Study of Single and Multipass f-rGO Inkjet-Printed Structures with Various Concentrations: Electrical and Thermal Evaluation.

Reduced graphene oxide (rGO) is a derivative of graphene, which has been widely used as the conductive pigment of many water-based inks and is recognized as one of the most promising graphene-based materials for large-scale and low-cost production processes. In this work, we evaluate a custom functionalised reduced graphene oxide ink (f-rGO) via inkjet-printing technology. Test line structures were designed and fabricated by the inkjet printing process using the f-rGO ink on a pretreated polyimide substrate. For the electrical characterisation of these devices, two-point (2P) and four-point (4P) probe measurements were implemented. The results showed a major effect of the number of printed passes on the resulting resistance for all ink concentrations in both 2P and 4P cases. Interesting results can be extracted by comparing the obtained multipass resistance values that results to similar effective concentration with less passes. These measurements can provide the ground to grasp the variation in resistance values due to the different ink concentrations, and printing passes and can provide a useful guide in achieving specific resistance values with adequate precision. Accompanying topography measurements have been conducted with white-light interferometry. Furthermore, thermal characterisation was carried out to evaluate the operation of the devices as temperature sensors and heaters. It has been found that ink concentration and printing passes directly influence the performance of both the temperature sensors and heaters.

Conformal Integration of Efficient Conductive-Ink-Printed Antennas in Smart Suitcases for LPWAN-Based Luggage Tracking.

In the context of localization and sensing within the Internet of Things, new antenna manufacturing technologies, such as antennas printed with conductive inks on thin thermoplastic sheets, allow for seamless integration into plastic objects produced by an injection molding process. In this paper, we present printed sensor antennas for the [862-928] MHz band supporting LoRa and Sigfox and the [2.4-2.5] GHz band for WiFi, Bluetooth, and IEEE802.15.4 communication. To integrate them into smart suitcases, the antennas are printed, overmolded, tested, and measured, following a dedicated conformal integration strategy consisting of two design iterations. Additionally, as a more convenient connection to the printed antennas, printed transmission lines along with a dedicated transition to printed circuit board technologies are implemented and characterized, avoiding rigid coaxial connectors that exhibit fragile mounting on flexible substrates. The overmolded stand-alone antennas achieve fractional impedance bandwidths of 26% and 15% covering the [862-928] MHz and [2.4-2.5] GHz bands, respectively, with a substantial margin and with in-band simulated total efficiencies of 94% and 88%, respectively. Finally, the seamless integration of two antennas into a smart suitcase for tracing via Sigfox and WiFi demonstrates the potential of the proposed technique to realize high-performance antennas occupying virtually no real estate.

[Application of nanocellulose in flexible sensors].

The shortage of medical resources promotes medical treatment reform, and smart healthcare is a promising strategy to solve this problem. With the development of Internet, real-time health status is expected to be monitored at home by using flexible healthcare systems, which puts forward new demands on flexible substrates for sensors. Currently, the flexible substrates are mainly traditional petroleum-based polymers, which are not renewable. As a natural polymer, cellulose, owing to its wide range of sources, convenient processing, biodegradability and so on, is an ideal alternative. In this review, the application progress of nanocellulose in flexible sensors is summarized. The structure and the modification methods of cellulose and nanocellulose are introduced at first, and then the application of nanocellulose flexible sensors in real-time medical monitoring is summarized. Finally, the advantages and future challenges of nanocellulose in the field of flexible sensors are discussed.

Flexible and Transparent Polymer-Based Optical Humidity Sensor.

Thin spin-coated polymer films of amphiphilic copolymer obtained by partial acetalization of poly (vinyl alcohol) are used as humidity-sensitive media. They are deposited on polymer substrate (PET) in order to obtain a flexible humidity sensor. Pre-metallization of substrate is implemented for increasing the optical contrast of the sensor, thus improving the sensitivity. The morphology of the sensors is studied by surface profiling, while the transparency of the sensor is controlled by transmittance measurements. The sensing behavior is evaluated through monitoring of transmittance values at different levels of relative humidity gradually changing in the range 5-95% and the influence of up to 1000 bending deformations is estimated by determining the hysteresis and sensitivity of the flexible sensor after each set of deformations. The successful development of a flexible sensor for optical monitoring of humidity in a wide humidity range is demonstrated and discussed.

One-Step Fabrication of Bio-Compatible Coordination Complex Film on Diverse Substrates for Ternary Flexible Memory.

