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

Interfacial Engineering of Silicon Carbide Nanowire/Cellulose Microcrystal Paper toward High Thermal Conductivity

Polymer composites with high thermal conductivity have attracted much attention, along with the rapid development of electronic devices toward higher speed and better performance. However, high interfacial thermal resistance between fillers and matrix or between fillers and fillers has been one of the primary bottlenecks for the effective thermal conduction in polymer composites. Herein, we report on engineering interfacial structure of silicon carbide nanowire/cellulose microcrystal paper by generating silver nanostructures. We show that silver nanoparticle-deposited silicon carbide nanowires as fillers can effectively enhance the thermal conductivity of the matrix. The in-plane thermal conductivity of the resultant composite paper reaches as high as 34.0 W/m K, which is one order magnitude higher than that of conventional polymer composites. Fitting the measured thermal conductivity with theoretical models qualitatively demonstrates that silver nanoparticles bring the lower interfacial thermal resistances both at silicon carbide nanowire/cellulose microcrystal and silicon carbide nanowire/silicon carbide nanowire interfaces. This interfacial engineering approach provides a powerful tool for sophisticated fabrication of high-performance thermal-management materials.

Biodegradable Polycaprolactone as Ion Solvating Polymer for Solution-Processed Light-Emitting Electrochemical Cells

In this work, we demonstrate the use of the biodegradable polymer polycaprolactone (PCL) as the ion solvating polymer in solution-processed light-emitting electrochemical cells (LEC). We show that the inclusion of PCL in the active layer yields higher ionic conductivities and thus contributes to a rapid formation of the dynamic p-i-n junction and reduction of operating voltages. PCL shows no phase separation with the emitter polymer and reduces film roughness. The devices show light-emission at voltages as low as 3.2 V and lifetimes on the order of 30 h operating above 150 cd m⁻² with turn-on times <20 s and current and luminous efficacies of 3.2 Cd A⁻¹ and 1.5 lm W⁻¹ respectively.

Fluorimetric Mercury Test Strips with Suppressed "Coffee Stains" by a Bio-inspired Fabrication Strategy

A fluorimetric Hg²⁺ test strip has been developed using a lotus-inspired fabrication method for suppressing the “coffee stains” toward the uniform distribution of probe materials through creating a hydrophobic drying pattern for fast solvent evaporation. The test strips were first loaded with the model probes of fluorescent gold-silver nanoclusters and then dried in vacuum on the hydrophobic pattern. On the one hand, here, the hydrophobic constraining forces from the lotus surface-like pattern could control the exterior transport of dispersed nanoclusters on strips leading to the minimized “coffee stains”. On the other hand, the vacuum-aided fast solvent evaporation could boost the interior Marangoni flow of probe materials on strips to expect the further improved probe distribution on strips. High aqueous stability and enhanced fluorescence of probes on test strips were realized by the hydrophilic treatment with amine-derivatized silicane. A test strips-based fluorimetry has thereby been developed for probing Hg²⁺ ions in wastewater, showing the detection performances comparable to the classic instrumental analysis ones. Such a facile and efficient fabrication route for the bio-inspired suppression of “coffee stains” on test strips may expand the scope of applications of test strips-based “point-of-care” analysis methods or detection devices in the biomedical and environmental fields.

Semiconductor Photonic Nanocavity on a Paper Substrate

Direct integration of semiconductor photonic nanocavities with paper substrates is demonstrated for the first time. 1D photonic crystal nanocavities successfully show lasing action on paper substrates. The device has great synergy as a sensor because paper has good wicking ability while a photonic crystal cavity has high figure of merit. The research provides a platform for eco-friendly and sustainable devices.

Flexible bottom-gate graphene transistors on Parylene C substrate and the effect of current annealing

Flexible graphene transistors built on a biocompatible Parylene C substrate would enable active circuitry to be integrated into flexible implantable biomedical devices. An annealing method to improve the performance of a flexible transistor without damaging the flexible substrate is also desirable. Here, we present a fabrication method of a flexible graphene transistor with a bottom-gate coplanar structure on a Parylene C substrate. Also, a current annealing method and its effect on the device performance have been studied. The localized heat generated by the current annealing method improves the drain current, which is attributed to the decreased contact resistance between graphene and S/D electrodes. A maximum current annealing power in the Parylene C-based graphene transistor has been extracted to provide a guideline for an appropriate current annealing. The fabricated flexible graphene transistor shows a field-effect mobility, maximum transconductance, and a I_on/I_off ratio of 533.5 cm²/V s, 58.1 μS, and 1.76, respectively. The low temperature process and the current annealing method presented here would be useful to fabricate two-dimensional materials-based flexible electronics.

