Wearable and flexible electronics for continuous molecular monitoring

Flexible and Stretchable Electrochemical Sensors for Biological Monitoring

The rise of flexible and stretchable electronics has revolutionized biosensor techniques for probing biological systems. Particularly, flexible and stretchable electrochemical sensors (FSECSs) enable the in situ quantification of numerous biochemical molecules in different biological entities owing to their exceptional sensitivity, fast response, and easy miniaturization. Over the past decade, the fabrication and application of FSECSs have significantly progressed. This review highlights key developments in electrode fabrication and FSECSs functionalization. It delves into the electrochemical sensing of various biomarkers, including metabolites, electrolytes, signaling molecules, and neurotransmitters from biological systems, encompassing the outer epidermis, tissues/organs in vitro and in vivo, and living cells. Finally, considering electrode preparation and biological applications, current challenges and future opportunities for FSECSs are discussed.

Flexible thin film thermocouples: From structure, material, fabrication to application

Flexible thin-film thermocouples (TFTCs) have been garnering interest as temperature sensors due to the advantages of being flexible, ultrathin, and ultralight. Additionally, they have fast response times and enable detection of temperature. These properties have made them suitable for applications such as wearable electronics, healthcare, portable personal devices, and smart detection systems. This review presents the progress in the development of flexible TFTCs. The mechanism, structural design, materials, fabrication methods, and related applications of flexible TFTCs are also elaborated. Finally, future development directions of flexible TFTCs are discussed such as wide-range temperature measurement, multiple sensor integration, and achieving reliable cold-end compensation systems.

Wearable Flexible Perspiration Biosensors Using Laser-Induced Graphene and Polymeric Tape Microfluidics

Wearable biosensors promise real-time measurements of chemicals in human sweat, with the potential for dramatic improvements in medical diagnostics and athletic performance through continuous metabolite and electrolyte monitoring. However, sweat sensing is still in its infancy, and questions remain about whether sweat can be used for medical purposes. Wearable sensors are focused on proof-of-concept designs that are not scalable for multisubject trials, which could elucidate the utility of sweat sensing for health monitoring. Moreover, many wearable sensors do not include the microfluidics necessary to protect and channel consistent and clean sweat volumes to the sensor surface or are not designed to be disposable to prevent sensor biofouling and inaccuracies due to repeated use. Hence, there is a need to produce low-cost and single-use wearable sensors with integrated microfluidics to ensure reliable sweat sensing. Herein, we demonstrate the convergence of laser-induced graphene (LIG) based sensors with soft tape polymeric microfluidics to quantify both sweat metabolites (glucose and lactate) and electrolytes (sodium) for potential hydration and fatigue monitoring. Distinct LIG-electrodes were functionalized with glucose oxidase and lactate oxidase for selective sensing of glucose and lactate across physiological ranges found in sweat with sensitivities of 26.2 and 2.47 × 10-3 μA mM-1 cm-2, detection limits of 8 and 220 μM, and linear response ranges of 0-1 mM and 0-32 mM, respectively. LIG-electrodes functionalized with a sodium-ion-selective membrane displayed Nernstian sensitivity of 58.8 mV decade-1 and a linear response over the physiological range in sweat (10-100 mM). The sensors were tested in a simulated sweating skin microfluidic system and on-body during cycling tests in a multisubject trial. Results demonstrate the utility of LIG integrated with microfluidics for real-time, continuous measurements of biological analytes in sweat and help pave the way for the development of personalized wearable diagnostic tools.

Topological quantum devices: a review

The introduction of the concept of topology into condensed matter physics has greatly deepened our fundamental understanding of transport properties of electrons as well as all other forms of quasi particles in solid materials. It has also fostered a paradigm shift from conventional electronic/optoelectronic devices to novel quantum devices based on topology-enabled quantum device functionalities that transfer energy and information with unprecedented precision, robustness, and efficiency. In this article, the recent research progress in topological quantum devices is reviewed. We first outline the topological spintronic devices underlined by the spin-momentum locking property of topology. We then highlight the topological electronic devices based on quantized electron and dissipationless spin conductivity protected by topology. Finally, we discuss quantum optoelectronic devices with topology-redefined photoexcitation and emission. The field of topological quantum devices is only in its infancy, we envision many significant advances in the near future.

