Recent advances in liquid-metal-based wearable electronics and materials

Acid-Induced in Situ Phase Separation and Percolation for Constructing Bi-Continuous Phase Hydrogel Electrodes With Motion-Insensitive Property

Conducting polymer hydrogels have gained attention in the bioelectronics field due to their unique combination of biocompatibility and customizable mechanical properties. However, achieving both excellent conductivity and mechanical strength in a hydrogel remains a significant challenge, primarily because of the inherent conflict between the hydrophobic nature of conducting polymers and the hydrophilic characteristics of hydrogels. To address this issue, this work proposes a simple one-step acid-induced approach that not only promotes the gelation of hydrophilic polymers but also facilitates the in situ phase separation of hydrophobic conducting polymers under mild conditions. This results in a distinctive bi-continuous phase structure with exceptional electrical property (906 mS cm-1) and mechanical performance (fracture strain of 1103%). The hydrogel forms robust percolating networks that maintain structural integrity under mechanical stress due to their entropic elasticity, providing remarkable strain insensitivity, low mechanical hysteresis, and an impressive resilience (95%). Electrodes fabricated from the conductive hydrogel exhibit stable and minimal interfacial contact impedance with skin (1-6 kilohms at 1-100 Hz) and significantly lower noise power (4.9 µV2). This work believes that the motion-insensitive characteristics and mechanical robustness of this hydrogel will enable efficient and reliable monitoring of biological signals, establishing a new benchmark in the bioelectronics.

Stiffness and Interface Engineered Soft Electronics with Large-Scale Robust Deformability

Skin-like stretchable electronics emerge as promising human-machine interfaces but are challenged by the paradox between superior electronic property and reliable mechanical deformability. Here, a general strategy is reported for establishing robust large-scale deformable electronics by effectively isolating strains and strengthening interfaces. A copolymer substrate is designed to consist of mosaic stiff and elastic areas with nearly four orders of magnitudes modulus contrast and cross-linked interfaces. Electronic functional devices and stretchable liquid metal (LM) interconnects are conformally attached at the stiff and elastic areas, respectively, through hydrogen bonds. As a result, functional devices are completely isolated from strains, and resistances of LM conductors change by less than one time when the substrate is deformed by up to 550%. By this strategy, solar cells, wireless charging antenna, supercapacitors, and light-emitting diodes are integrated into a self-powered electronic skin that can laminate on the human body and exhibit stable performances during repeated multimode deformations, demonstrating an efficient path for realizing highly deformable energy autonomous soft electronics.

Sculpting liquid metal stabilized interfaces: a gateway to liquid electronics

Liquid electronics have potential applications in soft robotics, printed electronics, and healable electronics. The intrinsic shortcomings of solid-state electronics can be offset by liquid conductors. Alloys of gallium have emerged as transformative materials for liquid electronics owing to their intrinsic fluidity, conductivity, and low toxicity. However, sculpting liquid metal or its composites into a 3D architecture is a challenging task. To tackle this issue, herein, we explored the interfacial chemistry of metal ions and tannic acid (TA) complexation at a liquid-liquid interface. First, we established that an MIII-TA network at the liquid-liquid interface could structure liquid in liquid by jamming the interfacial film. The surface coverage of the droplet largely depends on the concentration of metal ions, oxidation state of metal ions and pH of the surrounding environment. Further extending the approach, we demonstrated that TA-functionalized gallium nanoparticles (Ga NPs) can also sculpt liquid droplets in the presence of transition metal ions. Finally, a mold-based free-standing 3D architecture is obtained using the interfacial reaction and interfacial crowding of a metal-phenolate network. Conductivity measurement reveals that these liquid constructs can be used for low-voltage electronic applications, thus opening the door for liquid electronics.

Advancements in Flexible Sensors for Monitoring Body Movements during Sleep: A Review

Sleep plays a role in maintaining our physical well-being. However, sleep-related issues impact millions of people globally. Accurate monitoring of sleep is vital for identifying and addressing these problems. While traditional methods like polysomnography (PSG) are commonly used in settings, they may not fully capture natural sleep patterns at home. Moreover, PSG equipment can disrupt sleep quality. In recent years, there has been growing interest in the use of sensors for sleep monitoring. These lightweight sensors can be easily integrated into textiles or wearable devices using technology. The flexible sensors can be designed for skin contact to offer continuous monitoring without being obtrusive in a home environment. This review presents an overview of the advancements made in flexible sensors for tracking body movements during sleep, which focus on their principles, mechanisms, and strategies for improved flexibility, practical applications, and future trends.

