High-performance green flexible electronics based on biodegradable cellulose nanofibril paper

Degradation Behavior, Biocompatibility, Electrochemical Performance, and Circularity Potential of Transient Batteries

Transient technology seeks the development of materials, devices, or systems that undergo controlled degradation processes after a stable operation period, leaving behind harmless residues. To enable externally powered fully transient devices operating for longer periods compared to passive devices, transient batteries are needed. Albeit transient batteries are initially intended for biomedical applications, they represent an effective solution to circumvent the current contaminant leakage into the environment. Transient technology enables a more efficient recycling as it enhances material retrieval rates, limiting both human and environmental exposures to the hazardous pollutants present in conventional batteries. Little efforts are focused to catalog and understand the degradation characteristics of transient batteries. As the energy field is a property-driven science, not only electrochemical performance but also their degradation behavior plays a pivotal role in defining the specific end-use applications. The state-of-the-art transient batteries are critically reviewed with special emphasis on the degradation mechanisms, transiency time, and biocompatibility of the released degradation products. The potential of transient batteries to change the current paradigm that considers batteries as harmful waste is highlighted. Overall, transient batteries are ready for takeoff and hold a promising future to be a frontrunner in the uptake of circular economy concepts.

Cellulose-based polymers

The presented chapter deals with structure, morphology, and properties aspects concerning cellulose-based polymers in both research and industrial production, such as cellulose fibers, cellulose membranes, cellulose nanocrystals, and bacterial cellulose, etc. The idea was to highlight the main cellulose-based polymers and cellulose derivatives, as well as the dissolution technologies in processing cellulose-based products. The structure and properties of cellulose are introduced briefly. The main attention has been paid to swelling and dissolution of cellulose in order to yield various kinds of cellulose derivatives through polymerization. The main mechanisms and methods are also presented. Finally, the environmental friendly and green cellulose-based polymers will be evaluated as one of the multifunctional and smart materials with significant progress.

Investigation on the Role of Ionic Liquids in the Output Signal Produced by Bacterial Cellulose-Based Mechanoelectrical Transducers

Green sensors are required for the realization of a sustainable economy. Biopolymer-derived composites are a meaningful solution to such a needing. Bacterial Cellulose (BC) is a green biopolymer, with significant mechanical and electrical properties. BC-based composites have been proposed to realize generating mechanoelectrical transductors. The transductors consist of a sheet of BC, impregnated of Ionic Liquids (ILs), and covered with two layers of Conducting Polymer (CP) as the electrodes. Charges accumulate at the electrodes when the transductor is bent. Generating sensors can produce either Open Circuit (OC) voltage or Short Circuit (SC) current. In the paper, the OC voltage and SC current, generated from BC-based composites, in a cantilever configuration and subjected to dynamic deformation are compared. The influence of ILs in the transduction performance, both in the case of OC voltage and SC current is investigated. Experimental investigations of structural, chemical, and mechanoelectrical transduction properties, when the composite is dynamically bent, are performed. The mechanoelectrical investigation has been carried on both in the time and in the frequency domains. Reported results show that no relevant changes can be obtained because of the use of IL when the OC voltage is considered. On the contrary, dramatic changes are observed for the case of SC current, whose value increases by about two orders of magnitude.

Paper-Embedded Roll-to-Roll Mass Printed Piezoelectric Transducers

The trend to a world with ubiquitous electronics has the need for novel concepts for sensors and actuators that are lightweight, flexible, low-cost, and also sustainable. Piezoelectric transducers on the basis of functional polymers can meet these expectations. In this work, a novel concept for paper-embedded large-area piezoelectric devices realized solely by means of roll-to-roll (R2R) mass printing and post printing technologies including inline poling are introduced. The device set-up, as well as the process technology, offers the great opportunity for a cost-efficient and environmentally friendly mass production of thin and flexible organic large-area piezoelectric devices. As the functional layers are embedded into paper by the hot lamination of two poly(vinylidene fluoride-co-trifluoroethylene) P(VDF-TrFE) layers, the printed electronics is protected and invisible. The paper gives insights to the R2R printing of a 500 m long web including R2R post printing processes and electrical and acoustic inline characterization. Fully R2R processed devices show a high remnant polarization of up to 78 mC m-2 and can be realized with high yield of >90%. Finally, a 360° surround-sound installation realized with a 387 cm long paper web consisting of 56 piezoelectric speakers including wiring is presented.

Wearable Biosensors: An Alternative and Practical Approach in Healthcare and Disease Monitoring

With the increasing prevalence of growing population, aging and chronic diseases continuously rising healthcare costs, the healthcare system is undergoing a vital transformation from the traditional hospital-centered system to an individual-centered system. Since the 20th century, wearable sensors are becoming widespread in healthcare and biomedical monitoring systems, empowering continuous measurement of critical biomarkers for monitoring of the diseased condition and health, medical diagnostics and evaluation in biological fluids like saliva, blood, and sweat. Over the past few decades, the developments have been focused on electrochemical and optical biosensors, along with advances with the non-invasive monitoring of biomarkers, bacteria and hormones, etc. Wearable devices have evolved gradually with a mix of multiplexed biosensing, microfluidic sampling and transport systems integrated with flexible materials and body attachments for improved wearability and simplicity. These wearables hold promise and are capable of a higher understanding of the correlations between analyte concentrations within the blood or non-invasive biofluids and feedback to the patient, which is significantly important in timely diagnosis, treatment, and control of medical conditions. However, cohort validation studies and performance evaluation of wearable biosensors are needed to underpin their clinical acceptance. In the present review, we discuss the importance, features, types of wearables, challenges and applications of wearable devices for biological fluids for the prevention of diseased conditions and real-time monitoring of human health. Herein, we summarize the various wearable devices that are developed for healthcare monitoring and their future potential has been discussed in detail.