Recently, resistance random access memories (RRAMs) have been studied extensively, because the demand for information storage is increasing. However, it remains challenging to obtain a flexible device because the active materials involved need to be nontoxic, nonpolluting, distortion-tolerable, and biodegradable as well adhesive to diverse flexible substrates. In this paper, tannic acid (TA) and an iron ion (FeIII ) coordination complex were employed as the active layer in a sandwich-like (Al/active layer/substrate) device to achieve memory performance. A nontoxic, biocompatible TA-FeIII coordination complex was synthesized by a one-step self-assembly solution method. The retention time of the TA-FeIII memory performance was up to 15 000 s, the yield up to 53 %. Furthermore, the TA-FeIII coordination complex can form a high-quality film and shows stable ternary memory behavior on various flexible substrates, such as polyethylene terephthalate (PET), polyimide (PI), printer paper, and leaf. The device can be degraded by immersing it in vinegar solution. Our work will broaden the application of organic coordination complexes in flexible memory devices with diverse substrates.

Programmable and reversible plasmon mode engineering.

Plasmonic nanostructures with enhanced localized optical fields as well as narrow linewidths have driven advances in numerous applications. However, the active engineering of ultranarrow resonances across the visible regime-and within a single system-has not yet been demonstrated. This paper describes how aluminum nanoparticle arrays embedded in an elastomeric slab may exhibit high-quality resonances with linewidths as narrow as 3 nm at wavelengths not accessible by conventional plasmonic materials. We exploited stretching to improve and tune simultaneously the optical response of as-fabricated nanoparticle arrays by shifting the diffraction mode relative to single-particle dipolar or quadrupolar resonances. This dynamic modulation of particle-particle spacing enabled either dipolar or quadrupolar lattice modes to be selectively accessed and individually optimized. Programmable plasmon modes offer a robust way to achieve real-time tunable materials for plasmon-enhanced molecular sensing and plasmonic nanolasers and opens new possibilities for integrating with flexible electronics.

Recent Developments in Printing Flexible and Wearable Sensing Electronics for Healthcare Applications.

Wearable biosensors attract significant interest for their capabilities in real-time monitoring of wearers' health status, as well as the surrounding environment. Sensor patches are embedded onto the human epidermis accompanied by data readout and signal conditioning circuits with wireless communication modules for transmitting data to the computing devices. Wearable sensors designed for recognition of various biomarkers in human epidermis fluids, such as glucose, lactate, pH, cholesterol, etc., as well as physiological indicators, i.e., pulse rate, temperature, breath rate, respiration, alcohol, activity monitoring, etc., have potential applications both in medical diagnostics and fitness monitoring. The rapid developments in solution-based nanomaterials offered a promising perspective to the field of wearable sensors by enabling their cost-efficient manufacturing through printing on a wide range of flexible polymeric substrates. This review highlights the latest key developments made in the field of wearable sensors involving advanced nanomaterials, manufacturing processes, substrates, sensor type, sensing mechanism, and readout circuits, and ends with challenges in the future scope of the field. Sensors are categorized as biological and fluidic, mounted directly on the human body, or physiological, integrated onto wearable substrates/gadgets separately for monitoring of human-body-related analytes, as well as external stimuli. Special focus is given to printable materials and sensors, which are key enablers for wearable electronics.

Implementation of Radiating Elements for Radiofrequency Front-Ends by Screen-Printing Techniques for Internet of Things Applications.

The advent of the Internet of Things (IoT) has led to embedding wireless transceivers into a wide range of devices, in order to implement context-aware scenarios, in which a massive amount of transceivers is foreseen. In this framework, cost-effective electronic and Radio Frequency (RF) front-end integration is desirable, in order to enable straightforward inclusion of communication capabilities within objects and devices in general. In this work, flexible antenna prototypes, based on screen-printing techniques, with conductive inks on flexible low-cost plastic substrates is proposed. Different parameters such as substrate/ink characteristics are considered, as well as variations in fabrication process or substrate angular deflection in device performance. Simulation and measurement results are presented, as well as system validation results in a real test environment in wireless sensor network communications. The results show the feasibility of using screen-printing antenna elements on flexible low-cost substrates, which can be embedded in a wide array of IoT scenarios.

Light-Emitting GaAs Nanowires on a Flexible Substrate.

