On-Demand Programming of Liquid Metal-Composite Microstructures through Direct Ink Write 3D Printing

Interface Engineering Assisted 3D Printing of Silicone Composites with Synergistically Optimized Impact Resistance and Electromagnetic Interference Shielding Effectiveness

Silicone composites have been universally employed in smart devices, flexible electronics, and mechanical metamaterials. However, it remained challenging to develop 3D printable silicone composites with desirable mechanical and electrical properties. Here, an interface engineering strategy is reported, developing heterointerfacial silver-coated hollow glass microspheres (SHGMs), which are integrated with polydimethylsiloxane (PDMS) for 3D printing of impact-resistant, highly conductive, and mechanically robust SHGMs-PDMS (SHP) composites. SHP simultaneously achieves high compression modulus (12.65 MPa), substantial energy dissipation density (1.58 × 106 N m-2 at 50% strain), excellent conductivity (2.55 × 103 S m-1), and long-period robustness. SHP presents extraordinary impact resistance under dynamic impacts, reaching a considerable energy dissipation of 1.91 kJ m-1 at an incident velocity of 192.3 m s-1. More importantly, SHP with 2 mm in thickness achieves an ultraefficient electromagnetic interference (EMI) effectiveness of 92.5 dB, which is among that of state-of-the-art silicone and its derivatives, and can maintain favorable shielding efficiency (>70 dB) after undergoing mechanical excitations. Moreover, the formability enables it to fabricate delicate structures with a negative Poisson's ratio, ensuring adaptive fit and thus providing complete protection for individuals. This work paves an effective way to rapidly manufacture silicone composites with expected functions for new-generation protective devices.

Liquid Metal Particles Decorated by Poly(imidazole-urea) as Versatile Adhesive and Recyclable Inks for Substrate-Irrelevant Direct Writing

Liquid metals (LMs) with fluidity and conductivity are widely applied in flexible electronics. However, the surface patterning of liquid metals (LMs) is restricted by the low adhesion effect on the substrates because of the intrinsic high surface tension. In this study, a versatile adhesive conductive poly(imidazole-urea)/eutectic Ga75.5In24.5 alloy (PIU/EGaIn) ink is proposed by wrapping the EGaIn particles with PIU through metal coordination to realize substrate-independent direct writing. The adhesion of PIU guarantees that the PIU/EGaIn ink can be written smoothly on different substrates, ranging from flexible to rigid and plane to camber. Complex patterns can also be stamped on the substrate by transfer printing. The maximum conductivity of the handwriting trace can reach as high as 1.3 × 106 S/m due to the highly efficient stability of EGaIn particles with a low content of residue PIU. The written circuit demonstrates high stability and maintains constant conductivity after 500 cycles of deformations (folding, bending, and stretching), thanks to the good adhesion effect of PIU with substrates. In addition, the resistance touch sensor was patterned to detect finger contact as a demonstration of potential application. The PIU/EGaIn ink waste can be recycled using an alkaline solution owing to the intrinsic degradability of PIU. This strategy offers a new choice for universal adhesive conductive ink that is suitable for environmentally friendly flexible electronics.

Processing Parameter-Performance Nexus in 3D Printing of Nanostructured Chiral Photonics

Precisely crafted hierarchical architectures found in naturally derived biomaterials underpin the exceptional performance and functionality showcased by the host organism. In particular, layered helical assemblies composed of cellulose, chitin, or collagen serve as the foundation for some of the most mechanically robust and visually striking natural materials. By utilizing structured materials in additive manufacturing techniques such as extrusion-based 3D printing, the intrinsic deformation process can be used to implement bottom-up design of printed constructs, offering the potential to create intricate macroscale geometries with embedded nanoscale functionality. In this study, comprehensive rheological and rheo-optical characterization of structurally colored, photocurable liquid crystalline inks based on hydroxypropyl cellulose (HPC) are carried out to define the structural dynamics of the system under flow and following flow cessation. It is shown that the processing parameters selected for 3D printing can induce order or disarray in the extruded ink's liquid crystal nanostructure. Low to intermediate shear rates order the chiral nematic domains to yield intense structural color. In contrast, high shear rates induce elastic instabilities that diminish the filament's photonic quality. After establishing the processing parameter-nanostructure relationship, the curing kinetics of these photocurable inks are tailored to facilitate the arrest of the liquid crystalline structure.