Fast and Facile Liquid Metal Printing via Projection Lithography for Highly Stretchable Electronic Circuits

Soft electronic circuits are crucial for wearable electronics, biomedical technologies, and soft robotics, requiring soft conductive materials with high conductivity, high strain limit, and stable electrical performance under deformation. Liquid metals (LMs) have become attractive candidates with high conductivity and fluidic compliance, while effective manufacturing methods are demanded. Digital light processing (DLP)-based projection lithography is a high-resolution and high-throughput printing technique for primarily polymers and some metals. If LMs can be printed with DLP as well, the entire soft devices can be fabricated by one printer in a streamlined and highly efficient process. Herein, fast and facile DLP-based LM printing is achieved. Simply with 5-10 s of patterned ultraviolet (UV)-light exposure, a highly conductive and stretchable pattern can be printed using a photo-crosslinkable LM particle ink. The printed eutectic gallium indium traces feature high resolution (≈20 µm), conductivity (3 × 106 S m-1), stretchability (≈2500%), and excellent stability (consistent performance at different deformation). Various patterns are printed in diverse material systems for broad applications including stretchable displays, epidermal strain sensors, heaters, humidity sensors, conformal electrodes for electrography, and multi-layer actuators. The facile and scalable process, excellent performance, and diverse applications ensure its broad impact on soft electronic manufacturing.

Flexible, Wearable Wireless-Charging Power System Incorporating Piezo-Ultrasonic Arrays and MXene-Based Solid-State Supercapacitors

With the continuous development of wearable electronics, higher requirements are put forward for flexible, detachable, stable output, and long service life power modules. Given the limited capacity of energy storage devices, the integration of energy capture and storage is a viable approach. Here, we present a flexible, wearable, wireless-charging power system that integrates a piezoelectric ultrasonic array harvester (PUAH) with MXene-based solid-state supercapacitors (MSSSs) in a soft wristband format for sustainable applications. The MSSS as the energy storage module is developed by using Ti3C2Tx nanosheet-loaded inserted finger-like carbon cloth skeletons as electrodes and poly(vinyl alcohol)/H3PO4 gel as electrolytes, with high energy density (58.74 Wh kg-1) and long cycle life (99.37%, 10,000 cycles). A two-dimensional stretchable piezoelectric array as a wireless-charging module hybridizes high-performance 1-3 composite units with serpentine electrodes, which allows wireless power via ultrasonic waves, with a maximum power density of 1.56 W cm-2 and an output voltage of 20.75 V. The overall PUAH-MSSS wireless energy supply system is 2 mm thick and offers excellent energy conversion/storage performance, cyclic stability, and mechanical flexibility. The results of this project will lay the foundation for the development of next-generation wearable electronics.

Diels-Alder Network Blends as Self-Healing Encapsulants for Liquid Metal-Based Stretchable Electronics

Two dynamic covalent networks based on the Diels-Alder reaction were blended to exploit the properties of the dissimilar polymer backbones. Furan-functionalized polyether amines based on poly(propylene oxide) (PPO) FD4000 and polydimethylsiloxane (PDMS) FS5000 were mixed in a common solvent and reversibly cross-linked with the same bismaleimide DPBM. The morphology of the phase-separated blends is primarily controlled by the concentration of backbones. Increasing the PDMS content of the blends results in a dilute droplet morphology at 25 wt %, with a growing size and concentration of droplets and the formation of two separate PPO- and PDMS-rich layers at 50 wt %. Further increasing the PDMS content to 75 wt % leads to larger droplets and a thicker layer of the secondary phase. The hydrophobic PDMS phase creates a barrier against water, while the more hydrophilic PPO phase enhances the resistance against oxygen diffusion. Lowering the maleimide-to-furan stoichiometric ratio resulted in a decrease in cross-link density and thus more flexible and stretchable encapsulants. Changes in the stoichiometric ratio also affected the phase morphology due to resulting changes in phase separation and network formation kinetics. Lowering the stoichiometric ratio also resulted in enhanced self-healing properties of 96% at room temperature as a consequence of the increased chain mobility in the blended networks. The self-healing blends were used to encapsulate liquid metal circuits to create stretchable strain sensors with a linear electro-mechanical response without much drift or hysteresis, which could be efficiently recovered by 90% after the damage-healing cycles.

