Pencil-paper on-skin electronics

Electrochemically Active Materials for Tissue-Interfaced Soft Biochemical Sensing

Tissue-interfaced soft biochemical sensing represents a crucial approach to personalized healthcare by employing electrochemically active materials to monitor biochemical signals at the tissue interface in real time, either noninvasively or through implantation. These soft biochemical sensors can be integrated with various biological tissues, such as neural, gastrointestinal, ocular, cardiac, skin, muscle, and bone, adapting to their unique mechanical and biochemical environments. Sensors employing materials like conductive polymers, composites, metals, metal oxides, and carbon-based nanomaterials have demonstrated capabilities in applications, such as continuous glucose monitoring, neural activity mapping, and real-time metabolite detection, enhancing diagnostics and treatment monitoring across a range of medical fields. Next-generation tissue-interfaced biosensors that enable multimodal and multiplexed measurement of biochemical markers and physiological parameters could be transformative for personalized medicine, allowing for high-resolution, time-resolved historical monitoring of an individual's health status. In this review, we summarize current trends in the field to provide insights into the challenges and future trajectory of tissue-interfaced soft biochemical sensors, highlighting their potential to revolutionize personalized medicine and improve patient outcomes.

Wearable bioelectronics based on emerging nanomaterials for telehealth applications

Nanomaterial-driven, soft wearable bioelectronics are transforming telemedicine by offering skin comfort, biocompatibility, and the capability for continuous remote monitoring of physiological signals. The devices, enabled by advanced zero-dimensional (0D), one-dimensional (1D), and two-dimensional (2D) nanomaterials, have achieved new levels in electrical stability and reliability, allowing them to perform effectively even under dynamic physical conditions. Despite their promise, significant challenges remain in the fabrication, integration, and practical deployment of nanoscale materials and devices. Critical challenges include ensuring the durability and stability of nanomaterial-based bioelectronics for extended wear and developing efficient integration strategies to support multifunctional sensing modalities. Telemedicine has revolutionized healthcare by enabling remote health monitoring. The integration of nanomaterials within wearable devices is a central factor driving this breakthrough, as these materials enhance sensor sensitivity, durability, and multifunctionality. These wearable sensors leverage various operating principles tailored to specific applications, such as intraocular pressure monitoring, electrophysiological signal recording, and biochemical marker tracking.

Paper-Based Sensors: Fantasy or Reality?

This article analyzes the prospects for the appearance of paper-based sensors on the sensor market. It is concluded that paper-based sensors are not a fantasy but a reality. It is shown that paper has properties that make it possible to develop a wide variety of paper-based sensors, such as SERS, colorimetric, fluorescent, conductometric, capacitive, fiber-optic, electrochemical, microfluidic, shape-deformation, microwave, and various physical sensors. The use of paper in the manufacturing of various sensors opens up new possibilities both in terms of new approaches to their manufacturing and in terms of new areas of their application. However, it must be recognized that for the widespread use of paper and the appearance of paper-based sensors on the sensor market, many obstacles must be overcome.

Multifunctional Porous Soft Bioelectronics

Soft bioelectronics, seamlessly interfacing with the human body to enable both recording and modulation of curvilinear biological tissues and organs, have significantly driven fields such as digital healthcare, human-machine interfaces, and robotics. Nonetheless, intractable challenges persist due to the onerous demand for imperceptible, burden-free, and user-centric comfortable bioelectronics. Porous soft bioelectronics is a new way to a library of imperceptible bioelectronic systems, that form natural interfaces with the human body. In this review, we provide an overview of the development and recent advances in multifunctional porous engineered soft bioelectronics, aiming to bridge the gap between living biotic and stiff abiotic systems. We first discuss strategies for fabricating porous, soft, and stretchable bioelectronic materials, emphasizing the concept of materials-level porous engineering for breathable and imperceptible bioelectronics. Next, we summarize wearable bioelectronics devices and multimodal systems with porous configurations designed for on-skin healthcare applications. Moving beneath the skin, we discuss implantable devices and systems enabled by porous bioelectronics with tissue-like compliance. Finally, existing challenges and translational gaps are also proposed to usher further research efforts towards realizing practical and clinical applications of porous bioelectronic systems; thus, revolutionizing conventional healthcare and medical practices and opening up unprecedented opportunities for long-term, imperceptible, non-invasive, and human-centric healthcare networks.

Breathable and Stretchable Epidermal Electronics for Health Management: Recent Advances and Challenges

Advanced epidermal electronic devices, capable of real-time monitoring of physical, physiological, and biochemical signals and administering appropriate therapeutics, are revolutionizing personalized healthcare technology. However, conventional portable electronic devices are predominantly constructed from impermeable and rigid materials, which thus leads to the mechanical and biochemical disparities between the devices and human tissues, resulting in skin irritation, tissue damage, compromised signal-to-noise ratio (SNR), and limited operational lifespans. To address these limitations, a new generation of wearable on-skin electronics built on stretchable and porous substrates has emerged. These substrates offer significant advantages including breathability, conformability, biocompatibility, and mechanical robustness, thus providing solutions for the aforementioned challenges. However, given their diverse nature and varying application scenarios, the careful selection and engineering of suitable substrates is paramount when developing high-performance on-skin electronics tailored to specific applications. This comprehensive review begins with an overview of various stretchable porous substrates, specifically focusing on their fundamental design principles, fabrication processes, and practical applications. Subsequently, a concise comparison of various methods is offered to fabricate epidermal electronics by applying these porous substrates. Following these, the latest advancements and applications of these electronics are highlighted. Finally, the current challenges are summarized and potential future directions in this dynamic field are explored.

Asymmetric Aramid Aerogel Composite with Durable and Covert Thermal Management via Janus Heat Transfer Structure

Warmth preservation in cold climates requires a long-term heat supply. Conventional thermal devices usually deliver excessive heat and have difficulty preventing heat loss. Herein, to achieve durable thermal comfort, an asymmetric composite (AAAC) is devised through vacuum-filtrating silver nanowires (AgNWs) onto the surface of a poly(ethylene glycol) (PEG)-infiltrated aramid nanofiber aerogel. AgNW e-skin can transfer strong Joule heat to the back side of the AAAC, where the infused PEG further stores thermal energy by phase transition and then releases it to the body constantly after the power is off. Base on the Janus structure, the AAAC can provide thermotherapy for human waists and knees, holding a suitable temperature range of 37-39 °C for 5-8 min without a heat source. In addition, low infrared emissivity (0.064-0.315) of e-skin allows the AAAC to conceal thermal targets while producing Joule heat, which achieves comfortable and safe thermal management in multiple occasions including daily life and military warfare.

