A highly sensitive printed humidity sensor based on a functionalized MWCNT/HEC composite for flexible electronics application

Eco-Friendly Textile-Based Wearable Humidity Sensor with Multinode Wireless Connectivity for Healthcare Applications

Textile-based wearable humidity sensors are of great interest for human healthcare monitoring as they can provide critical human-physiology information. The demand for wearable and sustainable sensing technology has significantly promoted the development of eco-friendly sensing solutions for potential real-world applications. Herein, a biodegradable cotton (textile)-based wearable humidity sensor has been developed using fabsil-treated cotton fabric coated with a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) sensing layer. The structural, chemical composition, hygroscopicity, and morphological properties are examined using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), contact angle measurement, and scanning electron microscopy (SEM) analysis. The developed sensor exhibited a nearly linear response (Adj. R-square value observed as 0.95035) over a broad relative humidity (RH) range from 25 to 91.5%RH displaying high sensitivity (26.1%/%RH). The sensor shows excellent reproducibility (on replica sensors with a margin of error ±1.98%) and appreciable stability/aging with time (>4.5 months), high flexibility (studied at bending angles 30°, 70°, 120°, and 150°), substantial response/recovery durations (suitable for multiple applications), and highly repeatable (multicyclic analysis) sensing performance. The prospective relevance of the developed humidity sensor toward healthcare applications is demonstrated via breathing rate monitoring (via a sensor attached to a face mask), distinguishing different breathing patterns (normal, deep, and fast), skin moisture monitoring, and neonatal care (diaper wetting). The multinode wireless connectivity is demonstrated using a Raspberry Pi Pico-based system for demonstrating the potential applicability of the developed sensor as a real-time humidity monitoring system for the healthcare sector. Further, the biodegradability analysis of the used textile is evaluated using the soil burial degradation test. The work suggests the potential applicability of the developed flexible and eco-friendly humidity sensor in wearable healthcare devices and other humidity sensing applications.

Recent Advances in Materials, Devices and Algorithms Toward Wearable Continuous Blood Pressure Monitoring

Continuous blood pressure (BP) tracking provides valuable insights into the health condition and functionality of the heart, arteries, and overall circulatory system of humans. The rapid development in flexible and wearable electronics has significantly accelerated the advancement of wearable BP monitoring technologies. However, several persistent challenges, including limited sensing capabilities and stability of flexible sensors, poor interfacial stability between sensors and skin, and low accuracy in BP estimation, have hindered the progress in wearable BP monitoring. To address these challenges, comprehensive innovations in materials design, device development, system optimization, and modeling have been pursued to improve the overall performance of wearable BP monitoring systems. In this review, we highlight the latest advancements in flexible and wearable systems toward continuous noninvasive BP tracking with a primary focus on materials development, device design, system integration, and theoretical algorithms. Existing challenges, potential solutions, and further research directions are also discussed to provide theoretical and technical guidance for the development of future wearable systems in continuous ambulatory BP measurement with enhanced sensing capability, robustness, and long-term accuracy.

Advances in Printed Electronic Textiles

Electronic textiles (e-textiles) have emerged as a revolutionary solution for personalized healthcare, enabling the continuous collection and communication of diverse physiological parameters when seamlessly integrated with the human body. Among various methods employed to create wearable e-textiles, printing offers unparalleled flexibility and comfort, seamlessly integrating wearables into garments. This has spurred growing research interest in printed e-textiles, due to their vast design versatility, material options, fabrication techniques, and wide-ranging applications. Here, a comprehensive overview of the crucial considerations in fabricating printed e-textiles is provided, encompassing the selection of conductive materials and substrates, as well as the essential pre- and post-treatments involved. Furthermore, the diverse printing techniques and the specific requirements are discussed, highlighting the advantages and limitations of each method. Additionally, the multitude of wearable applications made possible by printed e-textiles is explored, such as their integration as various sensors, supercapacitors, and heated garments. Finally, a forward-looking perspective is provided, discussing future prospects and emerging trends in the realm of printed wearable e-textiles. As advancements in materials science, printing technologies, and design innovation continue to unfold, the transformative potential of printed e-textiles in healthcare and beyond is poised to revolutionize the way wearable technology interacts and benefits.