Indoor Photovoltaic Fiber with an Efficiency of 25.53% under 1500 Lux Illumination

Photovoltaic devices represent an efficient electricity generation mode. Integrating them into textiles offers exciting opportunities for smart electronic textiles-with the ultimate goal of supplying power for wearable technology-which is poised to change how electronic devices are designed. Many human activities occur indoors, so realizing indoor photovoltaic fibers (IPVFs) that can be woven into textiles to power wearables is critical, although currently unavailable. Here, a dye-sensitized IPVF is constructed by incorporating titanium dioxide nanoparticles into aligned nanotubes to produce close contact and stable interfaces among active layers on a curved fiber substrate, thus presenting efficient charge transport and low charge recombination in the photoanode. With the combination of highly conductive core-sheath Ti/carbon nanotube fiber as a counter electrode, the IPVF shows a certified power conversion efficiency of 25.53% under 1500 lux illuminance. Its performance variation is below 5% after bending, twisting, or pressing for 1000 cycles. These IPVFs are further integrated with fiber batteries as self-charging power textiles, which are demonstrated to effectively supply electricity for wearables, solving the power supply problem in this important direction.

An autonomous wearable biosensor powered by a perovskite solar cell

Wearable sweat sensors can potentially be used to continuously and non-invasively monitor physicochemical biomarkers that contain information related to disease diagnostics and fitness tracking. However, the development of such autonomous sensors faces a number of challenges including achieving steady sweat extraction for continuous and prolonged monitoring, and addressing the high power demands of multifunctional and complex analysis. Here we report an autonomous wearable biosensor that is powered by a perovskite solar cell and can provide continuous and non-invasive metabolic monitoring. The device uses a flexible quasi-two-dimensional perovskite solar cell module that provides ample power under outdoor and indoor illumination conditions (power conversion efficiency exceeding 31% under indoor light illumination). We show that the wearable device can continuously collect multimodal physicochemical data - glucose, pH, sodium ions, sweat rate, and skin temperature - across indoor and outdoor physical activities for over 12 hours.

Nanostructured wearable electrochemical and biosensor towards healthcare management: a review

In recent years, there has been a rapid increase in demand for wearable sensors, particularly these tracking the surroundings, fitness, and health of people. Thus, selective detection in human body fluid is a demand for a smart lifestyle by quick monitoring of electrolytes, drugs, toxins, metabolites and biomolecules, proteins, and the immune system. In this review, these parameters along with the main features of the latest and mostly cited research work on nanostructured wearable electrochemical and biosensors are surveyed. This study aims to help researchers and engineers choose the most suitable selective and sensitive sensor. Wearable sensors have broad and effective sensing platforms, such as contact lenses, Google Glass, skin-patch, mouth gourds, smartwatches, underwear, wristbands, and others. For increasing sensor reliability, additional advancements in electrochemical and biosensor precision, stability in uncontrolled environments, and reproducible sample conveyance are necessary. In addition, the optimistic future of wearable electrochemical sensors in fields, such as remote and customized healthcare and well-being is discussed. Overall, wearable electrochemical and biosensing technologies hold great promise for improving personal healthcare and monitoring performance with the potential to have a significant impact on daily lives. These technologies enable real-time body sensing and the communication of comprehensive physiological information.

Pattern design of a liquid metal-based wearable heater for constant heat generation under biaxial strain

As the wearable heater is increasingly popular due to its versatile applications, there is a growing need to improve the tensile stability of the wearable heater. However, maintaining the stability and precise control of heating in resistive heaters for wearable electronics remains challenging due to multiaxial dynamic deformation with human motion. Here, we propose a pattern study for a circuit control system without complex structure or deep learning of the liquid metal (LM)-based wearable heater. The LM direct ink writing (DIW) method was used to fabricate the wearable heaters in various designs. Through the study about the pattern, the significance of input power per unit area for steady average temperature with tension was proven, and the directionality of the pattern was shown to be a factor that makes feedback control difficult due to the difference in resistance change according to strain direction. For this issue, a wearable heater with the same minimal resistance change regardless of the tension direction was developed using Peano curves and sinuous pattern structure. Lastly, by attaching to a human body model, the wearable heater with the circuit control system shows stable heating (52.64°C, with a standard deviation of 0.91°C) in actual motion.