Smart Gas Sensors: Recent Developments and Future Prospective

Gas sensor is an indispensable part of modern society with wide applications in environmental monitoring, healthcare, food industry, public safety, etc. With the development of sensor technology, wireless communication, smart monitoring terminal, cloud storage/computing technology, and artificial intelligence, smart gas sensors represent the future of gas sensing due to their merits of real-time multifunctional monitoring, early warning function, and intelligent and automated feature. Various electronic and optoelectronic gas sensors have been developed for high-performance smart gas analysis. With the development of smart terminals and the maturity of integrated technology, flexible and wearable gas sensors play an increasing role in gas analysis. This review highlights recent advances of smart gas sensors in diverse applications. The structural components and fundamental principles of electronic and optoelectronic gas sensors are described, and flexible and wearable gas sensor devices are highlighted. Moreover, sensor array with artificial intelligence algorithms and smart gas sensors in "Internet of Things" paradigm are introduced. Finally, the challenges and perspectives of smart gas sensors are discussed regarding the future need of gas sensors for smart city and healthy living.

Programmable and flexible wood-based origami electronics

Natural polymer substrates are gaining attention as substitutes for plastic substrates in electronics, aiming to combine high performance, intricate shape deformation, and environmental sustainability. Herein, natural wood veneer is converted into a transparent wood film (TWF) substrate. The combination of 3D printing and origami technique is established to create programmable wood-based origami electronics, which exhibit superior flexibility with high tensile strength (393 MPa) due to the highly aligned cellulose fibers and the formation of numerous intermolecular hydrogen bonds between them. Moreover, the flexible TWF electronics exhibit editable multiplexed configurations and maintain stable conductivity. This is attributed to the strong adhesion between the cellulose-based ink and TWF substrate by non-covalent bonds. Benefiting from its anisotropic structure, the programmability of TWF electronics is achieved through sequentially folding into predesigned shapes. This design not only promotes environmental sustainability but also introduces its customizable shapes with potential applications in sensors, microfluidics, and wearable electronics.

Ambient Moisture-Driven Self-Powered Iontophoresis Patch for Enhanced Transdermal Drug Delivery

Iontophoretic transdermal drug delivery (TDD) devices are known to enhance the transdermal transport of drugs. However, conventional transdermal iontophoretic devices require external power sources, wired connections, or mechanical parts, which reduce the comfort level for patients during extended use. In this work, a self-powered, wearable transdermal iontophoretic patch (TIP) is proposed by harvesting ambient humidity for energy generation, enabling controlled TDD. This patch primarily uses moist-electric generators (MEGs) as its power source, thus obviating the need for complex power management modules and mechanical components. A single MEG unit can produce an open-circuit voltage of 0.80 V and a short-circuit current of 11.65 µA under the condition of 80% relative humidity. Amplification of the electrical output is feasible by connecting multiple generator units in series and parallel, facilitating the powering of certain commercial electronic devices. Subsequently, the MEG array is integrated with the TDD circuit to create the wearable TIP. After 20 min of application, the depth of drug penetration through the skin is observed to increase threefold. The effective promotion effect of TIP on the transdermal delivery of ionized drugs is corroborated by simulations and experiments. This wearable TIP offers a simple, noninvasive solution for TDD.

Pressure-constrained sonication activation of flexible printed metal circuit

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

Multiwalled Carbon Nanotube-Templated Nickel Porphyrin Covalent Organic Framework for Pencil-Drawn Noninvasive Respiration Sensors

Paper-integrated configuration with miniaturized functionality represents one of the future main green electronics. In this study, a paper-based respiration sensor was prepared using a multiwalled carbon nanotube-templated nickel porphyrin covalent organic framework (MWCNTs@COFNiP-Ph) as an electrical identification component and pencil-drawn graphite electric circuits as interdigitated electrodes (IDEs). The MWCNTs@COFNiP-Ph not only inherited the high gas sensing performance of porphyrin and the aperture induction effect of COFs but also overcame the shielding effect between phases through the MWCNT template. Furthermore, it possessed highly exposed M-N4 metallic active sites and unique periodic porosity, thereby effectively addressing the key technical issue of room-temperature sensing for the respiration sensor. Meanwhile, the introduction of a pencil-drawing approach on common printing papers facilitates the inexpensive and simple manufacturing of the as-fabricated graphite IDE. Based on the above advantages, the MWCNTs@COFNiP-Ph respiration sensor had the characteristics of wide detection range (1-500 ppm), low detection limit (30 ppb), acceptable flexibility for toluene, and rapid response/recovery time (32 s/116 s). These advancements facilitated the integration of the respiration sensor into surgical masks and clothes with maximum functionality at a minimized size and weight. Moreover, the primary internal mechanism of COFNiP-Ph for this efficient toluene detection was investigated through in situ FTIR spectra, thereby directly elucidating that the chemisorption interaction of oxygen modulated the depletion layers, resulting in alterations in sensor resistance upon exposure to the target gas. The encouraging results revealed the feasibility of employing a paper-sensing system as a wearable platform in green electronics.

Multifunctional Conductive Hydrogel Interface for Bioelectronic Recording and Stimulation

The past few decades have witnessed the rapid advancement and broad applications of flexible bioelectronics, in wearable and implantable electronics, brain-computer interfaces, neural science and technology, clinical diagnosis, treatment, etc. It is noteworthy that soft and elastic conductive hydrogels, owing to their multiple similarities with biological tissues in terms of mechanics, electronics, water-rich, and biological functions, have successfully bridged the gap between rigid electronics and soft biology. Multifunctional hydrogel bioelectronics, emerging as a new generation of promising material candidates, have authentically established highly compatible and reliable, high-quality bioelectronic interfaces, particularly in bioelectronic recording and stimulation. This review summarizes the material basis and design principles involved in constructing hydrogel bioelectronic interfaces, and systematically discusses the fundamental mechanism and unique advantages in bioelectrical interfacing with the biological surface. Furthermore, an overview of the state-of-the-art manufacturing strategies for hydrogel bioelectronic interfaces with enhanced biocompatibility and integration with the biological system is presented. This review finally exemplifies the unprecedented advancement and impetus toward bioelectronic recording and stimulation, especially in implantable and integrated hydrogel bioelectronic systems, and concludes with a perspective expectation for hydrogel bioelectronics in clinical and biomedical applications.

