Paper electronics

Using airbrushes to pattern reagents for microarrays and paper-fluidic devices

We report using an airbrush to pattern a number of reagents, including small molecules, proteins, DNA, and conductive microparticles, onto a variety of mechanical substrates such as paper and glass. Airbrushing is more economical and easier to perform than many other patterning methods available (for example, inkjet printing). In this work, we investigated the controllable parameters that affect patterned line width and studied their mechanisms of action, and we provide examples of possible patterns. This airbrushing approach allowed us to pattern lines and dot arrays from hundreds of μm to tens of mm with length scales comparable to those of other patterning methods. Two applications, enzymatic assays and DNA hybridization, were chosen to demonstrate the compatibility of the method with biomolecules. This airbrushing method holds promise in making paper-based platforms less expensive and more accessible.

Vertically stacked nanocellulose tactile sensor

Paper-based electronic devices are attracting considerable attention, because the paper platform has unique attributes such as flexibility and eco-friendliness. Here we report on what is claimed to be the firstly fully integrated vertically-stacked nanocellulose-based tactile sensor, which is capable of simultaneously sensing temperature and pressure. The pressure and temperature sensors are operated using different principles and are stacked vertically, thereby minimizing the interference effect. For the pressure sensor, which utilizes the piezoresistance principle under pressure, the conducting electrode was inkjet printed on the TEMPO-oxidized-nanocellulose patterned with micro-sized pyramids, and the counter electrode was placed on the nanocellulose film. The pressure sensor has a high sensitivity over a wide range (500 Pa-3 kPa) and a high durability of 10⁴ loading/unloading cycles. The temperature sensor combines various materials such as poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS), silver nanoparticles (AgNPs) and carbon nanotubes (CNTs) to form a thermocouple on the upper nanocellulose layer. The thermoelectric-based temperature sensors generate a thermoelectric voltage output of 1.7 mV for a temperature difference of 125 K. Our 5 × 5 tactile sensor arrays show a fast response, negligible interference, and durable sensing performance.

Ultra-Fast Microwave Synthesis of ZnO Nanorods on Cellulose Substrates for UV Sensor Applications

In the present work, tracing and Whatman papers were used as substrates to grow zinc oxide (ZnO) nanostructures. Cellulose-based substrates are cost-efficient, highly sensitive and environmentally friendly. ZnO nanostructures with hexagonal structure were synthesized by hydrothermal under microwave irradiation using an ultrafast approach, that is, a fixed synthesis time of 10 min. The effect of synthesis temperature on ZnO nanostructures was investigated from 70 to 130 °C. An Ultra Violet (UV)/Ozone treatment directly to the ZnO seed layer prior to microwave assisted synthesis revealed expressive differences regarding formation of the ZnO nanostructures. Structural characterization of the microwave synthesized materials was carried out by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The optical characterization has also been performed. The time resolved photocurrent of the devices in response to the UV turn on/off was investigated and it has been observed that the ZnO nanorod arrays grown on Whatman paper substrate present a responsivity 3 times superior than the ones grown on tracing paper. By using ZnO nanorods, the surface area-to-volume ratio will increase and will improve the sensor sensibility, making these types of materials good candidates for low cost and disposable UV sensors. The sensors were exposed to bending tests, proving their high stability, flexibility and adaptability to different surfaces.

Tailored Approaches in Drug Development and Diagnostics: From Molecular Design to Biological Model Systems

Approaches to increase the efficiency in developing drugs and diagnostics tools, including new drug delivery and diagnostic technologies, are needed for improved diagnosis and treatment of major diseases and health problems such as cancer, inflammatory diseases, chronic wounds, and antibiotic resistance. Development within several areas of research ranging from computational sciences, material sciences, bioengineering to biomedical sciences and bioimaging is needed to realize innovative drug development and diagnostic (DDD) approaches. Here, an overview of recent progresses within key areas that can provide customizable solutions to improve processes and the approaches taken within DDD is provided. Due to the broadness of the area, unfortunately all relevant aspects such as pharmacokinetics of bioactive molecules and delivery systems cannot be covered. Tailored approaches within (i) bioinformatics and computer-aided drug design, (ii) nanotechnology, (iii) novel materials and technologies for drug delivery and diagnostic systems, and (iv) disease models to predict safety and efficacy of medicines under development are focused on. Current developments and challenges ahead are discussed. The broad scope reflects the multidisciplinary nature of the field of DDD and aims to highlight the convergence of biological, pharmaceutical, and medical disciplines needed to meet the societal challenges of the 21st century.