Sweat and odor in sportswear - A review

Sportswear worn next to the skin is easily soaked by sweat and may become a breeding ground for the microbiome, thus a source of malodor. Malodor can cause social embarrassment and discomfort to both wearer and others. Given the risks current deodorant products pose to nature and human life, the development of sustainable textiles for odor control comes to the forefront. This review introduces the odor-generating mechanism in clothing from the perspectives of perspiration composition and cutaneous microbiome. With the knowledge of the significant role of sweat in odor formation, the sweat distribution of the human body, measurement techniques, and advanced technologies developed for quick-dry function are presented in the second part. Lastly, odor management in sportswear is evaluated, covering the odor-assessing techniques, the effects of various textile materials, and emerging solutions in terms of antibacterial treatment, adsorbent materials, and photocatalytic degradations of odorous compounds. Overall, it is of both personal and social value to develop novel textile materials with odor-control functions by making use of natural materials and fabric designs.

Epidermal Wearable Biosensors for the Continuous Monitoring of Biomarkers of Chronic Disease in Interstitial Fluid

Healthcare technology has allowed individuals to monitor and track various physiological and biological parameters. With the growing trend of the use of the internet of things and big data, wearable biosensors have shown great potential in gaining access to the human body, and providing additional functionality to analyze physiological and biochemical information, which has led to a better personalized and more efficient healthcare. In this review, we summarize the biomarkers in interstitial fluid, introduce and explain the extraction methods for interstitial fluid, and discuss the application of epidermal wearable biosensors for the continuous monitoring of markers in clinical biology. In addition, the current needs, development prospects and challenges are briefly discussed.

Air-Bubble-Insensitive Microfluidic Lactate Biosensor for Continuous Monitoring of Lactate in Sweat

This study aimed to develop a lactate sensor with a microchannel that overcomes the issue of air bubbles interfering with the measurement of lactate levels in sweat and to evaluate its potential for continuous monitoring of lactate in sweat. To achieve continuous monitoring of lactate, a microchannel was used to supply and drain sweat from the electrodes of the lactate sensor. A lactate sensor was then developed with a microchannel that has an area specifically designed to trap air bubbles and prevent them from contacting the electrode. The sensor was evaluated by a person while exercising to test its effectiveness in monitoring lactate in sweat and its correlation with blood lactate levels. Furthermore, the lactate sensor with a microchannel in this study can be worn on the body for a long time and is expected to be used for the continuous monitoring of lactate in sweat. The developed lactate sensor with a microchannel effectively prevented air bubbles from interfering with the measurement of lactate levels in sweat. The sensor showed a concentration correlation ranging from 1 to 50 mM and demonstrated a correlation between lactate in sweat and blood. Additionally, the lactate sensor with a microchannel in this study can be worn on the body for an extended period and is expected to be useful for the continuous monitoring of lactate in sweat, particularly in the fields of medicine and sports.

Self-regulating bioinspired supramolecular photonic hydrogels based on chemical reaction networks for monitoring activities of enzymes and biofuels

Inspired by the way many living organisms utilize chemical/biological reactions to regulate their skin and respond to stimuli in the external environment, we have developed a self-regulating hydrogel design by incorporating chemical reaction networks (CRNs) into biomimetic photonic crystal hydrogels. In this hydrogel system, we used host-guest supramolecular non-covalent bonds between beta-cyclodextrin (β-CD) and ferrocene (Fc) as partial crosslinkers and designed a CRN involving enzyme-fuel couples of horseradish peroxidase (HRP)/H₂O₂ and glucose oxidase (GOD)/d-glucose, by which the responsive hydrogel was transformed into a glucose-driven self-regulating hydrogel. Due to the biomimetic structural color in the hydrogel, the progress of the chemical reaction was accompanied by a change in the color of the hydrogel. Based on this principle, the designed supramolecular photonic hydrogel (SPH) can not only achieve naked-eye detection of H₂O₂ and glucose concentrations with the assistance of a smartphone but also monitor the reactions of HRP and GOD enzymes and determine their activity parameters. The sensitivity and stability of the sensor have been proven. In addition, due to the reversibility of the chemical reaction network, the sensor can be reused, thus having the potential to serve as a low-cost point-of-care sensor. The SPH was ultimately used to detect glucose in human plasma and H₂O₂ in liver tumor tissue. The results are comparable with commercial assay kits. By redesigning the chemical reaction network in the hydrogel, it is expected to be used for detecting other enzymes or fuels.

