Paper electronics

Green Electrochemical Point-of-Care Devices: Transient Materials and Sustainable Fabrication Methods

The spread of point-of-care (PoC) diagnostic tests using electrochemical sensors poses a significant environmental challenge, especially in limited-resource settings due to the lack of waste management infrastructure. This issue is expected to intensify with the emergence of the Internet of Medical Things (IoMT), necessitating eco-friendly solutions for disposable devices. This review discusses efforts to develop green and sustainable PoC diagnostic devices, clarifying terms like biodegradability and transient electronics. It explores potential transient and biodegradable materials and fabrication technologies, emphasizing sustainable electronics with low-energy consumption and low-carbon footprint techniques, particularly favoring printing methods. The review highlights examples of necessary electronic components containing biodegradable materials for electrochemical PoC devices and discusses their role in device sustainability. Finally, it examines the feasibility of integrating these components and technologies into comprehensive biodegradable PoC devices, addressing the imminent need for eco-friendly solutions in diagnostic testing. This comprehensive discussion serves as a guide for researchers and developers striving to mitigate the environmental impact of PoC testing in the era of IoMT and personalized medicine.

Printed Lithography of Graphene-Perovskite Quantum Dot Hybrid Photodetectors on Paper Substrates

Paper is an ideal platform for creating flexible and eco-friendly electronic systems. Leveraging the synergistic integration of zero- and two-dimensional materials, it unfolds a broad spectrum of applications within the realm of the Internet of Things (IoT), spanning from wearable electronics to smart packaging solutions. However, for paper without a polymer coating, the rough and porous nature presents significant challenges as a substrate for electronics, and the absence of well-established fabrication methods further hinders its application in wearable electronics. In this study, we present photodetectors (PDs) on a paper substrate composed of graphene and CsPbBr3 perovskite quantum dots (PQDs). Hybrid structures that combine PQDs with graphene offer a promising approach for PDs. These structures benefit from robust quantum confinement in PQDs alongside improved light interaction, tunable spectra, high absorption coefficients, and an enhanced photoconductive gain mechanism in graphene, all at ambient conditions. We use a microplotter for the lithographic printing of graphene, silver electrodes, and PQDs, to fabricate PDs on paper. These PDs have an external responsivity of ∼82,000 AW-1 at 520 nm for an operating voltage ⩽1 V. The external responsivity is 3 orders of magnitude higher than state-of-the-art paper-based PDs. Under bending at L0/L = 1.15 (L0 is the arc length and L is the chord length) and after 600 bending cycles, the external responsivity is maintained up to 80%. Thus, the combination of zero- and two-dimensional materials via microplotting on a paper substrate shows promise for wearable and flexible applications.

Paper-Based Sensors: Fantasy or Reality?

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

Machine Learning Strategy for Optimizing Multiple Electrical Characteristics in Dual-Layer Oxide Thin Film Transistors

A machine learning (ML) strategy is suggested to optimize dual-layer oxide thin film transistor (TFTs) performance. In this study, Bayesian optimization (BO), an algorithm recognized for its efficiency in optimizing material design, is applied to guide the design of a channel layer composed of IZO and IGZO. The sputtering fabrication process, which has attracted attention as an oxide semiconductor channel layer deposition method, is fine-tuned using ML to enhance multiple electrical characteristics of transistors: field-effect mobility, threshold voltage, and subthreshold swing. Using BO, the sputtering conditions─plasma power, pressure, and gas ratio, which intricately influence device performance─were modified using 19 data sets of 84 scenarios. It reveals that the modulated process conditions improve field-effect mobility up to 46.7 cm2V-1s-1, achieving more than double the performance of conventional IGZO TFTs. Furthermore, it was observed that threshold voltage is optimized to zero voltage, and the subthreshold swing is considerably improved, contributing to reduced power consumption. This study demonstrates that leveraging ML to optimize TFTs design not only accelerates the design process but also improves device performance dramatically. Overall, this ML strategy manages complex correlations among process parameters, properties, and performance and sets a precedent for the expeditious optimization of semiconductor devices.

An Investigation of the Indentation Elastic Modulus for Metal Films on Flexible Substrates Considering the Substrate Effect

The accurate measurement of the elastic modulus of thin metal films on flexible substrates is critical for understanding the mechanical reliability of flexible electronics. However, conventional methods, such as the Oliver-Pharr model, often underestimate the modulus due to substrate effects, particularly with low-modulus substrates like polyimide (PI). In this study, we propose an improved weighting model that replaces the empirical weighting factor with a variable X to better account for substrate contributions. Nanoindentation experiments were performed on Cu and Al films with thicknesses of 0.5, 1, and 1.5 μm, deposited on PI and silicon substrates. The results show a significant underestimation of the elastic modulus when traditional methods were applied, especially on PI substrates, where values decreased by up to 95%. Using the proposed X-based model, the corrected elastic modulus aligned with the inherent properties of the films, with errors reduced to within 2%. A finite element analysis (FEA) validated the stress and displacement distributions, demonstrating the substrate's influence on indentation behavior. This study provides a robust framework for accurately measuring the elastic modulus of thin films on flexible substrates, paving the way for a more reliable mechanical characterization in flexible electronics.

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

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

Transforming industrial byproduct to eco-friendly functional material: Ground-granulated blast furnace slag reinforced paper for renewable energy storage

This study pioneered an eco-friendly approach for reutilizing Ground-granulated blast furnace slag (GGBFS) in paper production. This investigation is the first study focusing on the usage of paper production that presents both a new usage area of GGBFS and also a new sight. So, it can contribute to save the trees. Also, GGBFS gains economical value in paper production. 15-25 % integrated slag led to markedly enhanced brightness, density and smoothness accompanied by only minor mechanical strength decreases versus pure pulp. Significantly, the electrical analysis revealed a higher conductivity at higher frequency region reaching almost S value near to 1 which might be a good choice for electromagnetic shielding, thus; higher conductivity with increasing slag contents from pure paper's 10-11 S/cm up to 10-6 S/cm for 25 % addition which confirms the modified paper's usefulness as conductive slag agent. Although the higher addition of GGBFS has led to rising in relaxation time basically from 1.77e-4 to 2.95e-3 and based on Debye relaxation, the rising time in relaxation which was observed after the addition of GGBFS reveals better polarizability values 0.29-0.35 compared to control sample 0.26 by which both longer relaxation time and higher polarizability contribute to the ability of energy storage of modified papers. The conductive characteristics and improved qualities demonstrate these recyclable slag-modified papers present unique opportunities for emerging flexible, eco-friendly electronics, capacitors, electromagnetic shielding, and renewable energy storage applications. Overall, novel integration and characterization of slag waste for enhanced sustainable paper products pioneers an unexplored territory.