Electrochemical Paper-Based Microfluidics: Harnessing Capillary Flow for Advanced Diagnostics

Electrochemical paper-based microfluidics has attracted much attention due to the promise of transforming point-of-care diagnostics by facilitating quantitative analysis with low-cost and portable analyzers. Such devices harness capillary flow to transport samples and reagents, enabling bioassays to be executed passively. Despite exciting demonstrations of capillary-driven electrochemical tests, conventional methods for fabricating electrodes on paper impede capillary flow, limit fluidic pathways, and constrain accessible device architectures. This account reviews recent developments in paper-based electroanalytical devices and offers perspective by revisiting key milestones in lateral flow tests and paper-based microfluidics engineering. The study highlights the benefits associated with electrochemical sensing and discusses how the detection modality can be leveraged to unlock novel functionalities. Particular focus is given to electrofluidic platforms that embed electrodes into paper for enhanced biosensing applications. Together, these innovations pave the way for diagnostic technologies that offer portability, quantitative analysis, and seamless integration with digital healthcare, all without compromising the simplicity of commercially available rapid diagnostic tests.

Sustainable Sensing with Paper Microfluidics: Applications in Health, Environment, and Food Safety

This manuscript offers a concise overview of paper microfluidics, emphasizing its sustainable sensing applications in healthcare, environmental monitoring, and food safety. Researchers have developed innovative sensing platforms for detecting pathogens, pollutants, and contaminants by leveraging the paper's unique properties, such as biodegradability and affordability. These portable, low-cost sensors facilitate rapid diagnostics and on-site analysis, making them invaluable tools for resource-limited settings. This review discusses the fabrication techniques, principles, and applications of paper microfluidics, showcasing its potential to address pressing challenges and enhance human health and environmental sustainability.

Biocompatible Electronic Skins for Cardiovascular Health Monitoring

Cardiovascular diseases represent a significant threat to the overall well-being of the global population. Continuous monitoring of vital signs related to cardiovascular health is essential for improving daily health management. Currently, there has been remarkable proliferation of technology focused on collecting data related to cardiovascular diseases through daily electronic skin monitoring. However, concerns have arisen regarding potential skin irritation and inflammation due to the necessity for prolonged wear of wearable devices. To ensure comfortable and uninterrupted cardiovascular health monitoring, the concept of biocompatible electronic skin has gained substantial attention. In this review, biocompatible electronic skins for cardiovascular health monitoring are comprehensively summarized and discussed. The recent achievements of biocompatible electronic skin in cardiovascular health monitoring are introduced. Their working principles, fabrication processes, and performances in sensing technologies, materials, and integration systems are highlighted, and comparisons are made with other electronic skins used for cardiovascular monitoring. In addition, the significance of integrating sensing systems and the updating wireless communication for the development of the smart medical field is explored. Finally, the opportunities and challenges for wearable electronic skin are also examined.

Solution-Processed Flexible Temperature Sensor Array for Highly Resolved Spatial Temperature and Tactile Mapping Using ESN-Based Data Interpolation

High-performance flexible temperature sensors are crucial in various technological applications, such as monitoring environmental conditions and human healthcare. The ideal characteristics of these sensors for stable temperature monitoring include scalability, mechanical flexibility, and high sensitivity. Moreover, simplicity and low power consumption will be essential for temperature sensor arrays in future integrated systems. This study introduces a solution-based approach for creating a V2O5 nanowire network temperature sensor on a flexible film. Through optimization of the fabrication conditions, the sensor exhibits remarkable performance, sustaining long-term stability (>110 h) with minimal hysteresis and excellent sensitivity (∼-1.5%/°C). In addition, this study employs machine learning techniques for data interpolation among sensors, thereby enhancing the spatial resolution of temperature measurements and adding tactile mapping without increasing the sensor count. Introducing this methodology results in an improved understanding of temperature variations, advancing the capabilities of flexible-sensor arrays for various applications.

Bacterial cellulose/multi-walled carbon nanotube composite films for moist-electric energy harvesting

Generation of renewable and clean electricity energy from ubiquitous moisture for the power supply of portable electronic devices has become one of the most promising energy collection methods. However, the modest electrical output and transient power supply characteristics of existing moist-electric generator (MEG) severely limit its commercial application, leading to an urgent demand of developing a MEG with high electrical output and continuous power generation capacity. In this work, it is demonstrated that a flexible bacterial cellulose (BC)/Multi-walled carbon nanotube (MWCNT) double-layer (BM-dl) film prepared by vacuum filtration can maintain the moisture concentration difference in the film MEG. Unlike previous studies on cellulose based MEG, BM-dl film has a heterogeneous structure, resulting in a maximum output power density of 0.163 μW/cm2, an extreme voltage of 0.84 V, and current of 2.21 μA at RH = 90 %. BM-dl MEG can generate a voltage of 0.55 V continuously for 45 h in a natural environment (RH = 63-77 %, T = 26-27 °C), which is in a leading level among existing reported cellulose-based MEGs. In summary, this study provides new ideas for innovative design of MEG, which is highly competitive in terms of energy supply for the Internet of Things and wearable devices.

Self-Sustained Programmable Hygroelectronic Interfaces for Humidity-Regulated Hierarchical Information Encryption and Display

The emerging moisture-driven energy generation (MEG) technology opens up new possibilities for humidity-responsive materials, devices, and interdisciplinary opportunities in fields like information security. However, such potential remains untapped. Here, an original MEG structure with a hygroionic energy-conversion route by selective coating of ionic hygroscopic hydrogels on a carbon black surface is reported. The hygroionic route features a process in which the scavenged energy is stored in the electrical double layers formed at the interfaces between the ionic hydrogel and the carbon nanoparticles. The resultant electrical field developed across the hydrogel-coated wet carbon and the rest of the dry carbon area is thus durably lasted. Based on this unique structure, hygroelectronic information interfaces (HEII) for humidity-regulated information encryption and display are put forward by devising hydrogel patterns on a carbon platform. Further by tuning the hygroscopicity of the ionic hydrogels and incorporating encoding methods (e.g., Morse code), it is demonstrated that the HEII platform is programmable to carry different information in certain humidity ranges. Unlike those conventional anti-counterfeiting methods that optically reveal the hidden information once the required stimulus is provided, the new HEII serves as a hierarchical solution for high-security encryption and display.

