Highly stretchable multilayer electronic circuits using biphasic gallium-indium

Technology Roadmap for Flexible Sensors

Humans rely increasingly on sensors to address grand challenges and to improve quality of life in the era of digitalization and big data. For ubiquitous sensing, flexible sensors are developed to overcome the limitations of conventional rigid counterparts. Despite rapid advancement in bench-side research over the last decade, the market adoption of flexible sensors remains limited. To ease and to expedite their deployment, here, we identify bottlenecks hindering the maturation of flexible sensors and propose promising solutions. We first analyze challenges in achieving satisfactory sensing performance for real-world applications and then summarize issues in compatible sensor-biology interfaces, followed by brief discussions on powering and connecting sensor networks. Issues en route to commercialization and for sustainable growth of the sector are also analyzed, highlighting environmental concerns and emphasizing nontechnical issues such as business, regulatory, and ethical considerations. Additionally, we look at future intelligent flexible sensors. In proposing a comprehensive roadmap, we hope to steer research efforts towards common goals and to guide coordinated development strategies from disparate communities. Through such collaborative efforts, scientific breakthroughs can be made sooner and capitalized for the betterment of humanity.

Silver-Nanowire-Based Elastic Conductors: Preparation Processes and Substrate Adhesion

The production of flexible electronic systems includes stretchable electrical interconnections and flexible electronic components, promoting the research and development of flexible conductors and stretchable conductive materials with large bending deformation or torsion resistance. Silver nanowires have the advantages of high conductivity, good transparency and flexibility in the development of flexible electronic products. In order to further prepare system-level flexible systems (such as autonomous full-software robots, etc.), it is necessary to focus on the conductivity of the system's composite conductor and the robustness of the system at the physical level. In terms of conductor preparation processes and substrate adhesion strategies, the more commonly used solutions are selected. Four kinds of elastic preparation processes (pretensioned/geometrically topological matrix, conductive fiber, aerogel composite, mixed percolation dopant) and five kinds of processes (coating, embedding, changing surface energy, chemical bond and force, adjusting tension and diffusion) to enhance the adhesion of composite conductors using silver nanowires as current-carrying channel substrates were reviewed. It is recommended to use the preparation process of mixed percolation doping and the adhesion mode of embedding/chemical bonding under non-special conditions. Developments in 3D printing and soft robots are also discussed.

Graded Mxene-Doped Liquid Metal as Adhesion Interface Aiming for Conductivity Enhancement of Hybrid Rigid-Soft Interconnection

Hybrid rigid-soft electronic system combines the biocompatibility of stretchable electronics and the computing capacity of silicon-based chips, which has a chance to realize a comprehensive stretchable electronic system with perception, control, and algorithm in near future. However, a reliable rigid-soft interconnection interface is urgently required to ensure both the conductivity and stretchability under a large strain. To settle this demand, this paper proposes a graded Mxene-doped liquid metal (LM) method to achieve a stable solid-liquid composite interconnect (SLCI) between the rigid chip and stretchable interconnect lines. To overcome the surface tension of LM, a high-conductive Mxene is doped for the balance between adhesion and liquidity of LM. And the high-concentration doping could prevent the contact failure with chip pins, while the low-concentration doping tends to maintain the stretchability. Based on this dosage-graded interface structure, the solid light-emitting diode (LED) and other devices integrated into the stretchable hybrid electronic system could achieve an excellent conductivity insensitive to the exerted tensile strain. In addition, the hybrid electronic system is demonstrated for skin-mounted and tire-mounted temperature-test applications under the tensile strain up to 100%. This Mxene-doped LM method aims to obtain a robust interface between rigid components and flexible interconnects by attenuating the inherent Young's modulus mismatch between rigid and flexible systems and makes it a promising candidate for effective interconnection between solid electronics and soft electronics.

Liquid metal-based soft, hermetic, and wireless-communicable seals for stretchable systems

Soft materials tend to be highly permeable to gases, making it difficult to create stretchable hermetic seals. With the integration of spacers, we demonstrate the use of liquid metals, which show both metallic and fluidic properties, as stretchable hermetic seals. Such soft seals are used in both a stretchable battery and a stretchable heat transfer system that involve volatile fluids, including water and organic fluids. The capacity retention of the battery was ~72.5% after 500 cycles, and the sealed heat transfer system showed an increased thermal conductivity of approximately 309 watts per meter-kelvin while strained and heated. Furthermore, with the incorporation of a signal transmission window, we demonstrated wireless communication through such seals. This work provides a route to create stretchable yet hermetic packaging design solutions for soft devices.

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.

Ultrahigh Strain-Insensitive Integrated Hybrid Electronics Using Highly Stretchable Bilayer Liquid Metal Based Conductor

Human-interfaced electronic systems require strain-resilient circuits. However, present integrated stretchable electronics easily suffer from electrical deterioration and face challenges in forming robust multilayered soft-rigid hybrid configurations. Here, a bilayer liquid-solid conductor (b-LSC) with amphiphilic properties is introduced to reliably interface with both rigid electronics and elastomeric substrates. The top liquid metal can self-solder its interface with rigid electronics at a resistance 30% lower than the traditional tin-soldered rigid interface. The bottom polar composite comprising liquid metal particles and polymers can not only reliably interface with elastomers but also help the b-LSC heal after breakage. The b-LSC can be scalably fabricated by printing and subsequent peeling strategies, showing ultra-high strain-insensitive conductivity (maximum 22 532 S cm^-1 ), extreme stretchability (2260%), and negligible resistance change under ultra-high strain (0.34 times increase under 1000% strain). It can act as stretchable vertical interconnect access for connecting multilayered layouts and can be scalably and universally fabricated on various substrates with a resolution of ≈200 µm. It is demonstrated that it can construct stretchable sensor arrays, multi-layered stretchable displays, highly integrated haptic user-interactive optoelectric E-skins, visualized heaters, robot touch sensing systems, and wireless powering for wearable electronics.

Stretchable conductors for stretchable field-effect transistors and functional circuits

Stretchable electronics have received intense attention due to their broad application prospects in many areas, and can withstand large deformations and form close contact with curved surfaces. Stretchable conductors are vital components of stretchable electronic devices used in wearables, soft robots, and human-machine interactions. Recent advances in stretchable conductors have motivated basic scientific and technological research efforts. Here, we outline and analyse the development of stretchable conductors in transistors and circuits, and examine advances in materials, device engineering, and preparation technologies. We divide the existing approaches to constructing stretchable transistors with stretchable conductors into the following two types: geometric engineering and intrinsic stretchability engineering. Finally, we consider the challenges and outlook in this field for delivering stretchable electronics.

