Inkjet-Printed Electrodes on A4 Paper Substrates for Low-Cost, Disposable, and Flexible Asymmetric Supercapacitors

Low-cost inkjet-printed nanostructured biosensor based on CRISPR/Cas12a system for pathogen detection

The escalating global incidence of infectious diseases caused by pathogenic bacteria, especially in developing countries, emphasises the urgent need for rapid and portable pathogen detection devices. This study introduces a sensitive and specific electrochemical biosensing platform utilising cost-effective electrodes fabricated by inkjet-printing gold and silver nanoparticles on a plastic substrate. The biosensor exploits the CRISPR/Cas12a system for detecting a specific DNA sequence selected from the genome of the target pathogen. Upon detection, the trans-activity of Cas12a/gRNA is triggered, leading to the cleavage of rationally designed single-strand DNA reporters (linear and hairpin) labelled with methylene blue (ssDNA-MB) and bound to the electrode surface. In principle, this sensing mechanism can be adapted to any bacterium by choosing a proper guide RNA to target a specific sequence of its DNA. The biosensor's performance was assessed for two representative pathogens (a Gram-negative, Escherichia coli, and a Gram-positive, Staphylococcus aureus), and results obtained with inkjet-printed gold electrodes were compared with those obtained by commercial screen-printed gold electrodes. Our results show that the use of inkjet-printed nanostructured gold electrodes, which provide a large surface area, in combination with the use of hairpin reporters containing a poly-T loop can increase the sensitivity of the assay corresponding to a signal variation of 86%. DNA targets amplified from various clinically isolated bacteria, have been tested and demonstrate the potential of the proposed platform for point-of-need applications.

Carbon Micro/Nano Machining toward Miniaturized Device: Structural Engineering, Large-Scale Fabrication, and Performance Optimization

With the rapid development of micro/nano machining, there is an elevated demand for high-performance microdevices with high reliability and low cost. Due to their outstanding electrochemical, optical, electrical, and mechanical performance, carbon materials are extensively utilized in constructing microdevices for energy storage, sensing, and optoelectronics. Carbon micro/nano machining is fundamental in carbon-based intelligent microelectronics, multifunctional integrated microsystems, high-reliability portable/wearable consumer electronics, and portable medical diagnostic systems. Despite numerous reviews on carbon materials, a comprehensive overview is lacking that systematically encapsulates the development of high-performance microdevices based on carbon micro/nano structures, from structural design to manufacturing strategies and specific applications. This review focuses on the latest progress in carbon micro/nano machining toward miniaturized device, including structural engineering, large-scale fabrication, and performance optimization. Especially, the review targets an in-depth evaluation of carbon-based micro energy storage devices, microsensors, microactuators, miniaturized photoresponsive and electromagnetic interference shielding devices. Moreover, it highlights the challenges and opportunities in the large-scale manufacturing of carbon-based microdevices, aiming to spark further exciting research directions and application prospectives.

Unleashing inkjet-printed nanostructured electrodes and battery-free potentiostat for the DNA-based multiplexed detection of SARS-CoV-2 genes

Following the global COVID-19 pandemic triggered by SARS-CoV-2, the need for rapid, specific and cost-effective point-of-care diagnostic solutions remains paramount. Even though COVID-19 is no longer a public health emergency, the disease still poses a global threat leading to deaths, and it continues to change with the risk of new variants emerging causing a new surge in cases and deaths. Here, we address the urgent need for rapid, cost-effective and point-of-care diagnostic solutions for SARS-CoV-2. We propose a multiplexed DNA-based sensing platform that utilizes inkjet-printed nanostructured gold electrodes and an inkjet-printed battery-free near-field communication (NFC) potentiostat for the simultaneous quantitative detection of two SARS-CoV-2 genes, the ORF1ab and the N gene. The detection strategy based on the formation of an RNA-DNA sandwich structure leads to a highly specific electrochemical output. The inkjet-printed nanostructured gold electrodes providing a large surface area enable efficient binding and increase the sensitivity. The inkjet-printed battery-free NFC potentiostat enables rapid measurements and real-time data analysis via a smartphone application, making the platform accessible and portable. With the advantages of speed (5 min), simplicity, sensitivity (low pM range, ∼450% signal gain) and cost-effectiveness, the proposed platform is a promising alternative for point-of-care diagnostics and high-throughput analysis that complements the COVID-19 diagnostic toolkit.

Binder-Free MOF-Based and MOF-Derived Nanoarrays for Flexible Electrochemical Energy Storage: Progress and Perspectives

The fast development of Internet of Things and the rapid advent of next-generation versatile wearable electronics require cost-effective and highly-efficient electroactive materials for flexible electrochemical energy storage devices. Among various electroactive materials, binder-free nanostructured arrays have attracted widespread attention. Featured with growing on a conductive and flexible substrate without using inactive and insulating binders, binder-free 3D nanoarray electrodes facilitate fast electron/ion transportation and rapid reaction kinetics with more exposed active sites, maintain structure integrity of electrodes even under bending or twisted conditions, readily release generated joule heat during charge/discharge cycles and achieve enhanced gravimetric capacity of the whole device. Binder-free metal-organic framework (MOF) nanoarrays and/or MOF-derived nanoarrays with high surface area and unique porous structure have emerged with great potential in energy storage field and been extensively exploited in recent years. In this review, common substrates used for binder-free nanoarrays are compared and discussed. Various MOF-based and MOF-derived nanoarrays, including metal oxides, sulfides, selenides, nitrides, phosphides and nitrogen-doped carbons, are surveyed and their electrochemical performance along with their applications in flexible energy storage are analyzed and overviewed. In addition, key technical issues and outlooks on future development of MOF-based and MOF-derived nanoarrays toward flexible energy storage are also offered.