Moisture-Enabled Electricity from Hygroscopic Materials: A New Type of Clean Energy

Water utilization is accompanied with the development of human beings, whereas gaseous moisture is usually regarded as an underexploited resource. The advances of highly efficient hygroscopic materials endow atmospheric water harvesting as an intriguing solution to convert moisture into clean water. The discovery of hygroelectricity, which refers to the charge buildup at a material surface dependent on humidity, and the following moisture-enabled electric generation (MEG) realizes energy conversion and directly outputs electricity. Much progress has been made since then to optimize MEG performance, pushing forward the applications of MEG into a practical level. Herein, the evolvement and development of MEG are systematically summarized in a chronological order. The optimization strategies of MEG are discussed and comprehensively evaluated. Then, the latest applications of MEG are presented, including high-performance powering units and self-powered devices. In the end, a perspective on the future development of MEG is given for inspiring more researchers into this promising area.

Well-defined in-textile photolithography towards permeable textile electronics

Textile-based wearable electronics have attracted intensive research interest due to their excellent flexibility and breathability inherent in the unique three-dimensional porous structures. However, one of the challenges lies in achieving highly conductive patterns with high precision and robustness without sacrificing the wearing comfort. Herein, we developed a universal and robust in-textile photolithography strategy for precise and uniform metal patterning on porous textile architectures. The as-fabricated metal patterns realized a high precision of sub-100 µm with desirable mechanical stability, washability, and permeability. Moreover, such controllable coating permeated inside the textile scaffold contributes to the significant performance enhancement of miniaturized devices and electronics integration through both sides of the textiles. As a proof-of-concept, a fully integrated in-textiles system for multiplexed sweat sensing was demonstrated. The proposed method opens up new possibilities for constructing multifunctional textile-based flexible electronics with reliable performance and wearing comfort.

Advances in Non-Electrochemical Sensing of Human Sweat Biomarkers: From Sweat Sampling to Signal Reading

Sweat, commonly referred to as the ultrafiltrate of blood plasma, is an essential physiological fluid in the human body. It contains a wide range of metabolites, electrolytes, and other biologically significant markers that are closely linked to human health. Compared to other bodily fluids, such as blood, sweat offers distinct advantages in terms of ease of collection and non-invasive detection. In recent years, considerable attention has been focused on wearable sweat sensors due to their potential for continuous monitoring of biomarkers. Electrochemical methods have been extensively used for in situ sweat biomarker analysis, as thoroughly reviewed by various researchers. This comprehensive review aims to provide an overview of recent advances in non-electrochemical methods for analyzing sweat, including colorimetric methods, fluorescence techniques, surface-enhanced Raman spectroscopy, and more. The review covers multiple aspects of non-electrochemical sweat analysis, encompassing sweat sampling methodologies, detection techniques, signal processing, and diverse applications. Furthermore, it highlights the current bottlenecks and challenges faced by non-electrochemical sensors, such as limitations and interference issues. Finally, the review concludes by offering insights into the prospects for non-electrochemical sensing technologies. By providing a valuable reference and inspiring researchers engaged in the field of sweat sensor development, this paper aspires to foster the creation of innovative and practical advancements in this domain.

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.

Facile Fabrication of "Tacky", Stretchable, and Aligned Carbon Nanotube Sheet-Based Electronics for On-Skin Health Monitoring

Point-of-care monitoring of physiological signals such as electrocardiogram, electromyogram, and electroencephalogram is essential for prompt disease diagnosis and quick treatment, which can be realized through advanced skin-worn electronics. However, it is still challenging to design an intimate and nonrestrictive skin-contact device for physiological measurements with high fidelity and artifact tolerance. This research presents a facile method using a "tacky" surface to produce a tight interface between the ACNT skin-like electronic and the skin. The method provides the skin-worn electronic with a stretchability of up to 70% strain, greater than that of most common epidermal electrodes. Low-density ACNT bundles facilitate the infiltration of adhesive and improve the conformal contact between the ACNT sheet and the skin, while dense ACNT bundles lessen this effect. The stretchability and conformal contact allow the ACNT sheet-based electronics to create a tight interface with the skin, which enables the high-fidelity measurement of physiological signals (the Pearson's coefficient of 0.98) and tolerance for motion artifacts. In addition, our method allows the use of degradable substrates to enable reusability and degradability of the electronics based on ACNT sheets, integrating "green" properties into on-skin electronics.

Recent Advances in Biodegradable Green Electronic Materials and Sensor Applications

As environmental issues have become the dominant agenda worldwide, the necessity for more environmentally friendly electronics has recently emerged. Accordingly, biodegradable or nature-derived materials for green electronics have attracted increased interest. Initially, metal-green hybrid electronics are extensively studied. Although these materials are partially biodegradable, they have high utility owing to their metallic components. Subsequently, carbon-framed materials (such as graphite, cylindrical carbon nanomaterials, graphene, graphene oxide, laser-induced graphene) have been investigated. This has led to the adoption of various strategies for carbon-based materials, such as blending them with biodegradable materials. Moreover, various conductive polymers have been developed and researchers have studied their potential use in green electronics. Researchers have attempted to fabricate conductive polymer composites with high biodegradability by shortening the polymer chains. Furthermore, various physical, chemical, and biological sensors that are essential to modern society have been studied using biodegradable compounds. These recent advances in green electronics have paved the way toward their application in real life, providing a brighter future for society.

Advanced Self-Powered Biofuel Cells with Capacitor and Nanogenerator for Biomarker Sensing

Self-powered biofuel cells (BFCs) have evolved for highly sensitive detection of biomarkers such as noncodon micro ribonucleic acids (miRNAs) in the presence of interfering substrates. Self-charging supercapacitive BFCs for in vivo and in vitro cellular microenvironments represent the most prevalent sensing mechanism for diagnosis. Therefore, self-powered biosensing (SPB) with a capacitor and contact separation with a triboelectric nanogenerator (TENG) offers electrochemical and colorimetric dual-mode detection via improved electrical signal intensity. In this review, we discuss three major components: stretchable self-powered BFC design, miRNA sensing, and impedance spectroscopy. A specific focus is given to 1) assembling of sensors for biomarkers, 2) electrical output signal intensification, and 3) role of supercapacitors and nanogenerators in SPBs. We outline the key features of stretchable SPBs and the sequence of miRNA sensing by SPBs. We have emphasized the need of a supercapacitor and nanogenerator for SPBs in the context of advanced assembly of the sensing unit. Finally, we outline the role of impedance spectroscopy in the detection and estimation of biomarkers. We highlight key challenges in SPBs for biomarker sensing, which needs improved sensing accuracy, integration strategies of electrochemical biosensing for in vitro and in vivo microenvironments, and the impact of miRNA sensing on cancer diagnostics. This article attempts a specific focus on the accuracy and limitations of sensing unit for miRNA biomarkers and associated tool for boosting electrical signal intensity for a potential big step further.

