Polymer-Assisted Metal Deposition (PAMD) for Flexible and Wearable Electronics: Principle, Materials, Printing, and Devices

Durable Metallized Liquid Crystal Polymer Fibers Enable Flexible and Tough Electrical Heaters

Fiber-shaped electrical heaters with high flexibility and excellent adaptability make an ideal candidate for the application of wearable electronics but still suffer from low strength and poor durability. Herein, an all-in-one Joule-heating fiber capable of outstanding mechanical properties, good heating efficiency, and long-term stability is reported by using polymer-assisted metal deposition to firmly coat Cu nanoparticles on high-performance liquid crystal polymer (LCP) fibers. Taking advantage of LCP, the resultant fibers exhibit a satisfying temperature threshold (up to 200 °C) and immense strength (2.94 GPa). By virtue of dense and continuous Cu film, these fibers show low electrical resistance (5.51 Ω/cm) and an ultrafast response rate (12.6 °C·s-1) at low supplied voltages (0.5-3.5 V). Benefiting from the levodopa/polyethyleneimine interface design, such fibers maintain nearly constant resistance after repeatable bending, folding, and even washing (50 cycles). Based on the above-mentioned merits, a wearable patch with a Joule-heating function is knitted by using as-made fibers to offer therapeutic benefits for human body joints. This work demonstrates prospective potential for enriching the challenging applications of fiber-shaped electrical heating systems.

GO-Enhanced MXene Sediment-Based Inks Achieve Remarkable Oxidation Resistance and High Conductivity

MXenes are emerging materials renowned for their exceptional conductivity, abundant functional groups, and excellent solution processability, making them highly promising as conductive-additive-free inks for flexible electronic devices. However, current preparation methods are hampered by low yields of MXene flakes so that substantial waste MXene sediments (MS) are generated. Here, we demonstrate a type of conductive ink with appropriate rheological properties, namely MG inks formulated using MS and graphene oxide (GO), for screen-printing frequency selective surface (FSS). GO facilitates interlayer interactions by covalently cross-linking with MXene flakes, resulting in a denser structure and significantly enhancing the conductivity of the best-performing MG-based ink to 849 S cm-1. Additionally, GO serves as a binder to considerably improve the rheological properties of MS, thus enabling high-quality printing on various substrates. The close stacking of MS and GO not only improves the oxidation resistance but also maintains conductivity above 97% even after 60 days. Furthermore, the MG-based FSS produced via straightforward screen printing demonstrates excellent performance and retains its functionality after 90 days of operation. This MS-based ink formulation represents a strategy of "turning trash into treasure" and highlights the potential of MS for the next generation of electronic devices.

A Highly Impact-Tolerant Textile-Based Lithium-Ion Battery

Textile-based lithium-ion batteries (LIBs) are in great demand to power wearable electronics. They currently face a key safety challenge, particularly concerning mechanical abuse that could trigger thermal runaway, causing harm to individuals. Here, we report on Kevlar-fabric-based LIBs that can afford high impact tolerance while offering excellent electrochemical performance comparable to metal-foil-based cells. The integration of Kevlar electrodes, known for their protective nature, with impact-tolerant shear thickening electrolytes (STEs) effectively dissipates the impact energy. It can be ascribed to the shear thickening effect and the induced yarn-to-yarn friction within Kevlar fabrics. This design mirrors the configuration of liquid body armor that consists of shear thickening fluid and Kevlar fabric. This work provides an alternative approach for developing highly impact-tolerant LIBs.

Preparation and Applications of Multifunctional MXene/Tussah Silk Fabric

The development of functional textiles has become a key focus in recent years, aiming to meet the diverse requirements of modern society. MXene has excellent conductivity, hydrophilicity, and UV resistance, and is widely used in electromagnetic shielding, sensors, energy storage, and photothermal conversion. Tussah silk (TS) is a unique natural textile raw material and has a unique jewelry luster, natural luxury, and a smooth and comfortable feel. However, there are relatively few studies on the functional finishing of TS fabric with Ti3C2Tx MXene. Here, we developed a multifunctional MXene/tussah silk (MXene/TS) fabric by the deposition of Ti3C2Tx MXene sheets on the surface of TS fabric through a simple padding-drying-curing process. The obtained MXene/TS fabric (five cycles) exhibited excellent conductivity (4.8 S/m), air permeability (313.6 mm/s), ultraviolet resistance (ultraviolet protection factor, UPF = 186.3), photothermal conversion (temperature increase of 11 °C), and strain sensing. Thanks to these superior properties, the MXene/TS fabric has broad application prospects in motion monitoring, smart clothing, flexible wearables, and artificial intelligence.