Inkjet-printed flexible planar Zn-MnO2 battery on paper substrate

Energy storage devices (ESD) which are intended to power electronic devices, used in close contact of human skin, are desirable to be safe and non-toxic. In light of this requirement, Zn based energy storage devices seem to provide a viable pathway as they mostly employ aqueous based electrolytes which are safe and non-toxic in their functioning. Additionally, having a flexible ESD will play a crucial role as it will enable the ESD to conform to the varying shapes and sizes of wearable electronics which they energize. In this work, we have developed an inkjet-printed Zinc ion battery (IPZIB) with planar electrode configuration over bond paper substrate. Zn has been used as the negative electrode, MnO2 is used as the positive electrode with Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) as the active binder. Conducting tracks of reduced graphene oxide (rGO) are used to construct the current collector on the paper substrate. The fabricated IPZIB delivered a high discharge capacity of 300.14 mAh g-1 at a current density of 200 mA g-1. The energy density of the IPZIB is observed as 330.15 Wh kg-1 at a power density of 220 W kg-1 and retains an energy density of 94.36 Wh kg-1 at a high power density of 1650 W kg-1. Finally, we have demonstrated the capability of the IPZIB to power a LED at various bending and folding conditions which indicates its potential to be used in the next generation flexible and wearable electronic devices.

Paper Supercapacitor Developed Using a Manganese Dioxide/Carbon Black Composite and a Water Hyacinth Cellulose Nanofiber-Based Bilayer Separator

Flexible and green energy storage devices have a wide range of applications in prospective electronics and connected devices. In this study, a new eco-friendly bilayer separator and primary and secondary paper supercapacitors based on manganese dioxide (MnO2)/carbon black (CB) are developed. The bilayer separator is prepared via a two-step fabrication process involving freeze-thawing and nonsolvent-induced phase separation. The prepared bilayer separator exhibits superior porosity of 46%, wettability of 46.5°, and electrolyte uptake of 194% when compared with a Celgard 2320 trilayer separator (39%, 55.58°, and 110%). Moreover, lower bulk resistance yields a higher ionic conductivity of 0.52 mS cm-1 in comparison to 0.22 mS cm-1 for the Celgard separator. Furthermore, the bilayer separator exhibits improved mean efficiency of 0.44% and higher specific discharge capacitance of 13.53%. The anodic and cathodic electrodes are coated on a paper substrate using MnO2/CB and zinc metal-loaded CB composites. The paper supercapacitor demonstrates a high specific capacitance of 34.1 mF cm-2 and energy and power density of 1.70 μWh cm-2 and 204.8 μW cm-2 at 500 μA, respectively. In summary, the concept of an eco-friendly bilayer cellulose separator with paper-based supercapacitors offers an environmentally friendly alternative to traditional energy storage devices.

Effect of the Standardized ZnO/ZnO-GO Filter Element Substrate driven Advanced Oxidation Process on Textile Industry Effluent Stream: Detailed Analysis of Photocatalytic Degradation Kinetics

A simple process of synthesizing coated filter element substrates (FES) containing zinc oxide (ZnO) nanorods and ZnO graphene-oxide nanocomposite for a pilot-scale industrial dye-effluent treatment plant is proposed. This work reports a detailed analysis of the photocatalysis mechanism on real industrial effluent streams containing a mixture of dyes. The analysis is very relevant for conducting advanced oxidation process-assisted effluent remediation at a field-level treatment operation. Estimation of the dye concentration shows nearly complete (≥98%) degradation from an initial dye sample concentration. A detailed study for the analysis of the initial reactive dyes and their degradation products was performed for quantification and identification of the degradation products through various spectral techniques. A design of the remediation mechanism through degradation pathways is proposed for characterizing the organic compounds in the degraded dye products. A regeneration and reusability study was performed on the FES presenting the durability of the FES-designed synthesis process originally for 11 cycles and regenerated FES for six cycles for achieving a threshold of 60% degradation efficiency. The experimental results demonstrate the efficacy of FES through the designed immobilized approach for the complete remediation of textile dye effluents for a 4 h treatment plant process and the consistent operability of the FES for the combined dye wastewater treatment operations.

Self-standing Fe3O4 decorated paper electrode as a binder-free trifunctional electrode for electrochemical ammonia synthesis and Zn-O2 batteries

The conversion of the abundant biodegradable material into electroactive electrode material can be a good resource for sustainable energy conversion and storage applications. Herein, we present a simple, cost-effective and green approach for the fabrication of a flexible cellulose paper electrode using an electroless-electrodeposition method. The one-step electroless deposition route is followed to induce conductivity into a non-conductive cellulose paper substrate without using any expensive activators or sensitisers. The Fe₃O₄ is then electro-deposited as an active catalyst over the conductive paper substrate for use in electrochemical activities. The as-fabricated paper electrode shows promising activity and stability during the dinitrogen reduction reaction (NRR) as well as oxygen bifunctional electrocatalysis. A faradaic efficiency of 4.32% with a yield rate of 245 μg h^-1 mg_cat^-1 at -0.1 V is achieved for NRR whereas a very small overpotential of 180 mV is required to reach 10 mA cm^-2 during OER, and the ORR reaction starts at the onset potential of 0.86 V. The practical applicability of the paper electrode is validated by assembling a Zn-O₂ battery showing a peak power density of 81 mW cm^-2 and a stability up to 35 h during charge-discharge cycles, which can power the NRR to produce NH₃ under full cell conditions.

