Ultrasonic-Enabled Nondestructive and Substrate-Independent Liquid Metal Ink Sintering

Liquid Metal Gel Ink with Self-Activating Conductivity for 3D Printing of Multifunctional Electronics

Liquid metal inks have emerged as promising conductive inks for the printing of soft circuits and multifunctional electronics. However, the printed patterns are typically nonconductive due to the native insulating oxide layer surrounding the liquid metal (LM) particles, which requires mechanical or chemical post-treatments to restore their electrical performance. In this study, the design and preparation of a self-activating LM gel ink are presented. This viscous gel ink consists of LM particles and supramolecular assemblies, which are formed by β-cyclodextrin (β-CD) and sodium dodecyl sulfate (SDS). These assemblies entangle to create a supramolecular gel network, which prevents the LM particles from settling and facilitates 3D printing. Moreover, the supramolecular assemblies are dissociated into host-guest complexes upon heating to 50 °C, thereby allowing the ink to transition its viscosity from ≈13 to ≈0.005 Pa·s at a shear rate of 1 s-1. This viscosity transition leads to the sedimentation of LM particles, resulting in the formation of a continuous liquid metal phase upon water evaporation, with a high electrical conductivity of 3.4 × 105 S m-1. The printed conductive patterns can subsequently be used in multifunctional devices, including stretchable displays, wireless power-transmission circuits, and fabric bioelectrodes.

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

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

Connecting the Dots: Sintering of Liquid Metal Particles for Soft and Stretchable Conductors

This review focuses on the sintering of liquid metal particles (LMPs). Here, sintering means the partial merging or connecting of particles (or droplets) to form a network of percolated and, thus, conductive electrical pathways. LMPs are attractive materials because they can be suspended in a carrier fluid to create printable inks or distributed in an elastomer to create soft, stretchable composites. However, films and traces of LMPs are not typically conductive as fabricated due to the native oxide that forms on the surface of the particles. In the case of composites, polymers can also get between particles, making sintering more challenging. Sintering can be done via a variety of ways, such as mechanical, thermal, and chemical processing. This review discusses the mechanisms to sinter these particles, patterning techniques that use sintering, unique properties of sintered LMPs, and their practical applications in fields such as stretchable electronics, soft robotics, and active materials.

Wetting and Spreading Behaviors of Impacting Metal Droplet Regulated by 2D Ultrasonic Field

The wetting and spreading behaviors of metal droplets on solid substrates are critical aspects of additive manufacturing. However, the inherent characteristics of metal droplets, including high surface tension, elevated viscosity, and extreme temperatures, pose significant challenges for wetting and spreading on nonwetting substrates. Herein, this work proposes a strategy that employs a two-dimensional (2D) orthogonal ultrasonic field to construct a vibration deposition substrate with radial vibration amplitude gradient, thereby enhancing the wettability and adhesive strength of impacting metal droplets ejected by a piezoelectric micro-jet device. First, a 2D ultrasonic vibration device is designed based on the combination of longitudinal vibration modes. Additionally, oblique and circular vibration trajectories are synthesized. The vibration amplitude distributions and trajectories of the deposition substrate are verified utilizing the finite element method. Subsequently, the experimental results demonstrate that the contact angle is decreased by 24.7%, the spreading diameter is increased by 10.3%, and the adhesive strength is enhanced by 5.4 times compared to deposition on a static substrate. The 2D ultrasonic field facilitates the transition of metal droplets from a non-wetting state to a wetting state on the nonwetting substrate, which highlights the versatility and adaptability of ultrasonic strategy for expanding the applications of metal droplets.

Disintegration of Thin Liquid Metal Films Engendered by Aluminum Corrosion

Liquid metals (LMs) illustrate a fantastic future. Thus, great endeavors are made to earn a comprehensive understanding of this fluid and carve it into a niche. Herein, by revisiting the combination of Ga-based LMs and aluminum (Al), a new phenomenon, namely the disintegration of LM films on encountering water, is identified. Deviating from previous investigations where the LM generally took the form of bulk puddles, the LM-Al slurry is spread as thin films here. In this case, Al debris embedded in the LM matrix hydrolyzes and therefore can exert disjoining pressure strong enough to split the thin film into countless tiny LM droplets. Based on this mechanism, transient circuits independent of substrate decomposition are realized. Furthermore, taking advantage of the portfolio strategy of pure LM and the LM-Al slurry, novel concepts of flood warning and information storage and encryption are demonstrated. Integrating these functions all in one demonstrates the versatility of the disintegration of thin LM films engendered by Al corrosion, which provides a scientific insight into ephemeral art and makes the Ga─Al combination more illuminating.