Highly flexible and degradable memory electronics comprised of all-biocompatible materials

Biocompatible materials have received increasing attention as one of the most important building blocks for flexible and transient memories. Herein, a fully biocompatible resistive switching (RS) memory electronic composed of a carbon dot (CD)-polyvinyl pyrrolidone (PVP) nanocomposite and a silver nanowire (Ag NW) network buried in a flexible gelatin film is introduced with promising nonvolatile RS characteristics for flexible and transient memory applications. The fabricated device exhibited a rewritable flash-type memory behavior, such as low operation voltage (≈-1.12 V), high ON/OFF ratio (>10²), long retention time (over 10⁴ s), and small bending radius (15 mm). As a proof of degradability, this transient memory can dissolve completely within 90 s after being immersed into deionized water at 55 °C; it can decompose naturally in soil within 6 days. This fully biocompatible memory electronic paves a novel way for flexible and wearable green electronics.

High-Efficiency Transfer Printing Using Droplet Stamps for Robust Hybrid Integration of Flexible Devices

Transfer printing has emerged as a deterministic assembly technique for moving thin-film semiconductors into desired layouts by using rubber stamps; however, replicating transfer printing for different semiconductors fails to achieve high efficiency, hindering the fast development of flexible hybrid electronics. In this work, a novel transfer printing technique using droplet stamps is developed based on Laplace pressure and surface tension. The working principle is explained by liquid bridge analysis and demonstrated by a 100% yield of transfer printing a batch of thin-film semiconductors with different materials, sizes, and shapes. Besides, the droplet stamps are used in fabricating epidermal hybrid optoelectronics for accurate blood pressure monitoring to verify their high working efficiency. Thus, taking advantage of eliminating Poisson effects and solving the incompatibility with conventional fabrication technologies, this technique will play an enabling role in hybrid integration and high-fidelity fabrication.

Flexible/Stretchable Supercapacitors with Novel Functionality for Wearable Electronics

With the miniaturization of personal wearable electronics, considerable effort has been expended to develop high-performance flexible/stretchable energy storage devices for powering integrated active devices. Supercapacitors can fulfill this role owing to their simple structures, high power density, and cyclic stability. Moreover, a high electrochemical performance can be achieved with flexible/stretchable supercapacitors, whose applications can be expanded through the introduction of additional novel functionalities. Here, recent advances in and future prospects for flexible/stretchable supercapacitors with innate functionalities are covered, including biodegradability, self-healing, shape memory, energy harvesting, and electrochromic and temperature tolerance, which can contribute to reducing e-waste, ensuring device integrity and performance, enabling device self-charging following exposure to surrounding stimuli, displaying the charge status, and maintaining the performance under a wide range of temperatures. Finally, the challenges and perspectives of high-performance all-in-one wearable systems with integrated functional supercapacitors for future practical application are discussed.

Advanced Materials and Systems for Biodegradable, Transient Electronics

Transient electronics refers to an emerging class of advanced technology, defined by an ability to chemically or physically dissolve, disintegrate, and degrade in actively or passively controlled fashions to leave environmentally and physiologically harmless by-products in environments, particularly in bio-fluids or aqueous solutions. The unusual properties that are opposite to operational modes in conventional electronics for a nearly infinite time frame offer unprecedented opportunities in research areas of eco-friendly electronics, temporary biomedical implants, data-secure hardware systems, and others. This review highlights the developments of transient electronics, including materials, manufacturing strategies, electronic components, and transient kinetics, along with various potential applications.

Hydroxypropyl Cellulose Adhesives for Transfer Printing of Carbon Nanotubes and Metallic Nanostructures

Transfer printing is one of the key nanofabrication techniques for the large-scale manufacturing of complex device architectures. It provides a cost-effective and high-throughput route for the integration of independently processed materials into spatially tailored architectures. Furthermore, this method enables the fabrication of flexible and curvilinear devices, paving the way for the fabrication of a new generation of technologies for optics, electronics, and biomedicine. In this work, hydroxypropyl cellulose (HPC) membranes are used as water soluble adhesives for transfer printing processes with improved performance and versatility compared to conventional silicone alternatives. The high-water solubility and excellent mechanical properties of HPC facilitate transfer printing with high yield for both metal and carbon nanotubes (CNTs) inks. In the case of metal inks, crack-free stripping of silver films and the simple fabrication of Moiré Plasmonic architectures of different geometries are demonstrated. Furthermore, HPC membranes are used to transfer print carbon nanotube films with different thicknesses and up to 77% transparency in the visible and near infrared region with potential applications as transparent conductive substrates. Finally, the use of prepatterned HPC membranes enables nanoscale patterning of CNT with feature resolution down to 1 µm.