Sweat as a source of non-invasive biomarkers for clinical diagnosis: An overview

Sweat has excellent potential as one of the sources of non-invasive biomarkers for clinical diagnosis. It is relatively easy to collect and process and may contain different disease-specific markers and drug metabolites, making it ideal for various clinical applications. This article discusses the anatomy of sweat glands and their role in sweat production, as well as the history and development of multiple sweat sample collection and analysis techniques. Another primary focus of this article is the application of sweat detection in clinical disease diagnosis and other life scenarios. Finally, the limitations and prospects of sweat analysis are discussed.

Femtosecond Laser Fabrication of High-Linearity Liquid Metal-Based Flexible Strain Sensor

Liquid metal (LM) is widely used in flexible electronic devices due to its excellent metallic conductivity and ductility. However, the fabrication of LM flexible strain sensors with high sensitivity and linearity is still a huge challenge, since the resistance of LM does not change much with strain. Here, a highly sensitive and linear fully flexible strain sensor with a resistive sensing function is proposed. The sensor comprises an Fe-doped liquid metal (Fe-LM) electrode for enhanced performance. The design and manufacturing of flexible strain sensors are based on the technology of controlling surface wettability by femtosecond laser micro/nano-processing. A supermetalphobic microstructure is constructed on a polydimethylsiloxane (PDMS) substrate to achieve the selection adhesion of Fe-LM on the PDMS substrate. The Fe-LM-based flexible strain sensor has high sensitivity and linearity, a gauge factor (GF) up to 1.18 in the strain range of 0-100%, excellent linearity with an R2 of 0.9978, a fast response time of 358 ms, and an excellent durability of more than 2400 load cycles. Additionally, the successful monitoring of human body signals demonstrates the potential of our developed flexible strain sensor in wearable monitoring applications.

Recent Advances in Wearable Healthcare Devices: From Material to Application

In recent years, the proliferation of wearable healthcare devices has marked a revolutionary shift in the personal health monitoring and management paradigm. These devices, ranging from fitness trackers to advanced biosensors, have not only made healthcare more accessible, but have also transformed the way individuals engage with their health data. By continuously monitoring health signs, from physical-based to biochemical-based such as heart rate and blood glucose levels, wearable technology offers insights into human health, enabling a proactive rather than a reactive approach to healthcare. This shift towards personalized health monitoring empowers individuals with the knowledge and tools to make informed decisions about their lifestyle and medical care, potentially leading to the earlier detection of health issues and more tailored treatment plans. This review presents the fabrication methods of flexible wearable healthcare devices and their applications in medical care. The potential challenges and future prospectives are also discussed.

Flexible and Stretchable Light-Emitting Diodes and Photodetectors for Human-Centric Optoelectronics

Optoelectronic devices with unconventional form factors, such as flexible and stretchable light-emitting or photoresponsive devices, are core elements for the next-generation human-centric optoelectronics. For instance, these deformable devices can be utilized as closely fitted wearable sensors to acquire precise biosignals that are subsequently uploaded to the cloud for immediate examination and diagnosis, and also can be used for vision systems for human-interactive robotics. Their inception was propelled by breakthroughs in novel optoelectronic material technologies and device blueprinting methodologies, endowing flexibility and mechanical resilience to conventional rigid optoelectronic devices. This paper reviews the advancements in such soft optoelectronic device technologies, honing in on various materials, manufacturing techniques, and device design strategies. We will first highlight the general approaches for flexible and stretchable device fabrication, including the appropriate material selection for the substrate, electrodes, and insulation layers. We will then focus on the materials for flexible and stretchable light-emitting diodes, their device integration strategies, and representative application examples. Next, we will move on to the materials for flexible and stretchable photodetectors, highlighting the state-of-the-art materials and device fabrication methods, followed by their representative application examples. At the end, a brief summary will be given, and the potential challenges for further development of functional devices will be discussed as a conclusion.