2D PtSe2 Enabled Wireless Wearable Gas Monitoring Circuits with Distinctive Strain-Enhanced Performance

The application of 2D materials-based flexible electronics in wearable scenarios is limited due to performance degradation under strain fields. In contrast to its negative role in existing transistors or sensors, herein, we discover a positive effect of strain to the ammonia detection in 2D PtSe₂. Linear modulation of sensitivity is achieved in flexible 2D PtSe₂ sensors via a customized probe station with an in situ strain loading apparatus. For trace ammonia absorption, a 300% enhancement in room-temperature sensitivity (31.67% ppm^-1) and an ultralow limit of detection (50 ppb) are observed under 1/4 mm^-1 curvature strain. We identify three types of strain-sensitive adsorption sites in layered PtSe₂ and pinpoint that basal-plane lattice distortion contributes to better sensing performance resulting from reduced absorption energy and larger charge transfer density. Furthermore, we demonstrate state-of-the-art 2D PtSe₂-based wireless wearable integrated circuits, which allow real-time gas sensing data acquisition, processing, and transmission through a Bluetooth module to user terminals. The circuits exhibit a wide detection range with a maximum sensitivity value of 0.026 V·ppm^-1 and a low energy consumption below 2 mW.

Flexible and Wearable Biosensors for Monitoring Health Conditions

Flexible and wearable biosensors have received tremendous attention over the past decade owing to their great potential applications in the field of health and medicine. Wearable biosensors serve as an ideal platform for real-time and continuous health monitoring, which exhibit unique properties such as self-powered, lightweight, low cost, high flexibility, detection convenience, and great conformability. This review introduces the recent research progress in wearable biosensors. First of all, the biological fluids often detected by wearable biosensors are proposed. Then, the existing micro-nanofabrication technologies and basic characteristics of wearable biosensors are summarized. Then, their application manners and information processing are also highlighted in the paper. Massive cutting-edge research examples are introduced such as wearable physiological pressure sensors, wearable sweat sensors, and wearable self-powered biosensors. As a significant content, the detection mechanism of these sensors was detailed with examples to help readers understand this area. Finally, the current challenges and future perspectives are proposed to push this research area forward and expand practical applications in the future.

Wafer-patterned, permeable, and stretchable liquid metal microelectrodes for implantable bioelectronics with chronic biocompatibility

Implantable bioelectronics provide unprecedented opportunities for real-time and continuous monitoring of physiological signals of living bodies. Most bioelectronics adopt thin-film substrates such as polyimide and polydimethylsiloxane that exhibit high levels of flexibility and stretchability. However, the low permeability and relatively high modulus of these thin films hamper the long-term biocompatibility. In contrast, devices fabricated on porous substrates show the advantages of high permeability but suffer from low patterning density. Here, we report a wafer-scale patternable strategy for the high-resolution fabrication of supersoft, stretchable, and permeable liquid metal microelectrodes (μLMEs). We demonstrate 2-μm patterning capability, or an ultrahigh density of ~75,500 electrodes/cm2, of μLME arrays on a wafer-size (diameter, 100 mm) elastic fiber mat by photolithography. We implant the μLME array as a neural interface for high spatiotemporal mapping and intervention of electrocorticography signals of living rats. The implanted μLMEs have chronic biocompatibility over a period of eight months.