Shellac-based materials: Structures, properties, and applications

Shellac stands out among natural polymers as the sole animal-derived resin, boasting a complex polyester composition comprising polyhydroxy long-chain fatty acids and sesquiterpene acids. Its unique attributes include biocompatibility, non-toxicity, distinctive amphiphilicity, superb film-forming and adhesive properties, excellent dielectric properties, rapid drying, and solubility in alkaline solutions while resisting acidic ones. These exceptional qualities have propelled shellac beyond its traditional role as a varnish and decorative material, positioning it as a viable option for diverse applications such as food packaging, pharmaceutical formulations, electronic devices, fiber dyeing, and wood restoration. Furthermore, shellac serves as a crucial carbon source for graphene materials. This review comprehensively explores shellac's contributions to prolonging food shelf life, enhancing the carbon sourcing of graphene materials, facilitating the delivery of active substances, boosting the performance of organic field-effect transistors, enabling environmentally friendly textile dyeing, and providing protective coatings for wood. Additionally, it delves into the current limitations and future directions of shellac's applications. By disseminating this knowledge, we aim to deepen researchers' comprehension of shellac and inspire further exploration, thereby fostering sustainable advancements across various industries.

Versatile Papertronics: Photo-Induced Synapse and Security Applications on Papers

Paper is a readily available material in nature. Its recyclability, eco-friendliness, portability, flexibility, and affordability make it a favored substrate for researchers seeking cost-effective solutions. Electronic devices based on solution process are fabricated on paper and banknotes using PVK and SnO2 nanoparticles. The devices manufactured on paper substrates exhibit photosynaptic behavior under ultraviolet pulse illumination, stemming from numerous interactions on the surface of the SnO2 nanoparticles. A light-modulated artificial synapse device is realized on a paper at a low voltage bias of -0.01 V, with an average recognition rate of 91.7% based on the Yale Face Database. As a security device on a banknote, 400 devices in a 20 × 20 array configuration exhibited random electrical characteristics owing to the local morphology of the SnO2 nanoparticles and differences in the depletion layer width at the SnO2/PVK interface. The security Physically Unclonable Functions (PUF) key based on the current distribution extracted at -1 V show unpredictable reproducibility with 50% uniformity, 48.7% inter-Hamming distance, and 50.1% bit-aliasing rates. Moreover, the device maintained its properties for more than 210 days under a curvature radius of 8.75 mm and bias and UV irradiation stress conditions.

Recent advances in implantable sensors and electronics using printable materials for advanced healthcare

This review article focuses on the recent printing technological progress in healthcare, underscoring the significant potential of implantable devices across diverse applications. Printing technologies have widespread use in developing health monitoring devices, diagnostic systems, and surgical devices. Recent years have witnessed remarkable progress in fabricating low-profile implantable devices, driven by advancements in printing technologies and nanomaterials. The importance of implantable biosensors and bioelectronics is highlighted, specifically exploring printing tools using bio-printable inks for practical applications, including a detailed examination of fabrication processes and essential parameters. This review also justifies the need for mechanical and electrical compatibility between bioelectronics and biological tissues. In addition to technological aspects, this article delves into the importance of appropriate packaging methods to enhance implantable devices' performance, compatibility, and longevity, which are made possible by integrating cutting-edge printing technology. Collectively, we aim to shed light on the holistic landscape of implantable biosensors and bioelectronics, showcasing their evolving role in advancing healthcare through innovative printing technologies.

Paper-based material with hydrophobic and antimicrobial properties: Advanced packaging materials for food applications

The environmental challenges posed by plastic pollution have prompted the exploration of eco-friendly alternatives to disposable plastic packaging and utensils. Paper-based materials, derived from renewable resources such as wood pulp, non-wood pulp (bamboo pulp, straw pulp, reed pulp, etc.), and recycled paper fibers, are distinguished by their recyclability and biodegradability, making them promising substitutes in the field of plastic food packaging. Despite their merits, challenges like porosity, hydrophilicity, limited barrier properties, and a lack of functionality have restricted their packaging potential. To address these constraints, researchers have introduced antimicrobial agents, hydrophobic substances, and other functional components to improve both physical and functional properties. This enhancement has resulted in notable improvements in food preservation outcomes in real-world scenarios. This paper offers a comprehensive review of recent progress in hydrophobic antimicrobial paper-based materials. In addition to outlining the characteristics and functions of commonly used antimicrobial substances in food packaging, it consolidates the current research landscape and preparation techniques for hydrophobic paper. Furthermore, the paper explores the practical applications of hydrophobic antimicrobial paper-based materials in agricultural produce, meat, and seafood, as well as ready-to-eat food packaging. Finally, challenges in production, application, and recycling processes are outlined to ensure safety and efficacy, and prospects for the future development of antimicrobial hydrophobic paper-based materials are discussed. Overall, the emergence of hydrophobic antimicrobial paper-based materials stands out as a robust alternative to plastic food packaging, offering a compelling solution with superior food preservation capabilities. In the future, paper-based materials with antimicrobial and hydrophobic functionalities are expected to further enhance food safety as promising packaging materials.

On paper characterisation of droplet and evaporation study using impedance spectroscopy

The development of paper-based devices has drawn a significant amount of attention, ranging from the creation of paper electronics to microfluidic devices. The flow of fluids through the paper substrate can be controlled by establishing a variety of barriers, which can be accomplished by either cutting or producing layers that are hydrophobic. Through the utilisation of this feature, a number of investigations, including mixing, modifying, and analytical studies, have been carried out on the paper substrate. However, because of the difficulties associated with its wettability, it is seldom investigated for the purpose of conducting evaporation studies of droplets. Traditionally, evaporation studies are carried out on a solid substrate like glass or silicon. Here we report a paper chip employing an impedance method to determine the characteristics of the droplet. It is also possible to determine the identity of the droplet by utilising the dielectric property of the liquid on a paper chip. A comparison is made between the traditional method of evaporation and the usage of the paper chip for the purpose of studying the evaporation of various liquids, ranging from ionic chemicals to volatile compounds. A subsequent step involves the utilisation of an electrical equivalent circuit in order to acquire the complex system attribute of the evaporation of the cellulose fibres. Finally, this reveals that paper chips have a significant amount of promise for use in scientific applications regarding evaporation analysis.

Sustainable paper electronics and neuromorphic paper chip

Paper electronics have received a lot of attention due to their special properties of mechanical flexibility/foldability, sustainability, biodegradability, light weight, and low cost. It provides a superb on-chip prototype with simple modular design and feasible energy-autonomous features, which can surpass the problems of inconvenience and possible pollution caused by conventional power sources by integrating different functional modules. Commonly, the sustainable operation of integrated paper electronics can be guaranteed by the basic components, including energy-harvesting devices, energy-storage devices, and low-power-consuming functional circuits/devices. Furthermore, sustainable paper electronics are possible to be further extended to develop energy-efficient neuromorphic paper chip by utilizing cutting-edge neuromorphic components based on traditional paper-based transistors, memories, and logic gates toward potential in-memory computing applications. The working process of the sustainable paper electronics implies an energy cycling of surrounding energy conversion, electrochemical energy storage, and energy utilization in functional circuits (in the form of photonic, thermal, electromagnetic, or mechanical energy). Sustainable paper electronics provide a promising path for achieving efficient, cost-effective, and customizable integrated electronics and self-powered systems with complementary features.