Hybrid boron nitride-natural fiber composites for enhanced thermal conductivity

Thermal conductivity was dramatically increased after adding natural fiber into hexagonal boron nitride (hBN)/epoxy composites. Although natural fiber does not show high-thermal conductivity itself, this study found that the synergy of natural fiber with hBN could significantly improve thermal conductivity, compared with that solely using hBN. A design of mixtures approach using constant fibers with increasing volume fractions of hBN was examined and compared. The thermal conductivity of the composite containing 43.6% hBN, 26.3% kenaf fiber and 30.1% epoxy reached 6.418 W m⁻¹ K⁻¹, which was 72.3% higher than that (3.600 W m⁻¹ K⁻¹) of the 69.0% hBN and 31.0% epoxy composite. Using the scanning electron microscope (SEM) and micro computed tomography (micro-CT), it was observed that the hBN powders were well distributed and ordered on the fiber surfaces enhancing the ceramic filler’s interconnection, which may be the reason for the increase in thermal conductivity. Additionally, the results from mechanical and dynamic mechanical tests showed that performances dramatically improved after adding kenaf fibers into the hBN/epoxy composite, potentially benefiting the composite’s use as an engineered material.

The formal electronic recycling industry: Challenges and opportunities in occupational and environmental health research

BACKGROUND

E-waste includes electrical and electronic equipment discarded as waste without intent of reuse. Informal e-waste recycling, typically done in smaller, unorganized businesses, can expose workers and communities to serious chemical health hazards. It is unclear if formalization into larger, better-controlled electronics recycling (e-recycling) facilities solves environmental and occupational health problems.

OBJECTIVES

To systematically review the literature on occupational and environmental health hazards of formal e-recycling facilities and discuss challenges and opportunities to strengthen research in this area.

METHODS

We identified 37 publications from 4 electronic databases (PubMed, Web of Science, Environmental Index, NIOSHTIC-2) specific to chemical exposures in formal e-recycling facilities.

DISCUSSION

Environmental and occupational exposures depend on the degree of formalization of the facilities but further reduction is needed. Reported worker exposures to metals were often higher than recommended occupational guidelines. Levels of brominated flame-retardants in worker's inhaled air and biological samples were higher than those from reference groups. Air, dust, and soil concentrations of metals, brominated flame-retardants, dioxins, furans, polycyclic-aromatic hydrocarbons, or polychlorinated biphenyls found inside or near the facilities were generally higher than reference locations, suggesting transport into the environment. Children of a recycler had blood lead levels higher than public health recommended guidelines.

CONCLUSIONS

With mounting e-waste, more workers, their family members, and communities could experience unhealthful exposures to metals and other chemicals. We identified research needs to further assess exposures, health, and improve controls. The long-term solution is manufacturing of electronics without harmful substances and easy-to-disassemble components.

Rapid Cellulose-Mediated Microwave Sintering for High-Conductivity Ag Patterns on Paper

Cellulose-based paper is essential in everyday life, but it also has further potentials for use in low-cost, printable, disposable, and eco-friendly electronics. Here, a method is developed for the cellulose-mediated microwave sintering of Ag patterns on conventional paper, in which the paper plays a significant role both as a flexible insulating substrate for the conductive Ag pattern and as a lossy dielectric media for rapid microwave heating. The anisotropic dielectric properties of the cellulose fibers mean that a microwave electric field applied parallel to the paper substrate provides sufficient heating to produce Ag patterns with a conductivity 29-38% that of bulk Ag in a short period of time (∼1 s) at 250-300 °C. Significantly, there is little thermal degradation of the substrate during this process. The microwave-sintered Ag patterns exhibit good mechanical stability against 10 000 bending cycles and can be easily soldered with lead-free solder. Therefore, cellulose-mediated microwave sintering presents a promising means of achieving short processing times and high electrical performance in flexible paper electronics.

A highly sensitive, highly transparent, gel-gated MoS2 phototransistor on biodegradable nanopaper

Transition metal dichalcogenides hold great promise for a variety of novel electrical, optical and mechanical devices and applications. Among them, molybdenum disulphide (MoS2) is gaining increasing attention as gate dielectrics and semiconductive channels for high-performance field effect transistors. Here we report on the first MoS2 phototransistor built on a flexible, transparent and biodegradable substrate with an electrolyte gate dielectric. We have carried out systematic studies on its electrical and optoelectronic properties. The MoS2 phototransistor exhibited an excellent photoresponsivity of ∼1.5 kA W(-1), about two times higher compared to typical back-gated devices reported in previous studies. The device is highly transparent at the same time with an average optical transmittance of 82%. Successful fabrication of phototransistors on flexible cellulose nanopaper with excellent performance and transparency suggests that it is feasible to achieve an ecofriendly and biodegradable phototransistor with great photoresponsivity, broad spectral range and durable flexibility.