3D Soft Architectures for Stretchable Thermoelectric Wearables with Electrical Self-Healing and Damage Tolerance

Flexible thermoelectric devices (TEDs) exhibit adaptability to curved surfaces, holding significant potential for small-scale power generation and thermal management. However, they often compromise stretchability, energy conversion, or robustness, thus limiting their applications. Here, the implementation of 3D soft architectures, multifunctional composites, self-healing liquid metal conductors, and rigid semiconductors is introduced to overcome these challenges. These TEDs are extremely stretchable, functioning at strain levels as high as 230%. Their unique design, verified through multiphysics simulations, results in a considerably high power density of 115.4 µW cm⁻2 at a low-temperature gradient of 10 °C. This is achieved through 3D printing multifunctional elastomers and examining the effects of three distinct thermal insulation infill ratios (0%, 12%, and 100%) on thermoelectric energy conversion and structural integrity. The engineered structure is lighter and effectively maintains the temperature gradient across the thermoelectric semiconductors, thereby resulting in higher output voltage and improved heating and cooling performance. Furthermore, these thermoelectric generators show remarkable damage tolerance, remaining fully functional even after multiple punctures and 2000 stretching cycles at 50% strain. When integrated with a 3D-printed heatsink, they can power wearable sensors, charge batteries, and illuminate LEDs by scavenging body heat at room temperature, demonstrating their application as self-sustainable electronics.

Versatile Patterning of Liquid Metal via Multiphase 3D Printing

This paper presents a scalable and straightforward technique for the immediate patterning of liquid metal/polymer composites via multiphase 3D printing. Capitalizing on the polymer's capacity to confine liquid metal (LM) into diverse patterns. The interplay between distinctive fluidic properties of liquid metal and its self-passivating oxide layer within an oxidative environment ensures a resilient interface with the polymer matrix. This study introduces an inventive approach for achieving versatile patterns in eutectic gallium indium (EGaIn), a gallium alloy. The efficacy of pattern formation hinges on nozzle's design and internal geometry, which govern multiphase interaction. The interplay between EGaIn and polymer within the nozzle channels, regulated by variables such as traverse speed and material flow pressure, leads to periodic patterns. These patterns, when encapsulated within a dielectric polymer polyvinyl alcohol (PVA), exhibit an augmented inherent capacitance in capacitor assemblies. This discovery not only unveils the potential for cost-effective and highly sensitive capacitive pressure sensors but also underscores prospective applications of these novel patterns in precise motion detection, including heart rate monitoring, and comprehensive analysis of gait profiles. The amalgamation of advanced materials and intricate patterning techniques presents a transformative prospect in the domains of wearable sensing and comprehensive human motion analysis.

3D Printing of TPU-Liquid Metal Composite Inks for the Preparation of Flexible Sensing Electronics

Direct 3D printing of liquid metal is difficult to form and easy to destroy. In this paper, we developed a 3D printed composite material consisting of a thermoplastic polyurethane (TPU) matrix and liquid metal (LM) dispersed droplets, and introduced the method for realizing 3D printed devices with this composite material: First, the LM is added to 10~50wt %TPU at 190~200 °C through ultrasonic blending to prepare blended ink. After solid cooling, the LM-TPU composite fiber with a diameter of 600 μm was prepared by Wellzoom desktop extruder at 190 °C at an extrusion speed of 400 mm/min. It has excellent elasticity, with a tensile limit of 0.637 N/m2, and the TPU could evenly wrap LM droplets. Finally, the LM-TPU fiber is 3D printed at 240 °C by using a 3D printer, and 2D/3D flexible electronic devices with heating and conductive functions could be prepared. The microcircuit has good electrical conductivity; after adding voltage, the circuit has heat release; it could be used as heating equipment to keep warm and used in various flexible wearable electronic products.

Printable Liquid Metal Foams That Grow When Watered

Pastes and "foams" containing liquid metal (LM) as the continuous phase (liquid metal foams, LMFs) exhibit metallic properties while displaying paste or putty-like rheological behavior. These properties enable LMFs to be patterned into soft and stretchable electrical and thermal conductors through processes conducted at room temperature, such as printing. The simplest LMFs, featured in this work, are made by stirring LM in air, thereby entraining oxide-lined air "pockets" into the LM. Here, it is reported that mixing small amounts of water (as low as 1 wt%) into such LMFs gives rise to significant foaming by harnessing known reactions that evolve hydrogen and produce oxides. The resulting structures can be ≈4-5× their original volume and possess a fascinating combination of attributes: porosity, electrical conductivity, and responsiveness to environmental conditions. This expansion can be utilized for a type of 4D printing in which patterned conductors "grow," fill cavities, and change shape and density with respect to time. Excessive exposure to water in the long term ultimately consumes the metal in the LMF. However, when exposure to water is controlled, the metallic properties of porous LMFs can be preserved.