Bipolar Textile Composite Electrodes Enabling Flexible Tandem Solid-State Lithium Metal Batteries

A majority of flexible and wearable electronics require high operational voltage that is conventionally achieved by serial connection of battery unit cells using external wires. However, this inevitably decreases the energy density of the battery module and may cause additional safety hazards. Herein, a bipolar textile composite electrode (BTCE) that enables internal tandem-stacking configuration to yield high-voltage (6 to 12 V class) solid-state lithium metal batteries (SSLMBs) is reported. BTCE is comprised of a nickel-coated poly(ethylene terephthalate) fabric (NiPET) core layer, a cathode coated on one side of the NiPET, and a Li metal anode coated on the other side of the NiPET. Stacking BTCEs with solid-state electrolytes alternatively leads to the extension of output voltage and decreased usage of inert package materials, which in turn significantly boosts the energy density of the battery. More importantly, the BTCE-based SSLMB possesses remarkable capacity retention per cycle of over 99.98% over cycling. The composite structure of BTCE also enables outstanding flexibility; the battery keeps stable charge/discharge characteristics over thousands of bending and folding. BTCE shows great promise for future safe, high-energy-density, and flexible SSLMBs for a wide range of flexible and wearable electronics.

Zebra-Patterned Stretchable Helical Yarn for Triboelectric Self-Powered Multifunctional Sensing

Smart textiles capable of both energy harvesting and multifunctional sensing are highly desirable for next-generation portable electronics. However, there are still challenges that need to be conquered, such as the innovation of an energy-harvesting model and the optimization of interface bonding between fibers and active materials. Herein, inspired by the spiral structure of natural vines, a highly stretchable triboelectric helical yarn (TEHY) was manufactured by twisting the carbon nanotube/polyurethane nanofiber (CNT/PU NF) Janus membrane. The TEHY had a zebra-stripe-like design that was composed of black interval conductive CNTs and white insulative PU NFs. Due to the different electron affinity, the zebra-patterned TEHY realized a self-frictional triboelectric effect because the numerous microscopic CNT/PU triboelectric interfaces generated an alternating current in the external conductive circuit without extra external friction layers. The helical geometry combined with the elastic PU matrix endowed TEHY with superelastic stretchability and outstanding output stability after 1000 cycles of the stretch-release test. By virtue of the robust mechanical and electrical stability, the TEHY can not only be used as a high-entropy mechanical energy harvester but also serve as a self-powered sensor to monitor the stretching or deforming stimuli and human physiological activities in real time. These merits manifested the versatile applications of TEHY in smart fabrics, wearable power supplies, and human-machine interactions.

Recent Research on Preparation and Application of Smart Joule Heating Fabrics

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

Porous Conductive Textiles for Wearable Electronics

Over the years, researchers have made significant strides in the development of novel flexible/stretchable and conductive materials, enabling the creation of cutting-edge electronic devices for wearable applications. Among these, porous conductive textiles (PCTs) have emerged as an ideal material platform for wearable electronics, owing to their light weight, flexibility, permeability, and wearing comfort. This Review aims to present a comprehensive overview of the progress and state of the art of utilizing PCTs for the design and fabrication of a wide variety of wearable electronic devices and their integrated wearable systems. To begin with, we elucidate how PCTs revolutionize the form factors of wearable electronics. We then discuss the preparation strategies of PCTs, in terms of the raw materials, fabrication processes, and key properties. Afterward, we provide detailed illustrations of how PCTs are used as basic building blocks to design and fabricate a wide variety of intrinsically flexible or stretchable devices, including sensors, actuators, therapeutic devices, energy-harvesting and storage devices, and displays. We further describe the techniques and strategies for wearable electronic systems either by hybridizing conventional off-the-shelf rigid electronic components with PCTs or by integrating multiple fibrous devices made of PCTs. Subsequently, we highlight some important wearable application scenarios in healthcare, sports and training, converging technologies, and professional specialists. At the end of the Review, we discuss the challenges and perspectives on future research directions and give overall conclusions. As the demand for more personalized and interconnected devices continues to grow, PCT-based wearables hold immense potential to redefine the landscape of wearable technology and reshape the way we live, work, and play.

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.

Biomimetic Ion Channel Regulation for Temperature-Pressure Decoupled Tactile Perception

The perception of temperature and pressure of skin plays a vital role in joint movement, hand grasp, emotional expression, and self-protection of human. Among many biomimetic materials, ionic gels are uniquely suited to simulate the function of skin due to its ionic transport mechanism. However, both the temperature and pressure sensing are heavily dependent on the changes in ionic conductivity, making it impossible to decouple the temperature and pressure signals. Here, a pressure-insensitive and temperature-modulated ion channel is designed by synergistic strategies for gel skeleton's compact packing and ultra-thin structure, mimicking the function of the temperature ion channel in human skin. This ion-confined gel can completely suppress the pressure response of the temperature sensing layer. Furthermore, a temperature-pressure decoupled ionic sensor is fabricated and it is demonstrated that the ionic sensor can sense complex signals of temperature and pressure. This novel and effective approach has great potential to overcome one of the current barriers in developing ionic skin and extending its applications.