Carbon-Based Composites with Biodegradable Matrix for Flexible Paper Electronics

The authors explore the development of paper-based electronics using carbon-based composites with a biodegradable matrix based on ethyl cellulose and dibasic ester solvent. The main focus is on screen-printing techniques for creating flexible, eco-friendly electronic devices. This research evaluates the printability with the rheological measurements, electrical properties, flexibility, and adhesion of these composites, considering various compositions, including graphene, graphite, and carbon black. The study finds that certain compositions offer sheet resistance below 1 kΩ/sq and good adhesion to paper substrates with just one layer of screen printing, demonstrating the potential for commercial applications, such as single-use electronics, flexible heaters, etc. The study also shows the impact of cyclic bending on the electrical parameters of the prepared layers. This research emphasizes the importance of the biodegradability of the matrix, contributing to the field of sustainable electronics. Overall, this study provides insights into developing environmentally friendly, flexible electronic components, highlighting the role of biodegradable materials in this evolving industry.

MXene Nanosheets-Decorated Paper as a Green Electronics Material for Biosensing

This research delves into the development and optimization of MXene nanosheet-based paper electrodes, emphasizing their adaptability in green electronics and diverse applications. Xuan paper, a cellulose-based material, was identified as an ideal substrate for its mechanical attributes and capacity to accommodate MXene, further yielding outstanding electrical conductivity. The MXene paper electrode demonstrated consistent performance under various conditions, showing its potential in the field of wearable electronics and medical devices. Notably, its impressive electrothermal capabilities and environmentally conscious decomposition mechanism make it a promising candidate for future green electronic applications. Overall, this study underscores the electrode's harmonization of performance and environmental sustainability, paving the way for its integration into futuristic electronic solutions.

Printable devices for neurotechnology

Printable electronics for neurotechnology is a rapidly emerging field that leverages various printing techniques to fabricate electronic devices, offering advantages in rapid prototyping, scalability, and cost-effectiveness. These devices have promising applications in neurobiology, enabling the recording of neuronal signals and controlled drug delivery. This review provides an overview of printing techniques, materials used in neural device fabrication, and their applications. The printing techniques discussed include inkjet, screen printing, flexographic printing, 3D printing, and more. Each method has its unique advantages and challenges, ranging from precise printing and high resolution to material compatibility and scalability. Selecting the right materials for printable devices is crucial, considering factors like biocompatibility, flexibility, electrical properties, and durability. Conductive materials such as metallic nanoparticles and conducting polymers are commonly used in neurotechnology. Dielectric materials, like polyimide and polycaprolactone, play a vital role in device fabrication. Applications of printable devices in neurotechnology encompass various neuroprobes, electrocorticography arrays, and microelectrode arrays. These devices offer flexibility, biocompatibility, and scalability, making them cost-effective and suitable for preclinical research. However, several challenges need to be addressed, including biocompatibility, precision, electrical performance, long-term stability, and regulatory hurdles. This review highlights the potential of printable electronics in advancing our understanding of the brain and treating neurological disorders while emphasizing the importance of overcoming these challenges.

Transparent Structures for ZnO Thin Film Paper Transistors Fabricated by Pulsed Electron Beam Deposition

Thin film transistors on paper are increasingly in demand for emerging applications, such as flexible displays and sensors for wearable and disposable devices, making paper a promising substrate for green electronics and the circular economy. ZnO self-assembled thin film transistors on a paper substrate, also using paper as a gate dielectric, were fabricated by pulsed electron beam deposition (PED) at room temperature. These self-assembled ZnO thin film transistor source-channel-drain structures were obtained in a single deposition process using 200 and 300 µm metal wires as obstacles in the path of the ablation plasma. These transistors exhibited a memory effect, with two distinct states, "on" and "off", and with a field-effect mobility of about 25 cm2/Vs in both states. For the "on" state, a threshold voltage (Vth on = -1.75 V) and subthreshold swing (S = 1.1 V/decade) were determined, while, in the "off" state, Vth off = +1.8 V and S = 1.34 V/decade were obtained. A 1.6 μA maximum drain current was obtained in the "off" state, and 11.5 μA was obtained in the "on" state of the transistor. Due to ZnO's non-toxicity, such self-assembled transistors are promising as components for flexible, disposable smart labels and other various green paper-based electronics.

Low-dimensional nanostructures for monolithic 3D-integrated flexible and stretchable electronics

Flexible/stretchable electronics, which are characterized by their ultrathin design, lightweight structure, and excellent mechanical robustness and conformability, have garnered significant attention due to their unprecedented potential in healthcare, advanced robotics, and human-machine interface technologies. An increasing number of low-dimensional nanostructures with exceptional mechanical, electronic, and/or optical properties are being developed for flexible/stretchable electronics to fulfill the functional and application requirements of information sensing, processing, and interactive loops. Compared to the traditional single-layer format, which has a restricted design space, a monolithic three-dimensional (M3D) integrated device architecture offers greater flexibility and stretchability for electronic devices, achieving a high-level of integration to accommodate the state-of-the-art design targets, such as skin-comfort, miniaturization, and multi-functionality. Low-dimensional nanostructures possess small size, unique characteristics, flexible/elastic adaptability, and effective vertical stacking capability, boosting the advancement of M3D-integrated flexible/stretchable systems. In this review, we provide a summary of the typical low-dimensional nanostructures found in semiconductor, interconnect, and substrate materials, and discuss the design rules of flexible/stretchable devices for intelligent sensing and data processing. Furthermore, artificial sensory systems in 3D integration have been reviewed, highlighting the advancements in flexible/stretchable electronics that are deployed with high-density, energy-efficiency, and multi-functionalities. Finally, we discuss the technical challenges and advanced methodologies involved in the design and optimization of low-dimensional nanostructures, to achieve monolithic 3D-integrated flexible/stretchable multi-sensory systems.