Porous liquid metal-elastomer composites with high leakage resistance and antimicrobial property for skin-interfaced bioelectronics

Liquid metal-elastomer composite is a promising soft conductor for skin-interfaced bioelectronics, soft robots, and others due to its large stretchability, ultrasoftness, high electrical conductivity, and mechanical-electrical decoupling. However, it often suffers from deformation-induced leakage, which can smear skin, deteriorate device performance, and cause circuit shorting. Besides, antimicrobial property is desirable in soft conductors to minimize microbial infections. Here, we report phase separation-based synthesis of porous liquid metal-elastomer composites with high leakage resistance and antimicrobial property, together with large stretchability, tissue-like compliance, high and stable electrical conductivity over deformation, high breathability, and magnetic resonance imaging compatibility. The porous structures can minimize leakage through damping effects and lower percolation thresholds to reduce liquid metal usage. In addition, epsilon polylysine is loaded into elastic matrices during phase separation to provide antimicrobial property. The enabled skin-interfaced bioelectronics can monitor cardiac electrical and mechanical activities and offer electrical stimulations in a mechanically imperceptible and electrically stable manner even during motions.

Reliability of printed stretchable electronics based on nano/micro materials for practical applications

Recent decades have witnessed the booming development of stretchable electronics based on nano/micro composite inks. Printing is a scalable, low-cost, and high-efficiency fabrication tool to realize stretchable electronics through additive processes. However, compared with conventional flexible electronics, stretchable electronics need to experience more severe mechanical deformation which may cause destructive damage. Most of the reported works in this field mainly focus on how to achieve a high stretchability of nano/micro composite conductors or single working modules/devices, with limited attention given to the reliability for practical applications. In this minireview, we summarized the failure modes when printing stretchable electronics using nano/micro composite ink, including dysfunction of the stretchable interconnects, the stress-concentrated rigid-soft interfaces for hybrid electronics, the vulnerable vias upon stretching, thermal accumulation, and environmental instability of stretchable materials. Strategies for tackling these challenges to realize reliable performances are proposed and discussed. Our review provides an overview on the importance of reliable, printable, and stretchable electronics, which are the key enablers in propelling stretchable electronics from fancy demos to practical applications.

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.

A wearable cardiac ultrasound imager

Continuous imaging of cardiac functions is highly desirable for the assessment of long-term cardiovascular health, detection of acute cardiac dysfunction and clinical management of critically ill or surgical patients1-4. However, conventional non-invasive approaches to image the cardiac function cannot provide continuous measurements owing to device bulkiness5-11, and existing wearable cardiac devices can only capture signals on the skin12-16. Here we report a wearable ultrasonic device for continuous, real-time and direct cardiac function assessment. We introduce innovations in device design and material fabrication that improve the mechanical coupling between the device and human skin, allowing the left ventricle to be examined from different views during motion. We also develop a deep learning model that automatically extracts the left ventricular volume from the continuous image recording, yielding waveforms of key cardiac performance indices such as stroke volume, cardiac output and ejection fraction. This technology enables dynamic wearable monitoring of cardiac performance with substantially improved accuracy in various environments.

Solvent-Assisted Filling of Liquid Metal and Its Selective Dewetting for the Multilayered 3D Interconnect in Stretchable Electronics

As stretchable electronics are rapidly developing and becoming complex, the requirement for stretchable, multilayered, and large-area printed circuit boards (PCBs) is emerging. This demands a stretchable electrode and its vertical interconnect access (via) for 3-dimensional (3D) connectivity between layers. Here, we demonstrate solvent-assisted liquid metal (LM) filling into the submicrometer channel (∼400 nm), including via-hole filling and selective dewetting of LM. We provide the theoretical background of solvent-assisted LM filling and selective dewetting and reveal the osmotic pressure arising from anomalous mass transport phenomena, case II diffusion, which drives negative pressure, the spontaneous pulling of LM into the open channel. Also, we suggest design criteria for the geometry and dimension of LM interconnects to obtain structural stability without dewetting, based on the theoretical and computational background. We demonstrate a simple stretchable near-field communication (NFC) device including transferred micrometer-size light-emitting diodes (LEDs) with only 230 μm to the stretchable liquid metal PCB, without any soldering process. The device operates stably under repetitive stretching and releasing (∼50% uniaxial strain) due to the stable connection through the LM via between the upper and lower layers. Finally, we propose a concept for modular-type stretchable electronics, based on the cohesive liquid nature of LM. As a building block, the functional module can be easily removed from a mainframe, and replaced by another functional module, to suit user demand.

A crack compensation strategy for highly stretchable conductors based on liquid metal inclusions

Crack control strategies have been proven very useful for enhancing the stretchability of metal film-based stretchable conductors. However, existing strategies often suffer from the drawbacks of complicated preparation and predefined effective directions. Here, we propose a crack compensation strategy for preparing conductors featured with high stretchability by using liquid metal microparticles (LMMPs)-embedded polydimethylsiloxane (PDMS) as the substrate with a thin film of gold (Au) sputtered on the surface. LMMPs can be elongated to connect the cracked Au film upon stretching, which can form a conductive "island-tunnel" (IT) architecture to compensate for the cracks and maintain the conductivity. The high performance of the stretchable conductor is demonstrated by using it as electrodes to record surface electromyography of human brachioradialis and monitor electrocorticography signals of a rat in normal and epileptic states. The developed strategy shows the potential to provide a new perspective for the fabrication of flexible electronics.

Recent advances in electronic skins: material progress and applications

Electronic skins are currently in huge demand for health monitoring platforms and personalized medicine applications. To ensure safe monitoring for long-term periods, high-performance electronic skins that are softly interfaced with biological tissues are required. Stretchability, self-healing behavior, and biocompatibility of the materials will ensure the future application of electronic skins in biomedical engineering. This mini-review highlights recent advances in mechanically active materials and structural designs for electronic skins, which have been used successfully in these contexts. Firstly, the structural and biomechanical characteristics of biological skins are described and compared with those of artificial electronic skins. Thereafter, a wide variety of processing techniques for stretchable materials are reviewed, including geometric engineering and acquiring intrinsic stretchability. Then, different types of self-healing materials and their applications in electronic skins are critically assessed and compared. Finally, the mini-review is concluded with a discussion on remaining challenges and future opportunities for materials and biomedical research.

