A Self-Powered Breath Analyzer Based on PANI/PVDF Piezo-Gas-Sensing Arrays for Potential Diagnostics Application

Exhale-Focused Thermal Image Segmentation Using Optical Flow-Based Frame Filtering and Transformer-Aided Deep Networks

Since the COVID-19 pandemic, interest in non-contact diagnostic technologies has grown, leading to increased research into remote biosignal monitoring. The respiratory rate, widely used in previous studies, offers limited insight into pulmonary volume. To redress this, we propose a thermal imaging-based framework for respiratory segmentation aimed at estimating non-invasive pulmonary function. The proposed method uses an optical flow magnitude-based thresholding technique to automatically extract exhalation frames and segment them into frame sequences. A TransUNet-based network, combining a Convolutional Neural Network (CNN) encoder-decoder architecture with a Transformer module in the bottleneck, is trained on these sequences. The model's Accuracy, Precision, Recall, IoU, Dice, and F1-score were 0.9832, 0.9833, 0.9830, 0.9651, 0.9822, and 0.9831, respectively, which results demonstrate high segmentation performance. The method enables the respiratory volume to be estimated by detecting exhalation behavior, suggesting its potential as a non-contact tool to monitor pulmonary function and estimate lung volume. Furthermore, research on thermal imaging-based respiratory volume analysis remains limited. This study expands upon conventional respiratory rate-based approaches to provide a new direction for respiratory analysis using vision-based techniques.

Flexible Pressure Sensors Based on Polyvinylidene Fluoride: A Critical Review

With the advent of the intelligent era, flexible piezoelectric tactile sensors, as key components for sensing information and transmitting signals, have received worldwide attention. However, piezoelectric pressure sensors are still currently limited, which severely restricts their practical applications. Furthermore, the demonstrations conducted in labs are not accurate to real-world scenarios. Thus, there is an urgent need to further optimize the intrinsic piezoelectric performance and usage characteristics to meet application requirements. As a representative piezoelectric, polyvinylidene fluoride (PVDF) exhibits significant advantages in terms of excellent flexibility, chemical stability, high electromechanical conversion, low cost, and appropriate acoustic impedance, which allow it to serve as the core matrix in flexible pressure sensors. This paper aims to summarize very recent progress in flexible piezoelectric sensors based on PVDF, including their composition modulation, structure optimization, and applications. Based on a comprehensive summary of recent representative studies, we propose rational perspectives and strategies regarding PVDF-based piezoelectric sensors and provide some new insights for the research and industrial communities.

Classification of Chronic Obstructive Pulmonary Disease (COPD) Through Respiratory Pattern Analysis

Background: This study proposes a classification system for predicting chronic obstructive pulmonary disease (COPD) patients and non-patients based on image and text data. Method: This study measured the respiratory volume based on thermal images, stored the respiratory data, and derived features related to respiratory patterns, including the total respiratory volume, average distance between expirations, average distance between inspirations, and total respiratory rate. The data for each feature were stored in text format. The four features saved as text were scaled using Z-score normalization and expressed as scores through weighted summation. These scores were compared to a threshold based on the ROC curve values, classifying participants as patients if the score exceeded the threshold and as non-patients if it fell below. Results: The proposed method achieved an accuracy of 82.5%. To validate the proposed approach, precision, recall, and F1-score were utilized, confirming the high classification performance of the model. The results of this study demonstrate the potential for future applications in non-contact medical examinations and diagnoses of respiratory diseases.

Self-Powered Wearable Displacement Sensor for Continuous Respiratory Monitoring and Human-Machine Synchronous Control

Flexible wearable electronic devices play a vital role in daily monitoring, medical diagnosis, and human-computer interaction, and such devices have a great demand for portability, integration, comfort, and self-power. In this study, a triboelectric nanogenerator integrated into a flexible chest belt is proposed as a displacement sensor to monitor the displacement and frequency of thoracic expansion. Based on three parallel interpolation electrode structures with phase differences, the Triboelectric Nanogenerators's(TENG) output signal pulse number can characterize the sliding displacement, with a resolution of more than 1 mm and a durability of more than 700,000 cycles. Based on the flexible printed circuit processing technology, the volume of the sensor is less than 8.5 cm3, and the weight is less than 3.2 g, which improves the portability of the device. Based on wireless radio frequency technology, the collected signals are transmitted to the upper computer, and then the monitoring of respiratory physiological signals and the human-machine synchronous control of the ventilator are achieved within the overshoot of 1.5% and the control error of 5% through a simulation machine. This work provides a sensing method for daily and medical respiratory monitoring and demonstrates the enormous potential of frictional electric sensors in intelligent medical applications.

Unleashing the Potential of Electroactive Hybrid Biomaterials and Self-Powered Systems for Bone Therapeutics

The incidence of large bone defects caused by traumatic injury is increasing worldwide, and the tissue regeneration process requires a long recovery time due to limited self-healing capability. Endogenous bioelectrical phenomena have been well recognized as critical biophysical factors in bone remodeling and regeneration. Inspired by bioelectricity, electrical stimulation has been widely considered an external intervention to induce the osteogenic lineage of cells and enhance the synthesis of the extracellular matrix, thereby accelerating bone regeneration. With ongoing advances in biomaterials and energy-harvesting techniques, electroactive biomaterials and self-powered systems have been considered biomimetic approaches to ensure functional recovery by recapitulating the natural electrophysiological microenvironment of healthy bone tissue. In this review, we first introduce the role of bioelectricity and the endogenous electric field in bone tissue and summarize different techniques to electrically stimulate cells and tissue. Next, we highlight the latest progress in exploring electroactive hybrid biomaterials as well as self-powered systems such as triboelectric and piezoelectric-based nanogenerators and photovoltaic cell-based devices and their implementation in bone tissue engineering. Finally, we emphasize the significance of simulating the target tissue's electrophysiological microenvironment and propose the opportunities and challenges faced by electroactive hybrid biomaterials and self-powered bioelectronics for bone repair strategies.

