Epoxy Resin-Assisted Cu Catalytic Printing for Flexible Cu Conductors on Smooth and Rough Substrates.

Flexible copper conductors have been extensively utilized in flexible and wearable electronics. They can be fabricated by using a variety of patterning techniques such as vacuum deposition, photolithography, and various printing techniques. However, vacuum deposition and photolithography are costly and result in material wastage. Moreover, traditional printing inks require posttreatment, which can damage flexible substrates, or grafting polymers, which involve complex processes to adhere to flexible substrates. Therefore, this study proposes a facile method of fabricating flexible metal patterns with high electrical conductivities and remarkable bonding forces on a diverse range of flexible substrates. Catalytic ink was prepared by using a mixture of epoxy resin, copper nanopowder, and nanosilica. The ink was applied to a variety of flexible substrates, including a poly(ethylene terephthalate) (PET) film, polyimide film, and filter paper, using screen printing to establish a bridge layer for subsequent electroless deposition (ELD). The catalytic efficiency was significantly improved by treating the cured ink patterns with air plasma. The fabricated flexible metals exhibited excellent adhesion and desirable electrical conductivity. The sheet resistance of the copper layer on the PET substrate decreased to 9.2 mΩ/□ after 150 min of ELD. The resistance of the flexible metal on the PET substrate increased by only 3.125% after 5000 bending cycles. The flexible metals prepared in this study demonstrated good foldability, and the samples with filter paper and PET substrates failed after 40 and 70 folds, respectively. A pressure sensor with a bottom electrode consisting of a copper interdigital electrode on a PET substrate displayed favorable sensing performance.

Mussel-Inspired Wet-Adhesive Multifunctional Organohydrogel with Extreme Environmental Tolerance for Wearable Strain Sensor.

As a flexible artificial material, the conductive hydrogel has broad application prospects in flexible wearable electronics, soft robotics, and biomedical monitoring. However, traditional hydrogels still face many challenges, such as long-term stability, availability in extreme environments, and long-lasting adhesion to the skin surface under sweaty or humid conditions. To circumvent the above issues, one kind of ionic conductive hydrogel was prepared by a simple one-pot method that dissolved chitosan (CS), 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS), tannic acid (TA), and 2-methoxy-ethyl acrylate (MEA) into dimethyl sulfoxide (DMSO)/H2O solvent. The resulting hydrogel showed excellent tensile properties (1440%), extreme environmental tolerance (-40-60 °C), adhesion (72 KPa at porcine skin), ionic conductivity (0.87 S m-1), and high-efficiency antibacterial property. Furthermore, the produced organohydrogel strain sensor exhibited high strain sensitivity (GF = 4.07), excellent signal sensing capabilities (human joint movement, microexpression, and sound signals), and long-term cyclic stability (400 cycles). Looking beyond, this work provides a simple and promising strategy for using hydrogel sensors in extreme environments for e-skin, health monitoring, and wearable electronic devices.

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.

Printable All-Paper Pressure Sensors with High Sensitivity and Wide Sensing Range.

With the rapid development of flexible electronics, a large amount of electronic waste is becoming a global concern. Because of the biodegradable and environment-friendly properties, cellulose paper as flexible substrates is an alternative pathway to effectively address the electronic pollution. Recently, paper-based piezoresistive pressure sensors with a simple structure and easy signal detection have been widely used in health monitoring, soft robots, and so forth. However, the low sensitivity and narrow working range of paper-based sensors limit their practical applications. Here, an all paper-based piezoresistive pressure sensor is successfully constructed by assembling a bottom electrode with a screen-printed interdigital Cu electrode on paper and a top sensing electrode. The top electrode is simply fabricated using a one-step impregnation method to coat a thin poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) layer on air-laid paper. The constructed all-paper sensor displays a maximum sensitivity of 768.07 kPa-1, a wide detection range (up to 250 kPa), and excellent cycle stability (5000 cycles). Furthermore, the sensor can clearly respond from low pressure (such as wrist pulse) to high pressure (finger tapping). The outstanding performance can be attributed to the surface and interface design of rough and fiber-structured paper and the high conductivity of copper and PEDOT:PSS. Finally, based on the printing technology, array sensors are fabricated to identify spatial pressure distributions, demonstrating the capability of low-cost and large-area fabrication for the practical production applications. This printable all-paper sensor with excellent sensing performance exhibits great potential for use in new-generation green and portable electronics.