Use of nanocellulose in printed electronics: a review

Since the last decade, interest in cellulose nanomaterials known as nanocellulose has been growing. Nanocellulose has various applications ranging from composite reinforcement to rheological modifiers. Recently, nanocellulose has been shown to have great potential in flexible printed electronics applications. The property of nanocellulose to form self-standing thermally stable films has been exploited for producing transparent and smooth substrates for printed electronics. However, other than substrates, the field of printed electronics involves the use of inks, various processing methods and the production of flexible electronic devices. This review aims at providing an overview of the use and potential of nanocellulose throughout the printed electronics field.

Flexible and printable paper-based strain sensors for wearable and large-area green electronics

Paper-based (PB) green electronics is an emerging and potentially game-changing technology due to ease of recycling/disposal, the economics of manufacture and the applicability to flexible electronics. Herein, new-type printable PB strain sensors (PPBSSs) from graphite glue (graphite powder and methylcellulose) have been fabricated. The graphite glue is exposed to thermal annealing to produce surface micro/nano cracks, which are very sensitive to compressive or tensile strain. The devices exhibit a gauge factor of 804.9, response time of 19.6 ms and strain resolution of 0.038%, all performance indicators attaining and even surpassing most of the recently reported strain sensors. Due to the distinctive sensing properties, flexibility and robustness, the PPBSSs are suitable for monitoring of diverse conditions such as structural strain, vibrational motion, human muscular movements and visual control.

Chitin Nanofiber Transparent Paper for Flexible Green Electronics

A transparent paper made of chitin nanofibers (ChNF) is introduced and its utilization as a substrate for flexible organic light-emitting diodes is demonstrated. Given its promising macroscopic properties, biofriendly characteristics, and availability of the raw material, the utilization of the ChNF transparent paper as a structural platform for flexible green electronics is envisaged.

Highly Anisotropic, Highly Transparent Wood Composites

For the first time, two types of highly anisotropic, highly transparent wood composites are demonstrated by taking advantage of the macro-structures in original wood. These wood composites are highly transparent with a total transmittance up to 90% but exhibit dramatically different optical and mechanical properties.

Cotton-textile-enabled flexible self-sustaining power packs via roll-to-roll fabrication

With rising energy concerns, efficient energy conversion and storage devices are required to provide a sustainable, green energy supply. Solar cells hold promise as energy conversion devices due to their utilization of readily accessible solar energy; however, the output of solar cells can be non-continuous and unstable. Therefore, it is necessary to combine solar cells with compatible energy storage devices to realize a stable power supply. To this end, supercapacitors, highly efficient energy storage devices, can be integrated with solar cells to mitigate the power fluctuations. Here, we report on the development of a solar cell-supercapacitor hybrid device as a solution to this energy requirement. A high-performance, cotton-textile-enabled asymmetric supercapacitor is integrated with a flexible solar cell via a scalable roll-to-roll manufacturing approach to fabricate a self-sustaining power pack, demonstrating its potential to continuously power future electronic devices.

Three-Dimensional Organic Conductive Networks Embedded in Paper for Flexible and Foldable Devices

The fabrication of three-dimensional (3D) polypyrrole conductive tracks through the porous structure of paper is demonstrated by the first time. We combined paper microfluidics and gas-phase pyrrole monomers to chemically synthesize polypyrrole-conducting channels embedded in-between the cellulose fibers. By using this method, foldable conductive structures can be created across the whole paper structure, allowing the electrical connection between both sides of the substrate. As a proof of concept, top-channel-top (TCT) and top-channel-bottom (TCB) conductive interconnections as well as all-organic paper-based touch buttons are demonstrated.

Bioinspired Mechanical Gradients in Cellulose Nanofibril/Polymer Nanopapers

Mechanical gradients are important as tough joints, for strain field engineering in printable electronics, for actuators, and for biological studies, yet they are difficult to prepare and quantitatively characterize. We demonstrate the additive fabrication of gradient bioinspired nanocomposites based on stiff, renewable cellulose nanofibrils that are bottom-up toughened via a tailor-made copolymer. Direct filament writing of different nanocomposite hydrogels in patterns, and subsequent healing of the filaments into continuous films while drying leads to a variety of linear, parabolic and striped bulk gradients. In situ digital image correlation under tensile deformation reveals important differences in the strain fields regarding asymmetry and step heights of the patterns. We envisage that merging top-down and bottom-up structuring of nanocellulose hybrids opens avenues for aperiodic and multiscale, bioinspired nanocomposites with optimized combinations of stiffness and toughness.

Evidence of Porphyrin-Like Structures in Natural Melanin Pigments Using Electrochemical Fingerprinting

Eumelanins are extended heterogeneous biopolymers composed of molecular subunits with ambiguous macromolecular topology. Here, an electrochemical fingerprinting technique is described, which suggests that natural eumelanin pigments contain indole-based tetramers that are arranged into porphyrin-like domains. Spectroscopy and density functional theory calculations suggest that sodium ions undergo occupancy-dependent stepwise insertion into the core of porphyrin-like tetramers in natural eumelanins at discrete potentials.