A new strategy to improve the dielectric properties of cellulose nanocrystals (CNCs): Surface modification of small molecules

Nanocellulose finds various applications in advanced electrical devices due to its impressive mechanical properties, thermal stability, and degradability. Cellulose nanocrystals (CNCs) with excellent dielectric properties may act as a fresh dielectric plastic. In this study, a new strategy of small molecule modification was used to improve the dielectric constant, breakdown strength, and band gap of the CNCs. The dipole moments, dipole density, and the anisotropic impact of surface groups on the dielectric constant were studied. The number of sulfates in the CNCs showed a gradient due to alkali treatment and sulfonation, which allowed for a controlled range of the dielectric constant of nanocellulose between 4.9 and 11.9. TEMPO oxidation (2,2,6,6-tetramethylpiperidine-1-oxyl) and cyanoethylation of the CNCs further increased the dielectric constant to 11.1 and 13.2, respectively, and the dielectric loss 10-1. By understanding and innovating organic polymer dielectrics, we can provide significant benefits to the electronics and device industries.

Recent Advances in Biodegradable Green Electronic Materials and Sensor Applications

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

The Critical Roles of Water in the Processing, Structure, and Properties of Nanocellulose

The cellulose industry depends heavily on water owing to the hydrophilic nature of cellulose fibrils and its potential for sustainable and innovative production methods. The emergence of nanocellulose, with its excellent properties, and the incorporation of nanomaterials have garnered significant attention. At the nanoscale level, nanocellulose offers a higher exposure of hydroxyl groups, making it more intimate with water than micro- and macroscale cellulose fibers. Gaining a deeper understanding of the interaction between nanocellulose and water holds the potential to reduce production costs and provide valuable insights into designing functional nanocellulose-based materials. In this review, water molecules interacting with nanocellulose are classified into free water (FW) and bound water (BW), based on their interaction forces with surface hydroxyls and their mobility in different states. In addition, the water-holding capacity of cellulosic materials and various water detection methods are also discussed. The review also examines water-utilization and water-removal methods in the fabrication, dispersion, and transport of nanocellulose, aiming to elucidate the challenges and tradeoffs in these processes while minimizing energy and time costs. Furthermore, the influence of water on nanocellulose properties, including mechanical properties, ion conductivity, and biodegradability, are discussed. Finally, we provide our perspective on the challenges and opportunities in developing nanocellulose and its interplay with water.

A Green Conformable Thermoformed Printed Circuit Board Sourced from Renewable Materials

Printed circuit boards (PCBs) physically support and connect electronic components to the implementation of complex circuits. The most widespread insulating substrate that also acts as a mechanical support in PCBs is commercially known as FR4, and it is a glass-fiber-reinforced epoxy resin laminate. FR4 has exceptional dielectric, mechanical, and thermal properties. However, it was designed without considering sustainability and end-of-life aspects, heavily contributing to the accumulation of electronic waste in the environment. Thus, greener alternatives that can be reprocessed, reused, biodegraded, or composted at the end of their function are needed. This work presents the development and characterization of a PCB substrate based on poly(lactic acid) and cotton fabric, a compostable alternative to the conventional FR4. The substrate has been developed by compression molding, a process compatible with the polymer industry. We demonstrate that conductive silver ink can be additively printed on the substrate's surface, as its morphology and wettability are similar to those of FR4. For example, the compostable PCB's water contact angle is 72°, close to FR4's contact angle of 64°. The developed substrate can be thermoformed to curved surfaces at low temperatures while preserving the conductivity of the silver tracks. The green substrate has a dielectric constant comparable to that of the standard FR4, showing a value of 5.6 and 4.6 at 10 and 100 kHz, respectively, which is close to the constant value of 4.6 of FR4. The substrate is suitable for microdrilling, a fundamental process for integrating electronic components to the PCB. We implemented a proof-of-principle circuit to control the blinking of LEDs on top of the PCB, comprising resistors, capacitors, LEDs, and a dual in-line package circuit timer. The developed PCB substrate represents a sustainable alternative to standard FR4 and could contribute to the reduction of the overwhelming load of electronic waste in landfills.

Construction of Poly(hydrophobic deep eutectic solvent)/Ethylcellulose Composite Films for Green Recyclable Tapes

Constructing green recyclable cellulose-based tapes with high transparency, mechanical robustness, and strong wet adhesion using natural components is highly desirable but challenging. Herein, novel cellulose-based self-adhesive tapes were reported by coating a polymerizable hydrophobic deep eutectic solvent (DES) on ethylcellulose followed by photopolymerization. The prepared ethylcellulose-based self-adhesive tape (ECSAT) exhibited an optical transmittance of up to ∼88% and could provide strong adhesion by interfacial intermolecular interactions without obstructing information. Due to the hydrophobic nature of the overall structure, ECSAT does not exhibit significant adhesive strength and mechanical degradation under water, acid, and alkali environments. Notably, ECSAT can be completely dissolved in the resultant DES and furthermore reused as a self-adhesive coating. The recycled ECSAT still maintained good optical transparency, mechanical strength, and wet adhesion. We believe that ECSATs with all-around performances have a wide range of applications in packaging and other engineering fields.

The Fast and One-Step Growth of ZnO Nanorods on Cellulose Nanofibers for Highly Sensitive Photosensors

Cellulose is the most abundant organic material on our planet which has a key role in our daily life (e.g., paper, packaging). In recent years, the need for replacing fossil-based materials has expanded the application of cellulose and cellulose derivatives including into electronics and sensing. The combination of nanostructures with cellulose nanofibers (CNFs) is expected to create new opportunities for the development of innovative electronic devices. In this paper, we report on a single-step process for the low temperature (<100 °C), environmentally friendly, and fully scalable CNF-templated highly dense growth of zinc oxide (ZnO) nanorods (NRs). More specifically, the effect of the degree of substitution of the CNF (enzymatic CNFs and carboxymethylated CNFs with two different substitution levels) on the ZnO growth and the application of the developed ZnO NRs/CNF nanocomposites in the development of UV sensors is reported herein. The results of this investigation show that the growth and nature of ZnO NRs are strongly dependent on the charge of the CNFs; high charge promotes nanorod growth whereas with low charge, ZnO isotropic microstructures are created that are not attached to the CNFs. Devices manufactured via screen printing/drop-casting of the ZnO NRs/CNF nanocomposites demonstrate a good photo-sensing response with a very stable UV-induced photocurrent of 25.84 µA. This also exhibits excellent long-term stability with fast ON/OFF switching performance under the irradiance of a UV lamp (15 W).

Debossed Contact Printing as a Patterning Method for Paper-Based Electronics

The proliferation of printed electronic devices is feeding the growth of the Internet of Things, with devices deployed everywhere to collect and communicate data. At the same time, the increase in low-cost disposable devices is a cause for serious environmental concern. In particular, widely used plastic substrates such as poly(ethylene terephthalate) are persistent hazards to the environment. Paper is promising as a greener substrate for printed electronics because it is biodegradable and sourced from renewable materials as well as being low cost and compatible with roll-to-roll printing. However, the porous microstructure of paper promotes wicking of functional inks, leading to poor electrical performance and printing resolution. Hydrophobic coatings applied to the surface of paper create a planarized, printable surface, but these materials may compromise biodegradability and/or recyclability. This paper describes a new resist-free patterning method for printed paper-based electronics that takes advantage of the porous structure of paper. Debossed contact printing uses the pressure from a debossing tip to compress the porous structure of paper and create a patterned relief structure. Printing functional inks with an unpatterned roller deposits ink only on the raised regions of the relief structure. We demonstrate debossed contact printing of silver, carbon black, and conducting polymer inks and show that this new fabrication method is suitable for the fabrication of printed devices with dense features. We demonstrate the fabrication of antennas and patterned electrodes for RFID and smart wallpaper applications, respectively.