Transparent, Ultra-Stretching, Tough, Adhesive Carboxyethyl Chitin/Polyacrylamide Hydrogel Toward High-Performance Soft Electronics

To date, hydrogels have gained increasing attentions as a flexible conductive material in fabricating soft electronics. However, it remains a big challenge to integrate multiple functions into one gel that can be used widely under various conditions. Herein, a kind of multifunctional hydrogel with a combination of desirable characteristics, including remarkable transparency, high conductivity, ultra-stretchability, toughness, good fatigue resistance, and strong adhesive ability is presented, which was facilely fabricated through multiple noncovalent crosslinking strategy. The resultant versatile sensors are able to detect both weak and large deformations, which owns a low detection limit of 0.1% strain, high stretchability up to 1586%, ultrahigh sensitivity with a gauge factor up to 18.54, as well as wide pressure sensing range (0-600 kPa). Meanwhile, the fabrication of conductive hydrogel-based sensors is demonstrated for various soft electronic devices, including a flexible human-machine interactive system, the soft tactile switch, an integrated electronic skin for unprecedented nonplanar visualized pressure sensing, and the stretchable triboelectric nanogenerators with excellent biomechanical energy harvesting ability. This work opens up a simple route for multifunctional hydrogel and promises the practical application of soft and self-powered wearable electronics in various complex scenes.

High-Quality Microprintable and Stretchable Conductors for High-Performance 5G Wireless Communication

With the advent of 5G wireless and Internet of Things technologies, flexible and stretchable printed circuit boards (PCBs) should be designed to address all the specifications necessary to receive signal transmissions, maintaining the signal integrity, and providing electrical connections. Here, we propose a silver nanoparticle (AgNP)/silver nanowire (AgNW) hybrid conductor and high-quality microprinting technology for fabricating flexible and stretchable PCBs in high-performance 5G wireless communication. A simple and low-cost reverse offset printing technique using a commercial adhesive hand-roller was adapted to ensure high-resolution and excellent pattern quality. The AgNP/AgNW micropatterns were fabricated in various line widths, from 5 μm to 5 mm. They exhibited excellent pattern qualities, such as fine line spacing, clear edge definition and outstanding pattern uniformity. After annealing via intense pulsed light irradiation, they showed outstanding electrical resistivity (15.7 μΩ cm). Moreover, they could withstand stretching up to a strain of 90% with a small change in resistance. As a demonstration of their practical application, the AgNP/AgNW micropatterns were used to fabricate 5G communication antennas that exhibited excellent wireless signal processing at operating frequencies in the C-band (4-8 GHz). Finally, a wearable sensor fabricated with these AgNP/AgNW micropatterns could successfully detected fine finger movements in real time with excellent sensitivity.

Surface-Embedded Liquid Metal Electrodes with Abrasion Resistance via Direct Magnetic Printing

Gallium-based liquid metals (LMs) featuring both high conductivity and fluidity are ideal conductors for soft and stretchable electronics. However, their liquid nature is a double-edged sword in many key applications since LMs are inherently prone to mechanical damage. Although additional encapsulation is frequently used for the protection of delicate LM electrodes, it hinders the electrical interfacing with other objects for interconnection, sensing, and stimulation. Here, different from conventional patterning methods that deposit LM on or inside substrates, we for the first time report a simple strategy to create surface-embedded LM of eutectic gallium-indium (EGaIn) circuits with mechanical damage endurance. This was achieved by using direct magnetic printing to overcome the high surface tension of LM, allowing it to be passively filled into the laser-patterned microgrooves on soft substrates. We show that the surface-embedded LM circuits are resistant to mechanical erasure, washing, and peeling. We also show the applications of our surface-embedded LM electrodes in respiration monitoring and electrical stimulation of nerves. This work provides a simple and efficient way to create mechanically reliable LM microelectrodes, holding great promise for wearable and implantable bioelectronics.

Transfer printing technologies for soft electronics

Soft electronics have received increasing attention in recent years, owing to their wide range of applications in dynamic nonplanar surface integration electronics that include skin electronics, implantable devices, and soft robotics. Transfer printing is a widely used assembly technology for micro- and nano-fabrication, which enables the integration of functional devices with flexible or elastomeric substrates for the manufacturing of soft electronics. Through advanced materials and process design, numerous impressive studies related to transfer printing strategies and applications have been proposed. Herein, a discussion of transfer printing technologies toward soft electronics in terms of mechanisms and example demonstrations is provided. Moreover, the perspectives on the potential challenges and future directions of this field are briefly discussed.

Universal assembly of liquid metal particles in polymers enables elastic printed circuit board

An elastic printed circuit board (E-PCB) is a conductive framework used for the facile assembly of system-level stretchable electronics. E-PCBs require elastic conductors that have high conductivity, high stretchability, tough adhesion to various components, and imperceptible resistance changes even under large strain. We present a liquid metal particle network (LMP Net ) assembled by applying an acoustic field to a solid-state insulating liquid metal particle composite as the elastic conductor. The LMP Net conductor satisfies all the aforementioned requirements and enables the fabrication of a multilayered high-density E-PCB, in which numerous electronic components are intimately integrated to create highly stretchable skin electronics. Furthermore, we could generate the LMP Net in various polymer matrices, including hydrogels, self-healing elastomers, and photoresists, thus showing their potential for use in soft electronics.

High-performance electrified hydrogel actuators based on wrinkled nanomembrane electrodes for untethered insect-scale soft aquabots

Hydrogels have diverse chemical properties and can exhibit reversibly large mechanical deformations in response to external stimuli; these characteristics suggest that hydrogels are promising materials for soft robots. However, reported actuators based on hydrogels generally suffer from slow response speed and/or poor controllability due to intrinsic material limitations and electrode fabrication technologies. Here, we report a hydrogel actuator that operates at low voltages (<3 volts) with high performance (strain > 50%, energy density > 7 × 10 5 joules per cubic meter, and power density > 3 × 10 4 watts per cubic meter), surpassing existing hydrogel actuators and other types of electroactive soft actuators. The enhanced performance of our actuator is due to the formation of wrinkled nanomembrane electrodes that exhibit high conductivity and excellent mechanical deformation through capillary-assisted assembly of metal nanoparticles and deswelling-induced wrinkled structures. By applying an electric potential through the wrinkled nanomembrane electrodes that sandwich the hydrogel, we were able to trigger a reversible and substantial electroosmotic water flow inside a hydrogel film, which drove the controlled swelling of the hydrogel. The high energy efficiency and power density of our wrinkled nanomembrane electrode–induced actuator enabled the fabrication of an untethered insect-scale aquabot integrated with an on-board control unit demonstrating maneuverability with fast locomotion speed (1.02 body length per second), which occupies only 2% of the total mass of the robot.