Strain-free GeSn nanomembranes enabled by transfer-printing techniques for advanced optoelectronic applications

GeSn alloys have emerged as promising materials for silicon-based optoelectronic devices. However, the epitaxy of pseudomorphic GeSn layers on a Ge buffer is susceptible to a significant compressive strain that significantly hinders the performance of GeSn-based photonic devices. Herein, we report on a new strategy to produce strain-free GeSn nanomembranes for advanced optoelectronic applications. The GeSn alloy was grown on a silicon-on-insulator substrate using Ge buffers, and it has a residual compressive strain. By transfer-printing the GeSn/Ge/Si multi-layers, followed by etching the Si template and the Ge buffer layers, respectively, the residual compressive strain was completely removed to achieve strain-free GeSn layers. A bandgap reduction was also observed as a result of strain relaxation. Furthermore, theoretical analysis was performed to evaluate the effect of strain relaxation on the GeSn-based optoelectronic devices. The proposed approach offers a practical and viable method for preparing strain-free GeSn alloys for advanced optoelectronic applications.

Flexible and Green Electronics Manufactured by Origami Folding of Nanosilicate-Reinforced Cellulose Paper

Today's consumer electronics are made from nonrenewable and toxic components. They are also rigid, bulky, and manufactured in an energy-inefficient manner via CO₂-generating routes. Though petroleum-based polymers such as polyethylene terephthalate and polyethylene naphthalate can address the rigidity issue, they have a large carbon footprint and generate harmful waste. Scalable routes for manufacturing electronics that are both flexible and ecofriendly (Fleco) could address the challenges in the field. Ideally, such substrates must incorporate into electronics without compromising device performance. In this work, we demonstrate that a new type of wood-based [nanocellulose (NC)] material made via nanosilicate (NS) reinforcement can yield flexible electronics that can bend and roll without loss of electrical function. Specifically, the NSs interact electrostatically with NC to reinforce thermal and mechanical properties. For instance, films containing 34 wt % of NS displayed an increased young's modulus (1.5 times), thermal stability (290 → 310 °C), and a low coefficient of thermal expansion (40 ppm/K). These films can also easily be separated and renewed into new devices through simple and low-energy processes. Moreover, we used very cheap and environmentally friendly NC from American Value Added Pulping (AVAP) technology, American Process, and therefore, the manufacturing cost of our NS-reinforced NC paper is much cheaper ($0.016 per dm^-2) than that of conventional NC-based substrates. Looking forward, the methodology highlighted herein is highly attractive as it can unlock the secrets of Fleco electronics and transform otherwise bulky, rigid, and "difficult-to-process" rigid circuits into more aesthetic and flexible ones while simultaneously bringing relief to an already-overburdened ecosystem.

Sensors Made of Natural Renewable Materials: Efficiency, Recyclability or Biodegradability-The Green Electronics

Nowadays, sensor devices are developing fast. It is therefore critical, at a time when the availability and recyclability of materials are, along with acceptability from the consumers, among the most important criteria used by industrials before pushing a device to market, to review the most recent advances related to functional electronic materials, substrates or packaging materials with natural origins and/or presenting good recyclability. This review proposes, in the first section, passive materials used as substrates, supporting matrixes or packaging, whether organic or inorganic, then active materials such as conductors or semiconductors. The last section is dedicated to the review of pertinent sensors and devices integrated in sensors, along with their fabrication methods.

The Evolution of Flexible Electronics: From Nature, Beyond Nature, and To Nature

The flourishing development of multifunctional flexible electronics cannot leave the beneficial role of nature, which provides continuous inspiration in their material, structural, and functional designs. During the evolution of flexible electronics, some originated from nature, some were even beyond nature, and others were implantable or biodegradable eventually to nature. Therefore, the relationship between flexible electronics and nature is undoubtedly vital since harmony between nature and technology evolution would promote the sustainable development. Herein, materials selection and functionality design for flexible electronics that are mostly inspired from nature are first introduced with certain functionality even beyond nature. Then, frontier advances on flexible electronics including the main individual components (i.e., energy (the power source) and the sensor (the electric load)) are presented from nature, beyond nature, and to nature with the aim of enlightening the harmonious relationship between the modern electronics technology and nature. Finally, critical issues in next-generation flexible electronics are discussed to provide possible solutions and new insights in prospective exploration directions.