Polyaniline/poly (2-acrylamido-2-methyl-1-propanesulfonic acid) modified cellulose as promising material for sensors design

A material based on cellulose coated with polyaniline/poly (2-acrylamido-2-methyl-1-propanesulfonic acid) (Cell/PANI-PAMPSA) was synthesized in a simple way starting from cellulose fibres, aniline and using PAMPSA as dopant. The morphology, mechanical properties, thermal stability, and electrical conductivity were investigated by means of several complementary techniques. The obtained results highlight the excellent features of the Cell/PANI-PAMPSA composite with respect to the Cell/PANI one. Based on the promising performance of this material, novel device functions and wearable applications have been tested. We focused on its possible single use as: i) humidity sensors and ii) disposable biomedical sensors to provide immediate diagnostic services as close to the patient as possible for heart rate or respiration activity monitoring. To our knowledge, this is the first time that Cell/PANI-PAMPSA system has been used for such applications.

Toward Photoactive Wallpapers Based on ZnO-Cellulose Nanocomposites

The quest for eco-friendly materials with anticipated positive impact for sustainability is crucial to achieve the UN sustainable development goals. Classical strategies of composite materials can be applied on novel nanomaterials and green materials. Besides the actual technology and applications also processing and manufacturing methods should be further advanced to make entire technology concepts sustainable. Here, they show an efficient way to combine two low-cost materials, cellulose and zinc oxide (ZnO), to achieve novel functional and "green" materials via paper-making processes. While cellulose is the most abundant and cost-effective organic material extractable from nature. ZnO is cheap and known of its photocatalytic, antibacterial, and UV absorption properties. ZnO nanowires are grown directly onto cellulose fibers in water solutions and then dewatered in a process mimicking existing steps of large-scale papermaking technology. The ZnO NW paper exhibits excellent photo-conducting properties under simulated sunlight with good ON/OFF switching and long-term stability (90 minutes). It also acts as an efficient photocatalyst for hydrogen peroxide (H2O2) generation (5.7 × 10-9 m s-1) with an envision the possibility of using it in buildings to enable large surfaces to spontaneously produce H2O2 at its outer surface. Such technology promise for fast degradation of microorganisms to suppress the spreading of diseases.

Paper-based diagnostic chips for viral detection

Viruses cause various diseases in humans, and pose serious health risks to individuals and populations worldwide. As a result, various diagnostic procedures and methods have been developed to prevent, manage, and reduce the burden of viral diseases, each with its own benefits and drawbacks. Among these, paper-based diagnostic chips are becoming increasingly common because of their speed, accuracy, convenience, and economical and environmental friendliness. These paper-based diagnostic tests have ideal point-of-care (POC) diagnostic applications, particularly in personalized healthcare. Paper-based diagnostics have emerged as innovative and low-cost solutions for diagnosing viral diseases in remote and underdeveloped regions where traditional diagnostic methods are not readily available. These tests are easy to use, require minimal equipment, and can be performed by nonspecialized personnel, making them accessible even in resource-constrained settings. In this review, we discuss recent developments in paper-based diagnostic chips, the importance of improved methods for identifying viral pathogens, drawbacks of traditional detection techniques, and challenges and prospects of paper-based diagnostic chips for the detection of viruses.

Foldable RF Energy Harvesting System Based on Vertically Layered Metal Electrodes within a Single Sheet of Paper

Radio frequency energy harvesting (RFEH) systems have emerged as a critical component for powering devices and replacing traditional batteries, with paper being one of the most promising substrates for use in flexible RFEH systems. However, previous paper-based electronics with optimized porosity, surface roughness, and hygroscopicity still face limitations in terms of the development of integrated foldable RFEH systems within a single sheet of paper. In the present study, a novel wax-printing control and water-based solution process are used to realize an integrated foldable RFEH system within a single sheet of paper. The proposed paper-based device includes vertically layered foldable metal electrodes, a via-hole, and stable conductive patterns with a sheet resistance of less than 1 Ω sq^-1 . The proposed RFEH system exhibits an RF/DC conversion efficiency of 60% and an operating voltage of 2.1 V in 100 s at a distance of 50 mm and a transmitted power of 50 mW. The integrated RFEH system also demonstrates stable foldability, with RFEH performance maintained up to a folding angle of 150°. The single-sheet paper-based RFEH system thus has the potential for use in practical applications associated with the remote powering of wearable and Internet-of-Things devices and in paper electronics.

Bio-Graphene Sensors for Monitoring Moisture Levels in Wood and Ambient Environment

Wood is an inherently hygroscopic material which tends to absorb moisture from its surrounding. Moisture in wood is a determining factor for the quality of wood being employed in construction, since it causes weakening, deformation, rotting, and ultimately leading to failure of the structures resulting in costs to the economy, the environment, and to the safety of residents. Therefore, monitoring moisture in wood during the construction phase and after construction is vital for the future of smart and sustainable buildings. Employing bio-based materials for the construction of electronics is one way to mitigate the environmental impact of such electronics. Herein, a bio-graphene sensor for monitoring the moisture inside and around wooden surfaces is fabricated using laser-induced graphitization of a lignin-based ink precursor. The bio-graphene sensors are used to measure humidity in the range of 10% up to 90% at 25 °C. Using laser induced graphitization, conductor resistivity of 18.6 Ω sq-1 is obtained for spruce wood and 57.1 Ω sq-1 for pine wood. The sensitivity of sensors fabricated on spruce and pine wood is 2.6 and 0.74 MΩ per % RH. Surface morphology and degree of graphitization are investigated using scanning electron microscopy, Raman spectroscopy, and thermogravimetric analysis methods.