Printing nanoparticle-based isotropic/anisotropic networks for directional electrical circuits

With the demand for integrated nanodevices, anisotropic conductive films are one type of interconnection structure for electronic components, which have been widely used for improving the integration of the system in printed circuit boards. This work presents a template-assisted printing strategy for the fabrication of nanoparticle-based networks with multi electrical properties. By manipulating the microfluid behavior under the guidance of the grid-shaped template, the continuity of liquid bridges can be precisely controlled in two directions. The isotropous circuits with crossbar paths, discrete paths as well as unidirectional paths are obtained, which achieve the switching of on/off states in the circuits. This work demonstrates a new type of directional circuits by the template-assisted printing method, which provides an effective fabrication strategy for electrical components and integrated systems.

Updated Perspectives on the Role of Biomechanics in COPD: Considerations for the Clinician

Patients with chronic obstructive pulmonary disease (COPD) demonstrate extra-pulmonary functional decline such as an increased prevalence of falls. Biomechanics offers insight into functional decline by examining mechanics of abnormal movement patterns. This review discusses biomechanics of functional outcomes, muscle mechanics, and breathing mechanics in patients with COPD as well as future directions and clinical perspectives. Patients with COPD demonstrate changes in their postural sway during quiet standing compared to controls, and these deficits are exacerbated when sensory information (eg, eyes closed) is manipulated. If standing balance is disrupted with a perturbation, patients with COPD are slower to return to baseline and their muscle activity is differential from controls. When walking, patients with COPD appear to adopt a gait pattern that may increase stability (eg, shorter and wider steps, decreased gait speed) in addition to altered gait variability. Biomechanical muscle mechanics (ie, tension, extensibility, elasticity, and irritability) alterations with COPD are not well documented, with relatively few articles investigating these properties. On the other hand, dyssynchronous motion of the abdomen and rib cage while breathing is well documented in patients with COPD. Newer biomechanical technologies have allowed for estimation of regional, compartmental, lung volumes during activity such as exercise, as well as respiratory muscle activation during breathing. Future directions of biomechanical analyses in COPD are trending toward wearable sensors, big data, and cloud computing. Each of these offers unique opportunities as well as challenges. Advanced analytics of sensor data can offer insight into the health of a system by quantifying complexity or fluctuations in patterns of movement, as healthy systems demonstrate flexibility and are thus adaptable to changing conditions. Biomechanics may offer clinical utility in prediction of 30-day readmissions, identifying disease severity, and patient monitoring. Biomechanics is complementary to other assessments, capturing what patients do, as well as their capability.

Conformally Adhesive, Large-Area, Solidlike, yet Transient Liquid Metal Thin Films and Patterns via Gelatin-Regulated Droplet Deposition and Sintering

Adhesion and spreading of liquid metals (LMs) on substrates are essential steps for the generation of flexible electronics and thermal management devices. However, the controlled deposition is limited by the high surface tension and peculiar wetting and adhesion behavior of LMs. Herein, we introduce gelatin-regulated LM droplet deposition and sintering (GLMDDS), for the upscalable production of conformally adhesive, solidlike, yet transient LM thin films and patterns on diverse substrates. This method involves four steps: homogeneous deposition of LM microdroplets, gelation of the LM-gelatin solution, toughening of the gelatin hydrogel by solvent displacement, and peeling-induced sintering of LM microdroplets. The LM thin film exhibits a three-layer structure, comprising an LM microdroplet-embedded tough organohydrogel adhesion layer, a continuous LM layer, and an oxide skin. The composite exhibits high stretchability and mechanical robustness, conformal adhesion to various substrates, high conductivity (4.35 × 10⁵ S·m^-1), and transience (86% LM recycled). Large-scale deposition (i.e., 5.6 dm²) and the potential for patterns on diverse substrates demonstrate its upscalability and broad suitability. Finally, the LM thin films and patterns are applied for flexible and wearable devices, i.e., pressure sensors, heaters, human motion tracking devices, and thermal management devices, illustrating the broad applicability of this strategy.

Monolithically Programmed Stretchable Conductor by Laser-Induced Entanglement of Liquid Metal and Metallic Nanowire Backbone

Owing to its low mechanical compliance, liquid metal is intrinsically suitable for stretchable electronics and future wearable devices. However, its invariable strain-resistance behavior according to the strain-induced geometrical deformation and the difficulty of circuit patterning limit the extensive use of liquid metal, especially for strain-insensitive wiring purposes. To overcome these limitations, herein, novel liquid-metal-based electrodes of fragmented eutectic gallium-indium alloy (EGaIn) and Ag nanowire (NW) backbone of which their entanglement is controlled by the laser-induced photothermal reaction to enable immediate and direct patterning of the stretchable electrode with spatially programmed strain-resistance characteristics are developed. The coexistence of fragmented EGaIn and AgNW backbone, that is, a biphasic metallic composite (BMC), primarily supports the uniform and durable formation of target layers on stretchable substrates. The laser-induced photothermal reaction not only promotes the adhesion between the BMC layer and substrates but also alters the structure of laser-irradiated BMC. By controlling the degree of entanglement between fragmented EGaIn and AgNW, the initial conductivity and local gauge factor are regulated and the electrode becomes effectively insensitive to applied strain. As the configuration developed in this study is compatible with both regimes of electrodes, it can open new routes for the rapid creation of complex stretchable circuitry through a single process.

Thermal Transfer-Enabled Rapid Printing of Liquid Metal Circuits on Multiple Substrates

Low-cost, rapid patterning of liquid metal on various substrates is a key processing step for liquid metal-based soft electronics. Current patterning methods rely on expensive equipment and specific substrates, which severely limit their widespread applications. Based on surface adhesion adjustment of liquid metal through thermal transferring toner patterns, we present a universal printing technique of liquid metal circuits. Without using any expensive processing steps or equipment, the circuit patterns can be printed quickly on thermal transfer paper using a desktop laser printer, and a toner on the thermal transfer paper can be transferred to various smooth substrates and polymer-coated rough substrates. The technique has yielded liquid metal circuits with a minimum linewidth of 50 μm fabricated on various smooth, rough, and three-dimensional substrates with complex morphology. The liquid metal circuits can maintain their functions even under an extreme strain of 800%. Various circuits such as LED arrays, multiple sensors, a flexible display, a heating circuit, a radiofrequency identification circuit, and a 12-lead electrocardiogram circuit on various substrates have been demonstrated, indicating the great potential of such a technique to rapidly achieve large-area flexible circuits for wearable health monitoring, internet of things, and consumer electronics at low cost and high efficiency.