Optimizing Ammonia Detection with a Polyaniline-Magnesia Nano Composite

Polyaniline-magnesia (PANI/MgO) composites with a fibrous nanostructure were synthesized via in situ oxidative polymerization, enabling uniform MgO integration into the polyaniline matrix. These composites were characterized using FTIR spectroscopy to analyze intermolecular bonding, XRD to assess crystallographic structure and phase purity, and SEM to examine surface morphology and topological features. The resulting PANI/MgO nanofibers were utilized to develop ammonia (NH3) gas-sensing probes with evaluations conducted at room temperature. The study addresses the critical challenge of achieving high sensitivity and selectivity in ammonia detection at low concentrations, which is a problem that persists in many existing sensor technologies. The nanofibers demonstrated high selectivity and optimal sensitivity for ammonia detection, which was attributed to the synergistic effects between the polyaniline and MgO that enhance gas adsorption. Furthermore, the study revealed that the MgO content critically influences both the morphology and the sensing performance, with higher MgO concentrations improving sensor response. This work underscores the potential of PANI/MgO composites as efficient and selective ammonia sensors, highlighting the importance of MgO content in optimizing material properties for gas-sensing applications.

Ingenious Structure Engineering to Enhance Piezoelectricity in Poly(vinylidene fluoride) for Biomedical Applications

The future development of wearable/implantable sensing and medical devices relies on substrates with excellent flexibility, stability, biocompatibility, and self-powered capabilities. Enhancing the energy efficiency and convenience is crucial, and converting external mechanical energy into electrical energy is a promising strategy for long-term advancement. Poly(vinylidene fluoride) (PVDF), known for its piezoelectricity, is an outstanding representative of an electroactive polymer. Ingeniously designed PVDF-based polymers have been fabricated as piezoelectric devices for various applications. Notably, the piezoelectric performance of PVDF-based platforms is determined by their structural characteristics at different scales. This Review highlights how researchers can strategically engineer structures on microscopic, mesoscopic, and macroscopic scales. We discuss advanced research on PVDF-based piezoelectric platforms with diverse structural designs in biomedical sensing, disease diagnosis, and treatment. Ultimately, we try to give perspectives for future development trends of PVDF-based piezoelectric platforms in biomedicine, providing valuable insights for further research.

Ionic liquid modified PVDF/BCZT nanocomposites for space charge induced mechanical energy harvesting performance

Mechanical energy harvesting performances of poly(vinylidene fluoride) (PVDF) based composites are most often correlated with their polar phase and the individual piezoelectricity of the used filler materials. Here we show that the significant enhancement of space charge polarization of the said composites can play the key dominant role in determining their mechanical energy harvesting performance regardless of their polar phase and individual piezoelectricity of the used fillers. For this purpose, ionic liquid has been incorporated into PVDF/0.5(Ba0.7Ca0.3)TiO3-0.5Ba(Ti0.8Zr0.2)O3(BCZT) composites which led to a huge enhancement in space charge polarization. The gradual addition of ionic liquid into 10 wt% BCZT loaded PVDF (PBCZT) has helped in extraordinarily enhancing the conductivity gradually which has confirmed the huge enhancement of space charge polarization. However, after a certain limit of ionic liquid addition, the polar phase of the composite films is decreased. Despite this, the output voltages from the piezoelectric and piezo-tribo hybrid nanogenerators (PENGs and HNGs, respectively) fabricated by using the developed films have been found to be increased gradually with the increase in the ionic liquid amount in PBCZT composite. As the amount of BCZT filler was kept fixed for all the films, this result has confirmed the key role of space charge polarization of PVDF-based composites in determining their mechanical energy harvesting performances compared to the effect of polar phase and individual piezoelectricity of filler. The optimized PENG and HNG devices have shown the output voltage as high as 52 and 167 V, respectively, with power densities ∼85 and 152μW cm-2which predicted their excellent usability in real life energy conversion devices. This work also shows that the effect of extraordinarily enhanced space charge polarization is effective in improving the performance of all types of mechanical energy harvesting devices regardless of their mechanisms (piezoelectric or hybrid).

From emerging modalities to advanced applications of hydrogel piezoelectrics based on chitosan, gelatin and related biological macromolecules: A review

The rapid development of functional materials and manufacturing technologies is fostering advances in piezoelectric materials (PEMs). PEMs can convert mechanical energy into electrical energy. Unlike traditional power sources, which need to be replaced and are inconvenient to carry, PEMs have extensive potential applications in smart wearable and implantable devices. However, the application of conventional PEMs is limited by their poor flexibility, low ductility, and susceptibility to fatigue failure. Incorporating hydrogels, which are flexible, stretchable, and self-healing, providing a way to overcome these limitations of PEMs. Hydrogel-based piezoelectric materials (H-PEMs) not only resolve the shortcomings of traditional PEMs but also provide biocompatibility and more promising application potential. This paper summarizes the working principle of H-PEMs. Recent advances in the use of H-PEMs as sensors and in vitro energy harvesting devices for smart wearable devices are described in detail, with emphasis on application scenarios in human body like fingers, wrists, ankles, and feet. In addition, the recent progress of H-PEMs in implantable medical devices, especially the potential applications in human body parts such as bones, skin, and heart, are also elaborated. In addition, challenges and potential improvements in H-PEMs are discussed.

