Gallium-Based Liquid–Solid Biphasic Conductors for Soft Electronics

Soft and stretchable electronics have diverse applications in the fields of compliant bioelectronics, textile-integrated wearables, novel forms of mechanical sensors, electronics skins, and soft robotics. In recent years, multiple material architectures have been proposed for highly deformable circuits that can undergo large tensile strains without losing electronic functionality. Among them, gallium-based liquid metals benefit from fluidic deformability, high electrical conductivity, and self-healing property. However, their deposition and patterning is challenging. Biphasic material architectures are recently proposed as a method to address this problem, by combining advantages of solid-phase materials and composites, with liquid deformability and self-healing of liquid phase conductors, thus moving toward scalable fabrication of reliable stretchable circuits. This article reviews recent biphasic conductor architectures that combine gallium-based liquid-phase conductors, with solid-phase particles and polymers, and their application in fabrication of soft electronic systems. In particular, various material combinations for the solid and liquid phases in the biphasic conductor, as well as methods used to print and pattern biphasic conductive compounds, are discussed. Finally, some applications that benefit from biphasic architectures are reviewed.     

Ion-Conducting Non-Flammable Liquid Crystal–Polymer Composites for High-Frequency Soft Actuators

High-frequency actuators are reported based on non-flammable lithium-ion conducting phosphate liquid crystal–polymer composite electrolytes, which exhibit a bending response at frequencies up to 80 Hz under an AC voltage of 2 V, owing to its high ionic conductivity reaching 10⁻⁴ S cm⁻¹ at room temperature. An equimolar complex of a phosphate-containing mesogenic molecule and lithium bis(trifluoromethylsulfonyl)imide through the ion-dipole interactions induced a room-temperature smectic A liquid-crystalline (LC) phase forming 2D ion-transport pathways comprising the 2D array of the phosphate moieties. A blend of 80 wt% LC electrolyte and 20 wt% polymers (poly(vinyl chloride) and poly(vinylidene fluoride-co-hexafluoropropylene)) formed a flexible, mechanically robust LC–polymer composite film. Scanning electron microscopy and white light interference microscopy revealed a microphase-segregated structure consisting of a continuous LC phase and a porous polymer matrix. In addition, the continuity of porous structure across the film is confirmed by permeation experiments of solvents thorough the membrane with a homemade filter in a dead-end filtration mode. The LC–polymer composite film sandwiched between two poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) electrodes is found to simultaneously exhibit high bending strain (0.63%) and high output force (0.35 mN), owing to the high ion migration into the composite electrolyte and electrode.     

Customizing Multifunctional Sulfur Host Materials Via a General Anion-Exchange Process with Metal–Organic Solid

Fe/Co/Ni-based discrete nanoparticles are often explored to chemically adsorb and catalytically convert polysulfides for high-performance Li–S batteries. Herein, by manipulating the anion-exchange process between Ni–Co acetate microcrystals and various organic anions, porous single- or multi-layered hierarchically functionalized carbon nanoshells are customized. Among them, the shell-in-shell composite, namely C–NiCoPi@C–Ni₂Co, which has an external Ni₂Co-decorated carbon nanoshell and an internal Ni₃P-anchored carbon nanoshell encapsulated within a 3D-structured continuous NiCo-phosphate (NiCoPi) layer, displays collective merits as a sulfur host. Particularly, compared with discrete nanoparticles, the network-like NiCoPi layer is structurally and inherently advantageous in chemical adsorption and catalytic conversion of polysulfides, as also supported by Density Functional Theory calculations, while the metallic Ni₂Co nanoparticles enable high local conductivity. Such S@C–NiCoPi@C–Ni₂Co displays an initial specific capacity of 1237 mAhg^–1 at 0.1 C and maintains 1010 mAhg^–1 after 100 cycles; at 0.5 C, its discharge specific capacity in the 1st cycle is as high as 1038 mAhg^–1 and maintains capacity retention as high as 86% after 500 cycles. This study provides a straightforward strategy in customizing multifunctional sulfur hosts for high-performance Li–S batteries.     