Semiconductor nanowire-based devices are among the most promising structures used to meet the current challenges of electronics, optics and photonics. Due to their high surface-to-volume ratio and excellent optical and electrical properties, devices with low power, high efficiency and high density can be created. This is of major importance for environmental issues and economic impact. Semiconductor nanowires have been used to fabricate high performance devices, including detectors, solar cells and transistors. Here, we demonstrate a technique for transferring large-area nanowire arrays to flexible substrates while retaining their excellent quantum efficiency in emission. Starting with a defect-free self-catalyzed molecular beam epitaxy (MBE) sample grown on a Si substrate, GaAs core-shell nanowires are embedded in a dielectric, removed by reactive ion etching and transferred to a plastic substrate. The original structural and optical properties, including the vertical orientation, of the nanowires are retained in the final plastic substrate structure. Nanowire emission is observed for all stages of the fabrication process, with a higher emission intensity observed for the final transferred structure, consistent with a reduction in nonradiative recombination via the modification of surface states. This transfer process could form the first critical step in the development of flexible nanowire-based light-emitting devices.

Flexible Gas Sensors Employing Octahedral Indium Oxide Films.

Indium oxide octahedral nanopowders were obtained from an ionic precursor compound after an oxidation process conducted under a low-oxygen atmosphere. This method was found to produce contamination-free indium oxide nanomaterial with very similar morphological and crystalline properties to the one produced by vapor-phase transport, but at significantly lower temperatures and higher yield. The as-synthesized indium oxide was mixed to an organic vehicle and microdrop deposited to form a film bridging the interdigitated silver electrodes patterned on top of a flexible, polyimide (Kapton®), substrate. The gas sensing properties of the flexible chemoresistors towards ammonia vapors, hydrogen, and nitrogen dioxide were investigated. It was found that these sensors were remarkably sensitive to nitrogen dioxide at a low operating temperature of 150 °C. These results are consistent with the performance of vapor-phase transport synthesized indium oxide octahedra sensors on rigid, ceramic substrates. Therefore, the results presented here pave the way for the mass production of inexpensive gas sensors onto flexible substrates via additive manufacturing.

Template-Free Mesoporous Electrochromic Films on Flexible Substrates from Tungsten Oxide Nanorods.

Low-temperature processed mesoporous nanocrystal thin films are platforms for fabricating functional composite thin films on flexible substrates. Using a random arrangement of anisotropic nanocrystals can be a facile solution to generate pores without templates. However, the tendency for anisotropic particles to spontaneously assemble into a compact structure must be overcome. Here, we present a method to achieve random networking of nanorods during solution phase deposition by switching their ligand-stabilized colloidal nature into a charge-stabilized nature by a ligand-stripping chemistry. Ligand-stripped tungsten suboxide (WO2.72) nanorods result in uniform mesoporous thin films owing to repulsive electrostatic forces preventing nanorods from densely packing. Porosity and pore size distribution of thin films are controlled by changing the aspect ratio of the nanorods. This template-free mesoporous structure, achieved without annealing, provides a framework for introducing guest components, therefore enabling our fabrication of inorganic nanocomposite electrochromic films on flexible substrates. Following infilling of niobium polyoxometalate clusters into pores and successive chemical condensation, a WOx-NbOx composite film is produced that selectively controls visible and near-infrared light transmittance without any annealing required. The composite shows rapid switching kinetics and can be stably cycled between optical states over 2000 times. This simple strategy of using anisotropic nanocrystals gives insight into mesoporous thin film fabrication with broader applications for flexible devices.

Spontaneously Embedded Scattering Structures in a Flexible Substrate for Light Extraction.

A flexible hazy substrate (FHS) with embedded air bubbles to increase light extraction efficiency of organic light-emitting diodes (OLEDs) is reported. In order to embed the air bubbles in the flexible substrate, micropatterned substrates are fabricated by plasma treatment, and then coated with a planarization layer. During the planarization layer coating, air bubbles are trapped between the substrate and the planarization layer. The haze of the FHS can be controlled from 1.7% to 68.4% by changing the size of micropatterns by adjusting the plasma treatment time. The FHS shows average haze of 68.4%, average total transmittance of 90.3%, and extremely flat surface with average roughness (R a ) of 1.2 nm. Rigorous coupled-wave analysis and finite-difference time-domain simulations are conducted to demonstrate that the air bubbles in the substrate can effectively extract photons that are trapped in the substrate. The FHS increases the power efficiency of OLEDs by 22% and further increases by 91% combined with an external extraction layer. Moreover, the FHS has excellent mechanical flexibility. No defect has been observed after 10 000 bending cycles at bending radius of 4 mm.

PDMS based multielectrode arrays for superior in-vitro retinal stimulation and recording.