Integrated Design and Fabrication of Pneumatic Soft Robot Actuators in a Single Casting Step

Bio-inspired soft robots have already shown the ability to handle uncertainty and adapt to unstructured environments. However, their availability is partially restricted by time-consuming, costly, and highly supervised design-fabrication processes, often based on resource-intensive iterative workflows. Here, we propose an integrated approach targeting the design and fabrication of pneumatic soft actuators in a single casting step. Molds and sacrificial water-soluble hollow cores are printed using fused filament fabrication. A heated water circuit accelerates the dissolution of the core's material and guarantees its complete removal from the actuator walls, while the actuator's mechanical operability is defined through finite element analysis. This enables the fabrication of actuators with non-uniform cross-sections under minimal supervision, thereby reducing the number of iterations necessary during the design and fabrication processes. Three actuators capable of bending and linear motion were designed, fabricated, integrated, and demonstrated as 3 different bio-inspired soft robots, an earthworm-inspired robot, a 4-legged robot, and a robotic gripper. We demonstrate the availability, versatility, and effectiveness of the proposed methods, contributing to accelerating the design and fabrication of soft robots. This study represents a step toward increasing the accessibility of soft robots to people at a lower cost.

Spongy Ag Foam for Soft and Stretchable Strain Gauges

Strain gauges, particularly for wearable sensing applications, require a high degree of stretchability, softness, sensitivity, selectivity, and linearity. They must also steer clear of challenges such as mechanical and electrical hysteresis, overshoot behavior, and slow response/recovery times. However, current strain gauges face challenges in satisfying all of these requirements at once due to the inevitable trade-offs between these properties. Here, we present an innovative method for creating strain gauges from spongy Ag foam through a steam-etching process. This method simplifies the traditional, more complex, and costly manufacturing techniques, presenting an eco-friendly alternative. Uniquely, the strain gauges crafted from this method achieve an unparalleled gauge factor greater than 8 × 103 at strains exceeding 100%, successfully meeting all required attributes without notable trade-offs. Our work includes systematic investigations that reveal the intricate structure-property-performance relationship of the spongy Ag foam with practical demonstrations in areas such as human motion monitoring and human-robot interaction. These breakthroughs pave the way for highly sensitive and selective strain gauges, showing immediate applicability across a wide range of wearable sensing applications.

Enhancing Electrical Conductivity of Stretchable Liquid Metal-Silver Composites through Direct Ink Writing

Structure-property-process relationships are a controlling factor in the performance of materials. This offers opportunities in emerging areas, such as stretchable conductors, to control process conditions during printing to enhance performance. Herein, by systematically tuning direct ink write (DIW) process parameters, the electrical conductivity of multiphase liquid metal (LM)-silver stretchable conductors is increased by a maximum of 400% to over 1.06 × 106 S·m-1. This is achieved by modulating the DIW print velocity, which enables the in situ elongation, coalescence, and percolation of these multiphase inclusions during printing. These DIW printed filaments are conductive as fabricated and are soft (modulus as low as 1.1 MPa), stretchable (strain limit >800%), and show strain invariant conductivity up to 80% strain. These capabilities are demonstrated through a set of electromagnetic induction coils that can transfer power wirelessly through air and water, even under deformation. This work provides a methodology to program properties in stretchable conductors, where the combination of material composition and process parameters leads to greatly enhanced performance. This approach can find use in applications such as soft robots, soft electronics, and printed materials for deformable, yet highly functional devices.

Gradient matters via filament diameter-adjustable 3D printing

Gradient matters with hierarchical structures endow the natural world with excellent integrity and diversity. Currently, direct ink writing 3D printing is attracting tremendous interest, and has been used to explore the fabrication of 1D and 2D hierarchical structures by adjusting the diameter, spacing, and angle between filaments. However, it is difficult to generate complex 3D gradient matters owing to the inherent limitations of existing methods in terms of available gradient dimension, gradient resolution, and shape fidelity. Here, we report a filament diameter-adjustable 3D printing strategy that enables conventional extrusion 3D printers to produce 1D, 2D, and 3D gradient matters with tunable heterogeneous structures by continuously varying the volume of deposited ink on the printing trajectory. In detail, we develop diameter-programmable filaments by customizing the printing velocity and height. To achieve high shape fidelity, we specially add supporting layers at needed locations. Finally, we showcase multi-disciplinary applications of our strategy in creating horizontal, radial, and axial gradient structures, letter-embedded structures, metastructures, tissue-mimicking scaffolds, flexible electronics, and time-driven devices. By showing the potential of this strategy, we anticipate that it could be easily extended to a variety of filament-based additive manufacturing technologies and facilitate the development of functionally graded structures.