Imbibition-induced selective wetting of liquid metal

Herein, we present the imbibition-induced, spontaneous, and selective wetting characteristics of gallium-based liquid metal alloys on a metallized surface with micro-scale topographical features. Gallium-based liquid metal alloys are fascinating materials that have enormous surface tension; therefore, they are difficult to pattern into films. The complete wetting of eutectic alloy of gallium and indium is realized on microstructured copper surfaces in the presence of HCl vapor, which removes the native oxide from the liquid metal alloy. This wetting is numerically explained based on the Wenzel’s model and imbibition process, revealing that the dimensions of the microstructures are critical for effective imbibition-driven wetting of the liquid metal. Further, we demonstrate that the spontaneous wetting of the liquid metal can be directed selectively along the microstructured region on the metallic surface to create patterns. This simple process enables the uniform coating and patterning of the liquid metal over large areas without an external force or complex processing. We demonstrate that the liquid metal-patterned substrates maintain electrical connection even in a stretched state and after repetitive stretching cycles.

Liquid metals that have enormous surface tension are difficult to pattern into films. Here, authors report the spontaneous and selective wetting of a gallium-based liquid metal, which is induced by imbibition on a micro-structured metallized substrate.

Mechanically Active Materials and Devices for Bio-Interfaced Pressure Sensors-A Review

Pressures generated by external forces or by internal body processes represent parameters of critical importance in diagnosing physiological health and in anticipating injuries. Examples span intracranial hypertension from traumatic brain injuries, high blood pressure from poor diet, pressure-induced skin ulcers from immobility, and edema from congestive heart failure. Pressures measured on the soft surfaces of vital organs or within internal cavities of the body can provide essential insights into patient status and progression. Challenges lie in the development of high-performance pressure sensors that can softly interface with biological tissues to enable safe monitoring for extended periods of time. This review focuses on recent advances in mechanically active materials and structural designs for classes of soft pressure sensors that have proven uses in these contexts. The discussions include applications of such sensors as implantable and wearable systems, with various unique capabilities in wireless continuous monitoring, minimally invasive deployment, natural degradation in biofluids, and/or multiplexed spatiotemporal mapping. A concluding section summarizes challenges and future opportunities for this growing field of materials and biomedical research.

Mechanical Gradients Enable Highly Stretchable Electronics Based on Nanofiber Substrates

Stretchable electronics play a pivotal role in the age of information and intelligence. Integrated circuit components are an integral part of high-performance and multifunctional stretchable electronic devices. Therefore, it is an ideal design concept for stretchable electronic devices to not only ensure the reliability of the connection between rigid inorganic electronic components and stretchable circuits but also maintain the stretchability of the device. In this work, we constructed a mechanical gradient strategy to fabricate high-performance stretchable electronic devices. Briefly, polyvinyl alcohol glue is used to fix integrated circuits on stretchable circuits, which are fabricated by printing liquid metal on a thermoplastic polyurethane nanofiber membrane. The strategy of integrated circuits (rigid)-polyvinyl alcohol glue (high elastic modulus)-thermoplastic polyurethane nanofiber membrane (low elastic modulus)-liquid metal (liquid) realizes the strain gradient during the stretching process of the device, thus ensuring the stability and reliability. Moreover, we explored the mechanism through experiments and finite element analysis. The flexible electronic devices fabricated by this scheme are not only ultra-stretchable (900%) but also have good stability and comfort. As proof, the application in stretchable sensors, human-computer interaction devices, and displays was realized.

A Personalized Electronic Tattoo for Healthcare Realized by On-the-Spot Assembly of an Intrinsically Conductive and Durable Liquid-Metal Composite

Conventional electronic (e-) skins are a class of thin-film electronics mainly fabricated in laboratories or factories, which is incapable of rapid and simple customization for personalized healthcare. Here a new class of e-tattoos is introduced that can be directly implemented on the skin by facile one-step coating with various designs at multi-scale depending on the purpose of the user without a substrate. An e-tattoo is realized by attaching Pt-decorated carbon nanotubes on gallium-based liquid-metal particles (CMP) to impose intrinsic electrical conductivity and mechanical durability. Tuning the CMP suspension to have low-zeta potential, excellent wettability, and high-vapor pressure enables conformal and intimate assembly of particles directly on the skin in 10 s. Low-cost, ease of preparation, on-skin compatibility, and multifunctionality of CMP make it highly suitable for e-tattoos. Demonstrations of electrical muscle stimulators, photothermal patches, motion artifact-free electrophysiological sensors, and electrochemical biosensors validate the simplicity, versatility, and reliability of the e-tattoo-based approach in biomedical engineering.

High-Stretchability, Ultralow-Hysteresis ConductingPolymer Hydrogel Strain Sensors for Soft Machines

Highly stretchable strain sensors based on conducting polymer hydrogel are rapidly emerging as a promising candidate toward diverse wearable skins and sensing devices for soft machines. However, due to the intrinsic limitations of low stretchability and large hysteresis, existing strain sensors cannot fully exploit their potential when used in wearable or robotic systems. Here, a conducting polymer hydrogel strain sensor exhibiting both ultimate strain (300%) and negligible hysteresis (<1.5%) is presented. This is achieved through a unique microphase semiseparated network design by compositing poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) nanofibers with poly(vinyl alcohol) (PVA) and facile fabrication by combining 3D printing and successive freeze-thawing. The overall superior performances of the strain sensor including stretchability, linearity, cyclic stability, and robustness against mechanical twisting and pressing are systematically characterized. The integration and application of such strain sensor with electronic skins are further demonstrated to measure various physiological signals, identify hand gestures, enable a soft gripper for objection recognition, and remote control of an industrial robot. This work may offer both promising conducting polymer hydrogels with enhanced sensing functionalities and technical platforms toward stretchable electronic skins and intelligent robotic systems.