Degradable self-adhesive epidermal sensors prepared from conductive nanocomposite hydrogel

Conductive hydrogel-based epidermal sensors are attracting significant interest due to their great potential in soft robotics, electronic skins, bioelectronics and personalized healthcare monitoring. However, the conventional conductive hydrogel-based epidermal sensors cannot be degraded, resulting in the significant problem of waste, which will gradually increase the burden on the environment. Herein, degradable adhesive epidermal sensors were assembled using conductive nanocomposite hydrogels, which were prepared via the conformal coating of cellulose nanofiber (CNF) networks and supramolecular interaction among CNF, polydopamine (PDA), Fe³⁺, and polyacrylamide (PAM). They exhibited superior mechanical properties, reliable degradability (30 days in water), and excellent self-adhesiveness. The obtained hydrogels could be assembled as self-adhesive, degradable epidermal sensors for real-time human motion monitoring. Air could be sucked into the hydrogels during their swelling process, thereby oxidizing the tris-catechol-Fe³⁺ complexes and releasing Fe³⁺. Finally, the polymer networks were degraded via a Fenton-like reaction dominated by S₂O₈^2- and Fe(ii/iii) with the help of the catechol groups of PDA. This work paves the way for the potential fabrication of degradable, and self-adhesive epidermal sensors for applications in human-machine interactions, implantable bioelectronics, and personalized healthcare monitoring.

Preparation and Properties of Cellulose-Based Films Regenerated from Waste Corrugated Cardboards Using [Amim]Cl/CaCl2

1-Ally-3-methylimidazolium chloride ([Amim]Cl), dimethyl sulfoxide (DMSO), and CaCl₂ were selected to construct dissolution systems to produce value-added products from pretreatment of waste corrugated cardboards (P-WCCs). The dissolution behaviors of P-WCCs before and after ball milling were studied in different dissolution systems. The regenerated cellulose films were quickly and efficiently prepared via dissolving, regenerating, and pressurized drying. When 4 wt % waste corrugated cardboard was dissolved in [Amim]Cl for 4 h at 90 °C, the regenerated cellulose films featured tensile strengths as high as 59.00 MPa. Adding 40% DMSO and 2 wt % CaCl₂ increased the tensile strength of the film to a maximum value of 85.86 MPa. This demonstrates that DMSO improves the ability of WCC to dissolve in ionic liquids; Ca²⁺ improves the tensile strength and thermal stability of the regenerated cellulose film but reduces its transparency. This work provides a new, simple, and highly efficient way to use WCCs for packaging and wrapping.

Flexible Capacitive Humidity Sensors Based on Ionic Conductive Wood-Derived Cellulose Nanopapers

With the growing requirements for the renewability and sustainability of electronic products, environmentally friendly cellulose-based materials have attracted immense research interests and gained increasing prominence for electronic devices. Humidity sensors play an essential role in industries, agriculture, climatology, medical services, and daily life. Here, for the first time, we fabricate capacitive humidity sensors based on ionic conductive wood-derived cellulose nanopapers (WCNs). The WCN-based humidity sensors exhibited ultrahigh sensitivity, fast response, small hysteresis, and more importantly, a wide working range of relative humidity (RH). The sensors showed >10⁴ times increase in the sensing signal over the 7-94% RH range at 20 Hz, while many reported humidity sensors with high sensitivity often have the working range limited to high RH levels. Our sensors can realize the distinction of nuances in humidity and exhibit outstanding noncontact skin humidity sensing properties. Flexible WCN-based humidity sensors were also fabricated, and they displayed excellent sensing properties with long-time stability, endowing them with multifunctional applications. The contrast humidity sensing experiment compared to the existing commercial humidity sensor further demonstrated the higher and faster response of our WCN-based sensors. Thus, this work provides effective guidance for the design of high-performance humidity sensors using nanopapers and opens a new dimension for a variety of future applications.

Polysaccharide-based triboelectric nanogenerators: A review

Triboelectric nanogenerators (TENGs) are versatile electronic devices used for environmental energy harvesting and self-powered electronics with a wide range of potential applications. The rapid development of TENGs has caused great concern regarding the environmental impacts of conventional electronic devices. Under this context, researching alternatives to synthetic and toxic materials in electronics are of major significance. In this review, we focused on TENGs based on natural polysaccharide materials. Firstly, a general overview of the working mechanisms and materials for high-performance TENGs were summarized and discussed. Then, the recent progress of polysaccharide-based TENGs along with their potential applications reported in the literature from 2015 to 2020 was reviewed. Here, we aimed to present polysaccharide polymers as a promising and viable alternative to the development of green TENGs and tackle the challenges of recycling e-wastes.

Optically Transparent Multiscale Composite Films for Flexible and Wearable Electronics

One of the key breakthroughs enabling flexible electronics with novel form factors is the deployment of flexible polymer films in place of brittle glass, which is one of the major structural materials for conventional electronic devices. Flexible electronics requires polymer films with the core properties of glass (i.e., dimensional stability and transparency) while retaining the pliability of the polymer, which, however, is fundamentally intractable due to the mutually exclusive nature of these characteristics. An overview of a transparent fiber-reinforced polymer, which is suggested as a potentially viable structural material for emerging flexible/wearable electronics, is provided. This includes material concept and fabrication and a brief review of recent research progress on its applications over the past decade.