Direct Induction of Porous Graphene from Mechanically Strong and Waterproof Biopaper for On-Chip Multifunctional Flexible Electronics

Graphene with a 3D porous structure is directly laser-induced on lignocellulosic biopaper under ambient conditions and is further explored for multifunctional biomass-based flexible electronics. The mechanically strong, flexible, and waterproof biopaper is fabricated by surface-functionalizing cellulose with lignin-based epoxy acrylate (LBEA). This composite biopaper shows as high as a threefold increase in tensile strength and excellent waterproofing compared with pure cellulose one. Direct laser writing (DLW) rapidly induces porous graphene from the biopaper in a single step. The porous graphene shows an interconnected carbon network, well-defined graphene domains, and high electrical conductivity (e.g., ≈3 Ω per square), which can be tuned by lignin precursors and loadings as well as lasing conditions. The biopaper in situ embedded with porous graphene is facilely fabricated into flexible electronics for on-chip and paper-based applications. The biopaper-based electronic devices, including the all-solid-state planer supercapacitor, electrochemical and strain biosensors, and Joule heater, show great performances. This study demonstrates the facile, versatile, and low-cost fabrication of multifunctional graphene-based electronics from lignocellulose-based biopaper.

Solvent-Free and Skin-Like Supramolecular Ion-Conductive Elastomers with Versatile Processability for Multifunctional Ionic Tattoos and On-Skin Bioelectronics

The development of stable and biocompatible soft ionic conductors, alternatives to hydrogels and ionogels, will open up new avenues for the construction of stretchable electronics. Here, a brand-new design, encapsulating a naturally occurring ionizable compound by a biocompatible polymer via high-density hydrogen bonds, resulting in a solvent-free supramolecular ion-conductive elastomer (SF-supra-ICE) that eliminates the dehydration problem of hydrogels and possesses excellent biocompatibility, is reported. The SF-supra-ICE with high ionic conductivity (>3.3 × 10-2  S m-1 ) exhibits skin-like softness and strain-stiffening behaviors, excellent elasticity, breathability, and self-adhesiveness. Importantly, the SF-supra-ICE can be obtained by a simple water evaporation step to solidify the aqueous precursor into a solvent-free nature. Therefore, the aqueous precursor can act as inks to be painted and printed into customized ionic tattoos (I-tattoos) for the construction of multifunctional on-skin bioelectronics. The painted I-tattoos exhibit ultraconformal and seamless contact with human skin, enabling long-term and high-fidelity recording of various electrophysiological signals with extraordinary immunity to motion artifacts. Human-machine interactions are achieved by exploiting the painted I-tattoos to transmit the electrophysiological signals of human beings. Stretchable I-tattoo electrode arrays, manufactured by the printing method, are demonstrated for multichannel digital diagnosis of the health condition of human back muscles and spine.

Ti3 C2 TX MXene Ink Direct Writing Flexible Sensors for Disposable Paper Toys

Flexible electronics have gained great attention in recent years owing to their promising applications in biomedicine, sustainable energy, human-machine interaction, and toys for children. Paper mainly produced from cellulose fibers is attractive substrate for flexible electronics because it is biodegradable, foldable, tailorable, and light-weight. Inspired by daily handwriting, the rapid prototyping of sensing devices with arbitrary patterns can be achieved by directly drawing conductive inks on flat or curved paper surfaces; this provides huge freedom for children to design and integrate "do-it-yourself (DIY)" electronic toys. Herein, viscous and additive-free ink made from Ti3 C2 TX MXene sediment is employed to prepare disposable paper electronics through a simple ball pen drawing. The as-drawn paper sensors possess hierarchical microstructures with interweaving nanosheets, nanoflakes, and nanoparticles, therefore exhibiting superior mechanosensing performances to those based on single/fewer-layer MXene nanosheets. As proof-of-concept applications, several popular children's games are implemented by the MXene-based paper sensors, including "You say, I guess," "Emotional expression," "Rock-Paper-Scissors," "Arm wrestling," "Throwing game," "Carrot squat," and "Grab the cup," as well as a DIY smart whisker for a cartoon mouse. Moreover, MXene-based paper sensors are safe and disposable, free from producing any e-waste and hazard to the environment.

Wearable bioelectronics fabricated in situ on skins

In recent years, wearable bioelectronics has rapidly expanded for diagnosing, monitoring, and treating various pathological conditions from the skin surface. Although the devices are typically prefabricated as soft patches for general usage, there is a growing need for devices that are customized in situ to provide accurate data and precise treatment. In this perspective, the state-of-the-art in situ fabricated wearable bioelectronics are summarized, focusing primarily on Drawn-on-Skin (DoS) bioelectronics and other in situ fabrication methods. The advantages and limitations of these technologies are evaluated and potential future directions are suggested for the widespread adoption of these technologies in everyday life.

Superelastic Radiative Cooling Metafabric for Comfortable Epidermal Electrophysiological Monitoring

Epidermal electronics with superb passive-cooling capabilities are of great value for both daytime outdoor dressing comfort and low-carbon economy. Herein, a multifunctional and skin-attachable electronic is rationally developed on a porous all-elastomer metafabric for efficient passive daytime radiative cooling (PDRC) and human electrophysiological monitoring. The cooling characteristics are realized through the homogeneous impregnation of polytetrafluoroethylene microparticles in the styrene-ethylene-butylene-styrene fibers, and the rational regulation of microporosity in SEBS/PTFE metafabrics, thus synergistically backscatter ultraviolet-visible-near-infrared light (maximum reflectance over 98.0%) to minimize heat absorption while efficiently emit human-body midinfrared radiation to the sky. As a result, the developed PDRC metafabric achieves approximately 17 °C cooling effects in an outdoor daytime environment and completely retains its passive cooling performance even under 50% stretching. Further, high-fidelity electrophysiological monitoring capability is also implemented in the breathable and skin-conformal metafabric through liquid metal printing, enabling the accurate acquisition of human electrocardiograph, surface electromyogram, and electroencephalograph signals for comfortable and lengthy health regulation. Hence, the fabricated superelastic PDRC metafabric opens a new avenue for the development of body-comfortable electronics and low-carbon wearing technologies.