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.

Room-Temperature Liquid Metals for Flexible Electronic Devices

Room-temperature gallium-based liquid metals (RT-GaLMs) have garnered significant interest recently owing to their extraordinary combination of fluidity, conductivity, stretchability, self-healing performance, and biocompatibility. They are ideal materials for the manufacture of flexible electronics. By changing the composition and oxidation of RT-GaLMs, physicochemical characteristics of the liquid metal can be adjusted, especially the regulation of rheological, wetting, and adhesion properties. This review highlights the advancements in the liquid metals used in flexible electronics. Meanwhile related characteristics of RT-GaLMs and underlying principles governing their processing and applications for flexible electronics are elucidated. Finally, the diverse applications of RT-GaLMs in self-healing circuits, flexible sensors, energy harvesting devices, and epidermal electronics, are explored. Additionally, the challenges hindering the progress of RT-GaLMs are discussed, while proposing future research directions and potential applications in this emerging field. By presenting a concise and critical analysis, this paper contributes to the advancement of RT-GaLMs as an advanced material applicable for the new generation of flexible electronics.

Review of Droplet Printing Technologies for Flexible Electronic Devices: Materials, Control, and Applications

Flexible devices have extensive applications in areas including wearable sensors, healthcare, smart packaging, energy, automotive and aerospace sectors, and other related fields. Droplet printing technology can be utilized to print flexible electronic components with micro/nanostructures on various scales, exhibiting good compatibility and wide material applicability for device production. This paper provides a comprehensive review of the current research status of droplet printing technologies and their applications across various domains, aiming to offer a valuable reference for researchers in related areas.

A Guide to Printed Stretchable Conductors

Printing of stretchable conductors enables the fabrication and rapid prototyping of stretchable electronic devices. For such applications, there are often specific process and material requirements such as print resolution, maximum strain, and electrical/ionic conductivity. This review highlights common printing methods and compatible inks that produce stretchable conductors. The review compares the capabilities, benefits, and limitations of each approach to help guide the selection of a suitable process and ink for an intended application. We also discuss methods to design and fabricate ink composites with the desired material properties (e.g., electrical conductance, viscosity, printability). This guide should help inform ongoing and future efforts to create soft, stretchable electronic devices for wearables, soft robots, e-skins, and sensors.

Pressure-Activatable Liquid Metal Composites Flexible Sensor with Antifouling and Drag Reduction Functional Surface

Flexible sensors produced through three-dimensional (3D) printing have exhibited promising results in the context of underwater sensing detection (for applications in navigational vehicles and human activities). However, underwater vehicles and activities such as swimming and diving are highly susceptible to drag, which can cause negative impacts such as reduced speed and increased energy consumption. Additionally, microbial adhesion can shorten the service life of these vehicles. However, natural organisms are able to circumvent such problems, with shark skin offering excellent barrier properties and ruffled papillae providing effective protection against fouling. Here, we show that a sandwich system consisting of a spraying layer, conductive elastomer composite, and encapsulation layer can be printed for multifunctional integrated underwater sensors. The modulated viscoelastic properties of liquid metal form the foundation for printing features, while its pressure-activated properties offer the potential for switchable sensors. An integrated drag reduction and antifouling layer were created by combining the shark skin surface shield scale structure with the lotus leaf surface papillae structure. A 3D-printed flexible sensor was designed using our approach to monitor attitude changes and strain in underwater environments, showcasing its capabilities. Our printed sensors can reduce biological attachment density by more than 50% and reduce underwater drag by 8.6-10.3%.