Recent advancements in the field of flexible/wearable enzyme fuel cells

This review article focusses on new advances in the field of enzyme fuel cells (EFCs), especially, on flexible materials which can be used to make flexible EFCs for wearable devices, three-dimensional (3D) printed structures to prepare electrodes for EFCs and micro/nano electromechanical structures (MEMS/NEMS) to fabricate micro-EFCs. Particular attention is given to newly developed approaches to obtain efficient electrodes for harvesting energy via EFCs. This review article explains the various attributes of these recently developing technologies and their ability to mitigate the energy requirements of flexible/wearable bioelectronic devices. Besides discussing key milestones achieved, this perspective review article also emphasizes the main hurdles that are currently impeding the realization of flexible/wearable EFCs. We have also emphasized on the major hurdles related to the flexible materials required to fabricate wearable EFCs, suitable 3D printing techniques required, and MEMS and NEMS to fabricate micro-EFCs. In all the recently developed techniques, there are some common issues like stability, low power output, mechanical strength and flexibility. This review article also provides various routes to mitigate these issues. The main aim of this perspective article is to develop curiosity among the researchers of various fields to team up in order to address the huge challenges that restrict the real-world application of flexible/wearable EFCs. Such collaboration is important to harness the full potential of EFCs. It is requested to read this review article with supporting information to get the complete essence.

Constructing MXene-PANI@MWCNTs heterojunction with high specific capacitance towards flexible micro-supercapacitor

Micro-supercapacitors (MSCs) are considered as the promising energy supply of miniaturized electronic devices. The electrode material, as one integral part, play a crucial role on the energy storage performance of MSCs. In our work, we constructed a heterojunction in MXene-PANI@MWCNTs (MPM) ternary composite, benefitting for the synergistic enhancement effect among MXene, polyaniline (PANI) and multiwall carbon nanotubes, an outstanding specific capacitance of 414 F g^-1(at 1 A g^-1) has been achieved. MPM shows high capacitance retention at large current density (86.7%, at 10 A g^-1) and long-term cycling stability of 90.4% for 10 000 cycles. Furthermore, we obtained MPM self-standing films, and constructed a flexible all-solid-state MSC based on the film electrode. A competitive charge storage capability of 30.2 mF cm^-2and long-term stability of 70.2% retention for 10 000 cycles was obtained in the MSC. Meanwhile, the MSC shows excellent flexibility, maintaining most capacitance under bending conditions. Moreover, using an integrated strategy, MSCs can obtain tunable voltages and currents that meet various practical requirements. All these results indicate that the MPM is an excellent charge storage material and will become a potential candidate for flexible energy-storage devices.

AIE-active macromolecules: designs, performances, and applications

Aggregation-induced emission (AIE) macromolecules as emerging luminescent materials gained increasing attention owing to their good processability, high brightness, wide functionality, and smart responsiveness, with great potential in many fields.

Nano-dendrite structured cobalt phosphide based hybrid supercapacitor with high energy storage and cycling stability

The supercapacitors possessing high energy storage and long serving period have strategic significance to solve the energy crisis issues. Herein, fluffy nano-dendrite structured cobalt phosphide (CoP) is grown on carbon cloth through simple hydrothermal and electrodeposition treatments (CoP/C-HE). Benefit from its excellent electrical conductivity and special structure, CoP/C-HE manifests a high specific capacity of 461.4 C g^-1at 1 A g^-1. Meanwhile, the capacity retention remains 92.8% over 10 000 cycles at 5 A g^-1, proving the superior cycling stability. The phase conversion of Co₂P during the activation process also contributes to the improved performance. The assembled two-electrode asymmetric supercapacitor demonstrates excellent performance in terms of energy density (42.4 W h kg^-1at a power density of 800.0 W kg^-1) and cycling stability (86.3% retention over 5000 cycles at 5 A g^-1), which is superior to many reported cobalt-based supercapacitors. Our work promotes the potential of transition metal phosphides for the applications in supercapacitors.

Hand-drawn electrode based disposable paper chip for artificial sweat analysis using impedance spectroscopy

Low cost, disposable paper based electrical sensor to examine the analyte concentration in an extremely small volume of sample solution is essential for environmental and healthcare applications. For the development of paper based devices, sophisticated instruments are essential to pattern electrode on the top surface of the paper. In most cases, such fabricated device results in direct contact with the analyte solution on the surface of the electrode during electrical detection and leads to high electrical double layer capacitance. In this work, we have focused to reduce the double layer capacitance by fabricating hand drawn electrode paper sensor utilising the reverse side of the paper. This design acts as a sample storage and facilitate impedimetric sensing of ionic concentration of analyte solution using a few microlitre. Droplet formation at the bottom of the paper in the confined area is visually monitored to reduce sample wastage. The interaction between two different electrode materials (graphite and silver) on the paper substrate with the different volume and concentration of the electrolyte is analysed to improve the robustness and sensitivity of the measurement. Simultaneously, we observed a reduction in the electrical double layer effect on the low sample volumes. The proposed paper based sensor shows the enhanced impedance stability on silver electrode patterned paper chip than graphite electrode paper chip to detect the different ionic concentration of artificial sweat sample. Finally, it demonstrates that paper chip has great potential as a disposable diagnostics sensor in healthcare applications.