Electroless deposition of silver nanoparticles on cellulose nanofibrils for electromagnetic interference shielding films

High-efficient electromagnetic interference (EMI) shielding films were successfully fabricated by electroless deposition of silver nanoparticles (AgNPs) on polydopamine (PDA) functionalized cellulose nanofibrils (CNFs) as building blocks according to a pressured-extrusion film-forming process. PDA was first functionalized on CNFs by oxidative self-polymerization of dopamine (DA) to synthesize CNFs@PDA, on which AgNPs were electrolessly deposited. The obtained CNFs@PDA@AgNPs composite films served as tightly-connected conductive network, so as to significantly improve the whole electrical conductivity of the EMI shielding films. Utilizing CNFs as green and soft matrix, the fabricated CNFs@PDA@AgNPs EMI films show an outstanding mechanical enduring performance. The CNFs@PDA@AgNPs films with weight ratios of CNFs:AgNO₃ = 1:24 exhibited superb electrical conductivity of 1,000,000 S m^-1 and remarkable EMI shielding effectiveness of 93.8 dB at 8.2 GHz in X band. This work provides a simple methodology to achieve surface metallization of insulating fibers to supersede traditional metals for highly efficient EMI shielding applications.

Nacre-Inspired Black Phosphorus/Nanofibrillar Cellulose Composite Film with Enhanced Mechanical Properties and Superior Fire Resistance

Natural nacre offers an optimized guiding principle for the assembly of lightweight and high-strength nanocomposites with excellent mechanical properties. Inspired by the "brick-and-mortar" layered structure of natural nacre, we present a cohort of bioinspired nanocomposites consisting of nanofibrillar cellulose (NFC) and few-layer hydroxyl functionalized black phosphorus (BP-OH) via a vacuum-assisted filtration self-assembly procedure. Owing to the well dispersed two-dimensional (2D) BP-OH in one-dimensional (1D) NFC and strong interfacial hydrogen bonding between them, these novel nacre-like BP-OHx/NFC composite films show excellent mechanical performance with tensile strength up to 214.0 MPa, 300% increase compared to pure NFC and tensile fracture strain up to 23.8%, 1.8 times higher than that of pure NFC. Moreover, these nacre-like composite films bare good fire resistance and high thermal stability. This nacre-inspired approach demonstrates a promising strategy for designing high-performance BP-OHx/NFC composite film, and the obtained bioinspired material could be a potential candidate in the application of flexible construction materials and flame retarded insulation materials.

Biodegradable Materials and Green Processing for Green Electronics

There is little question that the "electronic revolution" of the 20th century has impacted almost every aspect of human life. However, the emergence of solid-state electronics as a ubiquitous feature of an advanced modern society is posing new challenges such as the management of electronic waste (e-waste) that will remain through the 21st century. In addition to developing strategies to manage such e-waste, further challenges can be identified concerning the conservation and recycling of scarce elements, reducing the use of toxic materials and solvents in electronics processing, and lowering energy usage during fabrication methods. In response to these issues, the construction of electronic devices from renewable or biodegradable materials that decompose to harmless by-products is becoming a topic of great interest. Such "green" electronic devices need to be fabricated on industrial scale through low-energy and low-cost methods that involve low/non-toxic functional materials or solvents. This review highlights recent advances in the development of biodegradable materials and processing strategies for electronics with an emphasis on areas where green electronic devices show the greatest promise, including solar cells, organic field-effect transistors, light-emitting diodes, and other electronic devices.

Highly Electroconductive Nanopapers Based on Nanocellulose and Copper Nanowires: A New Generation of Flexible and Sustainable Electrical Materials

Nowadays, the development of sustainable high-performance functional nanomaterials is in the spotlight. In this work, we report the preparation of a new generation of flexible and high electroconductive nanopapers based on nanofibrillated cellulose (NFC) and copper nanowires (CuNWs). Homogeneous red brick color nanopapers (thickness 30.2-36.4 μm) were obtained by mixing different amounts of NFC aqueous suspensions and CuNWs (1, 5, 10, 20, and 50 wt %), followed by vacuum filtration and drying. scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) analysis confirmed the incorporation of the different amounts of CuNWs, and their uniform and random distribution. All of the nanomaterials displayed good mechanical properties, viz., Young's modulus = 2.62-4.72 GPa, tensile strength = 30.2-70.6 MPa, and elongation at break = 2.3-4.1% for the nanopapers with 50 and 1 wt % of CuNWs mass fraction, respectively. The electrical conductivity of these materials strongly depends on the CuNW content, attaining a value of 5.43 × 10⁴ S·m^-1 for the nanopaper with a higher mass fraction. This is one of the highest values reported so far for nanocellulose-based conductive materials. Therefore, these nanopapers can be seen as an excellent inexpensive and green alternative to the current electroconductive materials for applications in electronic devices, energy storage, or sensors.

High-performance water-borne fluorescent acrylic-based adhesive: synthesis and application

Water-borne adhesives have immense importance in cellulose-based materials, where their durability, handling, and strength remain to be a major concern. The present work demonstrates the development of three water-borne adhesives, namely, poly(1-vinyl-2-pyrrolidone-co-acrylic acid), poly(acrylonitrile-co-acrylic acid), and poly(1-vinyl-2-pyrrolidone-co-acrylonitrile-co-acrylic acid) applicable for cellulose-based materials. These acrylic-acid based adhesives were characterized by Fourier-transform infra-red spectroscopy, thermogravimetric analysis, X-ray diffraction, gel permeation chromatography, and universal testing machine. The synthesized polymer adhesives can be stored in the powder form for a longer period, thus utilizing less space. In order to use as adhesives, suitable formulations can be prepared in water. The adhesives show thermal stability up to 300 °C. Our studies show that poly(1-vinyl-2-pyrrolidone-co-acrylonitrile-co-acrylic acid) showed higher lap shear strength (ASTM D-906) than commercially available adhesives. In addition, these adhesives, being fluorescent in nature, can be detected under UV light and thus are applicable for the detection of fractured joints of any specimen. This property also helps in anti-counterfeiting applications, thus adding further to their utility.