Intermetallic Compound Formation Mechanisms for Cu-Sn Solid–Liquid Interdiffusion Bonding

Cu-Sn solid?Cliquid interdiffusion (SLID) bonding is an evolving technique for wafer-level packaging which features robust, fine pitch and high temperature tolerance. The mechanisms of Cu-Sn SLID bonding for wafer-level bonding and three-dimensional (3-D) packaging applications have been studied by analyzing the microstructure evolution of Cu-Sn intermetallic compounds (IMCs) at elevated temperature up to 400°C. The bonding time required to achieve a single IMC phase (Cu₃Sn) in the final interconnects was estimated according to the parabolic growth law with consideration of defect-induced deviation. The effect of predominantly Cu metal grain size on the Cu-Sn interdiffusion rate is discussed. The temperature versus time profile (ramp rate) is critical to control the morphology of scallops in the IMC. A low temperature ramp rate before reaching the bonding temperature is believed to be favorable in a SLID wafer-level bonding process.     

Liquid–Solid Triboelectric Probes for Real-Time Monitoring of Sucrose Fluid Status

Triboelectric probes have rapidly developed as an efficient tool for understanding contact electrification at liquid–solid interfaces. However, the liquid–solid electrification process is susceptible to interference from chemical components in mixed solutions, severely limiting the potential applications of triboelectric probes in various liquid environments. This study for the first time reports a triboelectric probe capable of sucrose solution concentration sensing, finding that the dissolution of sucrose destroys the hydrogen bond network between water molecules and forms sucrose–water hydrogen bonds, which alters the fluid mechanics characteristics of the solution and enhances its conductivity, thereby reducing the droplet size and causing an ion charge shielding effect that significantly affects the electron transfer in liquid–solid contact electrification. Owing to the feedback of the triboelectric probe on the sucrose concentration gradient-type sensing electrical signals, efficient sensing of sucrose solution was achieved (sensitivity of −0.0038%⁻¹, response time of 90 ms). The triboelectric probe is also used as a wireless smart terminal to enable real-time detection of sucrose solution. This work contributes to the understanding of the structure–function relationship between micro hydrogen bonding and macro performance, and provides a promising solution for building sustainable intelligent sensors.     

Hybrid Liquid–Solid Composite Electrolytes for Sulfide-Based Solid-State Batteries: Advantages and Limitation

All-solid-state batteries (SSBs) represent one of the most promising avenues for surpassing the energy density limitations of conventional lithium-ion batteries. However, the unstable interfacial contact between the solid-state electrolyte and the electrode poses a critical challenge for practical applications. To tackle this issue, a hybrid system incorporating both liquid electrolytes (LEs) and sulfide solid-state electrolytes may serve as a viable alternative. In this hybrid system, the LE facilitates the in situ formation of a solid electrolyte interphase layer, thereby enhancing the physical interface contact. Consequently, the electrochemical lifetime of the hybrid all-SSBs is significantly improved, as evidenced by the stable lithium plating behavior observed through analytical techniques such as in situ X-ray imaging. Nonetheless, the hybrid system exhibits clear limitations, and several issues that need to be addressed for its practical implementation are identified. In conclusion, potential solutions that could be employed to overcome these challenges are proposed.     

A Multifunctional Catalytic Interlayer for Propelling Solid–Solid Conversion Kinetics of Li2S2 to Li2S in Lithium–Sulfur Batteries

The theoretically high-energy-density lithium–sulfur batteries (LSBs) are seriously limited by the disadvantages including the shuttle effect of soluble lithium polysulfides (LiPSs) and the sluggish sulfur redox kinetics, especially for the most difficult solid–solid conversion of Li₂S₂ to Li₂S. Herein, a multifunctional catalytic interlayer to improve the performance of LSBs is tried to introduce, in which Fe_1–xS/Fe₃C nanoparticles are embedded in the N/S dual-doped carbon network (NSC) composed by nanosheets and nanotubes (the final product is named as FeSC@NSC). The well-designed 3D NSC network endows the interlayer with a satisfactory LiPSs capture-catalytic ability, thus ensuring fast redox reaction kinetics and suppressing LiPSs shuttling. The density functional theory calculations disclose the catalytic mechanisms that FeSC@NSC greatly improves the liquid–solid (LiPSs to Li₂S₂) conversion and unexpectedly the solid–solid (Li₂S₂ to Li₂S) one. As a result, the LSBs based on the FeSC@NSC interlayer can achieve a high specific capacity of 1118 mAh g⁻¹ at a current density of 0.2 C, and a relatively stable capacity of 415 mAh g⁻¹ at a large current density of 2.0 C after 700 cycles as well as superior rate performance.     