Liquid Metal Actuators: A Comparative Analysis of Surface Tension Controlled Actuation

Liquid metals, with their unique combination of electrical and mechanical properties, offer great opportunities for actuation based on surface tension modulation. Thanks to the scaling laws of surface tension, which can be electrochemically controlled at low voltages, liquid metal actuators stand out from other soft actuators for their remarkable characteristics such as high contractile strain rates and higher work densities at smaller length scales. This review summarizes the principles of liquid metal actuators and discusses their performance as well as theoretical pathways toward higher performances. The objective is to provide a comparative analysis of the ongoing development of liquid metal actuators. The design principles of the liquid metal actuators are analyzed, including low-level elemental principles (kinematics and electrochemistry), mid-level structural principles (reversibility, integrity, and scalability), and high-level functionalities. A wide range of practical use cases of liquid metal actuators from robotic locomotion and object manipulation to logic and computation is reviewed. From an energy perspective, strategies are compared for coupling the liquid metal actuators with an energy source toward fully untethered robots. The review concludes by offering a roadmap of future research directions of liquid metal actuators.

Materials, Designs, and Implementations of Wearable Antennas and Circuits for Biomedical Applications: A Review

The intersection of biomedicine and radio frequency (RF) engineering has fundamentally transformed self-health monitoring by leveraging soft and wearable electronic devices. This paradigm shift presents a critical challenge, requiring these devices and systems to possess exceptional flexibility, biocompatibility, and functionality. To meet these requirements, traditional electronic systems, such as sensors and antennas made from rigid and bulky materials, must be adapted through material science and schematic design. Notably, in recent years, extensive research efforts have focused on this field, and this review article will concentrate on recent advancements. We will explore the traditional/emerging materials for highly flexible and electrically efficient wearable electronics, followed by systematic designs for improved functionality and performance. Additionally, we will briefly overview several remarkable applications of wearable electronics in biomedical sensing. Finally, we provide an outlook on potential future directions in this developing area.

Scalable Electrodeposition of Liquid Metal from an Acetonitrile-Based Electrolyte for Highly Integrated Stretchable Electronics

The advancement of highly integrated stretchable electronics requires the development of scalable sub-micrometer conductor patterning. Eutectic gallium indium (EGaIn) is an attractive conductor for stretchable electronics, as its liquid metallic character grants it high electrical conductivity upon deformation. However, its high surface tension makes its patterning with sub-micrometer resolution challenging. In this work, this limitation is overcome by way of the electrodeposition of EGaIn. A non-aqueous acetonitrile-based electrolyte that exhibits high electrochemical stability and chemical orthogonality is used. The electrodeposited material leads to low-resistance lines that remain stable upon (repeated) stretching to a 100% strain. Because electrodeposition benefits from the resolution of mature nanofabrication methods used to pattern the base metal, the proposed "bottom-up" approach achieves a record-high density integration of EGaIn regular lines of 300 nm half-pitch on an elastomer substrate by plating on a gold seed layer prepatterned by nanoimprinting. Moreover, vertical integration is enabled by filling high-aspect-ratio vias. This capability is conceptualized by the fabrication of an omnidirectionally stretchable 3D electronic circuit, and demonstrates a soft-electronic analog of the stablished damascene process used to fabricate microchip interconnects. Overall, this work proposes a simple route to address the challenge of metallization in highly integrated (3D) stretchable electronics.

Surface Oxidation and Wetting Synergistic Effect of Liquid Metals

Various functions of liquid metals are closely related to their surface performances, among which oxidation and wetting are the two most important surface processes. The two processes of liquid metals are inseparable in most practical applications; however, the coupling of oxidation and wetting of liquid metals has received little attention. Here, we demonstrate the synergistic effect of oxidation and wetting of liquid metals through establishing a liquid system containing the copper ion acid solution. By modulating the concentrations of copper ions and hydrogen ions, three different modes of the liquid metal surface are presented, where the oxidation process and the wetting process are in a competitive relationship. Whichever of the two processes is dominant can determine the stability of copper particles produced on the surface of liquid metals, that is, affect whether the "phagocytosis" process can occur. It is revealed that the magnitude of current density on the surface of liquid metals, caused by galvanic corrosion behavior between liquid metals and copper particles, is the key factor influencing the dominance of different surface processes of liquid metals. Utilizing the synergistic effect, we prepare a liquid metal film with adjustable reflectivity, in which surface states can be changed repeatedly between the bright state and the darken state by simple solution immersion. The liquid metal film with different surface states can show obvious difference in optical performance, which has application potential in color camouflage. Understanding the surface synergistic effect will facilitate further exploration of the abundant exotic liquid metal interface phenomena.