Understanding of the neural response to electrical stimulation requires simultaneous recording from the various neurons of retina. Electrodes form the physical interface with the neural or retinal tissue. Successful retinal stimulation and recording demands conformal integration of these electrodes with the soft tissue to ensure establishment of proper electrical connection with the excitable tissue. Mechanical impedance of polydimethylsiloxane (PDMS) being compliant with that of retinal tissue, offers excellent potential as a substrate for metal electrodes. In this paper, Cr/Au micro electrodes with 200 µm diameter were fabricated on rigid and flexible PDMS substrates under crack free condition. Spontaneous buckling of thin films over PDMS substrates improved electrode performance circumventing the fabrication issues faced over a buckled surface. Individual electrodes from the multielectrode arrays (MEAs) were examined with electrochemical impedance spectroscopy and cyclic voltammetry. Controlled fabrication process as described here generates buckles in the metal films leading to increased electrode surface area that increases the charge storage capacity and decreases the interface impedance of the metal electrodes. At 1 kHz, impedance was reduced from 490 ± 27 kΩ to 246 ± 19 kΩ and charge storage capacity was increased from 0.40 ± 0.87 mC/cm2 to 2.1 ± 0.87 mC/cm2. Neural spikes recorded with PDMS based electrodes from isolated retina also contained less noise as indicated by signal to noise ratio analysis. The present study established that the use of PDMS as a substrate for MEAs can enhance the performance of any thin film metal electrodes without incorporation of any coating layers or nanomaterials.

A 24-GHz Front-End Integrated on a Multilayer Cellulose-Based Substrate for Doppler Radar Sensors.

This paper presents a miniaturized Doppler radar that can be used as a motion sensor for low-cost Internet of things (IoT) applications. For the first time, a radar front-end and its antenna are integrated on a multilayer cellulose-based substrate, built-up by alternating paper, glue and metal layers. The circuit exploits a distributed microstrip structure that is realized using a copper adhesive laminate, so as to obtain a low-loss conductor. The radar operates at 24 GHz and transmits 5 mW of power. The antenna has a gain of 7.4 dBi and features a half power beam-width of 48 degrees. The sensor, that is just the size of a stamp, is able to detect the movement of a walking person up to 10 m in distance, while a minimum speed of 50 mm/s up to 3 m is clearly measured. Beyond this specific result, the present paper demonstrates that the attractive features of cellulose, including ultra-low cost and eco-friendliness (i.e., recyclability and biodegradability), can even be exploited for the realization of future high-frequency hardware. This opens opens the door to the implementation on cellulose of devices and systems which make up the "sensing layer" at the base of the IoT ecosystem.

Measuring electro-mechanical properties of thin films on polymer substrates.

In order to advance flexible electronic technologies it is important to study the electrical properties of thin metal films on polymer substrates under mechanical load. At the same time, the observation of film deformation and fracture as well as the stresses that are present in the films during straining are also crucial to investigate. To address both the electromechanical and deformation behavior of metal films supported by polymer substrates, in-situ 4 point probe resistance measurements were performed with in-situ atomic force microscopy imaging of the film surface during straining. The 4 point probe resistance measurements allow for the examination of the changes in resistance with strain, while the surface imaging permits the visualization of localized thinning and crack formation. Furthermore, in-situ synchrotron tensile tests provide information about the stresses in the film and show the yield stress where the deformation initiates and the relaxation of the film during imaging. A thin 200 nm Cu film on 23 µm thick PET substrate will be used to illustrate the combined techniques. The combination of electrical measurements, surface imaging, and stress measurements allow for a better understanding of electromechanical behavior needed for the improvement and future success of flexible electronic devices.

Highly reliable ag nanowire flexible transparent electrode with mechanically welded junctions.

Deformation behavior of the Ag nanowire flexible transparent electrode under bending strain is studied and results in a novel approach for highly reliable Ag nanowire network with mechanically welded junctions. Bending fatigue tests up to 500,000 cycles are used to evaluate the in situ resistance change while imposing fixed, uniform bending strain. In the initial stages of bending cycles, the thermally annealed Ag nanowire networks show a reduction in fractional resistance followed by a transient and steady-state increase at later stages of cycling. SEM analysis reveals that the initial reduction in resistance is caused by mechanical welding as a result of applied bending strain, and the increase in resistance at later stages of cycling is determined to be due to the failure at the thermally locked-in junctions. Based on the observations from this study, a new methodology for highly reliable Ag nanowire network is proposed: formation of Ag nanowire networks with no prior thermal annealing but localized junction formation through simple application of mechanical bending strain. The non-annealed, mechanically welded Ag nanowire network shows significantly enhanced cyclic reliability with essentially 0% increase in resistance due to effective formation of localized wire-to-wire contact.