Rough-Surface-Enabled Capacitive Pressure Sensors with 3D Touch Capability

Fabrication strategies that pursue "simplicity" for the production process and "functionality" for a device, in general, are mutually exclusive. Therefore, strategies that are less expensive, less equipment-intensive, and consequently, more accessible to researchers for the realization of omnipresent electronics are required. Here, this study presents a conceptually different approach that utilizes the inartificial design of the surface roughness of paper to realize a capacitive pressure sensor with high performance compared with sensors produced using costly microfabrication processes. This study utilizes a writing activity with a pencil and paper, which enables the construction of a fundamental capacitor that can be used as a flexible capacitive pressure sensor with high pressure sensitivity and short response time and that it can be inexpensively fabricated over large areas. Furthermore, the paper-based pressure sensors are integrated into a fully functional 3D touch-pad device, which is a step toward the realization of omnipresent electronics.

Highly Deformable Origami Paper Photodetector Arrays

Flexible electronics will form the basis of many next-generation technologies, such as wearable devices, biomedical sensors, the Internet of things, and more. However, most flexible devices can bear strains of less than 300% as a result of stretching. In this work, we demonstrate a simple and low-cost paper-based photodetector array featuring superior deformability using printable ZnO nanowires, carbon electrodes, and origami-based techniques. With a folded Miura structure, the paper photodetector array can be oriented in four different directions via tessellated parallelograms to provide the device with excellent omnidirectional light harvesting capabilities. Additionally, we demonstrate that the device can be repeatedly stretched (up to 1000% strain), bent (bending angle ±30°), and twisted (up to 360°) without degrading performance as a result of the paper folding technique, which enables the ZnO nanowire layers to remain rigid even as the device is deformed. The origami-based strategy described herein suggests avenues for the development of next-generation deformable optoelectronic applications.

Review of recent research on flexible multifunctional nanopapers

Traditional paper and papermaking have struggled with a declining market during the last few decades. However, the incorporation of nanotechnology into papermaking has brought possibilities to develop low-cost, biocompatible and flexible products with sophisticated functionalities. The functionality of nanopapers emerges from the intrinsic properties of the nanofibrous network, the additional loading of specific nanomaterials (NMs), or the additional deposition and patterning of thin films of nanomaterials on the paper surface. A successful development of functional nanopapers requires understanding how the nanopaper matrix, nanofillers, nanocoating pigments, nanoprinting inks, processing additives and manufacturing processes all interact to provide the intended functionality. This review addresses the emerging area of functional nanopapers. This review discusses flexible and multifunctional nanopapers, NMs being used in nanopaper making, manufacturing techniques, and functional applications that provide new important possibilities to utilize papermaking technology. The interface where NM research meets traditional papermaking has important implications for food packaging, energy harvesting and energy storage, flexible electronics, low-cost devices for medical diagnostics, and numerous other areas.

Organic flash memory on various flexible substrates for foldable and disposable electronics

With the emergence of wearable or disposable electronics, there grows a demand for a flash memory realizable on various flexible substrates. Nevertheless, it has been challenging to develop a flash memory that simultaneously exhibits a significant level of flexibility and performance. This is mainly due to the scarcity of flexible dielectric materials with insulating properties sufficient for a flash memory, which involves dual dielectric layers, respectively, responsible for tunneling and blocking of charges. Here we report ultra-flexible organic flash memories based on polymer dielectrics prepared by initiated chemical vapor deposition. Using their near-ideal dielectric characteristics, we demonstrate flash memories bendable down to a radius of 300 μm that exhibits a relatively long-projected retention with a programming voltage on par with the present industrial standards. The proposed memory technology is then applied to non-conventional substrates, such as papers, to demonstrate its feasibility in a wide range of applications.

Flexible flash memory is crucial to modern electronics, but its fabrication is challenging in the absence of suitable dielectric materials. Here, Lee et al. realize organic memory with retention over 10 years using tunneling and blocking dielectric layers prepared by initiated chemical vapor deposition.

