Highly Responsive PEG/Gold Nanoparticle Thin-Film Humidity Sensor via Inkjet Printing Technology

Cellulose paper-based humidity power generator with high open circuit voltage based on zinc-air battery structure

The sensing mechanisms of common humidity sensors related to conductive active materials, which can be simply attributed to the variations in resistivity due to the separation of conductive materials and variations in polymer permittivity, are generally plagued by drawbacks such as cumbersome fabrication processes, high cost and low performance. Herein, we prepared Zn/Cellulose filter paper (CFP)/Nanoscale carbon ink (NCI)/Cu structure humidity power generators (ZHGs) based on the power generation principle of typical zinc-air batteries, using active metals with strong conductivity as electrodes, and the redox reactions that took place in the zinc-air battery can convert the chemical energy in the electrode into a stable electrical energy. The ZHG fabricated in this work can reach an extremely high open circuit voltage (Voc) of 0.803 V at 97 % RH and possesses an excellent power density of 312.24 μW/cm2, which has a good linear relationship (R2 = 0.9669) over a wide humidity range (20-97 % RH). In addition, 10 ZHGs in series can charge commercial capacitors up to 3.83 V. Finally, the proof of concept demonstrated that the humidity power generation sensor can be well applied in human respiratory monitoring, finger non-contact switch, and power supply for light emitting diode (LED).

Smart Home Sleep Respiratory Monitoring System Based on a Breath-Responsive Covalent Organic Framework

A smart home sleep respiratory monitoring system based on a breath-responsive covalent organic framework (COF) was developed and utilized to monitor the sleep respiratory behavior of real sleep apnea patients in this work. The capacitance of the interdigital electrode chip coated with COFTPDA-TFPy exhibits thousands-level reversible responses to breath humidity gases, with subsecond response time and robustness against environmental humidity. A miniaturized printed circuit board, an open-face-mask-based respiratory sensor, and a smartphone app were constructed for the wearable wireless smart home sleep respiratory monitoring system. Leveraging the sensitive and rapid reversible response of COFs, the COF-based respiratory monitoring system can effectively record normal breath, rapid breath, and breath apnea, enabling over a thousand cycles of hour-level continuous monitoring during daily wear. Next, we took the groundbreaking step of advancing the humidity sensor to the clinical trial stage. In clinical experiments on real sleep apnea patients, the COF-based respiratory monitoring system successfully recorded hour-level sleep respiratory data and differentiated the breathing behavior characteristics and severity of sleep apnea patients and subjects with normal sleep function and primary snoring patients. This work successfully advanced humidity sensors into clinical research for real patients and demonstrated the enormous application potential of COF materials in clinical diagnosis.

Promising New Horizons in Medicine: Medical Advancements with Nanocomposite Manufacturing via 3D Printing

Three-dimensional printing technology has fundamentally revolutionized the product development processes in several industries. Three-dimensional printing enables the creation of tailored prostheses and other medical equipment, anatomical models for surgical planning and training, and even innovative means of directly giving drugs to patients. Polymers and their composites have found broad usage in the healthcare business due to their many beneficial properties. As a result, the application of 3D printing technology in the medical area has transformed the design and manufacturing of medical devices and prosthetics. Polymers and their composites have become attractive materials in this industry because of their unique mechanical, thermal, electrical, and optical qualities. This review article presents a comprehensive analysis of the current state-of-the-art applications of polymer and its composites in the medical field using 3D printing technology. It covers the latest research developments in the design and manufacturing of patient-specific medical devices, prostheses, and anatomical models for surgical planning and training. The article also discusses the use of 3D printing technology for drug delivery systems (DDS) and tissue engineering. Various 3D printing techniques, such as stereolithography, fused deposition modeling (FDM), and selective laser sintering (SLS), are reviewed, along with their benefits and drawbacks. Legal and regulatory issues related to the use of 3D printing technology in the medical field are also addressed. The article concludes with an outlook on the future potential of polymer and its composites in 3D printing technology for the medical field. The research findings indicate that 3D printing technology has enormous potential to revolutionize the development and manufacture of medical devices, leading to improved patient outcomes and better healthcare services.