Stretchable Triboelectric Nanogenerator Based on Liquid Metal with Varying Phases

Stretchable triboelectric nanogenerators (TENGs) represent a new class of energy-harvesting devices for powering wearable devices. However, most of them are associated with poor stretchability, low stability, and limited substrate material choices. This work presents the design and demonstration of highly stretchable and stable TENGs based on liquid metalel ectrodes with different phases. The conductive and fluidic properties of eutectic gallium-indium (EGaIn) in the serpentine microfluidic channel ensure the robust performance of the EGaIn-based TENG upon stretching over several hundred percent. The bi-phasic EGaIn (bGaIn) from oxidation lowers surface tension and increases adhesion for printing on diverse substrates with high output performance parameters. The optimization of the electrode shapes in the bGaIn-based TENGs can reduce the device footprint and weight, while enhancing stretchability. The applications of the EGaIn- and bGaIn-based TENG include smart elastic bands for human movement monitoring and smart carpets with integrated data transmission/processing modules for headcount monitoring/control. Combining the concept of origami in the paper-based bGaIn TENG can reduce the device footprint to improve output performance per unit area. The integration of bGaIn-TENG on a self-healing polymer substrate with corrosion resistance against acidic and alkaline solutions further facilitates its use in various challenging and extreme environments.

Pressure-constrained sonication activation of flexible printed metal circuit

Metal micro/nanoparticle ink-based printed circuits have shown promise for promoting the scalable application of flexible electronics due to enabling superhigh metallic conductivity with cost-effective mass production. However, it is challenging to activate printed metal-particle patterns to approach the intrinsic conductivity without damaging the flexible substrate, especially for high melting-point metals. Here, we report a pressure-constrained sonication activation (PCSA) method of the printed flexible circuits for more than dozens of metal (covering melting points from room temperature to 3422 °C) and even nonmetallic inks, which is integrated with the large-scale roll-to-roll process. The PCSA-induced synergistic heat-softening and vibration-bonding effect of particles can enable multilayer circuit interconnection and join electronic components onto printed circuits without solder within 1 s at room temperature. We demonstrate PCSA-based applications of 3D flexible origami electronics, erasable and foldable double-sided electroluminescent displays, and custom-designed and large-area electronic textiles, thus indicating its potential for universality in flexible electronics.

Microconfined Assembly of High-Resolution and Mechanically Robust EGaIn Liquid Metal Stretchable Electrodes for Wearable Electronic Systems

Stretchable electrodes based on liquid metals (LM) are widely used in human-machine interfacing, wearable bioelectronics, and other emerging technologies. However, realizing the high-precision patterning and mechanical stability remains challenging due to the poor wettability of LM. Herein, a method is reported to fabricate LM-based multilayer solid-liquid electrodes (m-SLE) utilizing electrohydrodynamic (EHD) printed confinement template. In these electrodes, LM self-assembled onto these high-resolution templates, assisted by selective wetting on the electrodeposited Cu layer. This study shows that a m-SLE composed of PDMS/Ag/Cu/EGaIn exhibits line width of ≈20 µm, stretchability of ≈100%, mechanical stability ≈10 000 times (stretch/relaxation cycles), and recyclability. The multi-layer structure of m-SLE enables the adjustability of strain sensing, in which the strain-sensitive Ag part can be used for non-distributed detection in human health monitoring and the strain-insensitive EGaIn part can be used as interconnects. In addition, this study demonstrates that near field communication (NFC) devices and multilayer displays integrated by m-SLEs exhibit stable wireless signal transmission capability and stretchability, suggesting its applicability in creating highly-integrated, large-scale commercial, and recyclable wearable electronics.

Magnetically Responsive Gallium-Based Liquid Metal: Preparation, Property and Application

Magnetically responsive soft smart materials have garnered significant academic attention due to their flexibility, remote controllability, and reconfigurability. However, traditional soft materials used in the construction of these magnetically responsive systems typically exhibit low density and poor thermal and electrical conductivities. These limitations result in suboptimal performance in applications such as medical radiography, high-performance electronic devices, and thermal management. To address these challenges, magnetically responsive gallium-based liquid metals have emerged as promising alternatives. In this review, we summarize the methodologies for achieving magnetically responsive liquid metals, including the integration of magnetic agents into the liquid metal matrix and the utilization of induced Lorentz forces. We then provide a comprehensive discussion of the key physicochemical properties of these materials and the factors influencing them. Additionally, we explore the advanced and potential applications of magnetically responsive liquid metals. Finally, we discuss the current challenges in this field and present an outlook on future developments and research directions.