Advancement in Paper-Based Electrochemical Biosensing and Emerging Diagnostic Methods

The utilization of electrochemical detection techniques in paper-based analytical devices (PADs) has revolutionized point-of-care (POC) testing, enabling the precise and discerning measurement of a diverse array of (bio)chemical analytes. The application of electrochemical sensing and paper as a suitable substrate for point-of-care testing platforms has led to the emergence of electrochemical paper-based analytical devices (ePADs). The inherent advantages of these modified paper-based analytical devices have gained significant recognition in the POC field. In response, electrochemical biosensors assembled from paper-based materials have shown great promise for enhancing sensitivity and improving their range of use. In addition, paper-based platforms have numerous advantageous characteristics, including the self-sufficient conveyance of liquids, reduced resistance, minimal fabrication cost, and environmental friendliness. This study seeks to provide a concise summary of the present state and uses of ePADs with insightful commentary on their practicality in the field. Future developments in ePADs biosensors include developing novel paper-based systems, improving system performance with a novel biocatalyst, and combining the biosensor system with other cutting-edge tools such as machine learning and 3D printing.

Superhydrophobic, stretchable kirigami pencil-on-paper multifunctional device platform

Wearable electronics with applications in healthcare, human-machine interfaces, and robotics often explore complex manufacturing procedures and are not disposable. Although the use of conductive pencil patterns on cellulose paper provides inexpensive, disposable sensors, they have limited stretchability and are easily affected by variations in the ambient environment. This work presents the combination of pencil-on-paper with the hydrophobic fumed SiO2 (Hf-SiO2) coating and stretchable kirigami structures from laser cutting to prepare a superhydrophobic, stretchable pencil-on-paper multifunctional sensing platform. The resulting sensor exhibits a large response to NO2 gas at elevated temperature from self-heating, which is minimally affected by the variations in the ambient temperature and relative humidity, as well as mechanical deformations such as bending and stretching states. The integrated temperature sensor and electrodes with the sensing platform can accurately detect temperature and electrophysiological signals to alert for adverse thermal effects and cardiopulmonary diseases. The thermal therapy and electrical stimulation provided by the platform can also deliver effective means to battle against inflammation/infection and treat chronic wounds. The superhydrophobic pencil-onpaper multifunctional device platform provides a low-cost, disposable solution to disease diagnostic confirmation and early treatment for personal and population health.

Skin-interfaced electronics: A promising and intelligent paradigm for personalized healthcare

Skin-interfaced electronics (skintronics) have received considerable attention due to their thinness, skin-like mechanical softness, excellent conformability, and multifunctional integration. Current advancements in skintronics have enabled health monitoring and digital medicine. Particularly, skintronics offer a personalized platform for early-stage disease diagnosis and treatment. In this comprehensive review, we discuss (1) the state-of-the-art skintronic devices, (2) material selections and platform considerations of future skintronics toward intelligent healthcare, (3) device fabrication and system integrations of skintronics, (4) an overview of the skintronic platform for personalized healthcare applications, including biosensing as well as wound healing, sleep monitoring, the assessment of SARS-CoV-2, and the augmented reality-/virtual reality-enhanced human-machine interfaces, and (5) current challenges and future opportunities of skintronics and their potentials in clinical translation and commercialization. The field of skintronics will not only minimize physical and physiological mismatches with the skin but also shift the paradigm in intelligent and personalized healthcare and offer unprecedented promise to revolutionize conventional medical practices.

Carbon-Based Flexible Devices for Comprehensive Health Monitoring

Traditional public health systems suffer from incomprehensive, delayed, and inefficient medical services. Convenient and comprehensive health monitoring has been highly sought after recently. Flexible and wearable devices are attracting wide attention due to their potential applications in wearable human health monitoring and care systems. Using carbon materials with overall superiorities can facilitate the development of wearable and flexible devices with various functions and excellent performance, which can comprehensively and real-time monitor human health status and prevent diseases. Herein, the latest advances in the rational design and controlled fabrication of carbon materials for applications in health-related flexible and wearable electronics are reviewed. The fabrication strategies, working mechanism, performance, and applications in health monitoring of carbon-based flexible devices, including electromechanical sensors, temperature/humidity sensors, chemical sensors, and flexible conductive wires/electrodes, are reviewed. Furthermore, integrating multiple carbon-based devices into multifunctional wearable systems is discussed. Finally, the existing challenges and future opportunities in this field are also proposed.

Conductive Cellulose-Derived Carbon Nanofibrous Membranes with Superior Softness for High-Resolution Pressure Sensing and Electrophysiology Monitoring

Here, a strategy to overcome the stiff and brittle nature of cellulose-derived carbon nanofibrils (CCNFs) is proposed through a facile, low-cost, and scalable approach. Flexible and conformal CCNFs with a low bending rigidity below 55.4 mN and tunable conductivities of 0.14-45.5 S m^-1 are developed by introducing silanol as a multieffect additive in the electrospun hybrid nanofibrous network and subsequent carbonization at a relatively high temperature (900 °C) and chemical vapor deposition of polypyrrole (PPy) on the hybrid carbon nanofibril surface. Silica acts as a lubricant in each rigid carbon fiber to improve flexibility of the CCNF structure as well as a template during cellulose carbonization to prevent the melting of carbon nanofibrils. Meanwhile, the uniform coating of PPy leads to an improvement in electrical conductivity while conserving the porous structure and compressibility of the CCNF nets. These conductive hybrid CCNF films are evaluated as mechanoreceptors and physiological sensors, which are demonstrated to be applied in intelligent electronics including electronic skin, human-machine interfaces, and epidermic electrodes. The design or working principles of the hybrid CCNFs for achieving optimum applicable effects when applied in different scenarios are revealed.