Optimized ink jetted paper device for electroanalytical detection of picric acid

Picric acid (PA) is one of the essential components utilized in manufacturing of explosives. Therefore, the detection of trace amount of PA is critical in forensic science, criminal investigation, military security and environmental safety. Owing to these attributes, development of a simple, rapid and point-of-care (POC) analytical method for PA detection and quantification is crucial. Herein, a low-cost, POC, ink jetted paper device has been developed for electroanalytical detection of PA. Inkjet printing is an economic fabrication process used for extruding several nanomaterials with diversified applications. By improving the ink viscosity, inkjet printers can simplify the fabrication of paper-based electrochemical sensor, and provide easy, fast, environmental friendly and viable for large scale production sensors, thereby adding its commercialization potential. In this work, a commercially available circuit board printer and an inexpensive high viscosity carbon conductive ink were used to print an electrochemical paper device. The fabricated device was used for electrochemical detection of PA using cyclic voltammetry (CV) and wave voltammetry (SWV). Various parameters like effect of potential scan rate from 10 mVs^-1 to 300 mVs^-1, effect of variable PA concentration effect was studied. A linear concentration range of 4 μM to 60 μM was obtained. For a working electrode of 7 mm² surface area, the limit of detection (LOD) was 4.04 μM (922.56 ppb) which was less than the prescribed safe limit of 8 μM. Effect of interference with other chemicals was examined using SWV with the co-existing metals like zinc, lead, copper and mercury. Finally, real sample analysis for tap and lake water was successfully performed with the device. The developed cost-effective paper-based ink-jetted platform, with further fine-tuning and surface modifications, can be used for sensing various analytes as a point-of-care device.

A Review on Printed Electronics: Fabrication Methods, Inks, Substrates, Applications and Environmental Impacts

Innovations in industrial automation, information and communication technology (ICT), renewable energy as well as monitoring and sensing fields have been paving the way for smart devices, which can acquire and convey information to the Internet. Since there is an ever-increasing demand for large yet affordable production volumes for such devices, printed electronics has been attracting attention of both industry and academia. In order to understand the potential and future prospects of the printed electronics, the present paper summarizes the basic principles and conventional approaches while providing the recent progresses in the fabrication and material technologies, applications and environmental impacts.

Why Cellulose-Based Electrochemical Energy Storage Devices?

Recent findings demonstrate that cellulose, a highly abundant, versatile, sustainable, and inexpensive material, can be used in the preparation of very stable and flexible electrochemical energy storage devices with high energy and power densities by using electrodes with high mass loadings, composed of conducting composites with high surface areas and thin layers of electroactive material, as well as cellulose-based current collectors and functional separators. Close attention should, however, be paid to the properties of the cellulose (e.g., porosity, pore distribution, pore-size distribution, and crystallinity). The manufacturing of cellulose-based electrodes and all-cellulose devices is also well-suited for large-scale production since it can be made using straightforward filtration-based techniques or paper-making approaches, as well as utilizing various printing techniques. Herein, the recent development and possibilities associated with the use of cellulose are discussed, regarding the manufacturing of electrochemical energy storage devices comprising electrodes with high energy and power densities and lightweight current collectors and functional separators.

Development of hydrophobic paper substrates using silane and sol-gel based processes and deriving the best coating technique using machine learning strategies

Low energy surface coatings have found wide range of applications for generating hydrophobic and superhydrophobic surfaces. Most of the studies have been related to use of a single coating material over a single substrate or using a single technique. The degree of hydrophobicity is highly dependent on fabrication processes as well as materials being coated and as such warrants a high-level study using experimental optimization leading to the evaluation of the parametric behavior of coatings and their application techniques. Also, a single platform or system which can predict the required set of parameters for generating hydrophobic surface of required nature for given substrate is of requirement. This work applies the powerful machine learning algorithms (Levenberg Marquardt using Gauss Newton and Gradient methods) to evaluate the various processes affecting the anti-wetting behavior of coated printable paper substrates with the capability to predict the most optimized method of coating and materials that may lead to a desirable surface contact angle. The major application techniques used for this study pertain to dip coating, spray coating, spin coating and inkjet printing and silane and sol-gel base coating materials.

New Frontiers in 3D Structural Sensing Robots

Advanced robotics is the result of various contributions from complex fields of science and engineering and has tremendous value in human society. Sensing robots are highly desirable in practical settings such as healthcare and manufacturing sectors through sensing activities from human-robot interaction. However, there are still ongoing research and technical challenges in the development of ideal sensing robot systems. The sensing robot should synergically merge sensors and robotics. Geometrical difficulty in the sensor positioning caused by the structural complexity of sensing robots and their corresponding processing have been the main challenges in the production of sensing robots. 3D electronics integrated into 3D objects prepared by the 3D printing process can be the potential solution for designing realistic sensing robot systems. 3D printing provides the advantage to manufacture complex 3D structures in electronics in a single setup, allowing the ease of design flexibility, and customized functions. Therefore, the platform of 3D sensing systems is investigated and their expansion into sensing robots is studied further. The progress toward sensing robots from 3D electronics integrated into 3D objects and the advanced material strategies, used to overcome the challenges, are discussed.