A monolithically integrated in-textile wristband for wireless epidermal biosensing

Textile bioelectronics that allow comfortable epidermal contact hold great promise in noninvasive biosensing. However, their applications are limited mainly because of the large intrinsic electrical resistance and low compatibility for electronics integration. We report an integrated wristband that consists of multifunctional modules in a single piece of textile to realize wireless epidermal biosensing. The in-textile metallic patterning and reliable interconnect encapsulation contribute to the excellent electrical conductivity, mechanical robustness, and waterproofness that are competitive with conventional flexible devices. Moreover, the well-maintained porous textile architectures deliver air permeability of 79 mm s-1 and moisture permeability of 270 g m-2 day-1, which are more than one order of magnitude higher than medical tapes, thus ensuring superior wearing comfort. The integrated in-textile wristband performed continuous sweat potassium monitoring in the range of 0.3 to 40 mM with long-term stability, demonstrating its great potential for wearable fitness monitoring and point-of-care testing.

A Review of Structure Engineering of Strain-Tolerant Architectures for Stretchable Electronics

Stretchable electronics possess significant advantages over their conventional rigid counterparts and boost game-changing applications such as bioelectronics, flexible displays, wearable health monitors, etc. It is, nevertheless, a formidable task to impart stretchability to brittle electronic materials such as silicon. This review provides a concise but critical discussion of the prevailing structural engineering strategies for achieving strain-tolerant electronic devices. Not only the more commonly discussed lateral designs of structures such as island-bridge, wavy structures, fractals, and kirigami, but also the less discussed vertical architectures such as strain isolation and elastoplastic principle are reviewed. Future opportunities are envisaged at the end of the paper.

Boosting Sensitivity and Durability of Pressure Sensors Based on Compressible Cu Sponges by Strengthening Adhesion of "Rigid-Soft" Interfaces

The interface adhesion plays a key role between rigid metal and elastomer in compressible and stretchable conductors. However, the poor interfacial adhesion hinders their wide applications. To strengthen the interface adhesion, herein, a combination strategy of structure interlocking and polymer bridging is designed by introducing a method of subsurface-initiated atom transfer radical polymerization (sSI-ATRP). This method can make polymer brush root in polydimethylsiloxane (PDMS) subsurface, on this basis, metals further grow from subsurface to surface of PDMS via electroless deposition. As a result, the adhesive strength (≈2.5 MPa) between metal layer and PDMS elastomer is 4 times higher than that made by common polymer modification. As a demonstration, pressure sensor is constructed by using as-prepared compressible 3D Cu sponge as a top electrode and paper-based interdigited metal electrode as a bottom electrode. The device sensitivity can reach up to 961.2 kPa-1 and the durability can arrive at 3 000 cycles without degradation. Thus, this proposed interface-enhancement strategy for rigid-soft materials can significantly promote the performance of piezoresistive pressure sensors based on 3D conductive sponge. In the future, it would also be expanded to the fabrication of stretchable conductors and extensively applied in other flexible and wearable electronics.

E-Textile by Printing an All-through Penetrating Copper Complex Ink

Wearable electronics is an emerging field in academics and industry, in which electronic devices, such as smartwatches and sensors, are printed or embedded within textiles. The electrical circuits in electronics textile (e-textile) should withstand many cycles of bending and stretching. Direct printing of conductive inks enables the patterning of electrical circuits; however, while using conventional nanoparticle-based inks, printing onto the fabric results in a thin layer of a conductor, which is not sufficiently robust and impairs the reliability required for practical applications. Here, we present a new process for fabricating robust stretchable e-textile using a thermodynamically stable, solution-based copper complex ink, which is capable of full penetrating the fabric. After printing on knitted stretchable fabrics, they were heated, and the complex underwent an intermolecular self-reduction reaction. The continuously formed metallic copper was used as a seed layer for electroless plating (EP) to form highly conductive circuits. It was found that the stretching direction has a significant role in resistivity. This new approach enables fabricating e-textiles with high stretchability and durability, as demonstrated for wearable gloves, toward printing functional e-textile.

Metallic Glass-Fiber Fabrics: A New Type of Flexible, Super-Lightweight, and 3D Current Collector for Lithium Batteries

Current collectors are indispensable parts that provide electron transport and mechanical support of electrode materials in a battery. Nowadays, thin metal foils made of Cu and Al are used as current collectors of lithium batteries, but they do not contribute to the storage capacity. Therefore, decreasing the weight of current collectors can directly enhance the energy density of a battery. However, limited by the requirements of mechanical strength, it is difficult to reduce the weight of metal foils any further. Herein, a new type of current collectors made of 3D metallic glass-fiber fabrics (MGFs), which shows advantages of super-lightweight (2.9-3.2 mg cm⁻² ), outstanding electrochemical stability for cathodes and anodes of lithium-ion and lithium-metal batteries (LMBs), fire resistance, high strength, and flexibility suitable for roll-to-roll electrode fabrication is reported. The gravimetric energy densities of lithium batteries exhibit improvements of 9-18% by only replacing the metal foils with the MGFs. In addition, MGFs are suitable for the fabrication of flexible batteries. A high-energy-density flexible lithium battery with an outstanding figure of merit of flexible battery (fb_FOM ) and flexing stability is demonstrated.