Simple Synthesis of Cellulose-Based Nanocomposites as SERS Substrates for In Situ Detection of Thiram.

There is a growing interest in the use of flexible substrates for label-free and in situ Surface-enhanced Raman Spectroscopy (SERS) applications. In this study, a flexible SERS substrate was prepared using self-assembled Au/Ti3C2 nanocomposites deposited on a cellulose (CS) paper. The Au/Ti3C2 nanocomposites uniformly wrapped around the cellulose fibers to provide a three-dimensional plasma SERS platform. The limit of detection (LOD) of CS/Au/Ti3C2 was as low as 10-9 M for 4-mercaptobenzoic acid(4-MBA) and crystal violet (CV), demonstrating good SERS sensitivity. CS/Au/Ti3C2 was used for in situ SERS detection of thiram on apple surfaces by simple swabbing, and a limit of detection of 0.05 ppm of thiram was achieved. The results showed that CS/Au/Ti3C2 is a flexible SERS substrate that can be used for the detection of thiram on apple surfaces. These results demonstrate that CS/Au/Ti3C2 can be used for the non-destructive, rapid and sensitive detection of pesticides on fruit surfaces.

Enthalpy-Driven Self-Healing in Thin Metallic Films on Flexible Substrates.

Self-healing microelectronics are needed for costly applications with limited or without access. They are needed in fields such as space exploration to increase lifetime and decrease both costs and the environmental impact. While advanced self-healing mechanisms for polymers are numerous, practical ways for self-healing in metal films have yet to be found. A concept for an autonomous intrinsic self-healing metallic film system is developed, allowing the healing of cracks in metallic films on flexible substrates. The concept relies on stabilizing metastable thin films with high mixing enthalpy via segregation barriers. This allows the films to possess autonomous intrinsic self-healing capabilities triggered by cracking at temperatures not detrimental to flexible microelectronics. The effect will be shown on metastable Mo1-xAgx thin films, stabilized via a Mo segregation barrier. Without a segregation barrier, the system is known to exhibit spontaneous Ag particle formation on the surface. This property is controlled and directed to heal cracks and partially restore the electro-mechanical properties of the multilayer system. This mechanism opens up the field of self-healing thin metallic films that could profoundly impact the design of future microelectronics.

In situ assembly of silver nanoparticles throughout electrospun oriented alginate nanofibers for hazardous rust trace detection on bronze.

A convenient and reliable surface-enhanced Raman scattering (SERS) substrate has been developed for the surface corrosion analysis of bronze artifacts. The substrate consists of oriented alginate nanofiber membranes containing silver nanoparticles (Ag NPs), which were obtained through electrostatic spinning, ion exchange, and in-situ reduction. By controlling the reduction time, Ag/alginate nanofiber membranes with different contents, sizes, and distributions were obtained. The Ag/alginate nanofiber#20 membranes, obtained with a reduction time of 20 min, reached a detection limit of 10-12 M for R6G with an enhancement factor of 6.64 × 107. In the trace detection of bronze patina, the intensity of the characteristic peaks of harmful patina located at 513, 846, 911, and 974 cm-1 were increased by more than 500 %. This was due to the uniform loading of a large number of Ag NPs on the surface of the nanofiber membrane obtained by reduction for 20 min, and the formation of a large number of hot spots between the oriented nanofibers. This significantly improved the SERS performance of the flexible substrate layer under the joint action with the Ag NPs. These results indicate that the flexible substrate layer can greatly enhance the Raman characteristic peaks of alkali copper chloride and be effectively used for trace analysis of the surface composition of bronze artifacts.

Recent Advances in Wearable Healthcare Devices: From Material to Application.

In recent years, the proliferation of wearable healthcare devices has marked a revolutionary shift in the personal health monitoring and management paradigm. These devices, ranging from fitness trackers to advanced biosensors, have not only made healthcare more accessible, but have also transformed the way individuals engage with their health data. By continuously monitoring health signs, from physical-based to biochemical-based such as heart rate and blood glucose levels, wearable technology offers insights into human health, enabling a proactive rather than a reactive approach to healthcare. This shift towards personalized health monitoring empowers individuals with the knowledge and tools to make informed decisions about their lifestyle and medical care, potentially leading to the earlier detection of health issues and more tailored treatment plans. This review presents the fabrication methods of flexible wearable healthcare devices and their applications in medical care. The potential challenges and future prospectives are also discussed.