Electroactive properties and piezo-tribo hybrid energy harvesting performances of PVDF-AlFeO3 composites: role of crystal symmetry and agglomeration of fillers

Inorganic filler-loaded PVDF-based composites have been very widely used for electrical and energy harvesting applications in recent times. In this regard, the effects of different parameters of fillers like size, shape, chemical states, distribution, functional properties, and many others on the output performance of PVDF have been widely studied. However, the effect of another important parameter, namely the crystal symmetry of the filler, in tuning the energy harvesting performance of PVDF has been rarely explored. Therefore, to explore this fact, here we develop PVDF-based composite films by using two types of AlFeO3 fillers, one with rhombohedral R3̄c symmetry (AFRH) and another with an orthorhombic Pc21n structure. Ferrite-based oxides have been chosen here as fillers due to their good dielectric compatibility with PVDF. On the other hand, AlFeO3 has been chosen due to the simplicity of synthesizing it with both centrosymmetric and non-centrosymmetric crystal structures and the scarcity of reports exploring the energy-harvesting performance of AlFeO3-based polymer composites. A significant difference in particle agglomeration has also been observed here between the mentioned two types of AlFeO3 fillers which was mainly due to their specific synthesis conditions. The electroactive properties of PVDF have been observed to be mostly dependent on filler agglomeration. However, the crystal symmetry has shown a strong effect on the piezoelectric energy harvesting performances. As a result of these facts, the piezo-tribo hybrid energy harvesting performance, which depends on both the dielectric permittivity and piezoelectric activity, has been observed to be better for the AFRH5-based hybrid device (AFRH5H) (with ∼72 V open circuit voltage and ∼45 μW cm-2 power density) compared to that of the AFOR5-based hybrid device (AFOR5H). The real-life applications of all the energy harvesting devices have also been demonstrated here.

Fast-Response Non-Contact Flexible Humidity Sensor Based on Direct-Writing Printing for Respiration Monitoring

Respiratory monitoring is crucial for evaluating health status and identifying potential respiratory diseases such as respiratory failure, bronchitis, and pneumonia. Humidity sensors play a significant role in this regard, and efforts are being made to improve their performance. However, achieving ideal sensor parameters such as sensitivity, detection range, and response speed is challenging. In this work, we propose a flexible preparation method for a double-layer humidity sensor using PDMS as a substrate and a GNP/MWCNT composite material as a sensor element. This sensor exhibits high sensitivity (1.4 RH-1), a wide detection range (20-90%), ultra-fast response (0.35 s) and recovery (2.5 s), high repetitiveness (500 cycles), good long-term stability, and excellent flexibility. Due to these advantages, this sensor has potential applications in real-time clinical and home medical care, such as accurate human respiratory monitoring and non-invasive skin humidity monitoring. Hence, this humidity sensor can be a powerful tool to monitor respiratory moisture levels for diagnosing and treating respiratory diseases effectively.

The Preparation, Structural Design, and Application of Electroactive Poly(vinylidene fluoride)-Based Materials for Wearable Sensors and Human Energy Harvesters

Owing to their biocompatibility, chemical stability, film-forming ability, cost-effectiveness, and excellent electroactive properties, poly(vinylidene fluoride) (PVDF) and PVDF-based polymers are widely used in sensors, actuators, energy harvesters, etc. In this review, the recent research progress on the PVDF phase structures and identification of different phases is outlined. Several approaches for obtaining the electroactive phase of PVDF and preparing PVDF-based nanocomposites are described. Furthermore, the potential applications of these materials in wearable sensors and human energy harvesters are discussed. Finally, some challenges and perspectives for improving the properties and boosting the applications of these materials are presented.

Wearable Dual-Signal NH3 Sensor with High Sensitivity for Non-invasive Diagnosis of Chronic Kidney Disease

NH₃ gas in human exhaled breath contains abundant physiological information related to human health, especially chronic kidney disease (CKD). Unfortunately, up to now, most wearable NH₃ sensors show inevitable defects (low sensitivity, easy to be interfered by the environment, etc.), which may lead to misdiagnosis of CKD. To solve the above dilemma, a nanoporous, heterogeneous, and dual-signal (optical and electrical) wearable NH₃ sensor mask is developed successfully. More specifically, a polyacrylonitrile/bromocresol green (PAN/BCG) nanofiber film as a visual NH₃ sensor and a polyacrylonitrile/polyaniline/reduced graphene oxide (PAN/PANI/rGO) nanofiber film as a resistive NH₃ sensor are constructed. Due to the high specific surface area and abundant NH₃ binding sites of these two nanofiber films, they exhibit good NH₃ sensing performance. However, although the visual NH₃ sensor (PAN/BCG nanofiber film) is simple without the need of any detecting facilities and quite stable when temperature and humidity change, it shows poor sensitivity and resolution. In comparison, the resistive NH₃ sensor (PAN/PANI/rGO nanofiber film) is of high sensitivity, fast response, and good resolution, but its electrical signal is easily interfered by the external environment (such as humidity, temperature, etc.). Considering that the sensing principles between a visual NH₃ sensor and resistive NH₃ sensor are significantly different, a wearable dual-signal NH₃ sensor containing both a visual NH₃ sensor and resistive NH₃ sensor is further explored. Our data prove that the two sensing signals in this dual-signal NH₃ sensor mask can not only work well without interference with each other but also complement each other to improve the sensing accuracy, indicating its potential application in non-invasive diagnosis of CKD.