Unraveling the Steric Link to Copper Precursor Decomposition: A Multi-Faceted Study for the Printing of Flexible Electronics

The field of printed electronics strives for lower processing temperatures to move toward flexible substrates that have vast potential: from wearable medical devices to animal tagging. Typically, ink formulations are optimized using mass screening and elimination of failures; as such, there are no comprehensive studies on the fundamental chemistry at play. Herein, findings which describe the steric link to decomposition profile: combining density functional theory, crystallography, thermal decomposition, mass spectrometry, and inkjet printing, are reported. Through the reaction of copper(II) formate with excess alkanolamines of varying steric bulk, tris-co-ordinated copper precursor ions: "[CuL₃ ]," each with a formate counter-ion (1-3) are isolated and their thermal decomposition mass spectrometry profiles are collected to assess their suitability for use in inks (I^1-3 ). Spin coating and inkjet printing of I^1,2 provides an easily up-scalable method toward the deposition of highly conductive copper device interconnects (ρ = 4.7-5.3 × 10^-7 Ω m; ≈30% bulk) onto paper and polyimide substrates and forms functioning circuits that can power light-emitting diodes. The connection among ligand bulk, coordination number, and improved decomposition profile supports fundamental understanding which will direct future design.

Paper-Based Humidity Sensors as Promising Flexible Devices: State of the Art: Part 1. General Consideration

In the first part of the review article "General considerations" we give information about conventional flexible platforms and consider the advantages and disadvantages of paper when used in humidity sensors, both as a substrate and as a humidity-sensitive material. This consideration shows that paper, especially nanopaper, is a very promising material for the development of low-cost flexible humidity sensors suitable for a wide range of applications. Various humidity-sensitive materials suitable for use in paper-based sensors are analyzed and the humidity-sensitive characteristics of paper and other humidity-sensitive materials are compared. Various configurations of humidity sensors that can be developed on the basis of paper are considered, and a description of the mechanisms of their operation is given. Next, we discuss the manufacturing features of paper-based humidity sensors. The main attention is paid to the consideration of such problems as patterning and electrode formation. It is shown that printing technologies are the most suitable for mass production of paper-based flexible humidity sensors. At the same time, these technologies are effective both in the formation of a humidity-sensitive layer and in the manufacture of electrodes.

Papertronics: Marriage between Paper and Electronics Becoming a Real Scenario in Resource-Limited Settings

Integrating electronic applications with paper, placed next to or below printed images or graphics, can further expand the possible uses of paper substrates. Consuming paper as a substrate in the field of electronics can lead to significant innovations toward papertronics applications as paper comprises various advantages like being disposable, inexpensive, biodegradable, easy to handle, simple to use, and easily available. All of these advantages will definitely spur the advancement of the electronics field, but unfortunately, putting electronics on paper is not an easy task because, compared to plastics, the paper surface is not just rough but also porous. For example, in the case of lateral flow assay testing the sensor response is delayed if the pore size of the paper is enormous. This might be a disadvantage for most electrical devices printed directly on paper. Still, some methods make it compatible when fit with a rough, absorbent surface of the paper. Building electronic devices on a standard paper substrate have sparked much interest because of its lightweight, environmental friendliness, minimal cost, and simple fabrication. A slew of improvements have been achieved in recent years to make paper electronics perform better in various applications, including transistors, batteries, and displays. In addition, flexible electronics have gained much interest in human-machine interaction and wireless sensing. This review briefly examines the origins and fabrication of paper electronics and then moves on to applications and exciting possible paths for paper-based electronics.

Fabrication of practical deformable displays: advances and challenges

Display form factors such as size and shape have been conventionally determined in consideration of usability and portability. The recent trends requiring wearability and convergence of various smart devices demand innovations in display form factors to realize deformability and large screens. Expandable displays that are foldable, multi-foldable, slidable, or rollable have been commercialized or on the edge of product launches. Beyond such two-dimensional (2D) expansion of displays, efforts have been made to develop three dimensional (3D) free-form displays that can be stretched and crumpled for use in realistic tactile sensation, artificial skin for robots, and on-skin or implantable displays. This review article analyzes the current state of the 2D and 3D deformable displays and discusses the technological challenges to be achieved for industrial commercialization.

A mini-review on the dielectric properties of cellulose and nanocellulose-based materials as electronic components

Cellulose-based materials have the advantages of renewable, non-toxic, flexible, and strong mechanical properties, so it of is great significance to study the dielectric properties of cellulose-based materials. In this paper, we summarized the factors influencing the dielectric properties of cellulose and nanocellulose-based dielectric and the ways to change the dielectric properties, mainly exploring the methods to improve the dielectric constant of cellulose-based dielectric materials. Cellulose and nanocellulose-based dielectric need to improve the hygroscopic property, increase the flexibility and reduce dielectric loss of the composite materials. This review summarizes the current state-of-art progress of new dielectric materials for green energy storage and flexible electronic devices.

Screen Printed Reflective Electrochromic Displays for Paper and Other Opaque Substrates

Paper electronics is a viable alternative to traditional electronics, leading to more sustainable electronics. Many challenges still require solutions before paper electronics become mainstream. Here, we present a solution to enable the manufacturing of reflective all-printed organic electrochromic displays (OECDs) on paper substrates; devices that are usually printed on transparent substrates, for example, plastics. In order to operate on opaque paper substrates, an architecture for reversely printed OECDs (rOECDs) is developed. In this architecture, the electrochromic layer is printed as the last functional layer and can therefore be viewed from the print side. Square shaped 1 cm² rOECDs are successfully screen printed on paper, with a high manufacturing yield exceeding 99%, switching times <3 s and high color contrast (ΔE* > 27). Approximately 60% of the color is retained after 15 min in open-circuit mode. Compared to the conventional screen printed OECD architectures, the rOECDs recover approximately three times faster from storage in a dry environment, which is particularly important in systems where storage in low humidity atmosphere is required, for example, in many biosensing applications. Finally, a more complex rOECD with 9 individually addressable segments is successfully screen printed and demonstrated.

Stable water-floating transistor with recyclability

Electronic wastes from used devices containing environmentally hazardous materials are an immediate concern for the sustainable development of electronic and sensor industries. To address this, a highly controllable and dedicated electronic module should be devised, that allows systematic recollection of as many components from the original device for their reuse. Here, we report the total recycling of an electronic device, exploiting a water-floating system that is based on a water-compatible semiconductor as an active material. To do so, we developed a system for stable electronics on the water surface. The floating semiconductor features a tunable morphology on the water surface, and is constructed into a water-floating gated transistor (WFGT) and water floating sensor (WFS), exhibiting an on-current of 4.2 × 10^-5 A and an on/off ratio of ∼10³. The device showed high recyclability over 25 cycles, with an efficiency of 99 ± 0.9% within 1 cycle and 92 ± 0.7% within 30 cycles. Furthermore, the device was also found to be stable for over 10 days. Our system has the potential to be an eco-friendly, cost-effective, and scalable device that is fully recyclable, which can be applied in areas once thought of as being beyond the scope of current semiconductor technology.