Conformability of flexible sheets on spherical surfaces

Three-dimensional surface-conformable electronics is a burgeoning technology with potential applications in curved displays, bioelectronics, and biomimetics. Flexible electronics are notoriously difficult to fully conform to nondevelopable surfaces such as spheres. Although stretchable electronics can well conform to nondevelopable surfaces, they need to sacrifice pixel density for stretchability. Various empirical designs have been explored to improve the conformability of flexible electronics on spherical surfaces. However, no rational design guidelines exist. This study uses a combination of experimental, analytical, and numerical approaches to systematically investigate the conformability of both intact and partially cut circular sheets on spherical surfaces. Through the analysis of thin film buckling on curved surfaces, we identify a scaling law that predicts the conformability of flexible sheets on spherical surfaces. We also quantify the effects of radial slits on enhancing conformability and provide a practical guideline for using these slits to improve conformability from 40% to more than 90%.

Ga-In Alloy Segregation within a Porous Glass as Studied by SANS

Nanolattices can play the role of templates for metals and metallic alloys to produce functional nanocomposites with particular properties affected by nanoconfinement. To imitate the impact of nanoconfinement on the structure of solid eutectic alloys, we filled porous silica glasses with the Ga-In alloy, which is widely used in applications. Small-angle neutron scattering was observed for two nanocomposites, which comprised alloys of close compositions. The results obtained were treated using different approaches: the common Guinier and extended Guinier models, the recently suggested computer simulation method based on the initial formulae for neutron scattering, and ordinary estimates of the scattering hump positions. All of the approaches predicted a similar structure of the confined eutectic alloy. The formation of ellipsoid-like indium-rich segregates was demonstrated.

Recent Advances in Nanomaterials Used for Wearable Electronics

In recent decades, thriving Internet of Things (IoT) technology has had a profound impact on people's lifestyles through extensive information interaction between humans and intelligent devices. One promising application of IoT is the continuous, real-time monitoring and analysis of body or environmental information by devices worn on or implanted inside the body. This research area, commonly referred to as wearable electronics or wearables, represents a new and rapidly expanding interdisciplinary field. Wearable electronics are devices with specific electronic functions that must be flexible and stretchable. Various novel materials have been proposed in recent years to meet the technical challenges posed by this field, which exhibit significant potential for use in different wearable applications. This article reviews recent progress in the development of emerging nanomaterial-based wearable electronics, with a specific focus on their flexible substrates, conductors, and transducers. Additionally, we discuss the current state-of-the-art applications of nanomaterial-based wearable electronics and provide an outlook on future research directions in this field.

Shape-Programmable Liquid Metal Fibers

Conductive and stretchable fibers are the cornerstone of intelligent textiles and imperceptible electronics. Among existing fiber conductors, gallium-based liquid metals (LMs) featuring high conductivity, fluidity, and self-healing are excellent candidates for highly stretchable fibers with sensing, actuation, power generation, and interconnection functionalities. However, current LM fibers fabricated by direct injection or surface coating have a limitation in shape programmability. This hinders their applications in functional fibers with tunable electromechanical response and miniaturization. Here, we reported a simple and efficient method to create shape-programmable LM fibers using the phase transition of gallium. Gallium metal wires in the solid state can be easily shaped into a 3D helical structure, and the structure can be preserved after coating the wire with polyurethane and liquifying the metal. The 3D helical LM fiber offered enhanced stretchability with a high breaking strain of 1273% and showed invariable conductance over 283% strain. Moreover, we can reduce the fiber diameter by stretching the fiber during the solidification of polyurethane. We also demonstrated applications of the programmed fibers in self-powered strain sensing, heart rate monitoring, airflow, and humidity sensing. This work provided simple and facile ways toward functional LM fibers, which may facilitate the broad applications of LM fibers in e-skins, wearable computation, soft robots, and smart fabrics.

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.