Sensing-transducing coupled piezoelectric textiles for self-powered humidity detection and wearable biomonitoring

The performance of chemical sensors is dominated by the perception of the target molecules via sensitive materials and the conduction of sensing signals through transducers. However, sensing and transduction are spatially and temporally independent in most chemical sensors, which poses a challenge for device miniaturization and integration. Herein, we proposed a sensing-transducing coupled strategy by embedding the high piezoresponse Sm-PMN-PT ceramic (d₃₃ = ∼1500 pC N^-1) into a moisture-sensitive polyetherimide (PEI) polymer matrix via electrospinning to conjugate the humidity perception and signal transduction synchronously and sympatrically. Through phase-field simulation and experimental characterization, we reveal the principle of design of the composition and topological structure of sensing-transducing coupled piezoelectric (STP) textiles in order to modulate the recognition, conversion, and sensitive component utilization ratio of the prepared active humidity sensors, achieving high sensitivity (0.9%/RH%) and fast response (20 s) toward ambient moisture. The prepared STP textile can be worn on the human body to realize emotion recognition, exercise status monitoring, and physiological stress identification. This work offers unprecedented insights into the coupling mechanism between chemisorption-related interfacial state and energy conversion efficiency and opens up a new paradigm for developing autonomous, multifunctional and highly sensitive flexible chemical sensors.

A self-powered sound-driven humidity sensor for wearable intelligent dehydration monitoring system

Self-powered wearable sensing systems have attracted great attention for their application in continuous health monitoring, which can reveal real-time physiological information on the body. Here, an innovative self-powered sound-driven humidity sensor for wearable intelligent dehydration monitoring system has been proposed. The sensor is primarily comprised of PTFE membrane, ZnO nanoarrays and Ti thin film. The piezoelectric/triboelectric effect of ZnO nanoarrays/PTFE membrane is coupled with the humidity sensing process. Sound wave can drive PTFE membrane to vibrate, and the contact and separation between PTFE and ZnO can generate electrical signals through piezoelectric/triboelectric effect. At the same time, the surface of the nanostructures can absorb the water molecules, which will influence the electrical output of the device. The device can convert sound energy into electrical output without any external electricity power supply, and the outputting voltage decreases with increasing relative humidity, acting as the sensing signal. The sensor has been integrated with data processing unit and wireless transmission module to form a self-powered wearable intelligent dehydration monitoring system, which can actively monitor the humidity of exhaled breath and transmit the information to the mobile phone. The results can open a possible new direction for the development of sound-driven gas sensors and will further expand the scope for self-powered nanosystems.

Breath VOC analysis and machine learning approaches for disease screening: a review

Early disease detection is often correlated with a reduction in mortality rate and improved prognosis. Currently, techniques like biopsy and imaging that are used to screen chronic diseases are invasive, costly or inaccessible to a large population. Thus, a non-invasive disease screening technology is the need of the hour. Existing non-invasive methods like gas chromatography-mass spectrometry, selected-ion flow-tube mass spectrometry, and proton transfer reaction-mass-spectrometry are expensive. These techniques necessitate experienced operators, making them unsuitable for a large population. Various non-invasive sources are available for disease detection, of which exhaled breath is preferred as it contains different volatile organic compounds (VOCs) that reflect the biochemical reactions in the human body. Disease screening by exhaled breath VOC analysis can revolutionize the healthcare industry. This review focuses on exhaled breath VOC biomarkers for screening various diseases with a particular emphasis on liver diseases and head and neck cancer as examples of diseases related to metabolic disorders and diseases unrelated to metabolic disorders, respectively. Single sensor and sensor array-based (Electronic Nose) approaches for exhaled breath VOC detection are briefly described, along with the machine learning techniques used for pattern recognition.

From Piezoelectric Nanogenerator to Non-Invasive Medical Sensor: A Review

Piezoelectric nanogenerators (PENGs) not only are able to harvest mechanical energy from the ambient environment or body and convert mechanical signals into electricity but can also inform us about pathophysiological changes and communicate this information using electrical signals, thus acting as medical sensors to provide personalized medical solutions to patients. In this review, we aim to present the latest advances in PENG-based non-invasive sensors for clinical diagnosis and medical treatment. While we begin with the basic principles of PENGs and their applications in energy harvesting, this review focuses on the medical sensing applications of PENGs, including detection mechanisms, material selection, and adaptive design, which are oriented toward disease diagnosis. Considering the non-invasive in vitro application scenario, discussions about the individualized designs that are intended to balance a high performance, durability, comfortability, and skin-friendliness are mainly divided into two types: mechanical sensors and biosensors, according to the key role of piezoelectric effects in disease diagnosis. The shortcomings, challenges, and possible corresponding solutions of PENG-based medical sensing devices are also highlighted, promoting the development of robust, reliable, scalable, and cost-effective medical systems that are helpful for the public.