Materials and design strategies for stretchable electroluminescent devices

Stretchable displays have recently received increasing attention as input and/or output interfaces for next-generation human-friendly electronic systems. Stretchable electroluminescent (EL) devices are a core component of stretchable displays, and they can be classified into two types, structurally stretchable EL devices and intrinsically stretchable EL devices, according to the mechanism for achieving their stretchability. We herein present recent advances in materials and design strategies for stretchable EL devices. First, stretchable devices based on ultrathin EL devices are introduced. Ultrathin EL devices are mechanically flexible like thin paper, and they can become stretchable through various structural engineering methods, such as inducing a buckled structure, employing interconnects with stretchable geometries, and applying origami/kirigami techniques. Secondly, intrinsically stretchable EL devices can be fabricated by using inherently stretchable electronic materials. For example, light-emitting electrochemical cells and EL devices with a simpler structure using alternating current have been developed. Furthermore, novel stretchable semiconductor materials have been presented for the development of intrinsically stretchable light-emitting diodes. After discussing these two types of stretchable EL devices, we briefly discuss applications of deformable EL devices and conclude the review.

Flexible hybrid electronics: Enabling integration techniques and applications

The conventional electronic systems enabled by rigid electronic are prone to malfunction under deformation, greatly limiting their application prospects. As an emerging platform for applications in healthcare monitoring and human-machine interface (HMI), flexible electronics have attracted growing attention due to its remarkable advantages, such as stretchability, flexibility, conformability, and wearing comfort. However, to realize the overall electronic systems, rigid components are also required for functions such as signal acquisition and transmission. Therefore, flexible hybrid electronics (FHE), which simultaneously possesses the desirable flexibility and enables the integration of rigid components for functionality, has been emerging as a promising strategy. This paper reviews the enabling integration techniques for FHE, including technologies for two-dimensional/three-dimensional (2D/3D) interconnects, bonding of rigid integrated circuit (IC) chips to soft substrate, stress-isolation structures, and representative applications of FHE. In addition, future challenges and opportunities involved in FHE-based systems are also discussed.

Mechanics of tension-induced film wrinkling and restabilization: a review

Wrinkling of thin films under tension is omnipresent in nature and modern industry, a phenomenon which has aroused considerable attention during the past two decades because of its intricate nonlinear behaviours and intriguing morphology changes. Here, we review recent advancements in the mechanics of tension-induced film wrinkling and restabilization, by identifying three major stages of its progress: small-strain (less than 5 % ) wrinkling of stiff sheets, finite-strain (up to 30 % ) wrinkling and restabilization (isola-centre bifurcation) of soft films, and the effects of curved configurations and material properties on pattern formation. Growing demand for fundamental understanding, quantitative prediction and precise tracking of secondary bifurcation transitions in morphological evolution of thin films helps to advance finite-strain plate/shell theories and sophisticated modelling methods. This progress not only promotes our insightful understanding of complex instability behaviour but also reveals novel phenomena and sheds light on developing wrinkle-tunable membrane structures and functional surfaces.

Are Liquid Metals Bulk Conductors?

Stretchable electronics have potential in wide-reaching applications including wearables, personal health monitoring, and soft robotics. Many recent advances in stretchable electronics leverage liquid metals, particularly eutectic gallium-indium (EGaIn). A variety of EGaIn electromechanical behaviors have been reported, ranging from bulk conductor responses to effectively strain-insensitive responses. However, numerous measurement techniques have been used throughout the literature, making it difficult to directly compare the various proposed formulations. Here, the electromechanical responses of EGaIn found in the literature is reviewed and pure EGaIn is investigated using three electrical resistance measurement techniques: four point probe, two point probe, and Wheatstone bridge. The results indicate substantial differences in measured electromechanical behavior between the three methods, which can largely be accounted for by correcting for a fixed offset corresponding to the resistances of various parts of the measurement circuits. Yet, even accounting for several of these sources of experimental error, the average relative change in resistance of EGaIn is found to be lower than that predicted by the commonly used bulk conductor assumption, referred to as Pouillet's law. Building upon recent theories proposed in the literature, possible explanations for the discrepancies are discussed. Finally, suggestions are provided on experimental design to enable reproducible and interpretable research.

Strain-Driven Auto-Detachable Patterning of Flexible Electrodes

Flexible electrodes that are multilayer, multimaterial, and conformal are pivotal for multifunctional wearable electronics. Traditional electronic circuits manufacturing requires substrate-supported transfer printing, which limits their multilayer integrity and device conformability on arbitrary surfaces. Herein, a "shrinkage-assisted patterning by evaporation" (SHAPE) method is reported, by employing evaporation-induced interfacial strain mismatch, to fabricate auto-detachable, freestanding, and patternable electrodes. The SHAPE method utilizes vacuum-filtration of polyaniline/bacterial cellulose (PANI/BC) ink through a masked filtration membrane to print high-resolution, patterned, and multilayer electrodes. The strong interlayer hydrogen bonding ensures robust multilayer integrity, while the controllable evaporative shrinking property of PANI/BC induces mismatch between the strains of the electrode and filtration membrane at the interface and thus autodetachment of electrodes. Notably, a 500-layer substrateless micro-supercapacitor fabricated using the SHAPE method exhibits an energy density of 350 mWh cm^-2 at a power density of 40 mW cm^-2 , 100 times higher than reported substrate-confined counterparts. Moreover, a digital circuit fabricated using the SHAPE method functions stably on a deformed glove, highlighting the broad wearable applications of the SHAPE method.

Liquid Metal Nanoparticles as a Highly Efficient Photoinitiator to Develop Multifunctional Hydrogel Composites

Liquid metal (LM) composites are a class of emerging soft multifunctional materials that are promising for a variety of applications, yet the chemistry properties of LM have not been fully understood. Here, we report that LM nanoparticles can directly perform as a photoinitiator for radical polymerization and the in situ development of highly tough and multifunctional LM hydrogel composites. It is revealed that the photocatalytic activity of LM nanoparticles originates from the oxide layer on LM. Significantly, positively charged metal-organic framework (MOF) nanoparticles are used to stabilize LM nanoparticles in aqueous solutions, where the MOF can anchor on the surface of LM nanoparticles by electrostatic interaction while helping to preserve the unshielded oxide layer, therefore realizing the highly efficient photoinitiation and polymerization. The LM nanoparticle-initiated photopolymerization is shown to develop hydrogel composites featuring excellent stretchability, stimuli responsiveness, and sustained photocatalytic activity. The photocatalytic polymerization initiated by LM nanoparticles not only deepens the understanding on the semiconductor properties of the oxide skin on LM but also broadens the application scenarios of multifunctional LM/polymer composites in smart materials, wearable electronics, and soft robotics.