Chitosan-Based Highly Sensitive Viable Humidity Sensor for Human Health Monitoring

We report a sustainable resistive-type humidity sensor based on chitosan (CS) film deposited on an interdigitated Ti/Au electrode coated SiO2 substrate using a simple drop cast approach for human health monitoring. The sensor revealed remarkably high sensitivity (5.8 MΩ/%RH), fast response/recovery time (21 s/25 s), low hysteresis (∼9.3%), excellent reversibility, wide detecting range (11-95% RH), and high selectivity toward water vapor. The calculated associated uncertainty at different %RH indicates the excellent repeatability and stable performance of the sensor. The developed sensor is tested for different human breath patterns, and it is found that the sensor can clearly distinguish between the variations in rate and depth of respiration patterns during normal, fast, deep, and nasal breathing and can monitor for apnea-like situations. The sensor is also utilized to perform noncontact skin humidity sensing. Overall, the developed CS film humidity sensor provides a viable approach for the detection of respiratory disorders and human health issues, detected by skin moisture, in a noninvasive manner.

Recent progress and applications of cellulose and its derivatives-based humidity sensors: A review

Cellulose and its derivatives, which are low-cost, degradable, reproducible and highly hydrophilic, can serve as both substrate and humidity sensitive materials, making them more and more popular as ideal biomimetic materials for humidity sensors. Benefiting from these characteristics, cellulose-based humidity sensors cannot only exhibit high sensitivity, excellent mechanical performance, wide humidity response range, etc., but also can be applied to fields such as human health, medical care and agricultural product safety monitoring. Herein, cellulose-based humidity sensors are first classified according to the different conductive active materials, such as carbon nanotubes, graphene, electrolytes, metal compounds, and polymer materials, based on which the latest research progress is introduced, and the roles of different types of conductive materials in cellulose-based humidity sensors are analyzed and summarized. Besides, the similarities and differences in their working mechanisms are expounded. Finally, the application scenarios of cellulose-based humidity sensors in human movement respiration and skin surface humidity monitoring are discussed, which can make readers quickly familiarize the current preparation method, working mechanism and subsequent development trend of cellulose-based humidity sensors more effectively.

An All-Printed, Fast-Response Flexible Humidity Sensor Based on Hexagonal-WO3 Nanowires for Multifunctional Applications

The utilization of printing techniques for the development of high-performance humidity sensors holds immense significance for various applications in the fields of the Internet of Things, agriculture, human healthcare, and storage environments. However, the long response time and low sensitivity of current printed humidity sensors limit their practical applications. Herein, a series of high-sensing-performance flexible resistive-type humidity sensors is fabricated by the screen-printing method, and hexagonal tungsten oxide (h-WO3 ) is employed as the humidity-sensing material due to its low cost, strong chemical adsorption ability, and excellent humidity-sensing ability. The as-prepared printed sensors exhibit high sensitivity, good repeatability, outstanding flexibility, low hysteresis, and fast response (1.5 s) in a wide relative humidity (RH) range (11-95% RH). Furthermore, the sensitivity of humidity sensors can be easily adjusted by altering the manufacturing parameters of the sensing layer and interdigital electrode to meet the diverse requirements of specific applications. The printed flexible humidity sensors possess immense potential in various applications, including wearable devices, non-contact measurements, and packaging opening state monitoring.