Influence of dopant concentration on the ammonia sensing performance of citric acid-doped polyvinyl acetate nanofibers

The chemical modification of polymer nanofiber-based ammonia sensors by introducing dopants into the active layers has been proven as one of the low-cost routes to enhance their sensing performance. Herein, we investigate the influence of different citric acid (CA) concentrations on electrospun polyvinyl acetate (PVAc) nanofibers coated on quartz crystal microbalance (QCM) transducers as gravimetric ammonia sensors. The developed CA-doped PVAc nanofiber sensors are tested against various concentrations of ammonia vapors, in which their key sensing performance parameters (i.e., sensitivity, limit of detection (LOD), limit of quantification (LOQ), and repeatability) are studied in detail. The sensitivity and LOD values of 1.34 Hz ppm^-1 and 1 ppm, respectively, can be obtained during ammonia exposure assessment. Adding CA dopants with a higher concentration not only increases the sensor sensitivity linearly, but also prolongs both response and recovery times. This finding allows us to better understand the dopant concentration effect, which subsequently can result in an appropriate strategy for manufacturing high-performance portable nanofiber-based sensing devices.

Self-Powered Nitrogen Dioxide Sensor Based on Pd-Decorated ZnO/MoSe2 Nanocomposite Driven by Triboelectric Nanogenerator

This paper introduces a high-performance self-powered nitrogen dioxide gas sensor based on Pd-modified ZnO/MoSe₂ nanocomposites. Poly(vinyl alcohol) (PVA) nanofibers were prepared by high-voltage electrospinning and tribological nanogenerators (TENGs) were designed. The output voltage of TENG and the performance of the generator at different frequencies were measured. The absolute value of the maximum positive and negative voltage exceeds 200 V. Then, the output voltage of a single ZnO thin-film sensor, Pd@ZnO thin-film sensor and Pd@ZnO/MoSe₂ thin-film sensor was tested by using the energy generated by TENG at 5 Hz, when the thin-film sensor was exposed to 1-50 ppm NO₂ gas. The experimental results showed that the sensing response of the Pd@ZnO/MoSe₂ thin-film sensor was higher than that of the single ZnO film sensor and Pd@ZnO thin-film sensor. The TENG-driven response rate of the Pd@ZnO/MoSe₂ sensor on exposure to 50 ppm NO₂ gas was 13.8. At the same time, the sensor had good repeatability and selectivity. The synthetic Pd@ZnO/MoSe₂ ternary nanocomposite was an ideal material for the NO₂ sensor, with excellent structure and performance.

Emerging Development of Auto-Charging Sensors for Respiration Monitoring

In recent years, the development of biomedical monitoring systems, including respiration monitoring systems, has been accelerated. Wearable and implantable medical devices are becoming increasingly important in the diagnosis and management of disease and illness. Respiration can be monitored using a variety of biosensors and systems. Auto-charged sensors have a number of advantages, including low cost, ease of preparation, design flexibility, and a wide range of applications. It is possible to use the auto-charged sensors to directly convert mechanical energy from the airflow into electricity. The ability to monitor and diagnose one's own health is a major goal of auto-charged sensors and systems. Respiratory disease model output signals have not been thoroughly investigated and clearly understood. As a result, figuring out their exact interrelationship is a difficult and important research question. This review summarized recent developments in auto-charged respiratory sensors and systems in terms of their device principle, output property, detecting index, and so on. Researchers with an interest in auto-charged sensors can use the information presented here to better understand the difficulties and opportunities that lie ahead.

Fluorine-containing bio-inert polymers: Roles of intermediate water

Fluorine-containing polymers are used not only in industrial processes but also in medical applications, because they exhibit excellent heat, weather, and chemical resistance. As these polymers are not easily degraded in our body, it is difficult to use them in applications that require antithrombotic properties, such as artificial blood vessels. The material used for medical applications should not only be stable in vivo, but it should also be inert to biomolecules such as proteins or cells. In this review, this property is defined as "bio-inert," and previous studies in this field are summarized. Bio-inert materials are less recognized as foreign substances by proteins or cells in the living body, and they must be covered at interfaces designed with the concept of intermediate water (IW). On the basis of this concept, we present here the current understanding of bio-inertness and unusual blood compatibility found in fluoropolymers used in biomedical applications. IW is the water that interacts with materials with moderate strength and has been quantified by a variety of analytical methods and simulations. For example, by using differential scanning calorimetry (DSC) measurements, IW was defined as water frozen at around -40°C. To consider the role of the IW, quantification methods of the hydration state of polymers are also summarized. These investigations have been conducted independently because of the conflict between hydrophobic fluorine and bio-inert properties that require hydrophilicity. In recent years, not many materials have been developed that incorporate the good points of both aspects, and their properties have seldom been linked to the hydration state. This has been critically performed now. Furthermore, fluorine-containing polymers in medical use are reviewed. Finally, this review also describes the molecular design of the recently reported fluorine-containing bio-inert polymers for controlling their hydration state. STATEMENT OF SIGNIFICANCE: A material covered with a hydration layer known as intermediate water that interacts moderately with other objects is difficult to be recognized as a foreign substance and exhibits bio-inert properties. Fluoropolymers show high durability, but conflict with bio-inert characteristics requiring hydrophilicity as these research studies have been conducted independently. On the other hand, materials that combine the advantages of both hydrophobic and hydrophilic features have been developed recently. Here, we summarize the molecular architecture and analysis methods that control intermediate water and provide a guideline for designing novel fluorine-containing bio-inert materials.