A Metal Nanoparticle Thermistor with the Beta Value of 10 000 K

The thermistor, typically made from metallic oxides, is a type of resistor whose electrical resistance is dependent on its temperature. Despite the wide usage, the limitations of ceramic thermistors become increasingly apparent as devices with improved performances are sought and as new applications emerge. Herein, a thermistor that is showed with a beta (B) value of 10 000 K can be made exclusively from metal nanoparticles functionalized with charged organic ligands. This B value is hard to achieve for ceramic devices, which is due to the increase of effective counterion concentration and its mobility upon thermal activation. Importantly, the performance of the nanoparticle thermistor is maintained when it is fabricated on a flexible substrate and experiences reversible bending. Demos of thermistor arrays for heat transfer, distribution, and comparison of their performance with commercial products are also demonstrated. Owing to the low temperature and simple casting process, conformably flexible characteristics, stable solid states, and ultra-high sensitivities, this device is expected to be practically used soon.

Toward Sustainable Wearable Electronic Textiles

Smart wearable electronic textiles (e-textiles) that can detect and differentiate multiple stimuli, while also collecting and storing the diverse array of data signals using highly innovative, multifunctional, and intelligent garments, are of great value for personalized healthcare applications. However, material performance and sustainability, complicated and difficult e-textile fabrication methods, and their limited end-of-life processability are major challenges to wide adoption of e-textiles. In this review, we explore the potential for sustainable materials, manufacturing techniques, and their end-of-the-life processes for developing eco-friendly e-textiles. In addition, we survey the current state-of-the-art for sustainable fibers and electronic materials (i.e., conductors, semiconductors, and dielectrics) to serve as different components in wearable e-textiles and then provide an overview of environmentally friendly digital manufacturing techniques for such textiles which involve less or no water utilization, combined with a reduction in both material waste and energy consumption. Furthermore, standardized parameters for evaluating the sustainability of e-textiles are established, such as life cycle analysis, biodegradability, and recyclability. Finally, we discuss the current development trends, as well as the future research directions for wearable e-textiles which include an integrated product design approach based on the use of eco-friendly materials, the development of sustainable manufacturing processes, and an effective end-of-the-life strategy to manufacture next generation smart and sustainable wearable e-textiles that can be either recycled to value-added products or decomposed in the landfill without any negative environmental impacts.

Nanocellulose as a promising substrate for advanced sensors and their applications

Nanocellulose has broad and promising applications owing to its low density, large specific surface area, high mechanical strength, modifiability, renewability. Recently, nanocellulose has been widely used to fabricate flexible, durable and environmental-friendly sensor substrates. In this contribution, the construction and characteristics of nanocellulose-based sensors are comprehensively reviewed. Various nanocellulose-based sensors are summarized and divided into colorimetric, fluorescent, electronic, electrochemical and SERS types according to the sensing mechanism. This review also introduces the applications of nanocellulose-based sensors in the fields of biomedicine, environmental monitoring, food safety, and wearable devices.

Self-Powered Carbon Ink/Filter Paper Flexible Humidity Sensor Based on Moisture-Induced Voltage Generation

Cellulose paper-based materials are highly flexible, hydrophilic, low-cost, and environmentally friendly and are good substrates for use as humidity sensors. Therefore, developing a paper-based humidity sensor with facile fabrication, low cost, and high sensitivity is important for expanding its practical applications. Herein, we propose a CI/FP self-powered humidity sensor based on everyday items such as writing and drawing carbon ink (CI), cellulose filter paper (FP), and polyester conductive adhesive tape, which is fabricated with the help of facile dip-coating and pasting methods. This sensor is self-powered, and the paper-based material itself can absorb water molecules in a humid environment to generate humidity-related voltage and current, which can indirectly reflect the ambient humidity level. They are characterized by a wide relative humidity (RH) sensing range (11-98%), good linearity (R² = 0.97011), high response voltage (0.19 V), and excellent flexibility (over 1000 bends). This humidity sensor can be successfully applied to monitor human health (breathing, coughing), air humidity, and noncontact humidity sensing (skin, wet objects). This work not only proposes a low-cost and facile method for flexible humidity sensors but also provides a valuable strategy for the development of self-powered wearable electronics.