Terahertz Humidity Sensing Based on Surface-Modified Polymer Mesh Membranes with Photografting PEGMA Brush

A simple and compact intensity-interrogated terahertz (THz) relative humidity (RH) sensing platform is successfully demonstrated in experiments on the basis of combining a porous polymer sensing membrane and a continuous THz electronic system. The RH-sensing membrane is fabricated by surface modification of a porous polymer substrate with hydrophilic and photosensitive copolymer brushes via a UV-induced graft-polymerization process. The intensity interrogation sensing scheme indicated that the power reduction of the 0.4 THz wave is dependent on the grafting density of the copolymer brushes and proportional to the RH percent levels in the humidity-controlled air-sealed chamber. This finding was verified by the water contact angle measurement. Based on the slope of the proportional relation, the best sensitivity of the hydrophilic surface-modified sensing membrane was demonstrated at 0.0423 mV/% RH at the copolymer brush density of 1.57 mg/mm3 grafted on the single side of the sensing membrane. The sensitivity corresponds to a detection limit of approximately 1% RH. The THz RH sensing membrane was proven to exhibit the advantages of low loss, low cost, flexibility, high sensitivity, high RH resolution, and a wide RH working range of 25-99%. Thus, it is a good candidate for novel applications of wearable electronics, water- or moisture-related industrial and bio-sensing.

Wafer-Level, High-Performance, Flexible Sensors Based on Organic Nanoforests for Human-Machine Interactions

High-performance flexible sensors are essential for real-time information analysis and constructing noncontact communication modules for emerging human-machine interactions. In these applications, batch fabrication of sensors that exhibit high performance at the wafer level is in high demand. Here, we present organic nanoforest-based humidity sensor (NFHS) arrays on a 6 in. flexible substrate prepared via a facile, cost-effective manufacturing approach. Such an NFHS achieves state-of-the-art overall performance: high sensitivity and fast recovery time; the best properties are at a small device footprint. The high sensitivity (8.84 pF/% RH) and fast response time (5 s) of the as-fabricated organic nanoforests are attributed to the abundant hydrophilic groups, the ultra-large surface area with a huge number of nanopores, and the vertically distributed structures beneficial to the transfer of molecules up and down. The NFHS also exhibits excellent long-term stability (90 days), superior mechanical flexibility, and good performance repeatability after bending. With these superiorities, the NFHS is further applied as a smart noncontact switch, and the NFHS array is used as the motion trajectory tracker. The wafer-level batch fabrication capability of our NFHS provides a potential strategy for developing practical applications of such humidity sensors.

Superhydrophobic, stretchable kirigami pencil-on-paper multifunctional device platform

Wearable electronics with applications in healthcare, human-machine interfaces, and robotics often explore complex manufacturing procedures and are not disposable. Although the use of conductive pencil patterns on cellulose paper provides inexpensive, disposable sensors, they have limited stretchability and are easily affected by variations in the ambient environment. This work presents the combination of pencil-on-paper with the hydrophobic fumed SiO2 (Hf-SiO2) coating and stretchable kirigami structures from laser cutting to prepare a superhydrophobic, stretchable pencil-on-paper multifunctional sensing platform. The resulting sensor exhibits a large response to NO2 gas at elevated temperature from self-heating, which is minimally affected by the variations in the ambient temperature and relative humidity, as well as mechanical deformations such as bending and stretching states. The integrated temperature sensor and electrodes with the sensing platform can accurately detect temperature and electrophysiological signals to alert for adverse thermal effects and cardiopulmonary diseases. The thermal therapy and electrical stimulation provided by the platform can also deliver effective means to battle against inflammation/infection and treat chronic wounds. The superhydrophobic pencil-onpaper multifunctional device platform provides a low-cost, disposable solution to disease diagnostic confirmation and early treatment for personal and population health.

Pencil-on-Paper Humidity Sensor Treated with NaCl Solution for Health Monitoring and Skin Characterization

Although flexible humidity sensors are essential for human health monitoring, it is still challenging to achieve high sensitivity and easy disposal with simple, low-cost fabrication processes. This study presents the design and fabrication of highly reliable hand-drawn interdigital electrodes from pencil-on-paper treated with NaCl solution for highly sensitive hydration sensors working over a wide range of relative humidity (RH) levels from 5.6% to 90%. The applications of the resulting flexible humidity sensor go beyond the monitoring of respiratory rate and proximity to characterizations of human skin types and evaluations of skin barrier functions through insensible sweat measurements. The sensor array can also be integrated with a diaper to result in smart diapers to alert for an early diaper change. The design and fabrication strategies presented in this work could also be leveraged for the development of wearable, self-powered, and recyclable sensors and actuators in the future.