Simple and rapid fabrication of pencil-on-paper triboelectric nanogenerators with enhanced electrical performance

Paper and pencil have many advantages in triboelectric nanogenerators (TENGs) in terms of low-cost, light weight, and environment friendliness. In this work, a pencil-on-paper triboelectric nanogenerator (PP-TENG) with highly enhanced performance was introduced. In order to use paper as a friction layer and improve its triboelectric performance, a simple and rapid paper-coating process was utilized with polyvinylidene fluoride (PVDF), polyvinyledenedifluoride-trifluoroethylene (PVDF-TrFE), and poly(methyl methacrylate) (PMMA) solutions. The fabrication process of the PP-TENG was completed within 10 minutes via pencil drawing of an electrode followed by a solution coating. With an optimized electrode shape, the PP-TENG showed a maximum power density of 64 mW m^-2, which is more than 19 times higher than that of the uncoated paper TENG. The electrical performance of the PP-TENG was sufficient to drive a few hundred LEDs and charge various capacitors. It was maintained after the paper was folded or even crumpled. The proposed PP-TENG is expected to be utilized with other wearable electronic devices.

Review of Batteryless Wireless Sensors Using Additively Manufactured Microwave Resonators

The significant improvements observed in the field of bulk-production of printed microchip technologies in the past decade have allowed the fabrication of microchip printing on numerous materials including organic and flexible substrates. Printed sensors and electronics are of significant interest owing to the fast and low-cost fabrication techniques used in their fabrication. The increasing amount of research and deployment of specially printed electronic sensors in a number of applications demonstrates the immense attention paid by researchers to this topic in the pursuit of achieving wider-scale electronics on different dielectric materials. Although there are many traditional methods for fabricating radio frequency (RF) components, they are time-consuming, expensive, complicated, and require more power for operation than additive fabrication methods. This paper serves as a summary/review of improvements made to the additive printing technologies. The article focuses on three recently developed printing methods for the fabrication of wireless sensors operating at microwave frequencies. The fabrication methods discussed include inkjet printing, three-dimensional (3D) printing, and screen printing.

Cheap, Flexible, and Thermal-Sensitive Paper Sensor through Writing with Ionic Liquids Containing Pencil Leads

The flexible and portable paper-based sensors have a broad potential application in electronic detection and devices. In this work, a flexible thermoresponsive paper sensor was reported by writing on A4 paper with composite pencil leads which contain thermoresponsive pyrene-based ionic liquid [Pyrmim]⁺[Br]^-. The [Pyrmim]⁺[Br]^- was transferred onto the A4 paper surface with graphite by pencil writing for the facile preparation of thermal-sensitive paper chips. The as-prepared paper sensor was very sensitive to the NIR irradiation and warm objects. What is more, the pliable paper chip also had regular responses along with the varication of the folding angles, which could be employed for the angle goniometer of electronic robots.

Coffee-Driven Green Activation of Cellulose and Its Use for All-Paper Flexible Supercapacitors

Cellulose, which is one of the most-abundant and -renewable natural resources, has been extensively explored as an alternative substance for electrode materials such as activated carbons. Here, we demonstrate a new class of coffee-mediated green activation of cellulose as a new environmentally benign chemical-activation strategy and its potential use for all-paper flexible supercapacitors. A piece of paper towel is soaked in espresso coffee (acting as a natural activating agent) and then pyrolyzed to yield paper-derived activated carbons (denoted as "EK-ACs"). Potassium ions (K⁺), a core ingredient of espresso, play a viable role in facilitating pyrolysis kinetics and also in achieving a well-developed microporous structure in the EK-ACs. As a result, the EK-ACs show significant improvement in specific capacitance (131 F g^-1 at a scan rate of 1.0 mV s^-1) over control ACs (64 F g^-1) obtained from the carbonization of a pristine paper towel. All-paper flexible supercapacitors are fabricated by assembling EK-ACs/carbon nanotube mixture-embedded paper towels (as electrodes), poly(vinyl alcohol)/KOH mixture-impregnated paper towels (as electrolytes), and polydimethylsiloxane-infiltrated paper towels (as packaging substances). The introduction of the EK-ACs (as an electrode material) and the paper towel (as a deformable and compliant substrate) enables the resulting all-paper supercapacitor to provide reliable and sustainable cell performance as well as exceptional mechanical flexibility. Notably, no appreciable loss in the cell capacitance is observed after repeated bending (over 5000 cycles) or multiple folding. The coffee-mediated green activation of cellulose and the resultant all-paper flexible supercapacitors open new material and system opportunities for eco-friendly high-performance flexible power sources.