A stretchable, self-healing, okra polysaccharide-based hydrogel for fast-response and ultra-sensitive strain sensors

Self-healing conductive hydrogels have attracted widespread attention as a new generation of smart wearable devices and human motion monitoring sensors. To improve the biocompatibility and degradability of such strain sensors, we report a sensor with a sandwich structure based on a biomucopolysaccharide hydrogel. The sensor was constructed with a stretchable self-healing hydrogel composed of polyvinyl alcohol (PVA), okra polysaccharide (OP), borax, and a conductive layer of silver nanowires. The obtained OP/PVA/borax hydrogel exhibited excellent stretchability (~1073.7%) and self-healing ability (93.6% within 5 min), and the resultant hydrogel-based strain sensor demonstrated high sensitivity (gauge factor = 6.34), short response time (~20 ms), and good working stability. This study provides innovative ideas for the development of biopolysaccharide hydrogels for applications in the field of sensors.

A Printed Flexible Humidity Sensor with High Sensitivity and Fast Response Using a Cellulose Nanofiber/Carbon Black Composite

In the emerging Internet of Things (IoT) society, there is a significant need for low-cost, high-performance flexible humidity sensors in wearable devices. However, commercially available humidity sensors lack flexibility or require expensive and complex fabrication methods, limiting their application and widespread use. We report a high-performance printed flexible humidity sensor using a cellulose nanofiber/carbon black (CNF/CB) composite. The cellulose nanofiber enables excellent dispersion of carbon black, which facilitates the ink preparation and printing process. At the same time, its hydrophilic and porous nature provides high sensitivity and fast response to humidity. Significant resistance changes of 120% were observed in the sensor at humidity ranging from 30% RH to 90% RH, with a fast response time of 10 s and a recovery time of 6 s. Furthermore, the developed sensor also exhibited high-performance uniformity, response stability, and flexibility. A simple humidity detection device was fabricated and successfully applied to monitor human respiration and noncontact fingertip moisture as a proof-of-concept.

Highly sensitive, room temperature operated gold nanowire-based humidity sensor: adoptable for breath sensing

A novel, highly sensitive gold nanowire (AuNW) resistive sensor is reported here for humidity sensing in the relative humidity range of 11% to 92% RH as well as for breath sensing. Both humidity and breath sensors are widely needed. Despite a lot of research on humidity and breath sensors, there is a need for simple, inexpensive, reliable, sensitive and selective sensors, which will operate at room temperature. Here we have synthesized gold nanowires by a simple, wet chemical route. The nanowires synthesized by us are 4–7 nm in diameter and a few micrometers long. The nanowires are amine functionalized. The sensor was prepared by drop casting gold nanowires on an alumina substrate to form a AuNW layer with different thicknesses (10, 20, 30 μm). The AuNW sensor is highly selective towards humidity and shows minimum cross sensitivity towards other gases and organic vapors. At an optimum thickness of 20 μm, the humidity sensing performance of the AuNW sensor over 11% to 92% RH was found to be superior to that of 10 and 30 μm thick layers. The response time of the sensor is found to be 0.2 s and the recovery time is 0.3 s. The response of the AuNW sensor was 3.3 MΩ/% RH. Further, the AuNW sensor was tested for sensing human breathing patterns.

A novel, highly sensitive gold nanowire (AuNW) resistive sensor is reported here for humidity sensing in the relative humidity range of 11% to 92% RH as well as for breath sensing.

Harnessing selectivity in chemical sensing via supramolecular interactions: from functionalization of nanomaterials to device applications

Chemical sensing is a strategic field of science and technology ultimately aiming at improving the quality of our lives and the sustainability of our Planet. Sensors bear a direct societal impact on well-being, which includes the quality and composition of the air we breathe, the water we drink, and the food we eat. Pristine low-dimensional materials are widely exploited as highly sensitive elements in chemical sensors, although they suffer from lack of intrinsic selectivity towards specific analytes. Here, we showcase the most recent strategies on the use of (supra)molecular interactions to harness the selectivity of suitably functionalized 0D, 1D, and 2D low-dimensional materials for chemical sensing. We discuss how the design and selection of receptors via machine learning and artificial intelligence hold a disruptive potential in chemical sensing, where selectivity is achieved by the design and high-throughput screening of large libraries of molecules exhibiting a set of affinity parameters that dictates the analyte specificity. We also discuss the importance of achieving selectivity along with other relevant characteristics in chemical sensing, such as high sensitivity, response speed, and reversibility, as milestones for true practical applications. Finally, for each distinct class of low-dimensional material, we present the most suitable functionalization strategies for their incorporation into efficient transducers for chemical sensing.