Facile Fabrication of a Bio-Inspired Leaf Vein-Based Ultra-Sensitive Humidity Sensor with a Hygroscopic Polymer

Bio-inspired materials have received significant interest in the development of flexible electronics due to their natural grid structures, especially natural leaf vein networks. In this work, a bio-inspired leaf vein-based flexible humidity sensor is demonstrated. The proposed sensor is composed of a leaf/Al/glycerin/Ag paste. The Al-deposited leaf vein networks are used as a bottom electrode with a resistance of around 100 Ω. The humidity sensor responds well to relative humidity (RH) levels ranging from 15% to 70% at room temperature. The fabricated humidity sensor exhibits an ultra-sensitive response to different humidity conditions due to the biodegradable insulating hygroscopic polymer (glycerin), specifically the ionic conductivity reaction. To further verify the presence of ionic conduction, the device performance is tested by doping NaCl salt into the hygroscopic polymer sensing layer. In addition, both the repeatability and flexibility of the sensor are tested under different bending angles (0°, 90°, 180°, and 360°). The bioinspired ultrasensitive humidity sensor with a biocompatible and biodegradable sensing layer holds great potential, especially for health care applications (e.g., respiratory monitoring) without causing any body harm.

MWCNT supported V2O5 quantum dot nanoparticles decorated Bi2O3 nanosheets hybrid system: Efficient visible light driven photocatalyst for degradation of ciprofloxacin

A novel composite of multiwall carbon nanotube (MWCNT) supported V₂O₅ quantum dots decorated Bi₂O₃ hybrid was prepared by the simple wet-impregnation method, and the photocatalytic performance of the prepared samples was investigated against the photodegradation of ciprofloxacin (CIP). Herein, different samples of pristine, V₂O₅/Bi₂O₃ and MWCNT@V₂O₅/Bi₂O₃ hybrid photocatalyst were prepared and systematically characterized by various physicochemical techniques. The characterization results demonstrated that the introduction of MWCNT can change the energy band gap of V₂O₅/Bi₂O₃, and the band energies vary with a constituent of MWCNT@V₂O₅/Bi₂O₃ catalyst, in which MWCNT@V₂O₅/Bi₂O₃-5 (0.05 g@0.50 g:0.50 g) has the optimal band gap energy of 2.46 eV. The photocatalytic test demonstrates that the MWCNT@V₂O₅/Bi₂O₃-5 hybrid composites exhibited enhanced photocatalytic activity in CIP degradation compared to that pure and other photocatalyst and its degradation efficiency did not decrease significantly even after five cyclic experiments. The enhanced photocatalytic activity was due to the formation of heterojunction among MWCNT, V₂O₅ and Bi₂O₃, which distinctly improved the separation efficiency of the photogenerated charge carrier, thus increasing the degradation performance. This work gives a new approach to designing an efficient photocatalyst for contaminants degradation.

Garage-Fabricated, Ultrasensitive Capacitive Humidity Sensor Based on Tissue Paper

The role of humidity sensors in different industries and field applications, such as agriculture, food monitoring, biomedical equipment, heating, and ventilation, is well known. However, most commercially available humidity sensors are based on polymers or electronic materials that are not degradable and thus contribute to electronic waste. Here, we report a low-cost, flexible, easy-to-fabricate, and eco-friendly parallel-plate capacitive humidity sensor for field applications. The sensor is fabricated from copper tape and tissue paper, where copper tape is used to create the plates of the capacitor, and tissue paper is used as a dielectric sensing layer. Along with the low cost, the high sensitivity, better response and recovery times, stability, and repeatability make this sensor unique. The sensor was tested for relative humidity (RH), ranging from 40% to 99%, and the capacitance varied linearly with RH from 240 pF to 720 pF, as measured by an Arduino. The response time of the sensor is ~1.5 s, and the recovery time is ~2.2 s. The experiment was performed 4-5 times on the same sensor, and repeatable results were achieved with an accuracy of ±0.1%. Furthermore, the sensor exhibits a stable response when tested at different temperatures. Due to the above advantages, the presented sensor can find ready applications in different areas.