Surface Modification and Properties of Thin Ink Films with Added TiO2 and ZnO Nanoparticles Applied on Paperboard Substrates

In this study, the surface modification of thin ink films with added nanoparticles was used to improve the functional properties of ink applied on paperboard substrates. The surface modification was performed by additional exposure of the samples to xenon radiation. Anatase TiO₂, rutile TiO₂ and ZnO were added to the base ink. The effect of surface modification on the surface, structural, and mechanical properties of the printed ink films was determined by FTIR-ATR spectroscopy, calculating the surface free energy and adhesion parameters, performing the rub resistance test of the printed samples, and by measuring the resistance to bending. Color measurements on the ink films were performed in order to observe the optical properties of unmodified and modified samples. The results showed that surface modification significantly improved the adhesion properties of the thin ink films and the mechanical properties of the samples. The results obtained on uncoated and coated paperboard showed that the addition of rutile TiO₂ and ZnO nanoparticles had the greatest effect on the rub resistance of the ink films. The results of the color analysis showed that the addition of nanoparticles did not change the optical properties of the modified ink films and that rutile TiO₂ and ZnO nanoparticles improved the lightfastness of the applied ink films.

Highly Conductive and Compliant Silver Nanowire Nanocomposites by Direct Spray Deposition

The silver nanowire (Ag NW)/elastomer nanocomposite represents a prototypical form of a compliant conductor for flexible and stretchable electronic devices. The widespread implementations are currently hindered by the complicated procedures to effectively disperse Ag NWs into elastomer matrices. In this study, we report a facile and scalable coating process to create Ag NW nanocomposites on various flexible/stretchable substrates. As-synthesized Ag NWs from the high-yield polyol-reduction approach are homogeneously dispersed into a variety of dilute elastomer solutions, thereby enabling direct spray deposition into highly compliant conductors. The as-prepared nanocomposite exhibits excellent conductivity (∼11,000 S/cm) and high deformability to 100% strain. The stable electrical properties are largely retained under repetitive mechanical manipulations including stretching, bending, and folding. The patterned features of conductive nanocomposites are conveniently accessed using shadow masks or selective laser ablation. The practical suitability is demonstrated by the successful implementations of a stretchable sensing patch and a flexible light-emitting diode display.

All-Aerosol-Jet-Printed Carbon Nanotube Transistor with Cross-Linked Polymer Dielectrics

The printability of reliable gate dielectrics and their influence on the stability of the device are some of the primary concerns regarding the practical application of printed transistors. Major ongoing research is focusing on the structural properties of dielectric materials and deposition parameters to reduce interface charge traps and hysteresis caused by the dielectric-semiconductor interface and dielectric bulk. This research focuses on improving the dielectric properties of a printed polymer material, cross-linked polyvinyl phenol (crPVP), by optimizing the cross-linking parameters as well as the aerosol jet printability. These improvements were then applied to the fabrication of completely printed carbon nanotube (CNT)-based thin-film transistors (TFT) to reduce the gate threshold voltage (V_th) and hysteresis in V_th during device operation. Finally, a fully aerosol-jet-printed CNT device was demonstrated using a 2:1 weight ratio of PVP with the cross-linker poly(melamine-co-formaldehyde) methylated (PMF) in crPVP as the dielectric material. This device shows significantly less hysteresis and can be operated at a gate threshold voltage as low as -4.8 V with an on/off ratio of more than 10⁴.

Comparative Study on Preparation Methods for Transparent Conductive Films Based on Silver Nanowires

Silver nanowires, which have high optoelectronic properties, have the potential to supersede indium tin oxide in the field of electrocatalysis, stretchable electronic, and solar cells. Herein, four mainstream experimental methods, including Mayer-rod coating, spin coating, spray coating, and vacuum filtration methods, are employed to fabricate transparent conductive films based on the same silver nanowires to clarify the significance of preparation methods on the performance of the films. The surface morphology, conductive property, uniformity, and flexible stability of these four Ag NW-based films, are analyzed and compared to explore the advantages of these methods. The transparent conductive films produced by the vacuum filtration method have the most outstanding performance in terms of surface roughness and uniformity, benefitting from the stronger welding of NW-NW junctions after the press procedure. However, limited by the size of the membrane and the vacuum degree of the equipment, the small-size Ag films used in precious devices are appropriate to obtain through this method. Similarly, the spin coating method is suited to prepare Ag NWs films with small sizes, which shows excellent stability after the bending test. In comparison, much larger-size films could be obtained through Mayer-rod coating and spray coating methods. The pull-down speed and force among the Mayer-rod coating process, as well as the spray distance and traveling speed among the spray coating process, are essential to the uniformity of Ag NW films. After being treated with NaBH₄ and polymethyl methacrylate (PMMA), the obtained Ag NW/PMMA films show great potential in the field of film defogging due to the Joule heating effect. Taken together, based on the advantages of each preparation method, the Ag NW-based films with desired size and performances are easier to prepare, meeting the requirements of different application fields.

Physicochemical and Photocatalytic Properties of 3D-Printed TiO2/Chitin/Cellulose Composite with Ordered Porous Structures

In this study, we printed three-dimensional (3D) titanium dioxide (TiO₂)/chitin/cellulose composite photocatalysts with ordered interconnected porous structures. Chitin microparticles were mixed with cellulose in the N-methylmorpholine-N-oxide (NMMO) solution to prepare the printing "ink". TiO₂ nanoparticles were embedded on the chitin/cellulose composite in the NMMO removal process by water before the freeze-drying process to build the 3D cellulosic photocatalysts with well-defined porous structures. The 3D-printed TiO₂/chitin/cellulose composites were characterized by X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), and Energy Disperse Spectroscopy (EDS). The XRD and FTIR analyses showed that chitin had an interference effect on the crystal regeneration of cellulose and resulted in a large amount of amorphous phase. The SEM images show that the printed cellulosic strands had a hollow structure, and the EDS analysis showed that TiO₂ nanoparticles were embedded on the chitin/cellulose composite surfaces. In the photocatalytic degradation process of methylene blue (MB) dye in an aqueous solution, the TiO₂/chitin/cellulose 3D composite photocatalysts demonstrated efficient MB degradation activities with excellent reusability and stability, in which the chitin content performed the function of adjusting the MB degradation efficiency.

Amphiphilic Silver Nanoparticles for Inkjet-Printable Conductive Inks

The large-scale manufacturing of flexible electronics is nowadays based on inkjet printing technology using specially formulated conductive inks, but achieving adequate wetting of different surfaces remains a challenge. In this work, the development of a silver nanoparticle-based functional ink for printing on flexible paper and plastic substrates is demonstrated. Amphiphilic silver nanoparticles with narrow particle size distribution and good dispersibility were prepared via a two-step wet chemical synthesis procedure. First, silver nanoparticles capped with poly(acrylic acid) were prepared, followed by an amidation reaction with 3-morpholynopropylamine (MPA) to increase their lipophilicity. Density functional theory (DFT) calculations were performed to study the interactions between the particles and the dispersion medium in detail. The amphiphilic nanoparticles were dispersed in solvents of different polarity and their physicochemical and rheological properties were determined. A stable ink containing 10 wt% amphiphilic silver nanoparticles was formulated and inkjet-printed on different surfaces, followed by intense pulsed light (IPL) sintering. Low sheet resistances of 3.85 Ω sq^-1, 0.57 Ω sq^-1 and 19.7 Ω sq^-1 were obtained for the paper, coated poly(ethylene terephthalate) (PET) and uncoated polyimide (PI) flexible substrates, respectively. Application of the nanoparticle ink for printed electronics was demonstrated via a simple flexible LED circuit.