Biomolecular sensors for advanced physiological monitoring

Body-based biomolecular sensing systems, including wearable, implantable and consumable sensors allow comprehensive health-related monitoring. Glucose sensors have long dominated wearable bioanalysis applications owing to their robust continuous detection of glucose, which has not yet been achieved for other biomarkers. However, access to diverse biological fluids and the development of reagentless sensing approaches may enable the design of body-based sensing systems for various analytes. Importantly, enhancing the selectivity and sensitivity of biomolecular sensors is essential for biomarker detection in complex physiological conditions. In this Review, we discuss approaches for the signal amplification of biomolecular sensors, including techniques to overcome Debye and mass transport limitations, and selectivity improvement, such as the integration of artificial affinity recognition elements. We highlight reagentless sensing approaches that can enable sequential real-time measurements, for example, the implementation of thin-film transistors in wearable devices. In addition to sensor construction, careful consideration of physical, psychological and security concerns related to body-based sensor integration is required to ensure that the transition from the laboratory to the human body is as seamless as possible.

Recent Advancements in Physiological, Biochemical, and Multimodal Sensors Based on Flexible Substrates: Strategies, Technologies, and Integrations

Flexible wearable devices have been widely used in biomedical applications, the Internet of Things, and other fields, attracting the attention of many researchers. The physiological and biochemical information on the human body reflects various health states, providing essential data for human health examination and personalized medical treatment. Meanwhile, physiological and biochemical information reveals the moving state and position of the human body, and it is the data basis for realizing human-computer interactions. Flexible wearable physiological and biochemical sensors provide real-time, human-friendly monitoring because of their light weight, wearability, and high flexibility. This paper reviews the latest advancements, strategies, and technologies of flexibly wearable physiological and biochemical sensors (pressure, strain, humidity, saliva, sweat, and tears). Next, we systematically summarize the integration principles of flexible physiological and biochemical sensors with the current research progress. Finally, important directions and challenges of physiological, biochemical, and multimodal sensors are proposed to realize their potential applications for human movement, health monitoring, and personalized medicine.

Fast and sensitive coulometric signal transduction for ion-selective electrodes by utilizing a two-compartment cell

Here, we propose a fast and sensitive coulometric signal transduction method for ion-selective electrodes (ISEs) by utilizing a two-compartment cell. A potassium ion-selective electrode (K⁺-ISE) was connected as reference electrode (RE) and placed in the sample compartment. A glassy carbon (GC) electrode coated with poly(3,4-ethylenedioxythiophene) (GC/PEDOT), or reduced graphene oxide (GC/RGO), was connected as working electrode (WE) and placed in the detection compartment together with a counter electrode (CE). The two compartments were connected with an Ag/AgCl wire. The measured cumulated charge was amplified by increasing the capacitance of the WE. The observed slope of the cumulated charge with respect to the change of the logarithm of the K⁺ ion activity was linearly proportional to the capacitance of the GC/PEDOT and GC/RGO, estimated from impedance spectra. Furthermore, the sensitivity of the coulometric signal transduction using a commercial K⁺-ISE with internal filling solution as RE and GC/RGO as WE allowed to decrease the response time while still being able to detect a 0.2% change in K⁺ concentration. The coulometric method utilizing a two-compartment cell was found to be feasible for the determination of K⁺ concentrations in serum. The advantage of this two-compartment approach, compared to the coulometric transduction described earlier, was that no current passed through the K⁺-ISE that was connected as RE. Therefore, current-induced polarization of the K⁺-ISE was avoided. Furthermore, since the GCE/PEDOT and GCE/RGO (used as WE) had a low impedance, the response time of the coulometric response decreased from minutes to seconds.

A wearable sensor based on multifunctional conductive hydrogel for simultaneous accurate pH and tyrosine monitoring in sweat

Flexible and wearable sweat sensors have drawn extensive attention by virtue of their continuous and real-time monitoring of molecular level information. However, current sweat-based sensors still pose several challenges, such as low accuracy for analytes detection, susceptibility to microorganism and poor mechanical performance. Herein, we demonstrated a noninvasive wearable sweat sensing patch composed of an electrochemical sensing system, and a pilocarpine-based iontophoretic system to stimulate sweat secretion. The electrochemical sensor based on tannic acid-Ag-carbon nanotube-polyaniline (TA-Ag-CNT-PANI) composite hydrogel was designed for on-body detection of pH and tyrosine (Tyr), a disease marker associated with multiple disorders, such as tyrosinemia and bulimia nervosa. The wearable sweat sensor can not only monitor the pH and Tyr in sweat simultaneously, but also further calibrate Tyr detection results with the measured pH value, so as to eliminate the effect of Tyr response variance at different pH and enhance the accuracy of the sensor. Furthermore, the presence of tannic acid chelated-Ag nanoparticles (TA-Ag NPs) and carbon nanotubes (CNTs) significantly improved the conductivity and flexibility of the hydrogel and endowed the composite hydrogel with antibacterial capability. Of note, the constructed wearable sensor was capable of monitoring Tyr with enhanced accuracy in various sweats.