Recent progress of self-powered respiration monitoring systems

Wearable and implantable medical devices are playing more and more key roles in disease diagnosis and health management. Various biosensors and systems have been used for respiration monitoring. Among them, self-powered sensors have some special characteristics such as low-cost, easy preparation, highly designable, and diversified. The respiratory airflow can drive the self-powered sensors directly to convert mechanical energy of the airflow into electricity. One of the major goals of the self-powered sensors and systems is realizing health monitoring and diagnosis. The relationship between the output signals and the models of respiratory diseases has not been studied deeply and clearly. Therefore, how to find an accurate relationship between them is a challenging and significant research topic. This review summarized the recent progress of the self-powered respiratory sensors and systems from aspects of device principle, output property, detecting index and so on. The challenges and perspectives have also been discussed for reference to the researchers who are interested in the field of self-powered sensors.

Exhaled Breath Analysis for Diabetes Diagnosis and Monitoring: Relevance, Challenges and Possibilities

With the global population prevalence of diabetes surpassing 463 million cases in 2019 and diabetes leading to millions of deaths each year, there is a critical need for feasible, rapid, and non-invasive methodologies for continuous blood glucose monitoring in contrast to the current procedures that are either invasive, complicated, or expensive. Breath analysis is a viable methodology for non-invasive diabetes management owing to its potential for multiple disease diagnoses, the nominal requirement of sample processing, and immense sample accessibility; however, the development of functional commercial sensors is challenging due to the low concentration of volatile organic compounds (VOCs) present in exhaled breath and the confounding factors influencing the exhaled breath profile. Given the complexity of the topic and the skyrocketing spread of diabetes, a multifarious review of exhaled breath analysis for diabetes monitoring is essential to track the technological progress in the field and comprehend the obstacles in developing a breath analysis-based diabetes management system. In this review, we consolidate the relevance of exhaled breath analysis through a critical assessment of current technologies and recent advancements in sensing methods to address the shortcomings associated with blood glucose monitoring. We provide a detailed assessment of the intricacies involved in the development of non-invasive diabetes monitoring devices. In addition, we spotlight the need to consider breath biomarker clusters as opposed to standalone biomarkers for the clinical applicability of exhaled breath monitoring. We present potential VOC clusters suitable for diabetes management and highlight the recent buildout of breath sensing methodologies, focusing on novel sensing materials and transduction mechanisms. Finally, we portray a multifaceted comparison of exhaled breath analysis for diabetes monitoring and highlight remaining challenges on the path to realizing breath analysis as a non-invasive healthcare approach.

Construction of Bio-Piezoelectric Platforms: From Structures and Synthesis to Applications

Piezoelectric materials, with their unique ability for mechanical-electrical energy conversion, have been widely applied in important fields such as sensing, energy harvesting, wastewater treatment, and catalysis. In recent years, advances in material synthesis and engineering have provided new opportunities for the development of bio-piezoelectric materials with excellent biocompatibility and piezoelectric performance. Bio-piezoelectric materials have attracted interdisciplinary research interest due to recent insights on the impact of piezoelectricity on biological systems and their versatile biomedical applications. This review therefore introduces the development of bio-piezoelectric platforms from a broad perspective and highlights their design and engineering strategies. State-of-the-art biomedical applications in both biosensing and disease treatment will be systematically outlined. The relationships between the properties, structure, and biomedical performance of the bio-piezoelectric materials are examined to provide a deep understanding of the working mechanisms in a physiological environment. Finally, the development trends and challenges are discussed, with the aim to provide new insights for the design and construction of future bio-piezoelectric materials.

Compressible and Stretchable Magnetoelectric Sensors Based on Liquid Metals for Highly Sensitive, Self-Powered Respiratory Monitoring

Healthcare monitoring, especially for respiration, has attracted tremendous attention from academics considering the great significance of health information feedback. The respiratory rate, as a critical health indicator, has been used to screen and evaluate potential illness risks in early medical diagnoses. A self-powered sensing system for medical monitoring is critical and imperative due to needless battery replacement and simple assembly. However, the development of a self-powered respiratory sensor with highly sensitive performance is still a daunting challenge. In this work, a compressible and stretchable magnetoelectric sensor (CSMS) with an arch-shaped air gap is reported, enabling self-powered respiratory monitoring driven by exhaled/inhaled breath. The CSMS contains two key functional materials: liquid metals and magnetic powders both with low Young's modulus, allowing for sensing compressibility and stretchability simultaneously. More importantly, such a magnetoelectric sensor exhibits mechanoelectrical converting capacity under an external force, which has been verified by Maxwell numerical simulation. Owing to the air-layer introduction, the magnetoelectric sensors achieve high sensitivity (up to 17.73 kPa^-1), fast response, and long-term stability. The highly sensitive and self-powered magnetoelectric sensor can be further applied as a noninvasive, miniaturized, and portable respiratory monitoring system with the aim of warning for potential health risks. We anticipate that this technique will create an avenue for self-powered respiratory monitoring fields.