Paper-Based Electrodes for Flexible Energy Storage Devices

Paper-based materials are emerging as a new category of advanced electrodes for flexible energy storage devices, including supercapacitors, Li-ion batteries, Li-S batteries, Li-oxygen batteries. This review summarizes recent advances in the synthesis of paper-based electrodes, including paper-supported electrodes and paper-like electrodes. Their structural features, electrochemical performances and implementation as electrodes for flexible energy storage devices including supercapacitors and batteries are highlighted and compared. Finally, we also discuss the challenges and opportunity of paper-based electrodes and energy storage devices.

A review on wax printed microfluidic paper-based devices for international health

Paper-based microfluidics has attracted attention for the last ten years due to its advantages such as low sample volume requirement, ease of use, portability, high sensitivity, and no necessity to well-equipped laboratory equipment and well-trained manpower. These characteristics have made paper platforms a promising alternative for a variety of applications such as clinical diagnosis and quantitative analysis of chemical and biological substances. Among the wide range of fabrication methods for microfluidic paper-based analytical devices (μPADs), the wax printing method is suitable for high throughput production and requires only a commercial printer and a heating source to fabricate complex two or three-dimensional structures for multipurpose systems. μPADs can be used by anyone for in situ diagnosis and analysis; therefore, wax printed μPADs are promising especially in resource limited environments where people cannot get sensitive and fast diagnosis of their serious health problems and where food, water, and related products are not able to be screened for toxic elements. This review paper is focused on the applications of paper-based microfluidic devices fabricated by the wax printing technique and used for international health. Besides presenting the current limitations and advantages, the future directions of this technology including the commercial aspects are discussed. As a conclusion, the wax printing technology continues to overcome the current limitations and to be one of the promising fabrication techniques. In the near future, with the increase of the current interest of the industrial companies on the paper-based technology, the wax-printed paper-based platforms are expected to take place especially in the healthcare industry.

Stretchable and Soft Electronics using Liquid Metals

The use of liquid metals based on gallium for soft and stretchable electronics is discussed. This emerging class of electronics is motivated, in part, by the new opportunities that arise from devices that have mechanical properties similar to those encountered in the human experience, such as skin, tissue, textiles, and clothing. These types of electronics (e.g., wearable or implantable electronics, sensors for soft robotics, e-skin) must operate during deformation. Liquid metals are compelling materials for these applications because, in principle, they are infinitely deformable while retaining metallic conductivity. Liquid metals have been used for stretchable wires and interconnects, reconfigurable antennas, soft sensors, self-healing circuits, and conformal electrodes. In contrast to Hg, liquid metals based on gallium have low toxicity and essentially no vapor pressure and are therefore considered safe to handle. Whereas most liquids bead up to minimize surface energy, the presence of a surface oxide on these metals makes it possible to pattern them into useful shapes using a variety of techniques, including fluidic injection and 3D printing. In addition to forming excellent conductors, these metals can be used actively to form memory devices, sensors, and diodes that are completely built from soft materials. The properties of these materials, their applications within soft and stretchable electronics, and future opportunities and challenges are considered.

Paper as Active Layer in Inkjet-Printed Capacitive Humidity Sensors

An inkjet-printed relative humidity sensor based on capacitive changes which responds to different humidity levels in the environment is presented in this work. The inkjet-printed silver interdigitated electrodes configuration on the paper substrate allowed for the fabrication of a functional proof-of-concept of the relative humidity sensor, by using the paper itself as a sensing material. The sensor sensitivity in terms of relative humidity changes was calculated to be around 2 pF/RH %. The response time against different temperature steps from 3 to 85 °C was fairly constant (about 4–5 min), and it was considered fast for the aimed application, a smart label.