Technology Roadmap for Flexible Sensors

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

Elasto-Plastic Design of Ultrathin Interlayer for Enhancing Strain Tolerance of Flexible Electronics

The ability to tolerate large strains during various degrees of deformation is a core issue in the development of flexible electronics. Commonly used strategies nowadays to enhance the strain tolerance of thin film devices focus on the optimization of the device architecture and the increase of bonding at the materials interface. In this paper, we propose a strategy, namely elasto-plastic design of an ultrathin interlayer, to boost the strain tolerance of flexible electronics. We demonstrate that insertion of an ultrathin, stiff (high Young's modulus) and elastic (high yield strain) interlayer between an upper rigid film/device and a soft substrate, regardless of the substrate thickness or the interfacial bonding, can significantly reduce the actual strain applied on the film/device when the substrate is bent. Being independent of existing strategies, the elasto-plastic design strategy offers an effective method to enhance the device flexibility without redesigning the device structure or altering the material interface.

Destructive-Treatment-Free Rapid Polymer-Assisted Metal Deposition for Versatile Electronic Textiles

Highly conductive, durable, and breathable metal-coated textiles are critical building block materials for future wearable electronics. In order to enhance the metal adhesion on the textile surface, existing solution-based approaches to preparing these materials require time-consuming presynthesis and/or premodification processes, typically in the order of tens of minutes to hours, on textiles prior to metal plating. Herein, we report a UV-induced rapid polymer-assisted metal deposition (r-PAMD) that offers a destructive-treatment-free process to deposit highly conductive metals on a wide variety of textile materials, including cotton, polyester, nylon, Kevlar, glass fiber, and carbon cloth. In comparison to the state of the arts, r-PAMD significantly shortens the modification time to several minutes and is compatible with the roll-to-roll fabrication manner. Moreover, the deposited metals show outstanding adhesion, which withstands rigorous flexing, abrasion, and machine washing tests. We demonstrate that these metal-coated textiles are suitable for applications in two vastly different fields, being wearable and washable sensors, and lithium batteries.

Room-Temperature Annealing-Free Gold Printing via Anion-Assisted Photochemical Deposition

Metal patterning via additive manufacturing has been phasing-in to broad applications in many medical, electronics, aerospace, and automotive industries. While previous efforts have produced various promising metal-patterning strategies, their complexity and high cost have limited their practical application in rapid production and prototyping. Herein, a one-step gold printing technique based on anion-assisted photochemical deposition (APD), which can directly print highly conductive gold patterns (1.08 × 107 S m-1 ) under ambient conditions without post-annealing treatment, is introduced. Uniquely, the APD uses specific ion effects with projection lithography to pattern Au nanoparticles and simultaneously sinter them into tunable porous gold structures. The significant influence of kosmotropic or chaotropic anions in the precursor ink on tuning the morphologies and conductivities of the printed patterns by employing a series of different ions, including Cl- ions, in the printing process is presented. Additionally, the resistance stabilities and the electrochemical properties of the APD-printed gold patterns are carefully investigated. The high conductivity and excellent conformability of the printed Au electrodes are demonstrated with reliable performance in electrophysiological signal delivery and acquisition for biomedical applications. This work exploits the potential of photochemical-deposition-based metal patterning in flexible electronic manufacturing.

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.

Natural Polymer in Soft Electronics: Opportunities, Challenges, and Future Prospects

Pollution caused by nondegradable plastics has been a serious threat to environmental sustainability. Natural polymers, which can degrade in nature, provide opportunities to replace petroleum-based polymers, meanwhile driving technological advances and sustainable practices. In the research field of soft electronics, regenerated natural polymers are promising building blocks for passive dielectric substrates, active dielectric layers, and matrices in soft conductors. Here, the natural-polymer polymorphs and their compatibilization with a variety of inorganic/organic conductors through interfacial bonding/intermixing and surface functionalization for applications in various device modalities are delineated. Challenges that impede the broad utilization of natural polymers in soft electronics, including limited durability, compromises between conductivity and deformability, and limited exploration in controllable degradation, etc. are explicitly inspected, while the potential solutions along with future prospects are also proposed. Finally, integrative considerations on material properties, device functionalities, and environmental impact are addressed to warrant natural polymers as credible alternatives to synthetic ones, and provide viable options for sustainable soft electronics.

A Repeatable Self-Adhesive Liquid-Free Double-Network Ionic Conductor with Tunable Multifunctionality

Liquid-free ionic conductors (LFICs) have promising applications in flexible electronics because most ionic conductors currently suffer from ionic liquid leakage or water evaporation issues. However, it has been a formidable challenge for LFICs to achieve long-term repeated self-adhesion on different substrates, especially on soft biological tissues. Based on the double-network design concept, we first fabricate a series of repeatable self-adhesive liquid-free double-network ionic conductors (SALFDNICs), consisting of stretchable first poly(AA-ChCl)-type supramolecular deep eutectic polymer networks and stiff second polydopamine (PDA) networks, which can maintain sufficient dynamic hydrogen bonds and catechol groups in the ionic conductors by preventing the overoxidation of dopamine, thus balancing the contradiction between adhesion and cohesion in liquid-free ionic conductors. Therefore, SALFDNICs can instantly form various interface interaction forces with multiple substrates (adhesion strength up to 757 N/m) and firmly adhere to various substrates for 20 detachment-reattachment cycles with a reduction in adhesion strength of less than 15%. Furthermore, SALFDNICs also have other comprehensive properties, such as optimum self-healing properties (self-healing efficiency of 90%), good stretchability (strain at break of 1200%), and promising conductivity (2.31 × 10^-2 S m^-1). Therefore, we believe that the extraordinary performance of SALFDNICs is important for improving device integration and the further development of flexible electronics.