Advances in Materials, Sensors, and Integrated Systems for Monitoring Eye Movements

Eye movements show primary responses that reflect humans' voluntary intention and conscious selection. Because visual perception is one of the fundamental sensory interactions in the brain, eye movements contain critical information regarding physical/psychological health, perception, intention, and preference. With the advancement of wearable device technologies, the performance of monitoring eye tracking has been significantly improved. It also has led to myriad applications for assisting and augmenting human activities. Among them, electrooculograms, measured by skin-mounted electrodes, have been widely used to track eye motions accurately. In addition, eye trackers that detect reflected optical signals offer alternative ways without using wearable sensors. This paper outlines a systematic summary of the latest research on various materials, sensors, and integrated systems for monitoring eye movements and enabling human-machine interfaces. Specifically, we summarize recent developments in soft materials, biocompatible materials, manufacturing methods, sensor functions, systems' performances, and their applications in eye tracking. Finally, we discuss the remaining challenges and suggest research directions for future studies.

MycelioTronics: Fungal mycelium skin for sustainable electronics

Electronic devices are irrevocably integrated into our lives. Yet, their limited lifetime and often improvident disposal demands sustainable concepts to realize a green electronic future. Research must shift its focus on substituting nondegradable and difficult-to-recycle materials to allow either biodegradation or facile recycling of electronic devices. Here, we demonstrate a concept for growth and processing of fungal mycelium skins as biodegradable substrate material for sustainable electronics. The skins allow common electronic processing techniques including physical vapor deposition and laser patterning for electronic traces with conductivities as high as 9.75 ± 1.44 × 10⁴ S cm^-1. The conformal and flexible electronic mycelium skins withstand more than 2000 bending cycles and can be folded several times with only moderate resistance increase. We demonstrate mycelium batteries with capacities as high as ~3.8 mAh cm^-2 used to power autonomous sensing devices including a Bluetooth module and humidity and proximity sensor.

Construction of an Electrical Conductor, Strain Sensor, Electrical Connection and Cycle Switch Using Conductive Graphite Cotton Fabrics

Researchers in science and industry are increasingly interested in conductive textiles. In this article, we have successfully prepared conductive textiles by applying a graphite dispersion to cotton fabric using a simple brush-coating-drying method and the solvents of dimethyl sulfoxide, dimethyl formamide, and a solvent mixture of both. The sheet resistance of the resulting cotton fabrics could be influenced by the type of polar solvent used to prepare the graphite dispersion and the concentration of graphite. In addition, the graphite cotton fabrics showed semiconductive behavior upon studying the resistance at different temperatures. A flexible strain sensor was fabricated using these graphite cotton fabrics for human motion detection. Most importantly, the resulting strain sensor functions even after 100 bending cycles, indicating its excellent reproducibility. In addition, our results have also shown that these graphite cotton fabrics can be used as electrical interconnects in electrical circuits without any visible degradation of the conductive cotton. Finally, a cotton electrical cycle switch was made using the graphite cotton fabrics and worked in the on and off state.

Light-Driven Flying Balloons Based on Hybrids of Carbon Nanotubes and Cellulose Nanofibers

We have fabricated nanocarbon-based palm-sized cubic paper balloons that can be levitated by light irradiation. These paper balloons are composed of carbon nanotube (CNT) freestanding films and cellulose nanofiber (CNF) freestanding films. The number of CNT freestanding films (N_CNT) and the number of CNF freestanding films (6-N_CNT) among the six walls of the cube were varied. We investigated the effect of N_CNT on the levitation behaviors under light irradiation. We found that the balloons were levitated when N_CNT was greater than or equal to two. The levitation height was found to be increased by increasing N_CNT.

Broadband-tunable spectral response of perovskite-on-paper photodetectors using halide mixing

Paper offers a low-cost and widely available substrate for electronics. It possesses alternative characteristics to silicon, as it shows low density and high flexibility, together with biodegradability. Solution processable materials, such as hybrid perovskites, also present light and flexible features, together with a huge tunability of the material composition with varying optical properties. In this study, we combine paper substrates with halide-mixed perovskites for the creation of low-cost and easy-to-prepare perovskite-on-paper photodetectors with a broadband-tunable spectral response. From the bandgap tunability of halide-mixed perovskites we create photodetectors with a cut-off spectral onset that ranges from the NIR to the green region, by increasing the bromide content on MAPb(I_1-xBr_x)₃ perovskite alloys. The devices show a fast and efficient response. The best performances are observed for pure I and Br perovskite compositions, with a maximum responsivity of ∼400 mA W^-1 on the MAPbBr₃ device. This study provides an example of the wide range of possibilities that the combination of solution processable materials with paper substrates offers for the development of low-cost, biodegradable and easy-to-prepare devices.

A resistive sensor for humidity detection based on cellulose/polyaniline

Ambient humidity is an important parameter that affects the manufacturing and storage of several industrial and agricultural goods. In the view of the Internet of Things (IoT), single sensors could be associated with an object for smart monitoring enabling optimum conditions to be maintained. Nevertheless, the production of cost-effective humidity sensors for indoor and outdoor environmental monitoring currently represents the main bottleneck in the development of this technology. Herein we report the results obtained with sensors exclusively made of cellulose and polyaniline (cell/PANI) under strictly controlled relative humidity (30–50 RH%) and temperature (21 ± 1 °C) achieved with a climatic chamber that simulates the conditions of indoor air humidity, and at different RH% in a lab test chamber set-up. Cell/PANI sensors, prepared with a simple, inexpensive, and easily scalable industrial paper process, show a linear trend with a slope of 1.41 μA RH%⁻¹ and a percentage of sensitivity of 13%. Response time as well as percentage of sensitivity results are similar to those of a commercial digital-output relative humidity and temperature sensor (DHT22) employed in parallel for comparison. The commercial sensor DHT22 has a sensitivity of 14%. This low-cost sensor has potential applications in agriculture, food monitoring, and medical and industrial environments as a disposable sensor for humidity detection.

Preparation of highly conductive polyaniline-coated cellulose sheets for the fabrication of humidity sensors via a simple, inexpensive, and robust method. These sensors show a linear, rapid, and reliable response for humidity cycling.