Reversible Temperature-Sensitive Liquid–Solid Triboelectrification with Polycaprolactone Material for Wetting Monitoring and Temperature Sensing

Controlling liquid–solid triboelectrification is highly demanded in a wide range of applications, from electrostatic prevention to energy collection and utilization. Except for traditional unidirectional and irreversible ways, smart approaches are required urgently. Here, a novel temperature response liquid–solid triboelectric nanogenerator (TENG) is reported on the basis of a polycaprolactone (PCL) covered fluorinated alumina for tunable triboelectrification. The PCL conformation is regulated by temperature to endow the substrate controllable surface component and interfacial wettability to manipulate the liquid–solid triboelectricity flexibly. As the temperature rises from 20 to 40 °C, the short circuit current and the open-circuit voltage of the PCL-based TENG are reduced by more than 40 times. When the temperature drops to 20 °C, the electrical output can return to its original level again. Moreover, after one month, the electrical signal is still reversible and stable. In addition to water, the electrical output of organic liquid, such as ethylene glycol, also responds well to temperature. This work initially provides a new strategy for achieving the customizable manipulation of liquid–solid triboelectrification by polymer surface reorganization, gives a new idea for in situ monitoring the interfacial wettability changes, and configures the reconstruction of amphiphilic polymer using triboelectricity.     

Enhanced Mass Transfer of Oxygen through a Gas–Liquid–Solid Interface for Photocatalytic Hydrogen Peroxide Production

Solar-driven photocatalytic oxygen reduction is a potentially sustainable route for the production of hydrogen peroxide (H₂O₂). However, this approach suffers from the limited solubility and slow diffusion of oxygen in water. Another problem is that most photocatalytic oxygen reduction systems do not work well with just water. They often require the addition of sacrificial agents such as alcohols. Here, a covalent organic framework (COF)-based photocatalyst that can reduce O₂ to H₂O₂ efficiently in pure water under visible-light irradiation is reported. A solar-to-chemical conversion of 0.76% is achieved for H₂O₂ generation. More importantly, the hydrophobic and mesoporous properties of triphenylbenzene-dimethoxyterephthaldehyde-COF allow the formation of a triphase interface (gas–liquid–solid) when loading this catalyst onto a porous substrate. The H₂O₂ production rate reaches ≈2.9 mmol g_cat⁻¹ h⁻¹ at the triphase interface by overcoming the mass-transfer limitation of O₂ in water. Notably, this rate is 15 times higher than that in a diphase system (liquid–solid). The photoelectrochemical tests reveal that the increase in yield is closely related to the enhanced mass-transfer rate and the higher interfacial O₂ concentration. Furthermore, the triphenylbenzene part is identified as the reactive site based on theoretical calculations.     

Techniques and Apparatus for Measuring Rotational Core Losses of Soft Magnetic Materials

In many situations such as the cores of a rotating electrical machine and the T joints of a multiphase transformer, the local flux density varies with time in terms of both magnitude and direction, i.e. the flux density vector is rotating. Therefore, the magnetic properties of the core materials under the rotating flux density vector excitation should be properly measured, modeled and applied in the design and analysis of these electromagnetic devices. This paper presents an extensive review on the development of techniques and apparatus for measuring the rotational core losses of soft magnetic materials based on the experiences of various researchers in the last hundred years.     

Kinetics of Dissolution and Isothermal Solidification for Gold-Enriched Solid–Liquid Interdiffusion (SLID) Bonding

This paper presents the development and characterization of a fluxless die-attach soldering process based on gold-enriched solid–liquid interdiffusion (SLID). Eutectic Au-Sn and pure Au were deposited by jet vapor deposition (JVD) onto two substrates, assembled in a sandwiched structure, and processed in a vacuum furnace using different temperatures and times. Microstructural characterization, based on scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS) analysis, revealed the formation of sound joints governed by the interdiffusion of the main constituents. Kinetic studies for the dissolution and the isothermal solidification stages were conducted. Differential scanning calorimetry (DSC) revealed a solder joint that is thermally stable up to 498°C, thus demonstrating the effectiveness of using the SLID process for the production of joints which require a lower processing temperature compared with their remelting point. Based on these findings, the recommended final bonding parameters are processing temperature and time of 340°C (310°C < T _P < 340°C) and 5 min, respectively.     