Evaluating Different TiO2 Nanoflower-Based Composites for Humidity Detection

Unique three-dimensional (3D) titanium dioxide (TiO₂) nanoflowers (TFNA) have shown great potential for humidity sensing applications, due to their large surface area-to-volume ratio and high hydrophilicity. The formation of a composite with other materials could further enhance the performance of this material. In this work, the effect of different types of composites on the performance of a TNFA-based humidity sensor was examined. NiO, ZnO, rGO, and PVDF have been explored as possible composite pairing candidates with TiO₂ nanoflowers, which were prepared via a modified solution immersion method. The properties of the composites were examined using field emission electron spectroscopy (FESEM), X-ray diffractometry (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), current-voltage (I-V) analysis, Hall effect measurement, and contact angle measurement. The performance of the humidity sensor was assessed using a humidity sensor measurement system inside a humidity-controlled chamber. Based on the result, the combination of TiO₂ with rGO produced the highest sensor response at 39,590%. The achievement is attributed to the increase in the electrical conductivity, hydrophilicity, and specific surface area of the composite.

Aerosol-Printed MoS2 Ink as a High Sensitivity Humidity Sensor

Molybdenum disulfide (MoS₂) is attractive for use in next-generation nanoelectronic devices and exhibits great potential for humidity sensing applications. Herein, MoS₂ ink was successfully prepared via a simple exfoliation method by sonication. The structural and surface morphology of a deposited ink film was analyzed by scanning electron microscopy (SEM), Raman spectroscopy, and atomic force microscopy (AFM). The aerosol-printed MoS₂ ink sensor has high sensitivity, with a conductivity increase by 6 orders of magnitude upon relative humidity increase from 10 to 95% at room temperature. The sensor also has fast response/recovery times and excellent repeatability. Possible mechanisms for the water-induced conductivity increase are discussed. An analytical model that encompasses two ionic conduction regimes, with a percolation transition to an insulating state below a low humidity threshold, describes the sensor response successfully. In conclusion, our work provides a low-cost and straightforward strategy for fabricating a high-performance humidity sensor and fundamental insights into the sensing mechanism.

Highly Sensitive and Stable Humidity Sensor Based on the Bi-Layered PVA/Graphene Flower Composite Film

Two-dimensional (2D) materials and their composites have gained significant importance as the functional layer of various environmental sensors and nanoelectronics owing to their unique properties. This work reports for the first time a highly sensitive, fast, and stable humidity sensor based on the bi-layered active sensing area composed of graphene flower (GF) and poly (vinyl alcohol) PVA thin films for multifunctional applications. The GF/PVA humidity sensor exhibited stable impedance response over 15 days, for a relative humidity (RH) range of (40–90% RH) under ambient operating conditions. The proposed bi-layered humidity sensor also exhibited an ultra-high capacitive sensitivity response of the 29 nF/%RH at 10 kHz and fast transient response of 2 s and 3.5 s, respectively. Furthermore, the reported sensor also showed a good response towards multi-functional applications such as non-contact skin humidity and mouth breathing detection.

Improved thermoelectric properties of multi-walled carbon nanotubes/Ag2Se via controlling the composite ratio

MWCNTs/Ag2Se composites were synthesized via a facile hydrothermal method; higher electrical conductivity and lower thermal conductivity were simultaneously achieved compared with Ag2Se, resulting in enhanced thermoelectric performance.