Recent Development of Mechanical Stimuli Detectable Sensors, Their Future, and Challenges: A Review

By virtue of their wide applications in transportation, healthcare, smart home, and security, development of sensors detecting mechanical stimuli, which are many force types (pressure, shear, bending, tensile, and flexure) is an attractive research direction for promoting the advancement of science and technology. Sensing capabilities of various force types based on structural design, which combine unique structure and materials, have emerged as a highly promising field due to their various industrial applications in wearable devices, artificial skin, and Internet of Things (IoT). In this review, we focus on various sensors detecting one or two mechanical stimuli and their structure, materials, and applications. In addition, for multiforce sensing, sensing mechanism are discussed regarding responses in external stimuli such as piezoresistive, piezoelectric, and capacitance phenomena. Lastly, the prospects and challenges of sensors for multiforce sensing are discussed and summarized, along with research that has emerged.

Skin-Interfaced Wearable Sweat Sensors for Precision Medicine

Wearable sensors hold great potential in empowering personalized health monitoring, predictive analytics, and timely intervention toward personalized healthcare. Advances in flexible electronics, materials science, and electrochemistry have spurred the development of wearable sweat sensors that enable the continuous and noninvasive screening of analytes indicative of health status. Existing major challenges in wearable sensors include: improving the sweat extraction and sweat sensing capabilities, improving the form factor of the wearable device for minimal discomfort and reliable measurements when worn, and understanding the clinical value of sweat analytes toward biomarker discovery. This review provides a comprehensive review of wearable sweat sensors and outlines state-of-the-art technologies and research that strive to bridge these gaps. The physiology of sweat, materials, biosensing mechanisms and advances, and approaches for sweat induction and sampling are introduced. Additionally, design considerations for the system-level development of wearable sweat sensing devices, spanning from strategies for prolonged sweat extraction to efficient powering of wearables, are discussed. Furthermore, the applications, data analytics, commercialization efforts, challenges, and prospects of wearable sweat sensors for precision medicine are discussed.

Hydrogel-Based Flexible Electronics

Flexible electronics is an emerging field of research involving multiple disciplines, which include but not limited to physics, chemistry, materials science, electronic engineering, and biology. However, the broad applications of flexible electronics are still restricted due to several limitations, including high Young's modulus, poor biocompatibility, and poor responsiveness. Innovative materials aiming for overcoming these drawbacks and boost its practical application is highly desirable. Hydrogel is a class of 3D crosslinked hydrated polymer networks, and its exceptional material properties render it as a promising candidate for the next generation of flexible electronics. Here, the latest methods of synthesizing advanced functional hydrogels and the state-of-art applications of hydrogel-based flexible electronics in various fields are reviewed. More importantly, the correlation between properties of the hydrogel and device performance is discussed here, to have better understanding of the development of flexible electronics by using environmentally responsive hydrogels. Last, perspectives on the current challenges and future directions in the development of hydrogel-based multifunctional flexible electronics are provided.

High-Performance Stretchable Thermoelectric Generator for Self-Powered Wearable Electronics

Wearable thermoelectric generators (TEGs), which can convert human body heat to electricity, provide a promising solution for self-powered wearable electronics. However, their power densities still need to be improved aiming at broad practical applications. Here, a stretchable TEG that achieves comfortable wearability and outstanding output performance simultaneously is reported. When worn on the forehead at an ambient temperature of 15 °C, the stretchable TEG exhibits excellent power densities with a maximum value of 13.8 µW cm^-2 under the breezeless condition, and even as high as 71.8 µW cm^-2 at an air speed of 2 m s^-1 , being one of the highest values for wearable TEGs. Furthermore, this study demonstrates that this stretchable TEG can effectively power a commercial light-emitting diode and stably drive an electrocardiogram module in real-time without the assistance of any additional power supply. These results highlight the great potential of these stretchable TEGs for power generation applications.