Connecting liquid metals with sound

A stretchable conductive circuit is formed using a liquid metal-polymer composite

Ultra-Thin Flexible Encapsulating Materials for Soft Bio-Integrated Electronics

Recently, bioelectronic devices extensively researched and developed through the convergence of flexible biocompatible materials and electronics design that enables  more precise diagnostics and therapeutics in human health care and opens up the potential to expand into various fields, such as clinical medicine and biomedical research. To establish an accurate and stable bidirectional bio-interface, protection against the external environment and high mechanical deformation is essential for wearable bioelectronic devices. In the case of implantable bioelectronics, special encapsulation materials and optimized mechanical designs and configurations that provide electronic stability and functionality are required for accommodating various organ properties, lifespans, and functions in the biofluid environment. Here, this study introduces recent developments of ultra-thin encapsulations with novel materials that can preserve or even improve the electrical performance of wearable and implantable bio-integrated electronics by supporting safety and stability for protection from destruction and contamination as well as optimizing the use of bioelectronic systems in physiological environments. In addition, a summary of the materials, methods, and characteristics of the most widely used encapsulation technologies is introduced, thereby providing a strategic selection of appropriate choices of recently developed flexible bioelectronics.

Patterned Magnetofluids via Magnetic Printing and Photopolymerization for Multifunctional Flexible Electronic Sensors

Liquid conductor-based flexible sensors with high mechanical deformability and reliable electrical reversibility have aroused great interest in electronic skin, soft robotics, environmental monitoring, and other fields. Herein, we develop a novel strategy to fabricate liquid conductor-based flexible sensors by combining ionic liquid-based magnetofluids (IL-MFs), magnetic printing, and photopolymerization techniques. The as-prepared sensors exhibit excellent electromechanical properties, such as a wide detection range, low hysteresis, fast response time, good durability, etc. Moreover, the gauge factors (GFs) of the sensor could be easily adjusted by changing the modulators with different line widths or patterns, and the strain sensors can also be designed for anisotropic monitoring. Apart from serving as strain sensors, the magnetofluid-based flexible sensors can be used to detect external pressure, human activities, and changes in temperature, illumination, and magnetic field as well. This work provides a facile strategy to fabricate liquid conductor-based multifunctional sensors. Such a magnetofluid-based sensor has a great promising future.

Morphological Engineering of Sensing Materials for Flexible Pressure Sensors and Artificial Intelligence Applications

Various morphological structures in pressure sensors with the resulting advanced sensing properties are reviewed comprehensively. Relevant manufacturing techniques and intelligent applications of pressure sensors are summarized in a complete and interesting way. Future challenges and perspectives of flexible pressure sensors are critically discussed.

As an indispensable branch of wearable electronics, flexible pressure sensors are gaining tremendous attention due to their extensive applications in health monitoring, human –machine interaction, artificial intelligence, the internet of things, and other fields. In recent years, highly flexible and wearable pressure sensors have been developed using various materials/structures and transduction mechanisms. Morphological engineering of sensing materials at the nanometer and micrometer scales is crucial to obtaining superior sensor performance. This review focuses on the rapid development of morphological engineering technologies for flexible pressure sensors. We discuss different architectures and morphological designs of sensing materials to achieve high performance, including high sensitivity, broad working range, stable sensing, low hysteresis, high transparency, and directional or selective sensing. Additionally, the general fabrication techniques are summarized, including self-assembly, patterning, and auxiliary synthesis methods. Furthermore, we present the emerging applications of high-performing microengineered pressure sensors in healthcare, smart homes, digital sports, security monitoring, and machine learning-enabled computational sensing platform. Finally, the potential challenges and prospects for the future developments of pressure sensors are discussed comprehensively.

Intrinsically Stretchable Poly(3,4-ethylenedioxythiophene) Conducting Polymer Film for Flexible Electronics

The aim of this study was to synthesize an intrinsically stretchable conductive polymer (CP) by atom transfer radical polymerization (ATRP). For this purpose, poly(3,4-ethyilenedioxythiophene) (PEDOT) was synthesized as a backbone, while poly(acrylate-urethane) (PAU) was grafted onto the PEDOT backbone to form graft polymers PEDOT-g-PAU. Different concentrations of acrylate-urethane (AU) were used to synthesize PAU side chains of different lengths. The successful synthesis of the obtained intermediates and products (PEDOT-g-PAU) was confirmed by infrared spectroscopy and nuclear magnetic resonance. Thermal properties were evaluated by differential scanning calorimetry and thermogravimetric analysis, while conductivity was determined by four-point probe measurement. A simple tensile test was performed to characterize the ductility of the samples. PEDOT-g-PAU has shown high stretchability of up to 500% and, therefore, could potentially be used in skin-worn flexible electronics, while additional subsequent doping is required to improve the deterioration of electrical properties after the addition of the insulating urethane layer.