Prospects and Challenges of Flexible Stretchable Electrodes for Electronics

The application of flexible electronics in the field of communication has made the transition from rigid physical form to flexible physical form. Flexible electrode technology is the key to the wide application of flexible electronics. However, flexible electrodes will break when large deformation occurs, failing flexible electronics. It restricts the further development of flexible electronic technology. Flexible stretchable electrodes are a hot research topic to solve the problem that flexible electrodes cannot withstand large deformation. Flexible stretchable electrode materials have excellent electrical conductivity, while retaining excellent mechanical properties in case of large deformation. This paper summarizes the research results of flexible stretchable electrodes from three aspects: material, process, and structure, as well as the prospects for future development.

Flexible Metal Electrodes by Femtosecond Laser-Activated Deposition for Human-Machine Interfaces

Flexible metal electrodes are essential for flexible electronics, where the main challenge is to obtain mask-free patterned metals directly on substrates such as poly(dimethylsiloxane) (PDMS) at low cost. This work highlights a feasible strategy named femtosecond laser-activated metal deposition for electroless deposition of metals (Cu, Ni, Ag, and Au) on PDMS, which is suitable for maskless and low-cost fabrication of metal layers on PDMS and even on other materials of different natures including polyethylene terephthalate, paper, Si, and glass. The electrical conductivity of the PDMS/Cu electrode is comparable to that of bulk Cu. Moreover, robust bonding at the PDMS/Cu interface is evidenced by a scotch tape test and bending test of more than 20,000 cycles. Compared with previous studies using a nanosecond laser, the restriction on absorbing sensitizers could be alleviated, and catalysts could originate from precursors without polymer substrates under a femtosecond laser, which may be attributed to nonlinear absorption and ultrashort heating time with the femtosecond laser. Implementing a human-machine interface task is demonstrated by recognizing hand gestures via a multichannel electrode array with high fidelity to control a robot hand.

Functionalized Fiber-Based Strain Sensors: Pathway to Next-Generation Wearable Electronics

Wearable strain sensors are arousing increasing research interests in recent years on account of their potentials in motion detection, personal and public healthcare, future entertainment, man-machine interaction, artificial intelligence, and so forth. Much research has focused on fiber-based sensors due to the appealing performance of fibers, including processing flexibility, wearing comfortability, outstanding lifetime and serviceability, low-cost and large-scale capacity. Herein, we review the latest advances in functionalization and device fabrication of fiber materials toward applications in fiber-based wearable strain sensors. We describe the approaches for preparing conductive fibers such as spinning, surface modification, and structural transformation. We also introduce the fabrication and sensing mechanisms of state-of-the-art sensors and analyze their merits and demerits. The applications toward motion detection, healthcare, man-machine interaction, future entertainment, and multifunctional sensing are summarized with typical examples. We finally critically analyze tough challenges and future remarks of fiber-based strain sensors, aiming to implement them in real applications.

Stretchable Conductive Fabric Enabled By Surface Functionalization of Commercial Knitted Cloth

Textile-based stretchable electronic devices are one of the best candidates for future wearable applications, as they can simultaneously provide high compliance and wearing comfort to the human body. Stretchable conductive textile is the fundamental building block for constructing high-performance textile-based stretchable electronic devices. Here, we report a simple strategy for the fabrication of stretchable conductive fabric using commercial knitted cloth as a substrate. Briefly, we coated the fibers of the fabric with a thin layer of poly(styrene-block-butadiene-block-styrene) (SBS) by dip-coating. Then, silver nanoparticles (AgNPs) were loaded on the fabric by sequential absorption and in situ reduction. After loading AgNPs, the conductivity of the fabric could be as high as ∼800 S/m, while its maximal strain at break was higher than 540%. Meanwhile, such fabric also possesses excellent permeability, robust endurance to repeated stretching, long-time washing, and mechanical rubbing or tearing. We further approve that the fabric is less cytotoxic to mammalian skin and antibacterial to microbial, making it safe for on-skin applications. With these multifarious advantages, the fabric developed here is promising for on-skin wearable applications. As a proof-of-concept, we demonstrate its use as an electrode for collecting electrocardiograph signals and electrothermal therapy.