Synthesis and application of a water-borne fluorescent acrylic adhesive, which can be stored as a powder for long-term use.

Recyclable and Flexible Dual-Mode Electronics with Light and Heat Management

Realizing multiple functions and sustainable manufacturing within the same electronic device would be highly attractive from a design and fabrication perspective. Here we demonstrate a recyclable dual-mode thin-film device that can perform both light emission and heat management simultaneously. The device is composed of a dissolvable emitting layer sandwiched between two undissolvable conducting films. The vertical multilayered device enables a highly flexible and foldable multicolor electroluminescent emission ranging from yellow or blue to white, and the coplanar monolayered conductor achieves tunable Joule heat temperature setting. By utilizing selective dissolution and artificial reconstruction of each layered component, the parent device shows full recyclability and reconstructability without severe performance degradation after several recycles. The proof-of concept device provides an ideal strategy to construct a multifunctional film system with recyclability and makes a significant contribution to scientific and technological advancement in low-cost sustainable electronics and optoelectronics.

Heterogeneously integrated flexible microwave amplifiers on a cellulose nanofibril substrate

Low-cost flexible microwave circuits with compact size and light weight are highly desirable for flexible wireless communication and other miniaturized microwave systems. However, the prevalent studies on flexible microwave electronics have only focused on individual flexible microwave elements such as transistors, inductors, capacitors, and transmission lines. Thinning down supporting substrate of rigid chip-based monolithic microwave integrated circuits has been the only approach toward flexible microwave integrated circuits. Here, we report a flexible microwave integrated circuit strategy integrating membrane AlGaN/GaN high electron mobility transistor with passive impedance matching networks on cellulose nanofibril paper. The strategy enables a heterogeneously integrated and, to our knowledge, the first flexible microwave amplifier that can output 10 mW power beyond 5 GHz and can also be easily disposed of due to the use of cellulose nanofibril paper as the circuit substrate. The demonstration represents a critical step forward in realizing flexible wireless communication devices.

Though flexible microwave integrated circuits (MICs) are desirable for the construction of functional microwave amplifier circuits, realizing low cost III-V-based MMICs remains a challenge. Here, the authors report a heterogeneous integration strategy for the fabrication of flexible low-cost MICs.

Wafer-Scale Two-Dimensional MoS2 Layers Integrated on Cellulose Substrates Toward Environmentally Friendly Transient Electronic Devices

We explored the feasibility of wafer-scale two-dimensional (2D) molybdenum disulfide (MoS₂) layers toward futuristic environmentally friendly electronics that adopt biodegradable substrates. Large-area (> a few cm²) 2D MoS₂ layers grown on silicon dioxide/silicon (SiO₂/Si) wafers were delaminated and integrated onto a variety of cellulose-based substrates of various components and shapes in a controlled manner; examples of the substrates include planar papers, cylindrical natural rubbers, and 2,2,6,6-tetramethylpiperidine-1-oxyl-oxidized cellulose nanofibers. The integrated 2D layers were confirmed to well preserve their intrinsic structural and chemical integrity even on such exotic substrates. Proof-of-concept devices employing large-area 2D MoS₂ layers/cellulose substrates were demonstrated for a variety of applications, including photodetectors, pressure sensors, and field-effect transistors. Furthermore, we demonstrated the complete "dissolution" of the integrated 2D MoS₂ layers in a buffer solution composed of baking soda and deionized water, confirming their environmentally friendly transient characteristics. Moreover, the approaches to delaminate and integrate them do not demand any chemicals except for water, and their original substrates can be recycled for subsequent growths, ensuring excellent chemical benignity and process sustainability.

Nature-derived materials for the fabrication of functional biodevices

Nature provides an incredible source of inspiration, structural concepts, and materials toward applications to improve the lives of people around the world, while preserving ecosystems, and addressing environmental sustainability. In particular, materials derived from animal and plant sources can provide low-cost, renewable building blocks for such applications. Nature-derived materials are of interest for their properties of biodegradability, bioconformability, biorecognition, self-repair, and stimuli response. While long used in tissue engineering and regenerative medicine, their use in functional devices such as (bio)electronics, sensors, and optical systems for healthcare and biomonitoring is finding increasing attention. The objective of this review is to cover the varied nature derived and sourced materials currently used in active biodevices and components that possess electrical or electronic behavior. We discuss materials ranging from proteins and polypeptides such as silk and collagen, polysaccharides including chitin and cellulose, to seaweed derived biomaterials, and DNA. These materials may be used as passive substrates or support architectures and often, as the functional elements either by themselves or as biocomposites. We further discuss natural pigments such as melanin and indigo that serve as active elements in devices. Increasingly, combinations of different biomaterials are being used to address the challenges of fabrication and performance in human monitoring or medicine. Finally, this review gives perspectives on the sourcing, processing, degradation, and biocompatibility of these materials. This rapidly growing multidisciplinary area of research will be advanced by a systematic understanding of nature-inspired materials and design concepts in (bio)electronic devices.