Electrostatic self-assembly enabled flexible paper-based humidity sensor with high sensitivity and superior durability

Humidity sensors have been widely used for humidity monitoring in industrial fields. However, the application of conventional sensors is limited due to the structural rigidity, high cost, and time-consuming integration process. Owing to the good hydrophilicity, biodegradability, and low cost of cellulose, the sensors built on cellulose bulk materials are considered a feasible method to overcome these drawbacks while providing reasonable performance. Herein, we design a flexible paper-based humidity sensor based on conductive 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidized cellulose fibers/carbon nanotubes (TOCFs/CNTs) conformal fibers network. The CNTs are dispersed by cationic cetyl trimethyl ammonium bromide (CTAB), which introduces positive charges on the CNTs surface. The conductive fibers are achieved by an electrostatic self-assembly process that positively charged CNTs are adsorbed to the surface of negatively charged TOCFs. The vast number of hydrophilic hydroxyl groups on the surface of TOCFs provide more water molecules adsorption sites and facilitate the electron transfer from water molecules to CNTs, endowing the sensor with an excellent humidity responsive property. Besides, the swelling of the TOCFs greatly damages the conductive CNTs network and further promotes the humidity sensitive performance of the sensor. Benefiting from the unique structure, the obtained sensor exhibits a maximum response value of 87.0% (ΔI/I₀ , and the response limit is 100%), outstanding linearity (R² = 0.995) between 11 and 95% relative humidity (RH), superior bending (with a curvature of 2.1 cm^-1) and folding (up to 50 times) durability, and good long-time stability (more than 3 months). Finally, as a proof of concept, the sensor demonstrates an excellent responsive property to human breath, fingertip humidity, and the change of air humidity, indicating a great potential towards practical applications.

Application of P(VDF-TrFE) Glass Coating for Robust Harmonic Nanoparticles Characterization

Polyvinylidene fluoride and its copolymers are a well-known family of low-cost ferroelectric materials widely used for the fabrication of devices for a wide range of applications. A biocompatibility, high optical quality, chemical and mechanical durability of poly(vinylidene fluoride–trifluoroethylene), (P(VDF–TrFE)), makes it particularly attractive for designing of effective coating layers for different diagnostic techniques. In the present work, the nonlinear optical characterization of P(VDF-TrFE)-coating films deposited onto a glass substrate was done. Advantages of the coating application for cells/substrates in the field of multiphoton imaging the efficiency of such coating layer for long-duration characterization of so-called harmonic nanoparticles (HNPs) were shown. The influence of glass surface protection by P(VDF-TrFE) film from an effect of HNPs sticking to the walls of the flow-cell was analyzed for effective studying of the optical harmonics generation efficiency of HNPs making the analysis more robust.

A review of low-temperature H2S gas sensors: fabrication and mechanism

Reduced detection temperature of hazardous gases such as H2S can lower power consumption and increase the long-term stability. The decreased operating temperature can be achieved via physical and chemical modification of the sensing layer.

Nanosensor-Based Flexible Electronic Assisted with Light Fidelity Communicating Technology for Volatolomics-Based Telemedicine

Telemedicine provides an attractive vision for tele-monitoring human health conditions and, thus, offers the opportunity for timely preventing chronic disease. A key limitation of promoting telemedicine in clinic application is the lack of a noninvasive med-tech and effective monitoring platform, which should be wearable and capable of high-performance tele-monitoring of health risk. Here we proposed a volatolomics-based telemedicine for continuously and noninvasively assessing human health status through continuously tracking the variation of volatile markers derived from human breath or skin. Particularly, a nanosensor-based flexible electronic was specifically designed to serve as a powerful platform for implementing the proposed cost-effective healthcare. An all-flexible and highly packed makeup (all functional units were integrated in a 2*2*0.19 cm³ plate) enables an electronic, compact configuration and the capability of resisting negative impact derived from customers' daily movement. Notably, the nanosensor-based electronic demonstrates high specificity, quick response rate (t_90% = 4.5 s), and desirable low detection limit (down to 0.117 ppm) in continuous tele-monitoring chronic-disease-related volatile marker (e.g., acetone). Assisted by the power saved light fidelity (Li-Fi) communicating technology, a clinic proof on the specifically designed electronic for noninvasively and uninterrupted assessing potential health risk (e.g., diabetics) is successfully implemented, with the accuracy of around 81%. A further increase in the accuracy of prewarning is predicted by excluding the impact of individual differences such as the gender, age, and smoking status of the customer. These promising pilot results indicate a bright future for the tailor-made nanosensing-device-supported volatolomics-based telemedicine in preventing chronic diseases and increasing patients' survival rate.

Self-powered wearable electronics

Wearable electronics are an essential direction for the future development of smart wearables. Among them, the battery life of wearable electronics is a key technology that limits their development. The proposal of self-powered wearable electronics (SWE) provides a promising solution to the problem of long-term stable working of wearable electronics. This review has made a comprehensive summary and analysis of recent advances on SWE from the perspectives of energy, materials, and ergonomics methods. At the same time, some representative research work was introduced in detail. SWE can be divided into energy type SWE and sensor type SWE according to their working types. Both types of SWE are broadly applied in human-machine interaction, motion information monitoring, diagnostics, and therapy systems. Finally, this article summarizes the existing bottlenecks of SWE, and predicts the future development direction of SWE.

Studies of Langmuir and Langmuir-Schaefer Films of Poly(3-Hexylthiophene) and Poly(Vinylidene Fluoride)