Sand-Milling Fabrication of Screen-Printable Graphene Composite Inks for High-Performance Planar Micro-Supercapacitors

Rational engineering and simplified production of printable graphene inks are essential for building high-energy and flexible graphene micro-supercapacitors (MSCs). However, few graphene-based MSCs show impressive areal capacitance and energy density, especially based on additive-manufacturing, cost-effective, and printable inks. Herein, a new-style and solution-processable graphene composite ink is ingeniously formulated for scalable screen printing MSCs. More importantly, the as-formulated inks consist of interwoven two-dimensional graphene and activated carbon nanofillers, which are delaminated by one-step sand-milling turbulent flow exfoliation. Notably, embedding the activated carbon nanoplatelets into graphene layers drastically boosts the electrochemical performance of screen-printed micro-supercapacitors (denoted as Gr/AC-MSCs), such as an outstanding areal capacitance of 12.5 mF cm^-2 (about 20 times than pure graphene). The maximum energy density, maximum power density, and exceptional cyclability are 1.07 μW h cm^-2, 0.004 mW cm^-2, and 88.1% after 5000 cycles, respectively. As such, the as-printed MSCs on paper display high resolution and pronounced energy-storage performance. Furthermore, the packaged and optimized Gr/AC-MSCs showcase remarkable mechanical flexibility even under highly folded and excellent water resistance, maintaining 91.8% capacitance retention after being washed for 90 min. The versatile methodology highlights the promise of graphene and analogous 2D nanosheet functional inks for scalable fabrication of flexible energy-storage devices.

Synthesis of nickel-copper composite with controllable nanostructure through facile solvent control as positive electrode for high-performance supercapacitors

The surface characteristics of electrodes vary depending on the solvent used. Furthermore, electrochemical performance varies depending on the surface morphology of the electrode. In this study, we grew 3D binary NiCu-based composites on Ni foam, via a binder-free hydrothermal method, for use as a cathode in high-performance supercapacitors. We employed different solvents to prepare the electrodes by adjusting the ratio of deionized water (DI water) to methanol. The electrode prepared using DI water as the solvent had the largest surface area with a nanowire structure. This morphology allowed for good electrical performance by greatly improving the electrode and electrolyte contact area and shortening the ion diffusion path. The optimized deposition of NiCu(CO₃)(OH)₂ nanowires (50 mL of DI water as solvent) showed an excellent maximum specific capacity of 758.9 mA h g^-1 at a current density of 3 A g^-1, as well as outstanding cycling performance with 87.2% retention after 5000 cycles. In this work, we focused on the large specific surface area and suitable electrochemical properties of NiCu(CO₃)(OH)₂ electrodes with various solvents. As a result, the asymmetric supercapacitor (ASC) using the NiCu(CO₃)(OH)₂ electrode prepared with 50 ml of DI water as the solvent as the positive electrode and graphene as the negative electrode, exhibited an energy density of 26.7 W h kg^-1 at a power density of 2534 W kg^-1, and excellent cycling stability with 91.3% retention after 5000 cycles. The NiCu(CO₃)(OH)₂//graphene ASC could turn on an LED light and demonstrated better electrical performance than most previously reported nickel- and copper-based carbonate hydroxide ASCs. In addition, in the present scenario where many nanoscale studies are conducted, a method of controlling the nanostructure of a material through facile solvent control will be of great help to many researchers.

Doped photo-crosslinked polyesteramide hydrogels as solid electrolytes for supercapacitors

High-performance hydrogels play a crucial role as solid electrolytes for flexible electrochemical supercapacitors (ESCs). More specifically, all solid-state ESCs based on renewable, biodegradable and/or biocompatible hydrogels doped with inorganic salts as electrolytes are attractive not only because of their contribution to reducing resource consumption and/or the generation of electronic garbage, but also due to their potential applicability in the biomedical field. Here, computer simulations have been combined with experimental measurements to probe the outstanding capability as solid electrolytes of photo-crosslinked unsaturated polyesteramide hydrogels containing phenylalanine, butenediol and fumarate, and doped with NaCl (UPEA-Phe/NaCl). Atomistic molecular dynamics simulations have shown the influence of the hydrogel pore structure in the migration of Na⁺ and Cl^- ions, suggesting that UPEA-Phe/NaCl hydrogels prepared without completing the photo-crosslinking reaction will exhibit better behavior as solid electrolytes. Theoretical predictions have been confirmed by potentiodynamic and galvanostatic studies on ESCs fabricated using poly(3,4-ethylenedioxythiophene) electrodes and UPEA-Phe/NaCl hydrogels, which were obtained using different times of exposure to UV radiation (i.e. 4 and 8 h for incomplete and complete photo-crosslinking reaction). Moreover, the behavior as a solid electrolyte of the UPEA-Phe/NaCl hydrogel prepared using a photo-polymerization time of 4 h has been found to be significantly superior to those exhibited by different polypeptide and polysaccharide hydrogels, which were analyzed using ESCs with identical electrodes and experimental conditions.

Textile-based supercapacitors for flexible and wearable electronic applications

Electronic textiles have garnered significant attention as smart technology for next-generation wearable electronic devices. The existing power sources lack compatibility with wearable devices due to their limited flexibility, high cost, and environment unfriendliness. In this work, we demonstrate bamboo fabric as a sustainable substrate for developing supercapacitor devices which can easily integrate to wearable electronics. The work demonstrates a replicable printing process wherein different metal oxide inks are directly printed over bamboo fabric substrates. The MnO₂–NiCo₂O₄ is used as a positive electrode, rGO as a negative electrode, and LiCl/PVA gel as a solid-state electrolyte over the bamboo fabrics for the development of battery-supercapacitor hybrid device. The textile-based MnO₂–NiCo₂O₄//rGO asymmetric supercapacitor displays excellent electrochemical performance with an overall high areal capacitance of 2.12 F/cm² (1,766 F/g) at a current density of 2 mA/cm², the excellent energy density of 37.8 mW/cm³, a maximum power density of 2,678.4 mW/cm³ and good cycle life. Notably, the supercapacitor maintains its electrochemical performance under different mechanical deformation conditions, demonstrating its excellent flexibility and high mechanical strength. The proposed strategy is beneficial for the development of sustainable electronic textiles for wearable electronic applications.