Active Transiency: A Novel Approach to Expedite Degradation in Transient Electronics

Transient materials/electronics is an emerging class of technology concerned with materials and devices that are designed to operate over a pre-defined period of time, then undergo controlled degradation when exposed to stimuli. Degradation/transiency rate in solvent-triggered devices is strongly dependent on the chemical composition of the constituents, as well as their interactions with the solvent upon exposure. Such interactions are typically slow, passive, and diffusion-driven. In this study, we are introducing and exploring the integration of gas-forming reactions into transient materials/electronics to achieve expedited and active transiency. The integration of more complex chemical reaction paths to transiency not only expedites the dissolution mechanism but also maintains the pre-transiency stability of the system while under operation. A proof-of-concept transient electronic device, utilizing sodium-bicarbonate/citric-acid pair as gas-forming agents, is demonstrated and studied vs. control devices in the absence of gas-forming agents. While exhibiting enhanced transiency behavior, substrates with gas-forming agents also demonstrated sufficient mechanical properties and physical stability to be used as platforms for electronics.

Wood-Based Flexible Electronics

Next-generation electronics (e.g., substrate and conductor) need to be high performance, multifunctional, and environmentally friendly. Here, we report the creation of a fully wood-based flexible electronics circuit meeting these requirements, where the substrate, a strong, flexible and transparent wood film, is printed with a lignin-derived carbon nanofibers conductive ink. The wood film fabrication involves extensive removal of lignin and hemicellulose to tailor the nanostructure of the material followed by collapsing of the cell walls. This process preserves the original alignment of the cellulose nanofibers and promotes their binding. The film is flexible, yet strong in fiber direction with a Young's modulus and a tensile strength of 49.9 GPa and 469.9 MPa, respectively. Furthermore, a sustainable and bio-based conductive ink is formulated with lignin-derived carbon nanofibers. The bio-based ink is printed on transparent wood film, and a strain sensor application of the printed circuit is demonstrated. Combining the transparent wood film with the conductive ink produces environmental friendly and sustainable wood-based electronics for potential applications such as flexible circuits and sensors. Moreover, we envision the potential for a scalable and continuous fabrication process as well as end-of-life recyclability.

Flexible potentiometric pH sensors for wearable systems

There is a growing demand for developing wearable sensors that can non-invasively detect the signs of chronic diseases early on to possibly enable self-health management. Among these the flexible and stretchable electrochemical pH sensors are particularly important as the pH levels influence most chemical and biological reactions in materials, life and environmental sciences. In this review, we discuss the most recent developments in wearable electrochemical potentiometric pH sensors, covering the key topics such as (i) suitability of potentiometric pH sensors in wearable systems; (ii) designs of flexible potentiometric pH sensors, which may vary with target applications; (iii) materials for various components of the sensor such as substrates, reference and sensitive electrode; (iv) applications of flexible potentiometric pH sensors, and (v) the challenges relating to flexible potentiometric pH sensors.

Natural and Eco-Friendly Materials for Triboelectric Energy Harvesting

Triboelectric nanogenerators (TENGs) are promising electric energy harvesting devices as they can produce renewable clean energy using mechanical excitations from the environment. Several designs of triboelectric energy harvesters relying on biocompatible and eco-friendly natural materials have been introduced in recent years. Their ability to provide customizable self-powering for a wide range of applications, including biomedical devices, pressure and chemical sensors, and battery charging appliances, has been demonstrated. This review summarizes major advances already achieved in the field of triboelectric energy harvesting using biocompatible and eco-friendly natural materials. A rigorous, comparative, and critical analysis of preparation and testing methods is also presented. Electric power up to 14 mW was already achieved for the dry leaf/polyvinylidene fluoride-based TENG devices. These findings highlight the potential of eco-friendly self-powering systems and demonstrate the unique properties of the plants to generate electric energy for multiple applications.

Introducing Temperature-Controlled Phase Transition Elastin-like Polypeptides to Transient Electronics: Realization of Proactive Biotriggered Electronics with Local Transience

Transient electronics have dramatically changed inner-body therapy in health care. They stand out because of their harmless dissolution in the human body with no lingering electronic trash. However, high-precision biomedical implants require programmable and serial remedy operations, and controlling the whole-device destruction is not proactive and precise. Thus, a novel biotriggered and temperature-controlled transient electronics fabrication method using elastin-like polypeptides (ELPs) as triggers is proposed. Biocompatible ELPs simply mixed with trace silver nanowire (AgNW) can serve as the "switch" for the electronics to respond to local temperature changes in deionized water, exhibiting an agile response time. A ratio gradient experiment of the ELPs and AgNW shows that more programmable and precise transience properties (initial resistance, ready time, response time, and stable resistance) can be achieved by using a designated proportion. Further, we validated that the 3D-printing-based ELP-triggering transient electronics fabrication method is very simple yet effective for preparing transient wireless charging LEDs. Transient devices comprising ELPs-AgNW and PLGA-Ag respond within 160 s below 10 °C and degrade within a certain period.