Review of Flexible Temperature Sensing Networks for Wearable Physiological Monitoring

Physiological temperature varies temporally and spatially. Accurate and real-time detection of localized temperature changes in biological tissues regardless of large deformation is crucial to understand thermal principle of homeostasis, to assess sophisticated health conditions, and further to offer possibilities of building a smart healthcare and medical system. Additionally, continuous temperature mapping in flexible and stretchable formats opens up many other potential areas, such as artificially electronic skins and reflection of emotional changes. This review exploits a comprehensive investigation onto recent advances in flexible temperature sensors, stretchable sensor networks, and platforms constructed in soft and compliant formats for wearable physiological monitoring. The most recent examples of flexible temperature sensors are first discussed regarding to their materials, structures, electrical and mechanical properties; temperature sensing network technologies in new materials and structural designs are then presented based on platforms comprised of multiple physical sensors and stretchable electronics. Finally, wearable applications of the sensing network are described, such as detection of human activities, monitoring of health conditions, and emotion-related bodily sensations. Conclusions are made with emphasis on critical issues and new trends in the field of wearable temperature sensor network technologies.

Office paper decorated with silver nanostars - an alternative cost effective platform for trace analyte detection by SERS

For analytical applications in portable sensors to be used in the point-of-need, low-cost SERS substrates using paper as a base, are an alternative. In this work, SERS substrates were produced on two different types of paper: a high porosity paper (Whatman no. 1); and a low porosity paper (commercially available office paper, Portucel Soporcel). Solutions containing spherical silver nanoparticles (AgNPs) and silver nanostars (AgNSs) were separately drop-casted on hydrophilic wells patterned on the papers. The porosity of the paper was found to play a determinant role on the AgNP and AgNS distribution along the paper fibres, with most of the nanoparticles being retained at the illuminated surface of the office paper substrate. The highest SERS enhancements were obtained for the office paper substrate, with deposited AgNSs. A limit of detection for rhodamine-6G as low as 11.4 ± 0.2 pg could be achieved, with an analytical enhancement factor of ≈10⁷ for this specific analyte. The well patterning technique allowed good signal uniformity (RSD of 1.7%). Besides, these SERS substrates remained stable after 5 weeks of storage (RSD of 7.3%). Paper-induced aggregation of AgNPs was found to be a viable alternative to the classical salt-induced aggregation, to obtain a highly sensitive SERS substrates.

Ionic Gel Paper with Long-Term Bendable Electrical Robustness for Use in Flexible Electroluminescent Devices

Conductive paper has low-cost, lightweight, sustainability, easy scale-up, and tailorable advantages, allowing for its promising potential in flexible electronics, such as bendable supercapacitors, solar cells, electromagnetic shields, and actuators. Ionic gels, exhibiting a lower Young's modulus together with facile manufacturing, can fully serve as the conductive component to prepare conductive paper. Herein we report a low-cost (∼1.3 dollars/m²), continuous, and high-throughput (up to ∼30 m/min) fabrication of reliable and long-term (stable for more than two months) conductive paper. As-prepared conductive paper shows a high electrical durability with negligible bending-recovering signal changes over 5000 cycles. Using this ionic gel paper (IGP) as a key component, we build a variety of proof-of-principle demonstrations to show the capacity of IGP in constructing flexible electroluminescent devices with diverse patterns, including a square, an alphabetic string, and a laughing face. Our methodology has the potential to open a new powerful route to fabricate bendable conductive paper for a myriad of applications in future flexible electronics.

The rise of plastic bioelectronics

Plastic bioelectronics is a research field that takes advantage of the inherent properties of polymers and soft organic electronics for applications at the interface of biology and electronics. The resulting electronic materials and devices are soft, stretchable and mechanically conformable, which are important qualities for interacting with biological systems in both wearable and implantable devices. Work is currently aimed at improving these devices with a view to making the electronic-biological interface as seamless as possible.

Soft Actuators for Small-Scale Robotics

This review comprises a detailed survey of ongoing methodologies for soft actuators, highlighting approaches suitable for nanometer- to centimeter-scale robotic applications. Soft robots present a special design challenge in that their actuation and sensing mechanisms are often highly integrated with the robot body and overall functionality. When less than a centimeter, they belong to an even more special subcategory of robots or devices, in that they often lack on-board power, sensing, computation, and control. Soft, active materials are particularly well suited for this task, with a wide range of stimulants and a number of impressive examples, demonstrating large deformations, high motion complexities, and varied multifunctionality. Recent research includes both the development of new materials and composites, as well as novel implementations leveraging the unique properties of soft materials.