Crack Suppression in Conductive Film by Amyloid-Like Protein Aggregation toward Flexible Device

A fatal weakness in flexible electronics is the mechanical fracture that occurs during repetitive fatigue deformation; thus, controlling the crack development of the conductive layer is of prime importance and has remained a great challenge until now. Herein, this issue is tackled by utilizing an amyloid/polysaccharide molecular composite as an interfacial binder. Sodium alginate (SA) can take part in amyloid-like aggregation of the lysozyme, leading to the facile synthesis of a 2D protein/saccharide hybrid nanofilm over an ultralarge area (e.g., >400 cm2 ). The introduction of SA into amyloid-like aggregates significantly enhances the mechanical strength of the hybrid nanofilm, which, with the help of amyloid-mediated interfacial adhesion, effectively diminishes the microcracks in the hybrid nanofilm coating after repetitive bending or stretching. The microcrack-free hybrid nanofilm then shows high interfacial activity to induce electroless deposition of metal in a Kelvin model on a substrate, which noticeably suppresses the formation of microcracks and consequent conductivity loss during the bending and stretching of the metal-coated flexible substrates. This work underlines the significance of amyloid/polysaccharide nanocomposites in the design of interfacial binders for reliable flexible electronic devices and represents an important contribution to mimicking amyloid and polysaccharide-based adhesive cements created by organisms.

Flexible Conducting Composite Film with Reversible In-Plane Folding-Unfolding Property

Flexible conducting films in the forms of bendability or stretchability are developed as a key component to enable soft electronics. With the requirements of miniaturization and portability of modern electronics, conducting film that can endure in-plane shrinkage is urgently needed but still remains challenging. Here, a new type of conducting film achieving reversible in-plane folding-unfolding function with large deformation by infusing conductive liquids into hierarchically structured polymer films consisting of both nanostructured polymer nanofibers and microstructured frames is reported. Nanostructured polymer nanofibers that can be completely wetted by the conductive liquids provide capillary forces to gain reversible in-plane folding-unfolding property, while the microstructured frames greatly enhance the extent during folding-unfolding process. Conductivity of the produced films can be significantly improved by rationally tuning the composition of infused conductive liquids, which always keeps high values during the folding-unfolding deformation. It is believed that this work may serve as the basis for robust fabrication of flexible conducting films with reversible in-plane folding-unfolding function, and can also put one-step forward of modern soft electronics.

Polymer-Assisted Metallization of Mammalian Cells

Developing biotemplating techniques to translate microorganisms and cultured mammalian cells into metallic biocomposites is of great interest for biosensors, electronics, and energy. The metallization of viruses and microbial cells is successfully demonstrated via a genetic engineering strategy or electroless deposition. However, it is difficult to transform mammalian cells into metallic biocomposites because of the complicated genes and the delicate morphological features. Herein, "polymer-assisted cell metallization" (PACM) is reported as a general method for the transformation of mammalian cells into metallic biocomposites. PACM includes a first step of in situ polymerization of functional polymer on the surface and in the interior of the mammalian cells, and a subsequent electroless deposition of metal to convert the polymer-functionalized cells into metallic biocomposites, which retain the micro- and nanostructures of the mammalian cells. This new biotemplating method is compatible with different cell types and metals to yield a wide variety of metallic biocomposites with controlled structures and properties.

Printable elastomeric electrodes with sweat-enhanced conductivity for wearables

We rationally synthesized the thermoplastic and hydrophilic poly(urethane-acrylate) (HPUA) binder for a type of printable and stretchable Ag flakes-HPUA (Ag-HPUA) electrodes in which the conductivity can be enhanced by human sweat. In the presence of human sweat, the synergistic effect of Cl- and lactic acid enables the partial removal of insulating surfactant on silver flakes and facilitates sintering of the exposed silver flakes, thus the resistance of Ag-HPUA electrodes can be notably reduced in both relaxed and stretched state. The on-body data show that the resistance of one electrode has been decreased from 3.02 to 0.62 ohm during the subject's 27-min sweating activity. A stretchable textile sweat-activated battery using Ag-HPUA electrodes as current collectors and human sweat as the electrolyte was constructed for wearable electronics. The enhanced conductivity of the wearable wiring electrode from the reaction with sweat would provide meritorious insight into the design of wearable devices.

Rationally designed hierarchical C/TiO2/Ti multilayer core-sheath wires for high-performance energy storage devices

Fiber-shaped supercapacitors (FSCs) are promising power sources for wearable electronic devices due to their small size, excellent flexibility and deformability. The performance of FSCs has been severely affected by the framework of the fibrous electrodes and the interface between the electrode materials and current collector. Herein, we propose an ingenious strategy that combines anodizing etching and CVD methods to transform the less-active titanium wires into unique hierarchical carbon/TiO₂ nanotube/Ti (CTNT) core-sheath wires, which have high conductivity, good mechanical strength and porous structure on the surface. CTNT wires can be used not only as a high-performance electrode, but also as an ideal substrate for depositing active materials. We have demonstrated the deposition of MnO₂ and MoS₂ on the surface of CTNT to prepare MnO₂@CTNT and MoS₂@CTNT core-sheath composite wires through electrochemical deposition and hydrothermal reaction, respectively. The specific areal capacitance of a single wire (MoS₂@CTNT) can reach up to 557.83 mF cm^-2 in a three-electrode system. Two such wires were further used as electrodes for making an all-solid-state asymmetric fiber-shaped supercapacitor (AFSC). The prepared AFSC has a wide voltage window of 2.7 V, a large areal capacitance of 121.42 mF cm^-2 and an excellent energy density of 74.37 μW h cm^-2. It also shows good rate performance and stability, and even after 10 000 cycles of charging and discharging, a capacitance retention rate of 76.5% can be achieved.