Single-sided and integrated polyaniline/ poly(vinylidene fluoride) flexible membrane with micro/nanostructures as breathable, nontoxic and fast response wearable humidity sensor

Harmless and breathable flexible humidity sensor has important applications in continuous and real-time detection of human physiological activities. In this work, with hydrophobic poly (vinylidene fluoride) (PVDF) membrane as both the template and substrate and cetyltrimethylammonium bromide as a structure regulator, polyaniline (PANI) was unilaterally deposited on a PVDF microporous membrane to facilely fabricate a single-sided integrated flexible humidity sensor (IFHS). Such IFHS is featured with unique micro/nano structure and good air permeability. Moreover, it exhibits good humidity sensing properties at room temperature including fast response, small hysteresis and stable response even under bending deformation. The flexible sensor could realize non-contact monitoring of human respiration and speaking activities. Unilateral deposition of PANI and good breathability of IFHS avoids direct contact between PANI and human skin, thus averting harms to human and minimizing the deterioration of humidity sensing properties of PANI layer. The simple method is universal to the preparation of single-sided, integrated, breathable, nontoxic and fast response wearable humidity sensors based on PANI and hydrophobic microporous polymer membranes, offering useful references for the construction of advanced flexible sensors.

Fabrication and Materials Integration of Flexible Humidity Sensors for Emerging Applications

In the past decade, humidity measurements have ubiquitously gained consideration in the wide range of application paradigms such as industrial predictive maintenance, instrumentation, automation, agriculture, climate monitoring, healthcare, and semiconductor industries. Accurate humidity measurements and cost-effective fabrication processes for large-volume and high-performance sensors with flexible form factors are essential to meet the stringent performance requirements of the emerging application areas. To address this need, recent efforts focus on development of innovative sensing modalities, process technologies, and exploration and integration of new materials to enable low-cost, robust, and flexible humidity sensors with ultrahigh sensitivity and linearity, large dynamic range, low hysteresis, and fast response time. In this review paper, we present an overview of flexible humidity sensors based on distinct sensing mechanisms, employed processing techniques, and various functional sensing layers and substrate materials for specific applications. Furthermore, we present the critical device design parameters considered to be indicative of sensor performance such as relative humidity range, along with a discussion on some of the specific applications and use cases.

Design, construction, and testing of an accurate low-cost humidistat for laboratory-scale applications

Stable and precise control of humidity is imperative for a wide variety of experiments. However, commercially available humidistats (devices that maintain a constant humidity) are often prohibitively expensive. Here, we present a simple yet effective humidistat for laboratory-scale applications that can be easily and affordably (<€250) constructed based on an Arduino Uno as microcontroller, a set of proportional miniature solenoid valves, a gas washing bottle, and a humidity sensor. The microcontroller implements a PID controller that regulates the ratio of a dry and humid airflow. The design and implementation of the device, including a custom driver circuit for the solenoids, are described in detail, and the firmware is freely available online. Finally, we demonstrate its proper operation and performance through step response and long-term stability tests, which shows settling times of approx. 30 s and an attainable relative humidity range of 10-95.

Highly Sensitive Porous PDMS-Based Capacitive Pressure Sensors Fabricated on Fabric Platform for Wearable Applications