The synergistic use of blends of regioregular poly(3-hexylthiophene) (P3HT) and poly(vinylidene fluoride) (PVDF) or poly((vinylidene fluoride)-block-(methyl methacrylate)) (PVDF-PMMA) to form Langmuir and Langmuir-Schaefer (LS) films is reported. P3HT has wide applications in sensor devices because of its properties such as conductivity, luminescence, and chromism; however, the stiffness of the films and the difficulty in organizing the molecules may pose a problem in these applications. In this context, polymers based on PVDF can be used in the formation of thin P3HT films and present an alternative to improve the organization of P3HT molecules. In addition, PVDF acts as a plasticizer, making the film less rigid. The films were obtained from the blends of P3HT/PVDF and P3HT/PVDF-PMMA in a solution containing chloroform and DMAc (N,N-dimethylacetamide). Surface pressure isotherms, in situ ultraviolet-visible (UV-vis) spectroscopy, polarization-modulation infrared reflection-absorption spectroscopy, and Brewster angle microscopy techniques were used to analyze Langmuir films. The surface morphology of LS films was characterized by atomic force microscopy and UV-vis spectroscopy, and their degradation was analyzed by UV-vis spectroscopy after exposure to natural light under atmospheric conditions. The Langmuir films containing PVDF indicate a direct formation of the ferroelectric β phase, with dipoles parallel to the water surface. The Langmuir films formed by P3HT presented dipoles of side chains parallel and aromatic groups perpendicular to the water surface. P3HT and PVDF or PVDF-PMMA films show high molecular organization compared with pure P3HT films. The results suggest that these films could be used to improve the properties of P3HT in several device applications, such as in optical and electrical sensors.

Nanogenerator-Based Self-Powered Sensors for Wearable and Implantable Electronics

Wearable and implantable electronics (WIEs) are more and more important and attractive to the public, and they have had positive influences on all aspects of our lives. As a bridge between wearable electronics and their surrounding environment and users, sensors are core components of WIEs and determine the implementation of their many functions. Although the existing sensor technology has evolved to a very advanced level with the rapid progress of advanced materials and nanotechnology, most of them still need external power supply, like batteries, which could cause problems that are difficult to track, recycle, and miniaturize, as well as possible environmental pollution and health hazards. In the past decades, based upon piezoelectric, pyroelectric, and triboelectric effect, various kinds of nanogenerators (NGs) were proposed which are capable of responding to a variety of mechanical movements, such as breeze, body drive, muscle stretch, sound/ultrasound, noise, mechanical vibration, and blood flow, and they had been widely used as self-powered sensors and micro-nanoenergy and blue energy harvesters. This review focuses on the applications of self-powered generators as implantable and wearable sensors in health monitoring, biosensor, human-computer interaction, and other fields. The existing problems and future prospects are also discussed.

Skin-Inspired Humidity and Pressure Sensor with a Wrinkle-on-Sponge Structure

Sensors with multifunctions have attracted great attention for their extensive application value, among which humidity sensing and pressure sensing are necessary to electronics undoubtedly because of the complex physical environment we live in. Inspired by the structure of skin, in this article, we design a new method to combine wrinkle structure with porous sponge structure and achieve a novel, flexible, compressible, and bifunctional sensor based on carbon nanotube-polydimethylsiloxane (CNT-PDMS) with functions of humidity sensing and pressure sensing. The performance of the humidity sensing part can be controlled by the ultraviolet and ozone (UVO) treatment time and CNT concentration, while the sensitivity of the pressure sensing part can be controlled by the CNT concentration and grinding time of sugar granules. The bifunctional sensor can easily sense approaching and touching of a hand, which shows great potential of alarming and protecting some electronics. Moreover, the bifunctional sensor can also be used in detecting human joint motions and breath conditions as a wearable and flexible health monitor.

A water-evaporation-induced self-charging hybrid power unit for application in the Internet of Things

A self-charging hybrid power unit has been developed by integrating a water-evaporation-induced nanogenerator with a flexible nano-patterned supercapacitor. The nanogenerator can harvest environmental thermal energy and mechanical energy through the water evaporation process, and the supercapacitor can be charged simultaneously. The former offers stable electrical power as output, whereas the Ppy-based supercapacitor shows a capacitance of 12.497 mF/cm² with 96.42% retention after 4,000 cycles. After filling the power unit with water as the fuel, it can be fully charged in about 20 min. The power unit can be flexibly integrated with electronic devices such as sensor nodes and wireless transmitters employing the Internet of Things. This new approach can offer new possibilities in continuous future operation of randomly distributed electronic devices incorporated in the Internet of Things.

Development of low-cost hybrid multi-walled carbon nanotube-based ammonia gas-sensing strips with an integrated sensor read-out system for clinical breath analyzer applications

This work demonstrates the development of Ag@polyaniline/multi-walled carbon nanotube nanocomposite-based sensor strips and a suitable integrated electronic read-out system for the measurement of trace-level concentrations of ammonia (NH₃). The sensor is optimized under various operating conditions and the resulting sensor exhibited an enhanced response (32% for 2 ppm) with excellent selectivity. Stable performance was observed towards NH₃ in the presence of high concentrations of CO₂ (>40 000 ppm), simulated and real breath samples. A suitable electronic sensor read-out system has also been designed and developed based on multi-scale resistance-to-voltage conversion architecture, processed by a 32-bit microcontroller which is operatable over a wide range of sensor resistance (1 kΩ to 200 MΩ). As a proof of concept, integration of gas-sensing strips with the electronic read-out system was tested with various levels of NH₃ (<2 ppm as normal, >2 ppm as critical and 2 ppm as threshold), which is important for clinical breath analyzer applications. The developed prototype device can be readily incorporated into a portable, low-cost and non-invasive point-of-care breath NH₃ detection unit for portable pre-diagnostic breath analyzer applications.

Development of a potential LPG sensor based on a PANI–Co3O4 nanocomposite that functions at room temperature

Nanostructured Co₃O₄ was synthesized by a sol–gel technique while 30% & 40% Co₃O₄–PANI (polyaniline) nanocomposites were successfully prepared employing an in situ polymerization technique.