Multifunctional and Ultrasensitive-Reduced Graphene Oxide and Pen Ink/Polyvinyl Alcohol-Decorated Modal/Spandex Fabric for High-Performance Wearable Sensors

Sensitive and flexible sensors capable of monitoring physiological signals of human body for healthcare have been developed in recent years. It is still a challenge to fabricate a wearable sensor-integrated multifunctional performances and a good fit to human body. Here, an rGO and pen ink/PVA-layered strain-humidity sensor based on MS fabric is prepared through a cost-effective and scalable solution process. The conductive fabric as a strain sensor has a workable strain range (∼300%), ultrahigh sensitivity (maximum gauge factor of 492.8), great comfort, and long-term stability. Notably, a step increase in relative resistance variation will be achieved by controlling the coverage of an ink layer. Moreover, the reliable linear humidity-dependent resistance void of hysteresis and excellent repeatability renders conductive fabrics an opportunity as humidity sensors. Based on these superior multifunctions, the resultant conductive fabric can be applied to detect both human motions and skin humidity, showing potential in applications of wearable electronics for professional athletes.

Ultraelastic Yarns from Curcumin-Assisted ELD toward Wearable Human-Machine Interface Textiles

Intelligent human-machine interfaces (HMIs) integrated wearable electronics are essential to promote the Internet of Things (IoT). Herein, a curcumin-assisted electroless deposition technology is developed for the first time to achieve stretchable strain sensing yarns (SSSYs) with high conductivity (0.2 Ω cm-1) and ultralight weight (1.5 mg cm-1). The isotropically deposited structural yarns can bear high uniaxial elongation (>>1100%) and still retain low resistivity after 5000 continuous stretching-releasing cycles under 50% strain. Apart from the high flexibility enabled by helical loaded structure, a precise strain sensing function can be facilitated under external forces with metal-coated conductive layers. Based on the mechanics analysis, the strain sensing responses are scaled with the dependences on structural variables and show good agreements with the experimental results. The application of interfacial enhanced yarns as wearable logic HMIs to remotely control the robotic hand and manipulate the color switching of light on the basis of gesture recognition is demonstrated. It is hoped that the SSSYs strategy can shed an extra light in future HMIs development and incoming IoT and artificial intelligence technologies.

Textile-Based Capacitive Sensor for Physical Rehabilitation via Surface Topological Modification

Wearable sensor technologies, especially continuous monitoring of various human health conditions, are attracting increased attention. However, current rigid sensors present obvious drawbacks, like lower durability and poor comfort. Here, a strategy is proposed to efficiently yield wearable sensors using cotton fabric as an essential component, and conductive materials conformally coat onto the cotton fibers, leading to a highly electrically conductive interconnecting network. To improve the conductivity and durability of conductive coatings, a topographical modification approach is developed with genus-3 and genus-5 structures, and topological genus structures enable cage metallic seeds on the surface of substrates. A textile-based capacitive sensor with flexible, comfortable, and durable properties has been demonstrated. High sensitivity and convenience of signal collection have been achieved by the excellent electrical conductivity of this sensor. Based on results of deep investigation on capacitance, effects of distance and angles between two conductive fabrics contribute to the capacitive sensitivity. In addition, the textile-based capacitive sensor has successfully been used for real-time monitoring human breathing, speaking, blinking, and joint motions during physical rehabilitation exercises.

Interfacial nanoconnections and enhanced mechanistic studies of metallic coatings for molecular gluing on polymer surfaces

Interfacial adhesion has been identified as being key for realizing flexible devices. Here, strong interfacial nanoconnections involving metallic patterns on polymer surfaces were fabricated via a molecular bonding approach, which includes UV-assisted grafting and molecular self-assembly. The interfacial characteristics of conductive patterns on liquid crystal polymer substrates were observed via transmission electron microscopy and atomic force microscopy infrared spectroscopy. The interfacial molecular layers have a thickness of 10 nm. Due to the successful molecular bonding modifications, interfacial adhesion has been sufficiently improved; in particular, the peel-related breakage sites will be located in the modified layers on the plastic surface beneath the interface after the metallic coatings are peeled off. Integrating X-ray photoelectron spectroscopy, infrared spectroscopy, and scanning electron microscopy results, the molecular bonding mechanism has been revealed: UV-assisted grafting and self-assembly result in the construction of interfacial molecular architectures, which provide nanosized connecting bridges between the metallic patterns and polymer surfaces. Such in-depth interfacial studies can offer insight into interfacial adhesion, which will impact on the development of metal/polymer composite systems and continue to push the improvement of flexible devices.