A Review of MnO2 Composites Incorporated with Conductive Materials for Energy Storage.

Manganese dioxide (MnO2 ) has been widely used in the field of energy storage due to its high specific capacitance, low cost, natural abundance, and being environmentally friendly. However, suffering from poor electrical conductivity and high dissolvability, the performance of MnO2 can no longer meet the needs of rapidly growing technological development, especially for the application as electrode material in metal-ion batteries and supercapacitors. In this review, recent studies on the development of binary or multiple MnO2 -based composites with conductive components for energy storage are summarized. Firstly, general preparing methods for MnO2 -based composites are introduced. Subsequently, the binary and multiple MnO2 -based composites with carbon, conducting polymer, and other conductive materials are discussed respectively. The improvement in their performance is summarized as well. Finally, perspectives on the practical applications of MnO2 -based composites are presented.

Polydimethylsiloxane-Assisted Catalytic Printing for Highly Conductive, Adhesive, and Precise Metal Patterns Enabled on Paper and Textiles.

Paper and textile are two ideal carriers in wearable and printed electronics because of their flexibility and low price. However, the porous and fibrous structures restrain their use in printed electronics because the capillary effect results in ink diffusion. Especially, conventional metal ink needs to be post-treated at high temperatures (>150 °C), which is not compatible with paper and textile. To address problems involved in ink diffusion and avoid high-temperature treatment, herein, a new strategy is proposed: screen-printing of high-viscosity catalytic inks combined with electroless deposition of metal layers on paper and textile substrates. The ink consists of Ag nanoparticles, a polydimethylsiloxane (PDMS) prepolymer, and a curing agent. PDMS as a viscoelastic matrix of catalysts plays key roles in limiting ink diffusion, enhancing interfacial adhesion between the substrate and metal layer, keeping metal flexible. As a demonstration, metal Cu and Ni are printed, respectively. The printed precision (diffusion < 1% on filter paper) can be controlled by adjusting the Ag content in the PDMS matrix; interfacial adhesion can be enhanced by ink coating on substrate microfibers and metal embedding into the PDMS matrix. In addition, Cu on paper shows extremely low sheet resistance (0.29 mΩ/□), and Cu on nylon shows outstanding foldability with a resistance of less than five times of initial resistance during 5000 folding cycles.

Electrospinning Synthesis of Mesoporous MnCoNiOx@Double-Carbon Nanofibers for Sodium-Ion Battery Anodes with Pseudocapacitive Behavior and Long Cycle Life.

In this work, MnCoNiOx (denoted as MCNO) nanocrystals (with a size of less than 30 nm) finely encapsulated in double-carbon (DC, including reduced graphene oxide and amorphous carbon derived by polymer) composite nanofibers (MCNO@DC) were successfully synthesized via an electrospinning method followed by a sintering treatment. The as-obtained MCNO@DC nanofibers present superior sodium storage performance and retain an especially high specific capacity of 230 mAh g-1 with a large capacity retention of about 96% at 0.1 A g-1 after 500 cycles and a specific capacity of 107 mAh g-1 with capacity retention of about 89% at 1 A g-1 after 6500 cycles. The outstanding cycle characteristic is mainly due to the tiny MCNO nanoparticles, which shorten the ion migration distance, and the three-dimensional DC framework, which remarkably promotes the electronic transfer and efficiently limits the volume expansion during the progress of insertion and extraction of Na+ ions. Moreover, nitrogen doped in carbon is able to improve the electrochemical capability as well. Finally, kinetic analysis of the redox reactions is used to verify the pseudocapacitive mechanism in charge storage and the feasibility of using MCNO@DC composite nanofibers as an anode for sodium-ion batteries with the above-mentioned behavior.

Engineering the Electrochemical Capacitive Properties of Microsupercapacitors Based on Graphene Quantum Dots/MnO2 Using Ionic Liquid Gel Electrolytes.