An Origami Perovskite Photodetector with Spatial Recognition Ability

Flexible photodetectors are attracting substantial attention because of their promising applications in bendable display and smart clothes which cannot be fulfilled by the existing rigid counterparts. In this work, we demonstrate a newly designed photodetector constructed on the common printing paper. Pencil trace was applied as the graphite electrode. With such a simple and convenient method, the as-prepared photodetector exhibited a satisfactory responsivity of 4.4 mA/W, on/off current ratio of 32, coupled with a high response speed of <10 ms. It also demonstrated excellent mechanical flexibility and durability. Most inspiringly, by an ingenious origami, we created the first perovskite photodetector with a 3D configuration. The cubic photodetector array displayed an excellent spatial recognition ability which could not be achieved in all the previously reported 2D photodetectors. Such a fusion of materials science and the art of origami provides a robust strategy for the design of low-cost flexible electronics, especially for the applications in 3D configurations.

Microfluidization of Graphite and Formulation of Graphene-Based Conductive Inks

We report the exfoliation of graphite in aqueous solutions under high shear rate [∼ 10⁸ s^-1] turbulent flow conditions, with a 100% exfoliation yield. The material is stabilized without centrifugation at concentrations up to 100 g/L using carboxymethylcellulose sodium salt to formulate conductive printable inks. The sheet resistance of blade coated films is below ∼2Ω/□. This is a simple and scalable production route for conductive inks for large-area printing in flexible electronics.

Flexible and Highly Biocompatible Nanofiber-Based Electrodes for Neural Surface Interfacing

Polyimide (PI)-based electrodes have been widely used as flexible biosensors in implantable device applications for recording biological signals. However, the long-term quality of neural signals obtained from PI-based nerve electrodes tends to decrease due to nerve damage by neural tissue compression, mechanical mismatch, and insufficient fluid exchange between the neural tissue and electrodes. Here, we resolve these problems with a developed PI nanofiber (NF)-based nerve electrode for stable neural signal recording, which can be fabricated via electrospinning and inkjet printing. We demonstrate an NF-based nerve electrode that can be simply fabricated and easily applied due to its high permeability, flexibility, and biocompatibility. Furthermore, the electrode can record stable neural signals for extended periods of time, resulting in decreased mechanical mismatch, neural compression, and contact area. NF-based electrodes with highly flexible and body-fluid-permeable properties could enable future neural interfacing applications.

Ultrafast Paper Thermometers Based on a Green Sensing Ink

With the use of an ionic liquid as the ultrathermosensitive fluid, a paper thermometer is successfully developed with intrinsic ability of ultrafast response and high stability upon temperature change. The fluidic nature allows the ionic liquid to be easily deposited on paper by pen writing or inkjet printing, affording great promise for large-scale fabrication of low-cost paper sensors. Owing to the advantages of nonvolatilization, excellent continuity and deformability, the thermosensitive ink trapped within the cellulose fibers of paper matrix has no leakage or evaporation at open states, ensuring the excellent stability and repeatability of thermal sensing against arbitrary bending and folding operation. By shortening the heat exchange distance between ionic liquid and samples, it takes only 8 s for the thermometer to reach an electrical equilibrium at a given temperature. Moreover, the paper thermometer can be applied to remotely monitor temperature change with the combination of a wireless communication technology.

Attachable and flexible aluminum oxide resistive non-volatile memory arrays fabricated on tape as the substrate

We fabricated 8 × 8 arrays of non-volatile resistive memory devices on commercially available Scotch^® Magic^™ tape as a flexible substrate. The memory devices consist of double active layers of Al₂O₃ with a structure of Au/Al₂O₃/Au/Al₂O₃/Al (50 nm/20 nm/20 nm/20 nm/50 nm) on attachable tape substrates. Because the memory devices were fabricated using only dry and low temperature processes, the tape substrate did not suffer from any physical or chemical damage during the fabrication. The fabricated memory devices were turned to the low resistance state at ∼3.5 V and turned to the high resistance state at ∼10 V with a negative differential resistance region after ∼5 V, showing typical unipolar non-volatile resistive memory behavior. The memory devices on the tape substrates exhibited reasonable electrical performances including a high ON/OFF ratio of 10⁴, endurance over 200 cycles of reading/writing processes, and retention times of over 10⁴ s in both the flat and bent configurations.