Metal–Organic Framework-Integrated Composites for Bone Tissue Regeneration

Metal–organic frameworks (MOFs) exhibit exceptional characteristics, including high porosity and adjustable properties, rendering them highly suitable for biomedical applications. Recently, researchers have directed their attention toward the integration of MOFs into composites for bone tissue regeneration (BTE), presenting distinct advantages over conventional substitutes. In this comprehensive review, the authors delve into the latest advancements concerning MOF-integrated composites for BTE. This exploration encompasses an examination of the properties of MOFs and the various synthesis techniques employed to fabricate these materials. Furthermore, the diverse applications of MOF-integrated composites in BTE are investigated, encompassing areas such as antibacterial properties, osteogenic differentiation, angiogenesis, and immunomodulation. By providing a comprehensive overview of the ongoing research on MOF-integrated composites for BTE, this study holds the promise of yielding innovative solutions within the realm of orthopedics.     

Liquid–Metal-Superlyophilic and Conductivity–Strain-Enhancing Scaffold for Permeable Superelastic Conductors

Liquid metal (LM) has recently been used as an advanced stretchable material for constructing stretchable and wearable electronics. However, due to the poor wettability of LM and the large dimensional change during stretching, it remains very challenging to obtain a high conductivity with minimum resistance increase over large tensile strains. To address the challenge, an LM-superlyophilic and stretchable fibrous thin-film scaffold is reported, on which LM can be readily coated or printed to form permeable superelastic conductors. In contrast to conventional LM-based conductors where LM particles are filled into an elastic matrix or printed on the surface of an elastic thin film, the LM can quickly infuse into the LM-superlyophilic scaffold and form bi-continuous phases. The LM-superlyophilic scaffold shows unprecedented advantages of an extremely high uptake of the LM and a conductivity-enhancement characteristic when stretched. As a result, the LM-based conductor displays and ultrahigh conductivity of 155 900 S cm⁻¹ and a marginal resistance change by only 2.5 fold at 2 500% strain. The conductor also possesses a remarkable durability over a period of 220 000 cycles of stretching tests. The printing of LM onto the LM-superlyophilic scaffold for the fabrication of various permeable and wearable electronic devices is demonstrated.     

Soft Magnetic Alloy–Polymer Composite for High-Frequency Power Electronics Application

Soft magnetic alloys are limited to lower frequencies because of increased eddy-current losses at higher frequencies. A simple low-temperature solvent-based process was developed to coat permalloy powder with a benzocyclobutene insulating layer to reduce interparticle eddy-current loss. Low-signal measurements show that the permeability of the cured composite exhibits a bandwidth beyond 10 MHz. In contrast, the permeability of the pure powder rolled off well below 1 MHz with a corresponding increase in the imaginary permeability. Measurements of the core loss density at 5 MHz on pressed composite cores show a core loss of 300 mW/cm³ at more than 90 gauss, while the pure powder core achieved the same core loss density at just over 10 gauss. The results demonstrate that the polymer coating process is an effective way of reducing the interparticle eddy-current loss in powdered magnetic cores at high frequencies.     

PIN-PMN-PT Single-Crystal-Based 1–3 Piezoelectric Composites for Ultrasonic Transducer Applications

In this work, Pb(In_1/2Nb_1/2)O₃-Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (PIN-PMN-PT) single crystal/epoxy 1–3 composites with different thicknesses (400 μm to 825 μm) were fabricated using the conventional dice-and-fill method. Their properties were compared with the corresponding lead zirconate titanate (PZT) ceramic 1–3 composites. Excellent properties for ultrasonic transducer applications have been achieved, such as high electromechanical coupling coefficient (k _t ≈ 78% to 83%), high piezoelectric strain coefficient (d ₃₃ ≈ 1000 pm/V), and lower acoustic impedance (Z ≈ 20 Mrayl). The strain levels of PIN-PMN-PT composites were almost constant (1000 pm/V) with decreasing thickness, being much higher than those of PZT composites (650 pm/V). However, an increase in strain hysteresis was observed with decreasing thickness, reaching 25.3% for the 400-μm single-crystal 1–3 composite, which is lower than the corresponding PZT composites (44.1% for 350-μm PZT ceramic 1–3 composite). These results show that PIN-PMN-PT single-crystal 1–3 composites have great potential for use in advanced ultrasound transducer applications.     

Electro—optical Properties of Liquid Crystal Composite Materials

The liquid crystal composite materials consist of microdroplers of liquid crystals which are spontaneously formed in a matrix of a polymer at the time of its polymerization.The director configuration in liquid crystal droplets.the model of orientation of droplets,and the contrast ratios of a cell are investigated.Droplet size,spacing and distribution are readily controlled in these materials to allow optimization of displays based upon electrically controlled light scattering from the liquid crystal droplets.Preliminary experimental and theoretical studies of the light scattering and electro-optic response of new material show that these materials can offer new features suitable for large area displays and light valves.     