Recent Advances in Multifunctional Reticular Framework Nanoparticles: A Paradigm Shift in Materials Science Road to a Structured Future

Porous organic frameworks (POFs) have become a highly sought-after research domain that offers a promising avenue for developing cutting-edge nanostructured materials, both in their pristine state and when subjected to various chemical and structural modifications. Metal-organic frameworks, covalent organic frameworks, and hydrogen-bonded organic frameworks are examples of these emerging materials that have gained significant attention due to their unique properties, such as high crystallinity, intrinsic porosity, unique structural regularity, diverse functionality, design flexibility, and outstanding stability. This review provides an overview of the state-of-the-art research on base-stable POFs, emphasizing the distinct pros and cons of reticular framework nanoparticles compared to other types of nanocluster materials. Thereafter, the review highlights the unique opportunity to produce multifunctional tailoring nanoparticles to meet specific application requirements. It is recommended that this potential for creating customized nanoparticles should be the driving force behind future synthesis efforts to tap the full potential of this multifaceted material category.

Shaping a Soft Future: Patterning Liquid Metals

This review highlights the unique techniques for patterning liquid metals containing gallium (e.g., eutectic gallium indium, EGaIn). These techniques are enabled by two unique attributes of these liquids relative to solid metals: 1) The fluidity of the metal allows it to be injected, sprayed, and generally dispensed. 2) The solid native oxide shell allows the metal to adhere to surfaces and be shaped in ways that would normally be prohibited due to surface tension. The ability to shape liquid metals into non-spherical structures such as wires, antennas, and electrodes can enable fluidic metallic conductors for stretchable electronics, soft robotics, e-skins, and wearables. The key properties of these metals with a focus on methods to pattern liquid metals into soft or stretchable devices are summari.

A micro-vibration-driven direct ink write printing method of gallium-indium alloys

Combining liquid fluidity and metallic conductivity, gallium-indium (Ga-In) alloys are making a splash in areas such as stretchable electronic circuits and wearable medical devices. Due to high flexibility, direct ink write printing is already widely employed for printing Ga-In alloys. Currently, pneumatic extrusion is the main method of direct ink write printing, but the oxide skin and low viscosity of the Ga-In alloys make it challenging to control after extrusion. This work proposed a method for direct ink write printing of Ga-In alloys utilizing micro-vibration-driven extrusion. Micro-vibration reduces the surface tension of Ga-In alloy droplets and avoids the appearance of random droplets during printing. Under micro-vibration, the nozzle tip pierces the oxide skin to form small droplets which have a high moldability. The droplet growth process is significantly slowed down by optimizing suitable micro-vibration parameters. Therefore, the Ga-In alloy droplets with high moldability can be maintained at the nozzle for a long period, which improves printability. Furthermore, better printing outcomes were obtained with micro-vibrations by choosing the proper nozzle height and printing speed. Experiment results demonstrated the superiority of the method in terms of Ga-In alloys extrusion control. With this method, the printability of the liquid metals is enhanced.

Liquid Metal Patterning and Unique Properties for Next-Generation Soft Electronics

Room-temperature liquid metal (LM)-based electronics is expected to bring advancements in future soft electronics owing to its conductivity, conformability, stretchability, and biocompatibility. However, various difficulties arise when patterning LM because of its rheological features such as fluidity and surface tension. Numerous attempts are made to overcome these difficulties, resulting in various LM-patterning methods. An appropriate choice of patterning method based on comprehensive understanding is necessary to fully utilize the unique properties. Therefore, the authors aim to provide thorough knowledge about patterning methods and unique properties for LM-based future soft electronics. First, essential considerations for LM-patterning are investigated. Then, LM-patterning methods-serial-patterning, parallel-patterning, intermetallic bond-assisted patterning, and molding/microfluidic injection-are categorized and investigated. Finally, perspectives on LM-based soft electronics with unique properties are provided. They include outstanding features of LM such as conformability, biocompatibility, permeability, restorability, and recyclability. Also, they include perspectives on future LM-based soft electronics in various areas such as radio frequency electronics, soft robots, and heterogeneous catalyst. LM-based soft devices are expected to permeate the daily lives if patterning methods and the aforementioned features are analyzed and utilized.

Liquid Metal-Based Electronics for On-Skin Healthcare

Wearable devices are receiving growing interest in modern technologies for realizing multiple on-skin purposes, including flexible display, flexible e-textiles, and, most importantly, flexible epidermal healthcare. A 'BEER' requirement, i.e., biocompatibility, electrical elasticity, and robustness, is first proposed here for all the on-skin healthcare electronics for epidermal applications. This requirement would guide the designing of the next-generation on-skin healthcare electronics. For conventional stretchable electronics, the rigid conductive materials, e.g., gold nanoparticles and silver nanofibers, would suffer from an easy-to-fail interface with elastic substrates due to a Young's modulus mismatch. Liquid metal (LM) with high conductivity and stretchability has emerged as a promising solution for robust stretchable epidermal electronics. In addition, the fundamental physical, chemical, and biocompatible properties of LM are illustrated. Furthermore, the fabrication strategies of LM are outlined for pure LM, LM composites, and LM circuits based on the surface tension control. Five dominant epidermal healthcare applications of LM are illustrated, including electrodes, interconnectors, mechanical sensors, thermal management, and biomedical and sustainable applications. Finally, the key challenges and perspectives of LM are identified for the future research vision.