Inkjet Printability Assessment of Weakly Viscoelastic Fluid: A Semidilute Polyvinylpyrrolidone Solution Ink Case Study

Here, we present an integrated approach to the weakly viscoelastic fluid printability assessment by using global dimensionless criteria (DC). The problem was studied on a model semidiluted polyvinylpyrrolidone water-based ink. For the study purpose, the ink composition was kept as simple as possible. First, the solution density, viscosity, and surface tension were determined. Obtained data were used for testing limitations of DC printability diagrams already available for Newtonian fluids. A replotted version of the original Kim and Baek's map was developed emphasizing the importance of surface tension in the drop formation process. Another set of DC (e.g., Ec and De) was also used for a real evaluation of the viscoelasticity effect on both jetting conditions and drop formation. The polymer relaxation time as a crucial parameter for viscoelasticity was shown to be calculated using the Kuhn segment length rather than from Zimm and Rouse theories for diluted polymer systems. Then, a two-dimensional diagram using four DC (Oh and De with Ec and El as parameters) is proposed based on the famous McKinley's work. The diagram describes the interplay of possible forces responsible for filament thinning and breakup processes. Obtained results were supported by further experiments involving drop ejection and formation, determination of critical polymer concentration, and others. The proposed diagram promises a useful initial step in further investigations of viscoelasticity of polymer compounds by inkjet printing.

Monitoring Symptoms of Infectious Diseases: Perspectives for Printed Wearable Sensors

Infectious diseases possess a serious threat to the world's population, economies, and healthcare systems. In this review, we cover the infectious diseases that are most likely to cause a pandemic according to the WHO (World Health Organization). The list includes COVID-19, Crimean-Congo Hemorrhagic Fever (CCHF), Ebola Virus Disease (EBOV), Marburg Virus Disease (MARV), Lassa Hemorrhagic Fever (LHF), Middle East Respiratory Syndrome (MERS), Severe Acute Respiratory Syndrome (SARS), Nipah Virus diseases (NiV), and Rift Valley fever (RVF). This review also investigates research trends in infectious diseases by analyzing published research history on each disease from 2000-2020 in PubMed. A comprehensive review of sensor printing methods including flexographic printing, gravure printing, inkjet printing, and screen printing is conducted to provide guidelines for the best method depending on the printing scale, resolution, design modification ability, and other requirements. Printed sensors for respiratory rate, heart rate, oxygen saturation, body temperature, and blood pressure are reviewed for the possibility of being used for disease symptom monitoring. Printed wearable sensors are of great potential for continuous monitoring of vital signs in patients and the quarantined as tools for epidemiological screening.

Stimuli-Responsive Plasmonic Assemblies and Their Biomedical Applications

Among the diverse development of stimuli-responsive assemblies, plasmonic nanoparticle (NP) assemblies functionalized with responsive molecules are of a major interest. In this review, we outline a comprehensive and up-to-date overview of recently reported studies on in vitro and in vivo assembly/disassembly and biomedical applications of plasmonic NPs, wherein stimuli such as enzymes, light, pH, redox potential, temperature, metal ions, magnetic or electric field, and/or multi-stimuli were involved. Stimuli-responsive assemblies have been applied in various biomedical fields including biosensors, surfaced-enhanced Raman scattering (SERS), photoacoustic (PA) imaging, multimodal imaging, photo-activated therapy, enhanced X-ray therapy, drug release, stimuli-responsive aggregation-induced cancer therapy, and so on. The perspectives on the use of stimuli-responsive plasmonic assemblies are discussed by addressing future scientific challenges involving assembly/disassembly strategies and applications.