3D Smith charts scattering parameters frequency-dependent orientation analysis and complex-scalar multi-parameter characterization applied to Peano reconfigurable vanadium dioxide inductors

Recently, the field of Metal-Insulator-Transition (MIT) materials has emerged as an unconventional solution for novel energy efficient electronic functions, such as steep slope subthermionic switches, neuromorphic hardware, reconfigurable radiofrequency functions, new types of sensors, terahertz and optoelectronic devices. Employing radiofrequency (RF) electronic circuits with a MIT material like vanadium Dioxide, VO₂, requires appropriate characterization tools and fabrication processes. In this work, we develop and use 3D Smith charts for devices and circuits having complex frequency dependences, like the ones resulting using MIT materials. The novel foundation of a 3D Smith chart involves here the geometrical fundamental notions of oriented curvature and variable homothety in order to clarify first theoretical inconsistencies in Foster and Non Foster circuits, where the driving point impedances exhibit mixed clockwise and counter-clockwise frequency dependent (oriented) paths on the Smith chart as frequency increases. We show here the unique visualization capability of a 3D Smith chart, which allows to quantify orientation over variable frequency. The new 3D Smith chart is applied as a joint complex-scalar 3D multi-parameter modelling and characterization environment for reconfigurable RF design exploiting Metal-Insulator-Transition (MIT) materials. We report fabricated inductors with record quality factors using VO₂ phase transition to program multiple tuning states, operating in the range 4 GHz to 10 GHz.

Lignin Redistribution for Enhancing Barrier Properties of Cellulose-Based Materials

Renewable cellulose-based materials have gained increasing interest in food packaging because of its favorable biodegradability and biocompatibility, whereas the barrier properties of hydrophilic and porous fibers are inadequate for most applications. Exploration of lignin redistribution for enhancing barrier properties of paper packaging material was carried out in this work. The redistribution of nanolized alkali lignin on paper surface showed excellent water, grease, and water vapor barrier. It provided persisted water (contact angle decrease rate at 0.05°/s) and grease (stained area undetectable at 72 h) resistance under long-term moisture or oil direct contact conditions, which also inhibited the bacterial growth to certain degree. Tough water vapor transmission rate can be lowered 82% from 528 to 97 g/m2/d by lignin redistribution. The result suggests that alkali lignin, with multiple barrier properties, has great potential in bio-based application considering the biodegradability, biocompatibility, and recyclability.

"Return to the Soil" Nanopaper Sensor Device for Hyperdense Sensor Networks

A nanopaper sensor device that combines humidity sensing, wireless information transmission, and degradability has been fabricated using wood-derived nanopaper as the substrate and dielectric layers. The nanopaper shows excellent suitability for capacitor dielectric layers because of its high dielectric constant, insulating properties suitable for thin-film formation, and lamination properties. A wireless transmission circuit containing the nanopaper capacitor can transmit radio signals in the megahertz band, and the relative humidity change can be output as a change in the radio signal owing to the humidity sensitivity of the nanopaper capacitor. More than 95% of the total volume of the nanopaper sensor device decomposes in soil after 40 days. Because the nanopaper sensor device does not need to be recovered, it is expected to greatly contribute to a sustainable society through realization of hyperdense observation networks by mass installation of sensor devices.

Methods and applications of nanocellulose loaded with inorganic nanomaterials: A review

Nanocellulose obtained from natural renewable resources has attracted enormous interests owing to its unique morphological characteristics, excellent mechanical strength, biocompatibility and biodegradability for a variety of applications in many fields. The template structure, high specific surface area, and active surface groups make it feasible to conduct surface modification and accommodate various nano-structured materials via physical or chemical deposition. The review presented herein focuses on the methodologies of loading different nano-structured materials on nanocellulose, including metals, nanocarbons, oxides, mineral salt, quantum dots and nonmetallic elements; and further describes the applications of nanocellulose composites in the fields of catalysis, optical electronic devices, biomedicine, sensors, composite reinforcement, photoswitching, flame retardancy, and oil/water separation.

Ultraflexible and Lightweight Bamboo-Derived Transparent Electrodes for Perovskite Solar Cells

Wearable devices are mainly based on plastic substrates, such as polyethylene terephthalate and polyethylene naphthalate, which causes environmental pollution after use due to the long decomposition periods. This work reports on the fabrication of a biodegradable and biocompatible transparent conductive electrode derived from bamboo for flexible perovskite solar cells. The conductive bioelectrode exhibits extremely flexible and light-weight properties. After bending 3000 times at a 4 mm curvature radius or even undergoing a crumpling test, it still shows excellent electrical performance and negligible decay. The performance of the bamboo-based bioelectrode perovskite solar cell exhibits a record power conversion efficiency (PCE) of 11.68%, showing the highest efficiency among all reported biomass-based perovskite solar cells. It is remarkable that this flexible device has a highly bendable mechanical stability, maintaining over 70% of its original PCE during 1000 bending cycles at a 4 mm curvature radius. This work paves the way for perovskite solar cells toward comfortable and environmentally friendly wearable devices.