3D Printing of Liquid Metal Embedded Elastomers for Soft Thermal and Electrical Materials

Liquid metal embedded elastomers (LMEEs) are composed of a soft polymer matrix embedded with droplets of metal alloys that are liquid at room temperature. These soft matter composites exhibit exceptional combinations of elastic, electrical, and thermal properties that make them uniquely suited for applications in flexible electronics, soft robotics, and thermal management. However, the fabrication of LMEE structures has primarily relied on rudimentary techniques that limit patterning to simple planar geometries. Here, we introduce an approach for direct ink write (DIW) printing of a printable LMEE ink to create three-dimensional shapes with various designs. We use eutectic gallium-indium (EGaIn) as the liquid metal, which reacts with oxygen to form an electrically insulating oxide skin that acts as a surfactant and stabilizes the droplets for 3D printing. To rupture the oxide skin and achieve electrical conductivity, we encase the LMEE in a viscoelastic polymer and apply acoustic shock. For printed composites with a 80% LM volume fraction, this activation method allows for a volumetric electrical conductivity of 5 × 10⁴ S cm^-1 (80% LM volume)─significantly higher than what had been previously reported with mechanically sintered EGaIn-silicone composites. Moreover, we demonstrate the ability to print 3D LMEE interfaces that provide enhanced charge transfer for a triboelectric nanogenerator (TENG) and improved thermal conductivity within a thermoelectric device (TED). The 3D printed LMEE can be integrated with a highly soft TED that is wearable and capable of providing cooling/heating to the skin through electrical stimulation.

X-ray scattering as an effective tool for characterizing liquid metal composite morphology

Quantitative analysis of particle size and size distribution is crucial in establishing structure-property relationships of composite materials. An emerging soft composite architecture involves dispersing droplets of liquid metal throughout an elastomer, enabling synergistic properties of metals and soft polymers. The structure of these materials is typically characterized through real-space microscopy and image analysis; however, these techniques rely on magnified images that may not represent the global-averaged size and distribution of the droplets. In this study, we utilize ultra-small angle X-ray scattering (USAXS) as a reciprocal-space characterization technique that yields global-averaged dimensions of eutectic gallium indium (EGaIn) alloy soft composites. The Unified fit and Monte Carlo scattering methods are applied to determine the particle size and size distributions of the liquid metal droplets in the composites and are shown to be in excellent agreement with results from real-space image analysis. Additionally, all methods indicate that the droplets are getting larger as they are introduced into composites, suggesting that the droplets are agglomerating or possibly coalescing during dispersion. This work demonstrates the viability of X-ray scattering to elucidate structural information about liquid metal droplets for material development for applications in soft robotics, soft electronics, and multifunctional materials.

Soft and Stretchable Liquid Metal-Elastomer Composite for Wearable Electronics

Soft devices, especially capacitive stress (or strain) sensors, are important for applications, including wearable medical monitoring, electronic skin, and soft robotics. The incorporation of liquid metal particles (LMPs) into highly deformable elastomers as inclusions ameliorates the mechanical compliance caused by a rigid filler. The high dielectric constant and liquid feature of LMPs are suitable for soft sensors with high sensitivity and a large real-time dynamic detection range. Here, a class of LM-elastomer composites are introduced with elastic and high dielectric properties, making them uniquely suitable for the application of soft stress sensors. The prepared stretchable soft stress sensor can detect the bending degree of the finger, monitor physiological signals in real time, and distinguish the vibration from the pronunciation of different letters. The nanoscale X-ray computational tomography (nano-CT) measurements indeed detect the changes of LMPs under stress, i.e., LMPs in the matrix distribute from uneven to relatively uniform, agglomerate, and even connect each other to have a conduction path in the composition with high LMP contents, which cause the changes in the physical properties of devices under operation. The cognition of LMP changes in composites under stress is instructive for promoting their further applications in the field of soft devices.