Inkjet-Printing Technology for Supercapacitor Application: Current State and Perspectives

Inkjet-printing (IJP) technology is recognized as a significant breakthrough in manufacturing high-performance electrochemical energy storage systems. In comparison to conventional fabrication protocols, this printing technique offers various advantages, such as contact-less high-resolution patterning capability; low-cost, controlled material deposition; process simplicity; and compatibility with a variety of substrates. Due to these outstanding merits, significant research efforts have been devoted to utilizing IJP technology in developing electrochemical energy storage devices, particularly in supercapacitors (SCs). These attempts have focused on fabricating the key components of SCs, including electrode, electrolyte, and current collector, through rational formulation and patterning of functional inks. In an attempt to further expand the material design strategy and accelerate technology development, it is urgent and essential to obtain an in-depth insight into the recent developments of inkjet-printed SCs. Toward this aim, first, a general introduction to the fundamental principles of IJP technology is provided. After that, the latest achievements in IJP of capacitive energy storage devices are systematically summarized and discussed with a particular emphasis on the design of printable functional materials, the printing process, and capacitive performance of inkjet-printed SCs. To close, existing challenges and future research trends for developing state-of-the-art inkjet-printed SCs are proposed.

Functional Inks for Printable Energy Storage Applications based on 2 D Materials

Ubiquitous portable electronics and the ever-growing internet-of-things have necessitated the emergence of high-end miniaturized devices as well as associated sophisticated printing technologies. With excellent solution processability and tunable electronic properties, 2 D materials stand as a promising candidate for functional inks that are readily printable for energy-storage devices. In this Review, we outline the significance, status, and challenges that we are facing of the developments of 2 D materials-based functional inks. Then, general ink formulation and basic knowledge of printing techniques together with their rheological requirements and enabled applications in energy storage are introduced, providing guidelines for developing inks that match well with the present printing techniques. Last, but not least, we also propose the perspectives on the development of 2 D materials-based inks for energy-storage applications.

Biodegradable, Flexible, and Transparent Conducting Silver Nanowires/Polylactide Film with High Performance for Optoelectronic Devices

As a synthetic renewable and biodegradable material, the application of polylactide (PLA) in the green flexible electronics has attracted intensive attention due to the increasingly serious issue of electronic waste. Unfortunately, the development of PLA-based optoelectronic devices is greatly hindered by the poor heat resistance and mechanical property of PLA. To overcome these limitations, herein, we report a facile and promising route to fabricate silver nanowires/PLA (AgNW/PLA) film with largely improved properties by utilizing the stereocomplex (SC) crystallization between poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA). Through embedding the AgNW networks into the PLLA:PDLA blend matrix via a transfer method, the AgNW/PLLA:PDLA film with both high transparency and excellent conductivity was obtained. Compared with the AgNW/PLLA film, the formation of SC crystallites in the composites matrix could significantly enhance not only heat resistance but also mechanical strength of the AgNW/PLLA:PDLA film. Exceptionally, the AgNW/PLLA:PDLA film exhibited superior flexibility and could maintain excellent electrical conductivity stability even under the condition of 10,000 repeated bending cycles and 100 tape test cycles. In addition, the organic light-emitting diodes (OLEDs) with the AgNW/PLLA:PDLA films as electrodes were successfully fabricated in this work for the first time and they exhibited highly flexible, luminous, as well as hydrolytic degradation properties. This work could provide a low-cost and environment-friendly avenue towards fabricating high-performanced PLA-based biodegradable electronics.

Aerosol-Jet-Printed Graphene Immunosensor for Label-Free Cytokine Monitoring in Serum

Graphene-based inks are becoming increasingly attractive for printing low-cost and flexible electrical circuits due to their high electrical conductivity, biocompatibility, and manufacturing scalability. Conventional graphene printing techniques, such as screen and inkjet printing, are limited by stringent ink viscosity requirements properties and large as-printed line width that impedes the performance of printed biosensors. Here, we report an aerosol-jet-printed (AJP) graphene-based immunosensor capable of monitoring two distinct cytokines: interferon gamma (IFN-γ) and interleukin 10 (IL-10). Interdigitated electrodes (IDEs) with 40 μm finger widths were printed from graphene-nitrocellulose ink on a polyimide substrate. The IDEs were annealed in CO₂ to introduce reactive oxygen species on the graphene surface that act as chemical handles to covalently link IFN-γ and IL-10 antibodies to the graphene surfaces. The resultant AJP electrochemical immunosensors are capable of monitoring cytokines in serum with wide sensing range (IFN-γ: 0.1-5 ng/mL; IL-10: 0.1-2 ng/mL), low detection limit (IFN-γ: 25 pg/ml and IL-10: 46 pg/ml) and high selectivity (antibodies exhibited minimal cross-reactivity with each other and IL-6) without the need for sample prelabeling or preconcentration. Moreover, these biosensors are mechanically flexible with minimal change in signal output after 250 bending cycles over a high curvature (Φ = 5 mm). Hence, this technology could be applied to numerous electrochemical applications that require low-cost electroactive circuits that are disposable and/or flexible.