Printable inks and deformable electronic array devices

Deformable printed electronic array devices are expected to revolutionize next-generation electronics. However, although remarkable technological advances in printable inks and deformable electronic array devices have recently been achieved, technical challenges remain to commercialize these technologies. In this review article a brief introduction to printing methods highlighting significant research studies on ink formation for conductors, semiconductors, and insulators is provided, and the structural design and successful printing strategies of deformable electronic array devices are described. Successful device demonstrations are presented in the applications of passive- and active-matrix array devices. Finally, perspectives and technological challenges to be achieved are pointed out to print practically available deformable devices.

Healable, Recyclable, and Multifunctional Soft Electronics Based on Biopolymer Hydrogel and Patterned Liquid Metal

Recent years have witnessed the rapid development of sustainable materials. Along this line, developing biodegradable or recyclable soft electronics is challenging yet important due to their versatile applications in biomedical devices, soft robots, and wearables. Although some degradable bulk hydrogels are directly used as the soft electronics, the sensing performances are usually limited due to the absence of distributed conducting circuits. Here, sustainable hydrogel-based soft electronics (HSE) are reported that integrate sensing elements and patterned liquid metal (LM) in the gelatin-alginate hybrid hydrogel. The biopolymer hydrogel is transparent, robust, resilient, and recyclable. The HSE is multifunctional; it can sense strain, temperature, heart rate (electrocardiogram), and pH. The strain sensing is sufficiently sensitive to detect a human pulse. In addition, the device serves as a model system for iontophoretic drug delivery by using patterned LM as the soft conductor and electrode. Noncontact detection of nearby objects is also achieved based on electrostatic-field-induced voltage. The LM and biopolymer hydrogel are healable, recyclable, and degradable, favoring sustainable applications and reconstruction of the device with new functions. Such HSE with multiple functions and favorable attributes should open opportunities in next-generation electronic skins and hydrogel machines.

Adaptive Self-Organization of Nanomaterials Enables Strain-Insensitive Resistance of Stretchable Metallic Nanocomposites

Highly conductive and stretchable nanocomposites are promising material candidates for skin electronics. However, the resistance of stretchable metallic nanocomposites highly depends on external strains, often deteriorating the performance of fabricated electronic devices. Here, a material strategy for the highly conductive and stretchable nanocomposites comprising metal nanomaterials of various dimensions and a viscoelastic block-copolymer matrix is presented. The resistance of the nanocomposites can be well retained under skin deformations (<50% strain). It is demonstrated that silver nanomaterials can self-organize inside the viscoelastic media in response to external strain when their surface is conjugated with 1-decanethiol. Distinct self-organization behaviors associated with nanomaterial dimensions and strain conditions are found. Adopting the optimum composition of 0D/1D/2D silver nanomaterials can render the resistance of the nanocomposites insensitive to uniaxial or biaxial strains. As a result, the resistance can be maintained with a variance of < 1% during 1000 stretching cycles under uniaxial and biaxial strains of <50% while a high conductivity of ≈31 000 S cm^-1 is achieved.

Tissue Adhesive, Conductive, and Injectable Cellulose Hydrogel Ink for On-Skin Direct Writing of Electronics

Flexible and soft bioelectronics used on skin tissue have attracted attention for the monitoring of human health. In addition to typical metal-based rigid electronics, soft polymeric materials, particularly conductive hydrogels, have been actively developed to fabricate biocompatible electrical circuits with a mechanical modulus similar to biological tissues. Although such conductive hydrogels can be wearable or implantable in vivo without any tissue damage, there are still challenges to directly writing complex circuits on the skin due to its low tissue adhesion and heterogeneous mechanical properties. Herein, we report cellulose-based conductive hydrogel inks exhibiting strong tissue adhesion and injectability for further on-skin direct printing. The hydrogels consisting of carboxymethyl cellulose, tannic acid, and metal ions (e.g., HAuCl4) were crosslinked via multiple hydrogen bonds between the cellulose backbone and tannic acid and metal-phenol coordinate network. Owing to this reversible non-covalent crosslinking, the hydrogels showed self-healing properties and reversible conductivity under cyclic strain from 0 to 400%, as well as printability on the skin tissue. In particular, the on-skin electronic circuit printed using the hydrogel ink maintained a continuous electrical flow under skin deformation, such as bending and twisting, and at high relative humidity of 90%. These printable and conductive hydrogels are promising for implementing structurally complicated bioelectronics and wearable textiles.

Rapid meniscus-guided printing of stable semi-solid-state liquid metal microgranular-particle for soft electronics

Liquid metal is being regarded as a promising material for soft electronics owing to its distinct combination of high electrical conductivity comparable to that of metals and exceptional deformability derived from its liquid state. However, the applicability of liquid metal is still limited due to the difficulty in simultaneously achieving its mechanical stability and initial conductivity. Furthermore, reliable and rapid patterning of stable liquid metal directly on various soft substrates at high-resolution remains a formidable challenge. In this work, meniscus-guided printing of ink containing polyelectrolyte-attached liquid metal microgranular-particle in an aqueous solvent to generate semi-solid-state liquid metal is presented. Liquid metal microgranular-particle printed in the evaporative regime is mechanically stable, initially conductive, and patternable down to 50 μm on various substrates. Demonstrations of the ultrastretchable (~500% strain) electrical circuit, customized e-skin, and zero-waste ECG sensor validate the simplicity, versatility, and reliability of this manufacturing strategy, enabling broad utility in the development of advanced soft electronics.

In this article, meniscus-guided printing of polyelectrolyte-attached liquid metal particles to simultaneously achieve mechanical stability and initial electrical conductivity at high resolution is introduced.

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

Soft, elastically deformable composites with liquid metal (LM) droplets can enable new generations of soft electronics, robotics, and reconfigurable structures. However, techniques to control local composite microstructure, which ultimately governs material properties and performance, is lacking. Here a direct ink writing technique is developed to program the LM microstructure (i.e., shape, orientation, and connectivity) on demand throughout elastomer composites. In contrast to inks with rigid particles that have fixed shape and size, it is shown that emulsion inks with LM fillers enable in situ control of microstructure. This enables filaments, films, and 3D structures with unique LM microstructures that are generated on demand and locked in during printing. This includes smooth and discrete transitions from spherical to needle-like droplets, curvilinear microstructures, geometrically complex embedded inclusion patterns, and connected LM networks. The printed materials are soft (modulus < 200 kPa), highly deformable (>600 % strain), and can be made locally insulating or electrically conductive using a single ink by controlling the process conditions. These capabilities are demonstrated by embedding elongated LM droplets in a soft heat sink, which rapidly dissipates heat from high-power LEDs. These programmable microstructures can enable new composite paradigms for emerging technologies that demand mechanical compliance with multifunctional response.

