals, which is verified through numerous biomedical applications, holds immense potential in uses on the surface and inside of a living body. Here, the recent progress of liquid metal-based wearable electronic devices for healthcare with respect to the featured properties and the processing technologies is discussed. Representative examples of applications such as biosensors, neural interfaces, and a soft interconnection for devices are reviewed. The current challenges and prospects for further development are also discussed, and the future directions of advances in the latest research are explored.

Soft and Stretchable Liquid Metal Composites with Shape Memory and Healable Conductivity

Shape memory composites are fascinating materials with the ability to preserve deformed shapes that recover when triggered by certain external stimuli. Although elastomers are not inherently shape memory materials, the inclusion of phase-change materials within the elastomer can impart shape memory properties. When this filler changes the phase from liquid to solid, the effective modulus of the polymer increases significantly, enabling stiffness tuning. Using gallium, a metal with a low melting point (29.8 °C), it is possible to create elastomeric materials with metallic conductivity and shape memory properties. This concept has been used previously in core-shell (gallium-elastomer) fibers and foams, but here, we show that it can also be implemented in elastomeric films containing microchannels. Such microchannels are appealing because it is possible to control the geometry of the filler and create metallically conductive circuits. Stretching the solidified metal fractures the fillers; however, they can heal by body heat to restore conductivity. Such conductive, shape memory sheets with healable conductivity may find applications in stretchable electronics and soft robotics.

Microfluidics in Gas Sensing and Artificial Olfaction

Rapid, real-time, and non-invasive identification of volatile organic compounds (VOCs) and gases is an increasingly relevant field, with applications in areas such as healthcare, agriculture, or industry. Ideal characteristics of VOC and gas sensing devices used for artificial olfaction include portability and affordability, low power consumption, fast response, high selectivity, and sensitivity. Microfluidics meets all these requirements and allows for in situ operation and small sample amounts, providing many advantages compared to conventional methods using sophisticated apparatus such as gas chromatography and mass spectrometry. This review covers the work accomplished so far regarding microfluidic devices for gas sensing and artificial olfaction. Systems utilizing electrical and optical transduction, as well as several system designs engineered throughout the years are summarized, and future perspectives in the field are discussed.

All-Soft Supercapacitors Based on Liquid Metal Electrodes with Integrated Functionalized Carbon Nanotubes

Soft energy storage devices, such as supercapacitors, are an essential component for powering integrated soft microsystems. However, conventional supercapacitors are mainly manufactured using hard/brittle materials that easily crack and eventually delaminate from the current collector by mechanical deformation. Therefore, to realize all-soft supercapacitors, the electrodes should be soft, stretchable, and highly conductive without compromising the electrochemical performance. This paper presents all-soft supercapacitors for integrated soft microsystems based on gallium-indium liquid metal (eutectic gallium-indium alloy, EGaIn) electrodes with integrated functionalized carbon nanotubes (CNTs). Oxygen functional groups on the surface of the CNTs ensure strong adhesion between the functionalized CNTs and the thin native oxide layer on the surface of EGaIn, which enables delamination-free soft and stretchable electrodes even under mechanical deformation. The electrochemical performances of fabricated all-soft supercapacitors in a parallel-plate arrangement were investigated without and with applied mechanical deformation. The fabricated supercapacitors exhibit areal capacitances as high as 12.4 mF cm^-2 and show nearly unchanged performance under 30% applied strain. They maintain >95% of their original capacitance after >4200 charging and discharging cycles with a periodic applied strain of 30%. Finally, fabricated supercapacitors have been successfully integrated with a commercial light-emitting diode to demonstrate an integrated soft microsystem.

Soft and Deformable Sensors Based on Liquid Metals

Liquid metals are one of the most interesting and promising materials due to their electrical, fluidic, and thermophysical properties. With the aid of their exceptional deformable natures, liquid metals are now considered to be electrically conductive materials for sensors and actuators, major constituent transducers in soft robotics, that can experience and withstand significant levels of mechanical deformation. For the upcoming era of wearable electronics and soft robotics, we would like to offer an up-to-date overview of liquid metal-based soft (thus significantly deformable) sensors mainly but not limited to researchers in relevant fields. This paper will thoroughly highlight and critically review recent literature on design, fabrication, characterization, and application of liquid metal devices and suggest scientific and engineering routes towards liquid metal sensing devices of tomorrow.