Drying-Mediated Self-Assembly of Graphene for Inkjet Printing of High-Rate Micro-supercapacitors

Scalable fabrication of high-rate micro-supercapacitors (MSCs) is highly desired for on-chip integration of energy storage components. By virtue of the special self-assembly behavior of 2D materials during drying thin films of their liquid dispersion, a new inkjet printing technique of passivated graphene micro-flakes is developed to directly print MSCs with 3D networked porous microstructure. The presence of macroscale through-thickness pores provides fast ion transport pathways and improves the rate capability of the devices even with solid-state electrolytes. During multiple-pass printing, the porous microstructure effectively absorbs the successively printed inks, allowing full printing of 3D structured MSCs comprising multiple vertically stacked cycles of current collectors, electrodes, and sold-state electrolytes. The all-solid-state heterogeneous 3D MSCs exhibit excellent vertical scalability and high areal energy density and power density, evidently outperforming the MSCs fabricated through general printing techniques.

Recent advances and prospects of inkjet printing in heterogeneous catalysis

This review provides an insight into inkjet printing technology in the context of heterogeneous catalysis.

Direct Inkjet Printing of Aqueous Inks to Flexible All-Solid-State Graphene Hybrid Micro-Supercapacitors

In this article, the inkjet printing technique is demonstrated for the stacking of reduced graphene oxide (RGO) and molybdenum trioxide (MoO₃) nanosheets for flexible all-solid-state micro-supercapacitors. The ammonium molybdate tetrahydrate/graphene oxide ((NH₄)₆Mo₇O₂₄·4H₂O/GO) aqueous inks are facilely printed on polymide (PI) film and transformed to RGO/MoO₃ hybrids via thermal treatments at air atmosphere. The compound inks are water-based, inkjet-printable, and nontoxic for inkjet printing to form two-dimensional crystal materials. The physical properties of aqueous inks are optimized within a printable range characterized by the Ohnesorge number of 1 < Z < 14. The inkjet-printed symmetric micro-supercapacitors (MSCs) with poly(vinyl alcohol) (PVA)-H₂SO₄ gel electrolyte possess a wide voltage window of 0-0.8 V, excellent flexibility, a high volumetric specific capacitance of 22.5 F cm^-3 at 0.044 A cm^-3, as well as good cyclic stability due to the synergistic effect of RGO and MoO₃. Furthermore, the inkjet-printed composite MSCs delivered a maximum energy density of 2 mWh cm^-3 and a power density of 0.018 W cm^-3, and the capacity retention rate of inkjet-printed MSCs is still retained 82% even after 10 000 charge-discharge cycles, indicating good electrochemical properties. Above all, the as-designed inkjet printing technique shows potential for flexible and wearable energy storage electronics.

Reactive Conductive Ink Capable of In Situ and Rapid Synthesis of Conductive Patterns Suitable for Inkjet Printing

We report a fabrication method of the conductive pattern based on in situ reactive silver precursor inks by inkjet printing. The reactive silver precursor inks were prepared with ethylene glycol and deionized water mixture as the solvent, and silver nitrate as silver source. Sodium borohydride solution as the reducing agent was first coated on photographic paper by screen printing process, and then dried at 50 °C for 4 h. Furthermore, the reactive silver precursor inks were printed on a photographic paper coated with sodium borohydride using inkjet printing to form silver nanoparticles in situ due to redox reaction, and thus a conductive pattern was obtained. The effects of the reactive silver precursor ink concentration and printing layer number and treatment temperature on the electrical properties and microstructures of the printed patterns were investigated systematically. The size range of in situ-formed silver nanoparticles was 50–90 nm. When the reactive silver precursor ink concentration was 0.13 g/mL, the five-layer printed pattern exhibited a sheet resistance of 4.6 Ω/γ after drying at room temperature for 2 h; furthermore, the sheet resistance of the printed pattern decreased to 1.4 Ω/γ after drying at 130 °C for 2 h. In addition, the display function circuit was printed on the photographic paper to realize the display of the numbers 0–99. It provides new research ideas for the development of environmentally friendly and low-cost flexible paper-based circuits.

Layer-by-layer inkjet printing GO film and Ag nanoparticles supported nickel cobalt layered double hydroxide as a flexible and binder-free electrode for supercapacitors

Inkjet printing is an attractive technique in the field of flexible electronics due to the direct writing, digital controls and non-contact operation process. In this work, we successfully printed graphite oxide and Ag nanoparticles on the substrate of flexible carbon cloth to form a flexible, conductive and hydrophilic layer, which could be used as a new substrate with an electron transport layer of large surface area. In addition, Ni-Co LDH nanosheets as the main active materials were synthesized for improving the electrochemical activity via a convenient electrochemical deposition method. The binder-free Ni-Co LDH/Ag/rGO@CC electrode exhibits outstanding electrochemical performance along with a high capacity of 173 mA h g^-1 at 1 A g^-1. Moreover, an asymmetric supercapacitor (ASC) was assembled with Ni-Co LDH/Ag/rGO@CC electrode as the positive electrode materials and activated carbon coated CC as the negative electrode materials, showing a high capacity of 95 mA h g^-1 at 0.6 A g^-1, and maximum energy density of 76 Wh kg^-1 at a power density of 480 W kg^-1.