Specific Features of Vapor–Liquid–Solid Nanostructure Growth on the Surface of SnS Films during Plasma Treatment

The formation of nanostructure arrays on the surface of nanocrystalline tin sulfide (SnS) films during inductively coupled argon plasma treatment is investigated. The parameters of the nanostructures are studied by electron microscopy and energy-dispersive X-ray microanalysis and the main regularities of nanostructure growth are established. It is shown that the SnS nanostructure growth during plasma treatment is complex and involves the vapor–liquid–solid mechanism with self-assembled tin seeds.     

Liquid Alloy Enabled Solid-State Batteries for Conformal Electrode–Electrolyte Interfaces

Recent research efforts on solid-state alkali-metal batteries are pushing the limit of energy density to a higher level. However, the development of solid-state batteries is still hindered by many intrinsic limitations, among which the incompatibility between the solid electrolyte and the metal anode is a critical issue attracting massive research attention. A Na–K liquid alloy electrode is designed to form a conformal electrode–electrolyte interface with a solid electrolyte. Much enhanced electrode–electrolyte interfacial contact electrically and physically is observed with liquid metal anodes than solid alkali metals on solid electrolytes. Symmetric cells of the liquid metal electrolytes show much lower overpotential as well as better cyclability than the alkali-metal electrodes. Excellent cyclability over 500 cycles with reasonable capacity decay and good rate performance of full cells with the sodium rhodizonate and a ferricyanide potassium-ion cathode are both achieved. By adjusting salt and filler species in the polymer electrolyte, the wettability of liquid metal on the electrolyte can be further improved, and the raised ionic conductivity can further improve the battery performance of such a design.     

Low Resistance and High Stable Solid–Liquid Electrolyte Interphases Enable High-Voltage Solid-State Lithium Metal Batteries

Solid-state batteries (SSBs) with addition of liquid electrolytes are considered to possibly replace the current lithium-ion batteries (LIBs) because they combine the advantages of benign interfacial contact and strong barriers for unwanted redox shuttles. However, solid electrolyte and liquid electrolyte are generally (electro)-chemically incompatible and the resistance of the newly formed solid–liquid electrolyte interphase (SLEI) appears as an additional contribution to the overall battery resistance. Herein, a boron, fluorine-donating liquid electrolyte (B, F-LE) is introduced into the interface between the high-voltage cathode and ultrathin composite solid electrolyte (CSE), which is fabricated by adhering a high content of nanosized Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) with poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), to generate a low resistance and high stable SLEI in situ, giving a stable high-voltage output with a reinforced cathode|CSE interface. B, F-LE, consisting of a highly fluorinated electrolyte with a lithium bis(oxalato)borate additive, exhibits good chemical compatibility with CSE and enables rapid and uniform transportation of Li⁺, with its electrochemically and chemically stable interface for high-voltage cathode. Eventually, the B, F-LE assisted LiNi_0.6Co_0.2Mn_0.2O₂|Li battery displays the enhanced rate capability and high voltage cycling stability. The findings provide an interfacial engineering strategy to turn SLEI from a “real culprit” into the “savior” that may pave a brand-new way to manipulate SLEI chemistry in hybrid solid–liquid devices.     

Polymer Chemistry for Haptics,Soft Robotics,and Human–Machine Interfaces

Progress in the field of soft devices—that is, the types of haptic, robotic, and human-machine interfaces (HRHMIs) in which elastomers play a key role—has its basis in the science of polymeric materials and chemical synthesis. However, in examining the literature, it is found that most developments have been enabled by off-the-shelf materials used either alone or as components of physical blends and composites. A greater awareness of the methods of synthetic chemistry will accelerate the capabilities of HRHMIs. Conversely, an awareness of the applications sought by engineers working in this area may spark the development of new molecular designs and synthetic methodologies by chemists. Several applications of active, stimuli-responsive polymers, which have demonstrated or shown potential use in HRHMIs are highlighted. These materials share the fact that they are products of state-of-the-art synthetic techniques. The progress report is thus organized by the chemistry by which the materials are synthesized, including controlled radical polymerization, metal-mediated cross-coupling polymerization, ring-opening polymerization, various strategies for crosslinking, and hybrid approaches. These methods can afford polymers with multiple properties (i.e., conductivity, stimuli-responsiveness, self-healing, and degradable abilities, biocompatibility, adhesiveness, and mechanical robustness) that are of great interest to scientists and engineers concerned with soft devices for human interaction.