Surface Modification with Gallium Coating as Nonwetting Surfaces for Gallium-Based Liquid Metal Droplet Manipulation

We report gallium (Ga) coating as a simple approach to convert most common microfluidic substrates to nonwetting surfaces against surface-oxidized gallium-based liquid metal alloys. These alloys are readily oxidized in ambient air and adhere to almost all surfaces, which imposes significant challenges in mobilizing liquid metal droplets without leaving residue. Various flat substrates (e.g., PDMS, Si, SiO₂, SU-8, glass, and parylene-C coated PDMS) were coated with thin film (75-200 nm in thickness) of gallium by evaporation and the coated gallium formed nanoscale uneven and rough surface through Ostwald ripening with its surface covered with oxide shell. Static and dynamic contact angles of the gallium-coated surfaces were found to be greater than 160°, while dynamic contact angle measurements showed contact angle hysteresis in the range of 6.5-24.4°. Surface-oxidized gallium-based liquid metal alloy droplets were shown to bounce off and roll on the gallium-coated surfaces without leaving any residue which confirms the nonwettability of the gallium-coated flat surfaces. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) showed the gallium-coated flat substrates consist of nanoscale hemispherical structures with average surface roughness of 33.8 nm. Pneumatic actuation of surface-oxidized liquid metal droplets in PDMS microfluidic channels coated with gallium was conducted to confirm the feasibility of utilizing gallium coating as an effective surface modification for surface-oxidized gallium-based liquid metal droplet manipulation.

Liquid metals: fundamentals and applications in chemistry

Post-transition elements, together with zinc-group metals and their alloys belong to an emerging class of materials with fascinating characteristics originating from their simultaneous metallic and liquid natures. These metals and alloys are characterised by having low melting points (i.e. between room temperature and 300 °C), making their liquid state accessible to practical applications in various fields of physical chemistry and synthesis. These materials can offer extraordinary capabilities in the synthesis of new materials, catalysis and can also enable novel applications including microfluidics, flexible electronics and drug delivery. However, surprisingly liquid metals have been somewhat neglected by the wider research community. In this review, we provide a comprehensive overview of the fundamentals underlying liquid metal research, including liquid metal synthesis, surface functionalisation and liquid metal enabled chemistry. Furthermore, we discuss phenomena that warrant further investigations in relevant fields and outline how liquid metals can contribute to exciting future applications.

A Liquid-Metal Based Spiral Magnetohydrodynamic Micropump

A liquid-metal based spiral magnetohydrodynamic (MHD) micropump is proposed in this work. The micropump was fabricated in a polydimethylsiloxane (PDMS)-glass hybrid microfluidic chip. This pump utilized two parallel liquid-metal-filled channels as electrodes to generate a parallel electrical field across the pumping channel between the two electrodes. To prevent contact and cross contamination between the liquid metal in the electrode channel and the sample fluid in the pumping channel, a PDMS gap was designed between the liquid metal and the sample fluid. To minimize the chip size, the parallel electrode and pumping channels were designed in a spiral shape. To test pumping performance, NaCl aqueous solution containing fluorescent particles (0.5 μm in diameter) was filled into the pumping channel as the working sample fluid. When a pair of identical magnets (0.4 T) was placed onto both top and bottom surfaces of the chip, the pump was able to drive the sample fluid at a flow velocity of 233.26 μm/s at 3000 V. The pump has no moving parts, and the electrodes are easily fabricated, making the pump suitable for miniaturization and integration into microfluidic systems.

Customised spatiotemporal temperature gradients created by a liquid metal enabled vortex generator

Generating customised temperature gradients in miniaturised flow-free liquid chambers is challenging due to the dominance of diffusion. Inducing internal flows in the form of vortices is an effective strategy for overcoming the limitations of diffusion in such environments. Vortices can be produced by applying pressure, temperature and electric potential gradients via miniaturised actuators. However, the difficulties associated with the fabrication, integration, maintenance and operation of such actuators hinder their utility. Here, we utilise liquid metal enabled pumps to induce vortices inside a miniaturised liquid chamber. The configuration and rotational velocity of these vortices can be controlled by tuning the polarity and frequency of the energising electrical signal. This allows creation of customised spatial temperature gradients inside the chamber. The absence of conventional moving elements in the pumps facilitates the rapid reconfiguration of vortices. This enables quick transition from one temperature profile to another, and creates customised spatiotemporal temperature gradients. This allows temperature oscillation from 35 to 62 °C at the hot spot, and from 25 to 27 °C at the centre of the vortex within 15 seconds. Our liquid metal enabled vortex generator can be fabricated, integrated and operated easily, and offers opportunities for studying thermo-responsive materials and biological samples.