Flow reproducibility of whole blood and other bodily fluids in simplified no reaction lateral flow assay devices

The "no reaction" lateral flow assay (nrLFA) uses a simplified LFA structure with no conjugate pad and no stored reagents. In the nrLFA, the capillary-based transport time or distance is the key indicator, rather than the outcome of a biochemical reaction. Hence, the calibration and reproducibility of the nrLFA device are critical. The capillary flow properties of several membrane types (nitrocellulose, nylon, cellulose acetate, polyethersulfone, and polyvinylidene difluoride) are evaluated. Flow rate evaluations of MilliporeSigma Hi-Flow™ Plus (HF075, HF135 and HF180) nitrocellulose membranes on nrLFA are performed using bodily fluids (whole blood, blood plasma, and artificial sweat). The results demonstrate that fluids with lower viscosity travel faster, and membranes with slower flow rate exhibit higher capability to distinguish fluids with different viscosities. Reproducibility tests of nrLFA are performed on HF075, demonstrating excellent reproducibility. The coefficient of variation for blood coagulation tests performed with the nrLFA using induced coagulation was 5% for the plasma front and 2% for the RBC front. The effects of variation in blood hematocrit and sample volume are also reported. The overall results indicate that the nrLFA approach has a high potential to be commercially developed as a blood monitoring point-of-care device with simple calibration capability and excellent reproducibility.

Flexible Heteroepitaxy of CoFe2O4/Muscovite Bimorph with Large Magnetostriction

A bimorph composed of ferrimagnetic cobalt ferrite (CoFe₂O₄, CFO) and flexible muscovite was fabricated via van der Waals epitaxy. The combination of X-ray diffraction and transmission electron microscopy was conducted to reveal the heteroepitaxy of the CFO/muscovite system. The robust magnetic behaviors against mechanical bending were characterized by hysteresis measurements and magnetic force microscopy, which maintain a saturation magnetization (M_s) of ∼120-150 emu/cm³ under different bending states. The large magnetostrictive response of the CFO film was then determined by digital holographic microscopy, where the difference of magnetostrction coefficient (Δλ) is -104 ppm. The superior performance of this bimorph is attributed to the nature of weak interaction between film and substrate. Such a flexible CFO/muscovite bimorph provides a new platform to develop next-generation flexible magnetic devices.

Paper microchip with a graphene-modified silver nano-composite electrode for electrical sensing of microbial pathogens

Rapid and sensitive point-of-care diagnostics are of paramount importance for early detection of infectious diseases and timely initiation of treatment. Here, we present cellulose paper and flexible plastic chips with printed graphene-modified silver electrodes as universal point-of-care diagnostic tools for the rapid and sensitive detection of microbial pathogens or nucleic acids through utilizing electrical sensing modality and loop-mediated isothermal amplification (LAMP). We evaluated the ability of the developed paper-based assay to detect (i) viruses on cellulose-based paper microchips without implementing amplification in samples with viral loads between 106 and 108 copies per ml, and (ii) amplified HIV-1 nucleic acids in samples with viral loads between 10 fg μl-1 and 108 fg μl-1. The target HIV-1 nucleic acid was amplified using the RT-LAMP technique and detected through the electrical sensing of LAMP amplicons for a broad range of RNA concentrations between 10 fg μl-1 and 108 fg μl-1 after 40 min of amplification time. Our assay may be used for antiretroviral therapy monitoring where it meets the sensitivity requirement of the World Health Organization guidelines. Such a paper microchip assay without the amplification step may also be considered as a simple and inexpensive approach for acute HIV detection where maximum viral replication occurs.

Dry writing of highly conductive electrodes on papers by using silver nanoparticle-graphene hybrid pencils

The development of paper electronics would enable realization of extremely cheap devices for portable, disposable, and environmentally-benign electronics. Here, we propose a simple dry-writing tool similar to a pencil, which can be used to draw electrically conducting lines on paper for use in paper-based electronic devices. The fabricated pencil is composed of silver nanoparticles decorated on graphene layers to construct layered hybrid nanostructures. This pencil can draw highly conductive lines that are flexible and foldable on conventional papers. Electrodes drawn using this pencil on conventional copy paper are stable during repetitive mechanical folding and highly resistant to moisture/chemicals. This pencil can draw a conductive line where its resistance can be tuned by changing the amount of nanoparticles. A nonvolatile memory device is realized on papers by hand written lines with different resistance. All memory elements are composed of carbons on papers, so complete data security can be achieved by burning the memory papers. This work will provide a new opportunity to fabricate electronic devices on real papers with good conductivity as well as robust mechanical/chemical stability.