Controlling the Resistive Switching Behavior in Starch-Based Flexible Biomemristors

Implementation of biocompatible materials in resistive switching memory (ReRAM) devices provides opportunities to use them in biomedical applications. We demonstrate a robust, nonvolatile, flexible, and transparent ReRAM based on potato starch. We also introduce a biomolecular memory device that has a starch-chitosan composite layer. The ReRAM behavior can be controlled by mixing starch with chitosan in the resistive switching layer. Whereas starch-based biomemory devices which show abrupt changes in current level; the memory device with mixed biopolymers undergoes gradual changes. Both devices exhibit uniform and robust programmable memory properties for nonvolatile memory applications. The explicated source of the bipolar resistive switching behavior is assigned to formation and rupture of carbon-rich filaments. The gradual set/reset behavior in the memory device based on a starch-chitosan mixture makes it suitable for use in neuromorphic devices.

In Situ Carbonized Cellulose-Based Hybrid Film as Flexible Paper Anode for Lithium-Ion Batteries

Flexible free-standing carbonized cellulose-based hybrid film is integrately designed and served both as paper anode and as lightweight current collector for lithium-ion batteries. The well-supported heterogeneous nanoarchitecture is constructed from Li4Ti5O12 (LTO), carbonized cellulose nanofiber (C-CNF) and carbon nanotubes (CNTs) using by a pressured extrusion papermaking method followed by in situ carbonization under argon atmospheres. The in situ carbonization of CNF/CNT hybrid film immobilized with uniform-dispersed LTO results in a dramatic improvement in the electrical conductivity and specific surface area, so that the carbonized paper anode exhibits extraordinary rate and cycling performance compared to the paper anode without carbonization. The flexible, lightweight, single-layer cellulose-based hybrid films after carbonization can be utilized as promising electrode materials for high-performance, low-cost, and environmentally friendly lithium-ion batteries.

Effective dispersion of aqueous clay suspension using carboxylated nanofibrillated cellulose as dispersant

Carboxylated nanofibrillated cellulose extracted from wood fibers was used as a green dispersant to effectively disperse clay particles in water.

Conductivity of PEDOT:PSS on Spin-Coated and Drop Cast Nanofibrillar Cellulose Thin Films

Aqueous dispersion of conductive polymer poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (

PEDOT

PSS) was deposited on spin-coated and drop cast nanofibrillar cellulose (NFC)-glycerol (G) matrix on a glass substrate. A thin glycerol film was utilized on plasma-treated glass substrate to provide adequate adhesion for the NFC-glycerol (NFC-G) film. The effects of annealing temperature, the coating method of NFC-G, and the coating time intervals on the electrical performance of the

PEDOT

PSS were characterized.

PEDOT

PSS on drop cast NFC-G resulted in 3 orders of magnitude increase in the electrical conductivity compared to reference

PEDOT

PSS film on a reference glass substrate, whereas the optical transmission was only slightly decreased. The results point out the importance of the interaction between the

PEDOT

PSS and the NFC-G for the electrical and barrier properties for thin film electronics applications.

Cost-Effective and Highly Photoresponsive Nanophosphor-P3HT Photoconductive Nanocomposite for Near-Infrared Detection

The advent of flexible optoelectronic devices has accelerated the development of semiconducting polymeric materials. We seek to replace conventional expensive semiconducting photodetector materials with our cost-effective composite system. We demonstrate in this work the successful fabrication of a photoconductive composite film of poly(3-hexylthiophene-2,5-diyl) (P3HT) mixed with NaYF4:Yb,Er nanophosphors that exhibited a ultrahigh photoresponse to infrared radiation. The high photocurrent measured was enabled by the unique upconversion properties of NaYF4:Yb,Er nanophosphors, where low photon energy infrared excitations are converted to high photon energy visible emissions that are later absorbed by P3HT. Here we report, a significant 1.10 × 10(5) times increment of photocurrent from our photoconductive composite film upon infrared light exposure, which indicates high optical-to-electrical conversion efficiency. Our reported work lays the groundwork for the future development of printable, portable flexible and functional photonic composites for light sensing and harvesting, photonic memory devices, and phototransistors.

Effects of drying temperature and ethanol concentration on bipolar switching characteristics of natural Aloe vera-based memory devices

Natural Aloe vera provides a biodegradable, biocompatible, and renewable avenue for the sustainable development of electronics.

Surface modification of cellulose scaffold with polypyrrole for the fabrication of flexible supercapacitor electrode with enhanced capacitance

Green bioelectronics integrated the merits of biomaterial and conductive polymers have been prepared by in situ polymerization of conductive polymer monomer on porous structured cellulose matrix.