A novel porous polydimethylsiloxane (PDMS)-based capacitive pressure sensor was fabricated by optimizing the dielectric layer porosity for wide-range pressure sensing applications in the sports field. The pressure sensor consists of a porous PDMS dielectric layer and two fabric-based conductive electrodes. The porous PDMS dielectric layer was fabricated by introducing nitric acid (HNO₃) into a mixture of PDMS and sodium hydrogen bicarbonate (NaHCO₃) to facilitate the liberation of carbon dioxide (CO₂) gas, which induces the creation of porous microstructures within the PDMS dielectric layer. Nine different pressure sensors (PS1, PS2,..., PS9) were fabricated in which the porosity (pore size, thickness) and dielectric constant of the PDMS dielectric layers were varied by changing the curing temperature, the mixing proportions of the HNO₃/PDMS concentration, and the PDMS mixing ratio. The response of the fabricated pressure sensors was investigated for the applied pressures ranging from 0 to 1000 kPa. A relative capacitance change of ∼100, ∼323, and ∼485% was obtained by increasing the curing temperature from 110 to 140 to 170 °C, respectively. Similarly, a relative capacitance change of ∼170, ∼282, and ∼323% was obtained by increasing the HNO₃/PDMS concentration from 10 to 15 to 20%, respectively. In addition, a relative capacitance change of ∼94, ∼323, and ∼460% was obtained by increasing the PDMS elastomer base/curing agent ratio from 5:1 to 10:1 to 15:1, respectively. PS9 exhibited the highest sensitivity over a wide pressure sensing range (low-pressure ranges (<50 Pa), 0.3 kPa^-1; high-pressure ranges (0.2-1 MPa), 3.2 MPa^-1). From the results, it was observed that the pressure sensors with dielectric layers prepared at relatively higher curing temperatures, higher HNO₃ concentrations, and higher PDMS ratios resulted in increased porosity and provided the highest sensitivity. As an application demonstrator, a wearable fit cap was developed using an array of 16 pressure sensors for measuring and mapping the applied pressures on a player's head while wearing a helmet. The pressure mapping aids in observing and understanding the proper fit of the helmet in sports applications.

Customizing hydrothermal properties of inkjet printed sensitive films by functionalization of carbon nanotubes

Multiwalled carbon nanotubes (MWCNTs) are attractive materials for realizing sensors, owing to their high aspect ratio associated with excellent mechanical, electronic, and thermal properties. Moreover, their sensing properties can be tuned by introducing functional groups on their framework and adjusting the processing conditions. In this paper, we investigate the potential of functionalized CNTs for humidity and temperature sensing by optimization of the functionalization, the processing conditions and the printing conditions. The morphology of the differently functionalized MWCNTs is investigated by infrared spectroscopy (IR), scanning electron microscopy, thermogravimetry (TG) and TG-coupled mass-spectrometric studies. Using the functionalized MWCNTs, films were fabricated with different numbers of layers (4, 6, 8, 10 layers) via inkjet printing on a flexible polyimide substrate containing an interdigital microelectrode. The influence of hydrothermal effects was investigated. The sensitivity to humidity is higher for films prepared with MWCNTs functionalized with a high sonication amplitude and a bigger number of layers due to enhancements of hydrophilicity and water mobility. A higher sensitivity to temperature is achieved by a low sonication amplitude and a small number of layers. For the encapsulation of the temperature sensor against humidity, a Bectron layer is proposed, which reduces also the hysteresis effect. This study demonstrates the efficiency of carboxylic functionalized MWCNTs deposit by inkjet printing for realization of sensitive and cost-effective humidity and temperature sensors. It provides a real example for the interesting contribution of functionalization procedures to the sensing properties of MWCNTs films.

A Review on Humidity, Temperature and Strain Printed Sensors-Current Trends and Future Perspectives

Printing technologies have been attracting increasing interest in the manufacture of electronic devices and sensors. They offer a unique set of advantages such as additive material deposition and low to no material waste, digitally-controlled design and printing, elimination of multiple steps for device manufacturing, wide material compatibility and large scale production to name but a few. Some of the most popular and interesting sensors are relative humidity, temperature and strain sensors. In that regard, this review analyzes the utilization and involvement of printing technologies for full or partial sensor manufacturing; production methods, material selection, sensing mechanisms and performance comparison are presented for each category, while grouping of sensor sub-categories is performed in all applicable cases. A key aim of this review is to provide a reference for sensor designers regarding all the aforementioned parameters, by highlighting strengths and weaknesses for different approaches in printed humidity, temperature and strain sensor manufacturing with printing technologies.