Customizable, reconfigurable, and anatomically coordinated large-area, high-density electromyography from drawn-on-skin electrode arrays

Accurate anatomical matching for patient-specific electromyographic (EMG) mapping is crucial yet technically challenging in various medical disciplines. The fixed electrode construction of multielectrode arrays (MEAs) makes it nearly impossible to match an individual's unique muscle anatomy. This mismatch between the MEAs and target muscles leads to missing relevant muscle activity, highly redundant data, complicated electrode placement optimization, and inaccuracies in classification algorithms. Here, we present customizable and reconfigurable drawn-on-skin (DoS) MEAs as the first demonstration of high-density EMG mapping from in situ-fabricated electrodes with tunable configurations adapted to subject-specific muscle anatomy. The DoS MEAs show uniform electrical properties and can map EMG activity with high fidelity under skin deformation-induced motion, which stems from the unique and robust skin-electrode interface. They can be used to localize innervation zones (IZs), detect motor unit propagation, and capture EMG signals with consistent quality during large muscle movements. Reconfiguring the electrode arrangement of DoS MEAs to match and extend the coverage of the forearm flexors enables localization of the muscle activity and prevents missed information such as IZs. In addition, DoS MEAs customized to the specific anatomy of subjects produce highly informative data, leading to accurate finger gesture detection and prosthetic control compared with conventional technology.

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.

Recent Advances, Opportunities, and Challenges in Developing Nucleic Acid Integrated Wearable Biosensors for Expanding the Capabilities of Wearable Technologies in Health Monitoring

Wearable biosensors are becoming increasingly popular due to the rise in demand for non-invasive, real-time monitoring of health and personalized medicine. Traditionally, wearable biosensors have explored protein-based enzymatic and affinity-based detection strategies. However, in the past decade, with the success of nucleic acid-based point-of-care diagnostics, a paradigm shift has been observed in integrating nucleic acid-based assays into wearable sensors, offering better stability, enhanced analytical performance, and better clinical applicability. This narrative review builds upon the current state and advances in utilizing nucleic acid-based assays, including oligonucleotides, nucleic acid, aptamers, and CRISPR-Cas, in wearable biosensing. The review also discusses the three fundamental blocks, i.e., fabrication requirements, biomolecule integration, and transduction mechanism, for creating nucleic acid integrated wearable biosensors.

Wet-Adhesive On-Skin Sensors Based on Metal-Organic Frameworks for Wireless Monitoring of Metabolites in Sweat

Metal-organic frameworks (MOFs) with well-defined porous structures and tailored functionalities have been widely used in chemical sensing. However, the integration of MOFs with flexible electronic devices for wearable sensing is challenging because of their low electrical conductivity and fragile mechanical properties. Herein, a wearable sweat sensor for metabolite detection is presented by integrating an electrically conductive Ni-MOF with a flexible nanocellulose substrate. The MOF-based layered film sensor with inherent conductivity, highly porous structure, and active catalytic properties enables the selective and accurate detection of vitamin C and uric acid. More importantly, the lightweight sensor can conformably self-adhere to sweaty skin and exhibits high water-vapor permeability. Furthermore, a wireless epidermal nutrition tracking system for the in situ monitoring of the dynamics of sweat vitamin C is demonstrated, the results of which are comparable to those tested by high-performance liquid chromatography. This study opens a new avenue for integrating MOFs as the active layer in wearable electronic devices and holds promise for the future development of high-performance electronics with enhanced sensing, energy production, and catalytic capabilities through the implementation of multifunctional MOFs.

Natural gum-based electronic ink with water-proofing self-healing and easy-cleaning properties for directly on-skin electronics

On-skin electronic systems, which can facilitate noninvasive continuous acquisition of low-artifact physiological signals, are a promising technique for future wearable devices in healthcare. Inspired by the nature of Arabic gum (AG), we developed a costless, easy-to-prepare, easy-to-use, and environment-friendly electronic ink (E-ink) that can be used to construct multiform on-skin electronic systems through simple painting or stamping. In addition to its competitive electrical properties, the E-ink has the following advantages: waterproof (0.5 m/s water flushing for 10 s), self-healing (1.5 mm wide wound), and easy-cleaning (can be easily removed using cotton ball with 5% surfactant), making it environmentally tolerant and highly reliable for practical use. We demonstrated that our E-ink can act as electric wires for epidermal circuits, sensors to handle a variety of physiological data measurements. This research provides an effective strategy for direct integration of electronics and skin, which can accelerate the realization of the next generation of imperceptible, scalable, cost-effective and customized wearable devices.

Double-Sided Wearable Multifunctional Sensing System with Anti-interference Design for Human-Ambience Interface

Multifunctional sensing systems play important roles in a variety of applications, incluing health surveillance, intelligent prothetics, human-machine/ambinece interfaces, and many others. The richness of the signal and the decoupling among multiple parameters are essential for simultaneous, multimodal measurements. However, current multifunctional sensing fails to decouple interferences from various signals. Here, we propose a double-sided wearable system that both enables multifunctional sensing and avoids the interferences among multiple parameters. Specifically, the sensitivities of system modules to strain are controlled through customizing the pattern and morphology of sensing electrodes as well as the modification of active materials. Compensation of temperature drift and selection of sensing mechanisms ensure the thermal stability of the system. The encapsulation of modules resists the interferences of proximity, normal pressure, and gas molecules at the same time. A double-sided partition layout with serpentine interconnections reduces the effect of motion artifacts and ensures simultaneous operation of electrochemical-sensing modules. Cooperation among decoupled modules acts as the bridge between the perception of ambience changes and the timely feedback of the human body. In addition, to sense the signal at the interface, modules for energy harvesting and storage are also integrated into the system to broaden its application scenarios.

Drawn-on-Skin Sensors from Fully Biocompatible Inks toward High-Quality Electrophysiology

The need to develop wearable devices for personal health monitoring, diagnostics, and therapy has inspired the production of innovative on-demand, customizable technologies. Several of these technologies enable printing of raw electronic materials directly onto biological organs and tissues. However, few of them have been thoroughly investigated for biocompatibility of the raw materials on the cellular, tissue, and organ levels or with different cell types. In addition, highly accurate multiday in vivo monitoring using such on-demand, in situ fabricated devices has yet to be done. Presented herein is the first fully biocompatible, on-skin fabricated electronics for multiple cell types and tissues that can capture electrophysiological signals with high fidelity. While also demonstrating improved mechanical and electrical properties, the drawn-on-skin ink retains its properties under various writing conditions, which minimizes the variation in electrical performance. Furthermore, the drawn-on-skin ink shows excellent biocompatibility with cardiomyocytes, neurons, mice skin tissue, and human skin. The high signal-to-noise ratios of the electrophysiological signals recorded with the DoS sensor over multiple days demonstrate its potential for personalized, long-term, and accurate electrophysiological health monitoring.