Revolution in Flexible Wearable Electronics for Temperature and Pressure Monitoring—A Review

In the last few decades, technology innovation has had a huge influence on our lives and well-being. Various factors of observing our physiological characteristics are taken into account. Wearable sensing tools are one of the most imperative sectors that are now trending and are expected to grow significantly in the coming days. Externally utilized tools connected to any human to assess physiological characteristics of interest are known as wearable sensors. Wearable sensors range in size from tiny to large tools that are physically affixed to the user and operate on wired or wireless terms. With increasing technological capabilities and a greater grasp of current research procedures, the usage of wearable sensors has a brighter future. In this review paper, the recent developments of two important types of wearable electronics apparatuses have been discussed for temperature and pressure sensing (Psensing) applications. Temperature sensing (Tsensing) is one of the most important physiological factors for determining human body temperature, with a focus on patients with long-term chronic conditions, normally healthy, unconscious, and injured patients receiving surgical treatment, as well as the health of medical personnel. Flexile Psensing devices are classified into three categories established on their transduction mechanisms: piezoresistive, capacitive, and piezoelectric. Many efforts have been made to enhance the characteristics of the flexible Psensing devices established on these mechanisms.

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.

Liquid Metal-Based Devices: Material Properties, Fabrication and Functionalities

This paper reviews the material properties, fabrication and functionalities of liquid metal-based devices. In modern wireless communication technology, adaptability and versatility have become attractive features of any communication device. Compared with traditional conductors such as copper, the flow characteristics and lack of elastic limit of conductive fluids make them ideal alternatives for applications such as flexible circuits, soft electronic devices, wearable stretch sensors, and reconfigurable antennas. These fluid properties also allow for innovative manufacturing techniques such as 3-D printing, injecting or spraying conductive fluids on rigid/flexible substrates. Compared with traditional high-frequency switching methods, liquid metal (LM) can easily use micropumps or an electrochemically controlled capillary method to achieve reconfigurability of the device. The movement of LM over a large physical dimension enhances the reconfigurable state of the antenna, without depending on nonlinear materials or mechanisms. When LM is applied to wearable devices and sensors such as electronic skins (e-skins) and strain sensors, it consistently exhibits mechanical fatigue resistance and can maintain good electrical stability under a certain degree of stretching. When LM is used in microwave devices and paired with elastic linings such as polydimethylsiloxane (PDMS), the shape and size of the devices can be changed according to actual needs to meet the requirements of flexibility and a multistate frequency band. In this work, we discuss the material properties, fabrication and functionalities of LM.

Polydopamine Shell as a Ga3+ Reservoir for Triggering Gallium-Indium Phase Separation in Eutectic Gallium-Indium Nanoalloys

Low melting point eutectic systems, such as the eutectic gallium-indium (EGaIn) alloy, offer great potential in the domain of nanometallurgy; however, many of their interfacial behaviors remain to be explored. Here, a compositional change of EGaIn nanoalloys triggered by polydopamine (PDA) coating is demonstrated. Incorporating PDA on the surface of EGaIn nanoalloys renders core-shell nanostructures that accompany Ga-In phase separation within the nanoalloys. The PDA shell keeps depleting the Ga³⁺ from the EGaIn nanoalloys when the synthesis proceeds, leading to a Ga³⁺-coordinated PDA coating and a smaller nanoalloy. During this process, the eutectic nanoalloys turn into non-eutectic systems that ultimately result in the solidification of In when Ga is fully depleted. The reaction of Ga³⁺-coordinated PDA-coated nanoalloys with nitrogen dioxide gas is presented as an example for demonstrating the functionality of such hybrid composites. The concept of phase-separating systems, with polymeric reservoirs, may lead to tailored materials and can be explored on a variety of post-transition metals.