Multifunctional and Ultrasensitive-Reduced Graphene Oxide and Pen Ink/Polyvinyl Alcohol-Decorated Modal/Spandex Fabric for High-Performance Wearable Sensors

Sensitive and flexible sensors capable of monitoring physiological signals of human body for healthcare have been developed in recent years. It is still a challenge to fabricate a wearable sensor-integrated multifunctional performances and a good fit to human body. Here, an rGO and pen ink/PVA-layered strain-humidity sensor based on MS fabric is prepared through a cost-effective and scalable solution process. The conductive fabric as a strain sensor has a workable strain range (∼300%), ultrahigh sensitivity (maximum gauge factor of 492.8), great comfort, and long-term stability. Notably, a step increase in relative resistance variation will be achieved by controlling the coverage of an ink layer. Moreover, the reliable linear humidity-dependent resistance void of hysteresis and excellent repeatability renders conductive fabrics an opportunity as humidity sensors. Based on these superior multifunctions, the resultant conductive fabric can be applied to detect both human motions and skin humidity, showing potential in applications of wearable electronics for professional athletes.

Recent Progress in Manufacturing Techniques of Printed and Flexible Sensors: A Review

This review provides an outlook on some of the significant research work done on printed and flexible sensors. Printed sensors fabricated on flexible platforms such as paper, plastic and textiles have been implemented for wearable applications in the biomedical, defense, food, and environmental industries. This review discusses the materials, characterization methods, and fabrication methods implemented for the development of the printed and flexible sensors. The applications, challenges faced and future opportunities for the printed and flexible sensors are also presented in this review.

Low-cost and customizable inkjet printing for microelectrodes fabrication

Microelectrodes for detection of chemicals present several advantages over conventional sized electrodes. However, rapid and low-cost fabrication of microelectrodes is challenging due to high complexity of patterning equipment. We present the development of a low-cost, customizable inkjet printer for printing nanomaterials including carbon nanotubes for the fabrication of microelectrodes. The achieved spatial resolution of the inkjet printer is less than 20 µm, which is comparable to advanced commercially available inkjet printers, with the advantage of being low-cost and easily replicated.

Synthesis of a novel hexaazatriphenylene derivative for the selective detection of copper ions in aqueous solution

A hexaazatriphenylene (HAT) derivative, naphtho[2,3-h]naphtho[2′,3':7,8]quinoxalino[2,3-a]naphtho[2′,3′:7,8]quinoxalino[2,3-c]phenazine-5,10,15,20,25,30-hexaone (NQH) was synthesized, characterized, and found to have novel properties in being selective toward the detection of copper (Cu²⁺) ions. The capability of NQH to be employed as a colorimetric, chemo-fluorescence and electrochemical sensor for the detection of Cu²⁺ was demonstrated by performing UV-Vis absorbance, fluorescence intensity, and cyclic voltammetry (CV) measurements. The interaction between NQH and Cu²⁺ was initially observed with an obvious color change from yellow to brown upon the addition of Cu²⁺ ions to NQH. The interaction was also confirmed by UV-Vis absorbance, fluorescence intensity, and mass spectroscopy (MS/MS) measurements. UV absorbance, fluorescence and CV of NQH toward Cu²⁺ showed good linearity with a detection limit of 3.32 μM, 2.20 μM and 0.78 μM, respectively, which are lower than the toxicity levels of copper in drinking water (20–30 μM) set by the U.S. Environmental Protection Agency (EPA) and World Health Organization (WHO). A 1 : 2 stoichiometry complexation between NQH and Cu²⁺ was confirmed by Job's plot and MS/MS. In addition, the selectivity and sensitivity of the NQH compound towards Cu²⁺ ions were further confirmed by performing CV on a screen printed flexible and planar electrochemical sensor.

A hexaazatriphenylene (HAT) derivative, naphtho[2,3-h]naphtho[2′,3′:7,8]quinoxalino[2,3-a]naphtho[2′,3′:7,8]quinoxalino[2,3-c]phenazine-5,10,15,20,25,30-hexaone (NQH) was synthesized, characterized, and found to be selective to copper (Cu²⁺) ions.