Screen-Printing of a Highly Conductive Graphene Ink for Flexible Printed Electronics

Conductive inks for the future printed electronics should have the following merits: high conductivity, flexibility, low cost, and compatibility with wide range of substrates. However, the state-of-the-art conductive inks based on metal nanoparticles are high in cost and poor in flexibility. Herein, we reported a highly conductive, low cost, and super flexible ink based on graphene nanoplatelets. The graphene ink has been screen-printed on plastic and paper substrates. Combined with postprinting treatments including thermal annealing and compression rolling, the printed graphene pattern shows a high conductivity of 8.81 × 10⁴ S m^-1 and good flexibility without significant conductivity loss after 1000 bending cycles. We further demonstrate that the printed highly conductive graphene patterns can act as current collectors for supercapacitors. The supercapacitor with the printed graphene pattern as the current collector and printed activated carbon as the active material shows a good rate capability of up to 200 mV s^-1. This work potentially provides a promising route toward the large-scale fabrication of low cost yet flexible printed electronic devices.

The direct-writing of low cost paper based flexible electrodes and touch pad devices using silver nano-ink and ZnO nanoparticles

We report a novel and simple approach for the synthesis of silver nanoparticles capped with inositol (Ag NPs/Ino) by the reduction of silver salt with ascorbic acid under basic conditions. UV-vis, TEM, FTIR and TGA techniques were used to characterize the Ag NPs/Ino to determine the size, shape and surface modification of the NPs. Stable silver nano-ink was prepared in aqueous solution containing 1% PVP (stabilizer) and glycerol (cosolvent) and was used for the direct-writing of a paper electrode with a roller ball-point pen for electrochemical applications. The solvent, stabilizing agents, concentration of NPs (10%), paper substrate, sintering temperature (40 °C) and sintering time (15 min) were optimized to obtain a uniform coating of Ag NPs on the paper substrate. Further, the synthesis and fabrication of ZnO NPs on a paper substrate was put forward to design a touch pad device based on the piezoelectric effect. The preparation of paper based devices suggests a direction for the development of a simple, low cost and compatible approach for the direct-writing of paper based flexible electrodes and electronics for future applications.

Printed supercapacitors: materials, printing and applications

This review summarizes how printing methods can revolutionize the manufacturing of supercapacitors – promising energy storage devices for flexible electronics.

Non-Invasive Flexible and Stretchable Wearable Sensors With Nano-Based Enhancement for Chronic Disease Care

Advances in flexible and stretchable electronics, functional nanomaterials, and micro/nano manufacturing have been made in recent years. These advances have accelerated the development of wearable sensors. Wearable sensors, with excellent flexibility, stretchability, durability, and sensitivity, have attractive application prospects in the next generation of personal devices for chronic disease care. Flexible and stretchable wearable sensors play an important role in endowing chronic disease care systems with the capability of long-term and real-time tracking of biomedical signals. These signals are closely associated with human body chronic conditions, such as heart rate, wrist/neck pulse, blood pressure, body temperature, and biofluids information. Monitoring these signals with wearable sensors provides a convenient and non-invasive way for chronic disease diagnoses and health monitoring. In this review, the applications of wearable sensors in chronic disease care are introduced. In addition, this review exploits a comprehensive investigation of requirements for flexibility and stretchability, and methods of nano-based enhancement. Furthermore, recent progress in wearable sensors-including pressure, strain, electrophysiological, electrochemical, temperature, and multifunctional sensors-is presented. Finally, opening research challenges and future directions of flexible and stretchable sensors are discussed.

Facile Activation of Commercial Carbon Felt as a Low-Cost Free-Standing Electrode for Flexible Supercapacitors

High cost, low capacitance, and complicated synthesis process are still the key limitations for carbon-negative materials to meet their industrial production and application in high-energy-density asymmetric supercapacitors (ASCs). In this work, we demonstrate the facile preparation of ultrahigh-surface-area free-standing carbon material from low-cost industrial carbon felt (CF) and its application for flexible supercapacitor electrode with outstanding performance. Through a simple freeze-drying-assisted activation method, the as-prepared activated CF (ACF) was endowed with satisfactory flexibility, ultrahigh specific surface area of 2109 m² g^-1, good electric conductivity (311 S m^-1), and excellent wettability to aqueous electrolyte. Owing to these merits, the ACF expressed an ultrahigh areal capacitance of 1441 mF cm^-2, a high specific capacitance ( C_s) of 280 F g^-1 based on the mass of the whole electrode, and an impressive cycling stability (87% retention after 5000 cycles). When applied as a flexible freestanding electrode for MnO₂//ACF ASCs, the ACF-based device provided satisfactory areal energy densities of 0.283 and 0.104 mWh cm^-2 in aqueous and quasi-solid electrolytes, respectively. The values outperform many previously reported carbon-based electrochemical devices. The low cost of raw material and the facile fabrication process, together with the high electrochemical performance, make our ACF electrode highly applicable for the mass production of flexible energy-storage devices.

Printable Nanomaterials for the Fabrication of High-Performance Supercapacitors

In recent years, supercapacitors are attracting great attention as one kind of electrochemical energy storage device, which have a high power density, a high energy density, fast charging and discharging, and a long cycle life. As a solution processing method, printing technology is widely used to fabricate supercapacitors. Printable nanomaterials are critical to the fabrication of high-performance supercapacitors by printing technology. In this work, the advantages of printing technology are summarized. Moreover, various nanomaterials used to fabricate supercapacitors by printing technology are presented. Finally, the remaining challenges and broad research as well as application prospects in printing high-performance supercapacitors with nanomaterials are proposed.