All-solid-state microsupercapacitors (MSCs) have been receiving intense interest due to their potential as micro/nanoscale energy storage devices, but their low energy density has limited practical applications. It has been reported that gel electrolytes based on ionic liquids (ionogels) with large potential windows can be used as solid electrolytes to enhance the energy density of MSCs, but a systematic study on how to select and evaluate such ionogels for MSCs is rare. In this study, we construct a series of all-solid-state asymmetric MSCs on the interdigital finger electrodes, using graphene quantum dots (GQDs) as the negative electrode, MnO2 nanosheets as the positive electrode, and different ionogels as the solid electrolytes. Among them, the MSC using 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][NTF2]) with 4 wt % fumed SiO2 ionogel exhibited the best electrochemical performance, having excellent rate capability with the scan rate up to 2000 V s(-1), ultrafast frequency response (τ0 = 206.9 µs) and high energy density. The outstanding performance of this device mainly results from fast ion diffusion, high ion conductivity of the ionogel, and ionic liquid-matrix interactions. The results presented here provide guidance for picking out appropriate ionogels for use in high-performance all-solid-state MSCs to meet the growing requirement of micronanoscale energy storage devices. Additionally, the ultrafast frequency response of our MSCs suggests potential applications in ac line-filters.

Biomimicking Topographic Elastomeric Petals (E-Petals) for Omnidirectional Stretchable and Printable Electronics.

Elastomeric petals directly replicated from natural rose petal are new versatile substrates for stretchable and printable electronics. Compared with conventional flat polydimethylsiloxane substrates, elastomeric petals have biomimicking topographic surfaces that can effectively inhibit the propagation of microcracks formed in the conducting layer, which is deposited on top, regardless of the type of conductive materials and the deposition methods.

Full-solution processed flexible organic solar cells using low-cost printable copper electrodes.

Full-solution-processed flexible organic solar cells (OSCs) are fabricated using low-cost and high-quality printable Cu electrodes, which achieve a power conversion efficiency as high as 2.77% and show remarkable stability upon 1000 bending cycles. This device performance is thought to be the best among all full-solution-processed OSCs reported in the literature using the same active materials. This printed Cu electrode is promising for application in roll-to-roll fabrication of flexible OSCs.

Aqueous and air-compatible fabrication of high-performance conductive textiles.

This paper describes a fully aqueous- and air-compatible chemical approach to preparing high-performance conductive textiles. In this method, the surfaces of textile materials are first modified with an aqueous solution of double-bond-containing silane molecules to form a surface-anchoring layer for subsequent in situ free-radical polymerization of [2-(methacryloyloxy)ethyl]trimethylammonium chloride (METAC) in the air. Thin layers of poly-METAC (PMETAC) are therefore covalently grafted on top of the silane-modified textile surface. Cu- or Ni-coated textiles are finally fabricated by electroless deposition (ELD) onto the PMETAC-modified textiles. Parameters including polymerization time, temperature, and ELD conditions are studied to optimize the whole fabrication process. The as-made conductive textiles exhibit sheet resistance as low as 0.2â€…Ω sq(-1) , which makes them highly suitable for use as conductive wires and interconnects in flexible and wearable electronic devices. More importantly, the chemical method is fully compatible with the conventional "pad-dry-cure" fabrication process in the textile manufacturing industry, thus indicating that it is very promising for high-throughput and roll-to-roll fabrication of high-performance metal-coated conductive textiles in the future.

Three-dimensional compressible and stretchable conductive composites.

Three-dimensional (3D) conductive composites with remarkable flexibility, compressibility, and stretchability are fabricated by solution deposition of thin metal coatings on chemically modified, macroscopically continuous, 3D polyurethane sponges, followed by infiltration of the metallic sponges with polydimethylsiloxane (PDMS). These low-cost conductive composites are used as high-performance interconnects for flexible and stretchable light-emitting diode (LED) arrays, even with severe surface abrasion or cutting.

Matrix-assisted catalytic printing for the fabrication of multiscale, flexible, foldable, and stretchable metal conductors.

Matrix-assisted catalytic printing (MACP) is developed as a low-cost and versatile printing method for the fabrication of multiscale metal conductors on a wide variety of plastic, elastomeric, and textile substrates. Highly conductive Cu interconnects (2.0 × 108 S/m) fabricated by MACP at room temperature display excellent flexibility, foldability, and stretchability.