Enhanced aerosol-jet printing using annular acoustic field for high resolution and minimal overspray

Aerosol jet printing has the potential to fabricate fine features on various substrates due to its large stand-off distance. However, the presence of overspray and instability, particularly at high printing resolutions, has limited its widespread application. In this study, we introduce an efficient approach called annular acoustic focusing for aerosol jet printing. By determining the optimal focusing frequency (5.8 MHz) for silver nanoparticles using a particle ejection model, we achieve precise and stable printing. We also propose a modified print nozzle geometry, resulting in ultrafine traces (line width < 6 μm, overspray < 0.1 μm). Compared to printing without acoustic focusing, the line width of the traces decreases to 60 ± 5% while their conductivity increases to 180 ± 5%. Additionally, several 8 h experiments demonstrate excellent printing stability. This research opens up possibilities for the fabrication of conformal electronics with high precision and improved conductivity using aerosol jet printing.

Integrating High-Performance Flexible Wires with Strain Sensors for Wearable Human Motion Detection

Flexible electronics have revolutionized the field by overcoming the rigid limitations of traditional devices, offering superior flexibility and adaptability. Conductive ink performance is crucial, directly impacting the stability of flexible electronics. While metal filler-based inks exhibit excellent conductivity, they often lack mechanical stability. To address this challenge, we present a novel conductive ink utilizing a ternary composite filler system: liquid metal and two micron-sized silver morphologies (particles and flakes). We systematically investigated the influence of filler type, mass ratio, and sintering process parameters on the composite ink's conductivity and mechanical stability. Our results demonstrate that flexible wires fabricated with the liquid metal/micron silver particle/micron silver flake composite filler exhibit remarkable conductivity and exceptional bending stability. Interestingly, increasing the liquid metal content results in a trade-off, compromising conductivity while enhancing mechanical performance. After enduring 5000 bending cycles, the resistance change in wires formulated with a 4:1 mass ratio of micron silver particles to flakes is only half that of wires with a 1:1 ratio. This study further investigates the mechanism governing resistance variations during flexible wire bending. Additionally, we observed a positive correlation between sintering temperature and pressure with the conductivity of flexible wires. The significance of the sintering parameters on conductivity follows a descending order: sintering temperature, sintering pressure, and sintering time. Finally, we demonstrate the practical application of this technology by integrating the composite ink-based flexible wires with conductive polymer-based strain sensors. This combination successfully achieved the detection of human movements, including finger and wrist bending.

A Gradient Stiffness-Programmed Circuit Board by Spatially Controlled Phase-Transition of Supercooled Hydrogel for Stretchable Electronics Integration

Due to emerging demands in soft electronics, there is an increasing need for material architectures that support robust interfacing between soft substrates, stretchable electrical interconnects, and embedded rigid microelectronics chips. Though researchers have adopted rigid-island structures to solve the issue, this approach merely shifts stress concentrations from chip-conductor interfaces to rigid-island-soft region interfaces in the substrate. Here, a gradient stiffness-programmed circuit board (GS-PCB) that possesses high stretchability and stability with surface mounted chips is introduced. The board comprises a stiffness-programmed hydrogel substrate and a laser-patterned liquid metal conductor. The hydrogel simultaneously obtains a large stiffness disparity and robust interfaces between rigid-islands and soft regions. These seemingly contradictory conditions are accomplished by adopting a gradient stiffness structure at the interfaces, enabled by combining polymers with different interaction energies and a supercooled sodium acetate solution. By integrating the gel with laser-patterned liquid metal with exceptional properties, GS-PCB exhibits higher electromechanical stability than other rigid-island research. To highlight the practicality of this approach, a finger-sensor device that successfully distinguishes objects by direct physical contact is fabricated, demonstrating its stability under various mechanical disturbances.

Recent Research on Preparation and Application of Smart Joule Heating Fabrics

Multifunctional wearable heaters have attracted much attention for their effective applications in personal thermal management and medical therapy. Compared to passive heating, Joule heating offers significant advantages in terms of reusability, reliable temperature control, and versatile coupling. Joule-heated fabrics make wearable electronics smarter. This review critically discusses recent advances in Joule-heated smart fabrics, focusing on various fabrication strategies based on material-structure synergy. Specifically, various applicable conductive materials with Joule heating effect are first summarized. Subsequently, different preparation methods for Joule heating fabrics are compared, and then their various applications in smart clothing, healthcare, and visual indication are discussed. Finally, the challenges faced in developing these smart Joule heating fabrics and their possible solutions are discussed.