Flexible Electronics and Devices as Human-Machine Interfaces for Medical Robotics

Medical robots are invaluable players in non-pharmaceutical treatment of disabilities. Particularly, using prosthetic and rehabilitation devices with human-machine interfaces can greatly improve the quality of life for impaired patients. In recent years, flexible electronic interfaces and soft robotics have attracted tremendous attention in this field due to their high biocompatibility, functionality, conformability, and low-cost. Flexible human-machine interfaces on soft robotics will make a promising alternative to conventional rigid devices, which can potentially revolutionize the paradigm and future direction of medical robotics in terms of rehabilitation feedback and user experience. In this review, the fundamental components of the materials, structures, and mechanisms in flexible human-machine interfaces are summarized by recent and renowned applications in five primary areas: physical and chemical sensing, physiological recording, information processing and communication, soft robotic actuation, and feedback stimulation. This review further concludes by discussing the outlook and current challenges of these technologies as a human-machine interface in medical robotics.

Solution-Processed Electron-Transport Layer-free Organic Photovoltaics with Liquid Metal Cathodes

Flexible, lightweight, and large-area solar cells provide new power supply opportunities in the renewable energy field and facilitate the supply of power to internet-of-things devices and wearable devices. The choice of printing process technologies is a key parameter for such flexible power sources because of their energy-saving process technology and high throughput rate. In addition to selecting the appropriate printing method for the active and charge transport layers, the development of printed electrodes is critical. Numerous printable materials have been developed to replace conventional evaporated top electrodes. However, achieving fully solution-processed organic photovoltaics (OPVs) with power conversion efficiency (PCE) comparable to OPVs with vacuum-deposited transparent and top electrodes is challenging. This is because of the difficulty of forming a uniform interface between the top solution-processed electrode and the active layers while preventing deterioration. In this study, an electron transport layer-free, eutectic gallium-indium (EGaIn) top-cathode strategy was developed and a record PCE of 12.7% in fully solution-processed, flexible OPVs was achieved. Direct coating of EGaIn on the active layer, in a nitrogen atmosphere, is conducive for energy band matching and obtaining physically perfect interfaces without any penetrations or voids. An average PCE of 14.1% and enhanced operating stability, comparable to conventional OPVs, were achieved with indium tin oxide transparent electrodes by eliminating the electron-transport layer. The fully solution-processed flexible OPVs fabricated with the embedded silver nanowire strategy in ultrathin transparent polyimide, achieved an average PCE of 12.7%, representing a promising technique to meet green and high-throughput energy demands.

Highly stretchable and sensitive strain sensors with ginkgo-like sandwich architectures

The development of a strain sensor that can detect tensile strains exceeding 800% has been challenging. The non-conductive stretchable Eco-flex tape has been widely used in strain sensors due to its high elastic limit. In this work, an Eco-flex-based strain sensor that was conductive until occurrence of fracture was developed. The silver nanoparticles and carbon nanotubes constituted stretchable conductive paths in the Eco-flex matrix. The maximum tensile strain of this sensor was 867%, and the resistance change rate was higher than 10⁴, while the strain resolution was 7.9%. Moreover, the sensor is characterized by segmented logarithmic linearity. This excellent performance was attributed to the ginkgo-like pattern, the patterned strain-coordinating architecture (PSCL), and specific nanocomposites with micro-cracks. The deformation of the architecture and the evolution of the microcracks were studied. In addition, the application of this strain sensor on a wing-shaped aircraft was proposed and its feasibility was demonstrated.

Stress Dissipation Encoded Silk Fibroin Electrode for the Athlete-Beneficial Silk Bioelectronics

The kinetic body motions have guided the core-shell fabrics of wearable bioelectronics to be elastoplastic. However, the polymeric electrodes follow the trade-off relationship between toughness and stretchability. To this end, the stress dissipation encoded silk fibroin electrode is proposed as the core electrode of wearable bioelectronics. Significantly, the high degree of intrinsic stress dissipation is realized via an amino acid crosslink. The canonical phenolic amino acid (i.e., tyrosine) of silk fibroin is engineered to bridge the secondary structures. A sufficient crosslink network is constructed when tyrosine is exposed near the amorphous strand. The stress dissipative tyrosine crosslink affords 12.5-fold increments of toughness (4.72 to 58.9 MJ m^-3 ) and implements the elastoplastic silk fibroin. The harmony of elastoplastic core electrodes with shell fabrics enables the wearable bioelectronics to employ mechanical performance (elastoplasticity of 750 MJ m^-3 ) and stable electrical response. The proposed wearable is capable of assisting the effective workouts via triboelectricity. In principle, active mobility with suggested wearables potentially relieves muscular fatigues and severe injuries during daily fitness.

Applications of liquid metals in nanotechnology

Post-transition liquid metals (LMs) offer new opportunities for accessing exciting dynamics for nanomaterials. As entities with free electrons and ions as well as fluidity, LM-based nanomaterials are fundamentally different from their solid counterparts. The low melting points of most post-transition metals (less than 330 °C) allow for the formation of nanodroplets from bulk metal melts under mild mechanical and chemical conditions. At the nanoscale, these liquid state nanodroplets simultaneously offer high electrical and thermal conductivities, tunable reactivities and useful physicochemical properties. They also offer specific alloying and dealloying conditions for the formation of multi-elemental liquid based nanoalloys or the synthesis of engineered solid nanomaterials. To date, while only a few nanosized LM materials have been investigated, extraordinary properties have been observed for such systems. Multi-elemental nanoalloys have shown controllable homogeneous or heterogeneous core and surface compositions with interfacial ordering at the nanoscale. The interactions and synergies of nanosized LMs with polymeric, inorganic and bio-materials have also resulted in new compounds. This review highlights recent progress and future directions for the synthesis and applications of post-transition LMs and their alloys. The review presents the unique properties of these LM nanodroplets for developing functional materials for electronics, sensors, catalysts, energy systems, and nanomedicine and biomedical applications, as well as other functional systems engineered at the nanoscale.