A Focused Review on the Flexible Wearable Sensors for Sports: From Kinematics to Physiologies

As an important branch of wearable electronics, highly flexible and wearable sensors are gaining huge attention due to their emerging applications. In recent years, the participation of wearable devices in sports has revolutionized the way to capture the kinematical and physiological status of athletes. This review focuses on the rapid development of flexible and wearable sensor technologies for sports. We identify and discuss the indicators that reveal the performance and physical condition of players. The kinematical indicators are mentioned according to the relevant body parts, and the physiological indicators are classified into vital signs and metabolisms. Additionally, the available wearable devices and their significant applications in monitoring these kinematical and physiological parameters are described with emphasis. The potential challenges and prospects for the future developments of wearable sensors in sports are discussed comprehensively. This review paper will assist both athletic individuals and researchers to have a comprehensive glimpse of the wearable techniques applied in different sports.

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.

End-to-end design of wearable sensors

Wearable devices provide an alternative pathway to clinical diagnostics by exploiting various physical, chemical and biological sensors to mine physiological (biophysical and/or biochemical) information in real time (preferably, continuously) and in a non-invasive or minimally invasive manner. These sensors can be worn in the form of glasses, jewellery, face masks, wristwatches, fitness bands, tattoo-like devices, bandages or other patches, and textiles. Wearables such as smartwatches have already proved their capability for the early detection and monitoring of the progression and treatment of various diseases, such as COVID-19 and Parkinson disease, through biophysical signals. Next-generation wearable sensors that enable the multimodal and/or multiplexed measurement of physical parameters and biochemical markers in real time and continuously could be a transformative technology for diagnostics, allowing for high-resolution and time-resolved historical recording of the health status of an individual. In this Review, we examine the building blocks of such wearable sensors, including the substrate materials, sensing mechanisms, power modules and decision-making units, by reflecting on the recent developments in the materials, engineering and data science of these components. Finally, we synthesize current trends in the field to provide predictions for the future trajectory of wearable sensors.

Biodegradable Polymer Composites for Electrophysiological Signal Sensing

Electrophysiological signals are collected to characterize human health and applied in various fields, such as medicine, engineering, and pharmaceuticals. Studies of electrophysiological signals have focused on accurate signal acquisition, real-time monitoring, and signal interpretation. Furthermore, the development of electronic devices consisting of biodegradable and biocompatible materials has been attracting attention over the last decade. In this regard, this review presents a timely overview of electrophysiological signals collected with biodegradable polymer electrodes. Candidate polymers that can constitute biodegradable polymer electrodes are systemically classified by their essential properties for collecting electrophysiological signals. Moreover, electrophysiological signals, such as electrocardiograms, electromyograms, and electroencephalograms subdivided with human organs, are discussed. In addition, the evaluation of the biodegradability of various electrodes with an electrophysiology signal collection purpose is comprehensively revisited.

Surface Wettability for Skin-Interfaced Sensors and Devices

The practical applications of skin-interfaced sensors and devices in daily life hinge on the rational design of surface wettability to maintain device integrity and achieve improved sensing performance under complex hydrated conditions. Various bio-inspired strategies have been implemented to engineer desired surface wettability for varying hydrated conditions. Although the bodily fluids can negatively affect the device performance, they also provide a rich reservoir of health-relevant information and sustained energy for next-generation stretchable self-powered devices. As a result, the design and manipulation of the surface wettability are critical to effectively control the liquid behavior on the device surface for enhanced performance. The sensors and devices with engineered surface wettability can collect and analyze health biomarkers while being minimally affected by bodily fluids or ambient humid environments. The energy harvesters also benefit from surface wettability design to achieve enhanced performance for powering on-body electronics. In this review, we first summarize the commonly used approaches to tune the surface wettability for target applications toward stretchable self-powered devices. By considering the existing challenges, we also discuss the opportunities as a small fraction of potential future developments, which can lead to a new class of skin-interfaced devices for use in digital health and personalized medicine.

Laser-scribed conductive, photoactive transition metal oxide on soft elastomers for Janus on-skin electronics and soft actuators

Laser-assisted fabrication of conductive materials on flexible substrates has attracted intense interests because of its simplicity, easy customization, and broad applications. However, it remains challenging to achieve laser scribing of conductive materials on tissue-like soft elastomers, which can serve as the basis to construct bioelectronics and soft actuators. Here, we report laser scribing of metallic conductive, photoactive transition metal oxide (molybdenum dioxide) on soft elastomers, coated with molybdenum chloride precursors, under ambient conditions. Laser-scribed molybdenum dioxide (LSM) exhibits high electrical conductivity, biocompatibility, chemical stability, and compatibility with magnetic resonance imaging. In addition, LSM can be made on various substrates (polyimide, glass, and hair), showing high generality. Furthermore, LSM-based Janus on-skin electronics are developed to record information from human skin, human breath, and environments. Taking advantage of its outstanding photothermal effect, LSM-based soft actuators are developed to build light-driven reconfigurable three-dimensional architectures, reshapable airflow sensors, and smart robotic worms with bioelectronic sensors.

Moldable and Transferrable Conductive Nanocomposites for Epidermal Electronics

Skin-inspired soft and stretchable electronic devices based on functional nanomaterials have broad applications such as health monitoring, human-machine interface, and the Internet of things. Solution-processed conductive nanocomposites have shown great promise as a building block of soft and stretchable electronic devices. However, realizing conductive nanocomposites with high conductivity, electromechanical stability, and low modulus over a large area at sub-100 μm resolution remains challenging. Here, we report a moldable, transferrable, high-performance conductive nanocomposite comprised of an interpenetrating network of silver nanowires and poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate). The stacked structure of the nanocomposite synergistically integrates the complementary electrical and mechanical properties of the individual components. We patterned the nanocomposite via a simple, low-cost micromolding process and then transferred the patterned large-area electrodes onto various substrates to realize soft, skin-interfaced electrophysiological sensors. Electrophysiological signals measured using the nanocomposite electrodes exhibit a higher signal-to-noise ratio than standard gel electrodes. The nanocomposite design and fabrication approach presented here can be broadly employed for soft and stretchable electronic devices.