Development of a fiber-based membraneless hydrogen peroxide fuel cell

Polyvinylidene fluoride (PVDF) electrospun nano-fiber is suggested as the substrate material for developing biocompatible membraneless hydrogen peroxide fuel cells.

Pencil-on-Paper Capacitors for Hand-Drawn RC Circuits and Capacitive Sensing

Electronic capacitors were constructed via hand-printing on paper using pencil graphite. Graphite traces were used to draw conductive connections and capacitor plates on opposing sides of a sheet of standard notebook paper. The paper served as the dielectric separating the plates. Capacitance of the devices was generally < 1000 pF and scaled with surface area of the plate electrodes. By combining a pencil-drawn capacitor with an additional resistive pencil trace, an RC low-pass filter was demonstrated. Further utility of the pencil-on-paper devices was demonstrated through description of a capacitive force transducer and reversible chemical sensing. The latter was achieved for water vapor when the hygroscopic cellulose matrix of the paper capacitor’s dielectric adsorbed water. The construction and demonstration of pencil-on-paper capacitive elements broadens the scope of paper-based electronic circuits while allowing new opportunities in the rapidly expanding field of paper-based sensors.

Rapid and efficient intense pulsed light reduction of graphene oxide inks for flexible printed electronics

Inkjet-printed GO patterns without additives were reduced by IPL treatment and achieved resistance as low as 760.4 Ω and acceptable flexibility.

Graphene/polyaniline@carbon cloth composite as a high-performance flexible supercapacitor electrode prepared by a one-step electrochemical co-deposition method

A one-step electrochemical co-deposition method was used to prepare a graphene/polyaniline composite on carbon cloth for high-performance flexible supercapacitors.

Natural Materials Assembled, Biodegradable, and Transparent Paper-Based Electret Nanogenerator

Developing eco-friendly and low-cost electronics is an effective strategy to address the electronic waste issue. In this study, transparent cellulose nanopaper (T-paper) and polylactic acid (PLA) electret were used to construct a biodegradable and transparent paper-based electret nanogenerator. The nanogenerator could be assembled with paper products to form a self-powered smart packaging system without impairing the appearance, due to the high transparency and desirable output performance. Furthermore, the self-degradation property in the natural soil of the nanogenerator is demonstrated, indicating that the nanogenerator is recycled and will not pollute the environment. We anticipate that this study will provide new insights to develop eco-friendly power source and paper-based electronics.

Flexible and Disposable Sensing Platforms Based on Newspaper

The flexible sensing platform is a key component for the development of smart portable devices targeting healthcare, environmental monitoring, point-of-care diagnostics, and personal electronics. Herein, we demonstrate a simple, scalable, and cost-effective strategy for fabrication of a sensing electrode based on a waste newspaper with conformal coating of parylene C (P-paper). Thin polymeric layers over cellulose fibers allow the P-paper to possess improved mechanical and chemical stability, which results in high-performance flexible sensing platforms for the detection of pathogenic E. coli O157:H7 based on DNA hybridization. Moreover, P-paper electrodes have the potential to serve as disposable, flexible sensing platforms for point-of-care testing biosensors.

Disposable, Paper-Based, Inkjet-Printed Humidity and H₂S Gas Sensor for Passive Sensing Applications

An inkjet-printed, fully passive sensor capable of either humidity or gas sensing is presented herein. The sensor is composed of an interdigitated electrode, a customized printable gas sensitive ink and a specialized dipole antenna for wireless sensing. The interdigitated electrode printed on a paper substrate provides the base conductivity that varies during the sensing process. Aided by the porous nature of the substrate, a change in relative humidity from 18% to 88% decreases the electrode resistance from a few Mega-ohms to the kilo-ohm range. For gas sensing, an additional copper acetate-based customized ink is printed on top of the electrode, which, upon reaction with hydrogen sulphide gas (H₂S) changes, both the optical and the electrical properties of the electrode. A fast response time of 3 min is achieved at room temperature for a H₂S concentration of 10 ppm at a relative humidity (RH) of 45%. The passive wireless sensing is enabled through an antenna in which the inner loop takes care of conductivity changes in the 4-5 GHz band, whereas the outer-dipole arm is used for chipless identification in the 2-3 GHz band.