Dual Structural Design of Platinum-Nickel Hydrogels for Wearable Glucose Biosensing with Ultrahigh Stability

Wearable glucose sensors are of great significance and highly required in mobile health monitoring and management but suffering from limited long-term stability and wearable adaptability. Here a simultaneous component and structure engineering strategy is presented, which involves Pt with abundant Ni to achieve three-dimensional, dual-structural Pt-Ni hydrogels with interconnected networks of PtNi nanowires and Ni(OH)₂ nanosheets, showing prominent electrocatalytic activity and stability in glucose oxidation under neutral condition. Specifically, the PtNi_(1:3) dual hydrogels shows 2.0 and 270.6 times' activity in the glucose electro-oxidation as much as the pure Pt and Ni hydrogels. Thanks to the high activity, structural stability, good flexibility, and self-healing property, the PtNi_(1:3) dual gel-based non-enzymatic glucose sensing chip is endowed with high performance. It features a high sensitivity, an excellent selectivity and flexibility, and particularly an outstanding long-term stability over 2 months. Together with a pH sensor and a wireless circuit, an accurate, real-time, and remote monitoring of sweat glucose is achieved. This facile design of novel dual-structural metallic hydrogels sheds light to rationally develop new functional materials for high-performance wearable biosensors.

Smart Janus fabrics for one-way sweat sampling and skin-friendly colorimetric detection

Functionalized textiles with biofluid management capability have attracted tremendous attention in recent years due to their significant roles in health monitoring and dehydration prevention. Here we propose a one-way colorimetric sweat sampling and sensing system based on a Janus fabric using interfacial modification techniques. The opposite wettability of Janus fabric enables sweat to be quickly moved from the skin surface to the hydrophilic side and colorimetric patches. The unidirectional sweat-wicking performance of Janus fabric not only facilitates adequate sweat sampling but also inhibits the backflow of hydrated colorimetric regent from the assay patch toward the skin, eliminating potential epidermal contaminations. On this basis, visual and portable detection of sweat biomarkers including chloride, pH, and urea is also achieved. The results show that the true concentrations of chloride, pH, and urea in sweat are ∼10 mM, ∼7.2, and ∼10 mM, respectively. The detection limits of chloride and urea are 1.06 mM and 3.05 mM. This work bridges the gap between sweat sampling and a friendly epidermal microenvironment, providing a promising way for multifunctional textiles.

Flexible Ultrathin Chip-Film Patch for Electronic Component Integration and Encapsulation using Atomic Layer-Deposited Al2O3-TiO2 Nanolaminates

Plasma-enhanced atomic layer deposition (PEALD) is utilized to improve the barrier properties of an organic chip-film patch (CFP) when it is used as an implant to prevent moisture and ions from migrating into the embedded electronic circuits. For this purpose, surface condition and material properties of eight modifications of Al2O3-TiO2 nanolaminates sequentially deposited on polyimide PI-2611 films are evaluated in detail. The effect of stress-induced warpage of the deposited Al2O3-TiO2 on the wafer level is calculated with the Stoney equation and reveals higher tensile stress values while increasing the thickness of Al2O3-TiO2 nanolaminates from 20 up to 80 nm. Contact angle measurement and atomic force microscopy are used to investigate the surface energy and wettability, as well as the surface morphology of polyimide-Al2O3-TiO2 interfaces. We show that plasma treatment of pristine polyimide leads to an enhanced adhesion force of the PEAL-deposited layer by a factor of 1.3. The water vapor transmission rate (WVTR) is determined by exposing the coated polyimide films to 85% humidity and 23 °C and yields down to 1.58 × 10-3 g(H2O)/(m2 d). The data obtained are compared with alternative coating processes using the polymers parylene-C and benzocyclobutene (BCB). The latter shows higher WVTR values of 1.2 × 10-1 and 1.7 × 10-1 g(H2O)/(m2 d) compared to the PEALD-PI-2611 systems, indicating lower barrier properties. Two Al2O3-TiO2 modifications with low WVTR values have been chosen for encapsulating the CFP substrates and exposing them in a long-time experiment to chemical and mechanical loads in a chamber filled with phosphate-buffered saline at 37 °C, pH 7.3, and a cyclically applied pressure of 160 mbar (∼120 mm Hg). The electrical leakage behavior of the CFP systems is measured and reveals reliable electrical long-term stability far beyond 11 months, highlighting the great potential of PEALD-encapsulated CFPs.