Conformable self-assembling amyloid protein coatings with genetically programmable functionality

Functional coating materials have found broad technological applications in diverse fields. Despite recent advances, few coating materials simultaneously achieve robustness and substrate independence while still retaining the capacity for genetically encodable functionalities. Here, we report Escherichia coli biofilm-inspired protein nanofiber coatings that simultaneously exhibit substrate independence, resistance to organic solvents, and programmable functionalities. The intrinsic surface adherence of CsgA amyloid proteins, along with a benign solution-based fabrication approach, facilitates forming nanofiber coatings on virtually any surface with varied compositions, sizes, shapes, and structures. In addition, the typical amyloid structures endow the nanofiber coatings with outstanding robustness. On the basis of their genetically engineerable functionality, our nanofiber coatings can also seamlessly participate in functionalization processes, including gold enhancement, diverse protein conjugations, and DNA binding, thus enabling a variety of proof-of-concept applications, including electronic devices, enzyme immobilization, and microfluidic bacterial sensors. We envision that our coatings can drive advances in electronics, biocatalysis, particle engineering, and biomedicine.

Realizing Ultrahigh Mechanical Flexibility and >15% Efficiency of Flexible Organic Solar Cells via a "Welding" Flexible Transparent Electrode

The power conversion efficiencies (PCEs) of flexible organic solar cells (OSCs) still lag behind those of rigid devices and their mechanical stability is unable to meet the needs of flexible electronics at present due to the lack of a high-performance flexible transparent electrode (FTE). Here, a so-called "welding" concept is proposed to design an FTE with tight binding of the upper electrode and the underlying substrate. The upper electrode consisting of solution-processed Al-doped ZnO (AZO) and silver nanowire (AgNW) network is well welded by utilizing the capillary force effect and secondary growth of AZO, leading to a reduction of the AgNWs junction site resistance. Meanwhile, the poly(ethylene terephthalate) is modified by embedding the AgNWs, which are then used to link with the AgNWs in the upper hybrid electrode, thus enhancing the adhesion of the electrode to the substrate. By this welding strategy, critical bottleneck issues relating to the FTEs in terms of optoelectronic and mechanical properties are comprehensively addressed. The single-junction flexible OSCs based on this welded FTE show a high performance, achieving a record high PCE of 15.21%. In addition, the PCEs of the flexible OSCs are less influenced by the device area and display robust bending durability even under extreme test conditions.

Flexible Interface Design for Stress Regulation of a Silicon Anode toward Highly Stable Dual-Ion Batteries

Dual-ion batteries (DIBs) have attracted increasing attention due to their high working voltage, low cost, and environmental friendliness, yet their development is hindered by their limited energy density. Pairing silicon-a most promising anode due to its highest theoretical capacity (4200 mAh g^-1 )-with a graphite cathode is a feasible strategy to address the challenge. Nevertheless, the cycling stability of silicon is unsatisfactory due to the loss of electrical contact resulting from the high interface stress when using rigid current collectors. In this work, a flexible interface design to regulate the stress distribution is proposed, via the construction of a silicon anode on a soft nylon fabric modified with a conductive Cu-Ni transition layer, which endows the silicon electrode with remarkable flexibility and stability over 50 000 bends. Assembly of the flexible silicon anode with an expanded graphite cathode yields a silicon-graphite DIB (SGDIB), which possesses record-breaking rate performance (up to 150 C) and cycling stability over 2000 cycles at 10 C with a capacity retention of 97%. Moreover, the SGDIB shows a high capacity retention of ≈84% after 1500 bends and a low self-discharging voltage loss of 0.0015% per bend after 10 000 bends, suggesting high potential for high-performance flexible energy-storage applications.

Flexible Conductive Polymer Film Grafted with Azo-Moieties and Patterned by Light Illumination with Anisotropic Conductivity

In this work, we present the method for the creation of an anisotropic electric pattern on thin poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS) films through PSS grafting by azo-containing moieties followed by light-induced polymers redistribution. Thin PEDOT:PSS films were deposited on the flexible and biodegradable polylactic acid (PLLA) substrates. The light-sensitive azo-groups were grafted to PSS using the diazonium chemistry followed by annealing in methanol. Local illumination of azo-grafted PEDOT:PSS films through the lithographic mask led to the conversion of azo-moieties in Z-configuration and further creation of the lateral gradient of azo-isomers along the film surface. The concentration gradient led to the migration of PSS away from the illuminated area, increasing the PEDOT chains' concentration and the corresponding increase of local electrical conductivity in the illuminated place. Utilization of mask with linear pattern results in the appearance of conductive PEDOT-rich and non-conductive PSS-rich lines on the film surface, and final, lateral anisotropy of electric properties. Our work gives an optical lithography-based alternative to common methods for the creation of anisotropic electric properties, based on the spatial confinement of conductive polymer structures or their mechanical strains.