Experimental and Theoretical Studies on Sustainable Synthesis of Gold Sol Displaying Dichroic Effect

Gold nanoparticles depending on their shape and mixtures of multiple shapes can exhibit peculiar optical properties, including the dichroic effect typical of the Lycurgus cup, which has puzzled scientists for a long time. Such optical properties have been recently exploited in several fields such as paint technology, sensors, dichroic polarizers, display (LCD) devices, laser applications, solar cells and photothermal therapy among others. In this article, we have demonstrated a simple room temperature one-pot synthesis of gold sol displaying a dichroic effect using a slow reduction protocol involving only trisodium citrate as a reducing agent. We found that the dichroic gold sol can be easily formed at room temperature by reducing gold salt by trisodium citrate below a certain critical concentration. The sol displayed an orangish-brown color in scattered/reflected light and violet/blue/indigo/purple/red/pink in transmitted light, depending on the experimental conditions. With minor changes such as the introduction of a third molecule or replacing a small amount of water in the reaction mixture with ethanol, the color of the gold sol under transmitted light changed and a variety of shades of red, pink, cobalt blue, violet, magenta and purple were obtained. The main advantage of the proposed method lies in its simplicity, which involves the identification of the right ratio of the reactants, and simple mixing of reactants at room temperature with no other requirements. TEM micrographs displayed the formation of two main types of particles viz. single crystal gold nanoplates and polycrystalline faceted polyhedron nanoparticles. The mechanism of growth of the nanoplates and faceted polyhedron particles have been described by an enhanced diffusion limited aggregation numerical scheme, where it was assumed that both trisodium citrate and the gold ions in solution undergo a stochastic Brownian motion, and that the evolution of the entire system is regulated by a principle of energy minimization. The predictions of the model matched with the experiments with a good accuracy, indicating that the initial hypothesis is correct.

Ultrathin Porous Hydrocarbon Membranes Templated by Nanoparticle Assemblies

Porous polymer membranes are widely desired as catalyst supports, sensors, and active layers for separation membranes. We demonstrate that electron beam irradiation of freely suspended gold or Fe₃O₄ nanoparticle (NP) monolayer sheets followed by wet chemical etching is a high-fidelity strategy to template two-dimensional (2D) porous cross-linked hydrocarbon membranes. This approach, which relies on secondary electrons generated by the NP cores, can further be used to transform three-dimensional (3D) terraced gold NP supercrystals into 3D porous hydrocarbon membranes. We utilize electron tomography to show how the number of NP layers (monolayer to pentalayer) controls attenuation and scattering of the primary e-beam, which in turn determines ligand cross-link density and 3D pore structure. Electron tomography also reveals that many nanopores are vertically continuous because of preferential sintering of NPs. This work demonstrates new routes for the construction of functional nanoporous media.

Fully Printed Flexible Chemiresistors with Tunable Selectivity Based on Gold Nanoparticles

This study presents a method for printing flexible chemiresistors comprising thin film transducers based on cross-linked gold nanoparticles (GNPs). First, interdigitated silver paste electrodes are printed onto polyimide (PI) foil via dispenser printing. Second, coatings of GNPs and dithiol/monothiol blends are inkjet-printed onto these electrode structures. 1,9-Nonanedithiol (9DT) is used as cross-linking agent and a variety of monothiols are added to tune the sensors’ chemical selectivity. When dosing these sensors with different analyte vapors (n-octane, toluene, 4-methyl-2-pentanone, 1-butanol, 1-propanol, ethanol, water; concentration range: 25–2000 ppm) they show fully reversible responses with short response and recovery times. The response isotherms follow a first-order Langmuir model, and their initial slopes reveal sensitivities of up to 4.5 × 10−5 ppm−1. Finally, it is demonstrated that arrays of printed sensors can be used to clearly discern analytes of different polarity.

Local orientational order in self-assembled nanoparticle films: the role of ligand composition and salt

An X-ray cross-correlation study of the local orientational order in self-assembled films made from PEGylated gold nanoparticles is presented. The local structure of this model system is dominated by four- and sixfold order. Coadsorption of shorter ligands in the particle's ligand layer and variation of salt concentration in the suspension prior to self-assembly result in a change of local orientational order. The degree of sixfold order is reduced after salt addition. This decrease of order is less pronounced for the fourfold symmetry. The results presented here suggest complex symmetry-selective order formation upon ligand exchange and salt addition and demonstrate the versatility of X-ray cross-correlation methods for nanoparticle superlattices.