Technology Roadmap for Flexible Sensors

Humans rely increasingly on sensors to address grand challenges and to improve quality of life in the era of digitalization and big data. For ubiquitous sensing, flexible sensors are developed to overcome the limitations of conventional rigid counterparts. Despite rapid advancement in bench-side research over the last decade, the market adoption of flexible sensors remains limited. To ease and to expedite their deployment, here, we identify bottlenecks hindering the maturation of flexible sensors and propose promising solutions. We first analyze challenges in achieving satisfactory sensing performance for real-world applications and then summarize issues in compatible sensor-biology interfaces, followed by brief discussions on powering and connecting sensor networks. Issues en route to commercialization and for sustainable growth of the sector are also analyzed, highlighting environmental concerns and emphasizing nontechnical issues such as business, regulatory, and ethical considerations. Additionally, we look at future intelligent flexible sensors. In proposing a comprehensive roadmap, we hope to steer research efforts towards common goals and to guide coordinated development strategies from disparate communities. Through such collaborative efforts, scientific breakthroughs can be made sooner and capitalized for the betterment of humanity.

A stretchable wireless wearable bioelectronic system for multiplexed monitoring and combination treatment of infected chronic wounds

Chronic nonhealing wounds are one of the major and rapidly growing clinical complications all over the world. Current therapies frequently require emergent surgical interventions, while abuse and misapplication of therapeutic drugs often lead to an increased morbidity and mortality rate. Here, we introduce a wearable bioelectronic system that wirelessly and continuously monitors the physiological conditions of the wound bed via a custom-developed multiplexed multimodal electrochemical biosensor array and performs noninvasive combination therapy through controlled anti-inflammatory antimicrobial treatment and electrically stimulated tissue regeneration. The wearable patch is fully biocompatible, mechanically flexible, stretchable, and can conformally adhere to the skin wound throughout the entire healing process. Real-time metabolic and inflammatory monitoring in a series of preclinical in vivo experiments showed high accuracy and electrochemical stability of the wearable patch for multiplexed spatial and temporal wound biomarker analysis. The combination therapy enabled substantially accelerated cutaneous chronic wound healing in a rodent model.

Smartphone-based platforms implementing microfluidic detection with image-based artificial intelligence

The frequent outbreak of global infectious diseases has prompted the development of rapid and effective diagnostic tools for the early screening of potential patients in point-of-care testing scenarios. With advances in mobile computing power and microfluidic technology, the smartphone-based mobile health platform has drawn significant attention from researchers developing point-of-care testing devices that integrate microfluidic optical detection with artificial intelligence analysis. In this article, we summarize recent progress in these mobile health platforms, including the aspects of microfluidic chips, imaging modalities, supporting components, and the development of software algorithms. We document the application of mobile health platforms in terms of the detection objects, including molecules, viruses, cells, and parasites. Finally, we discuss the prospects for future development of mobile health platforms.

Screen-Printed Corrosion-Resistant and Long-Term Stable Stretchable Electronics Based on AgAu Microflake Conductors

High-throughput production methods such as screen printing can bring stretchable electronics out of the lab into the market. Most stretchable conductor inks for screen printing are based on silver nanoparticles or flakes due to their favorable performance-to-cost ratio, but silver is prone to tarnishing and corrosion, thereby limiting the stability of such conductors. Here, we report on a cost-efficient and scalable approach to resolve this issue by developing screen printable inks based on silver flakes chemically coated by a thin layer of gold. The printed stretchable AgAu conductors reach a conductivity of 8500 S cm^-1, remain conductive up to 250% strain, show excellent corrosion and tarnishing stability, and are used to demonstrate wearable LED and NFC circuits. The reported approach is attractive for smart clothing, as the long-term functionality of such devices is expected in a variety of environments.

A collagen-based theranostic wound dressing with visual, long-lasting infection detection capability

Continuous wound monitoring is one strategy to minimise infection severity and inform prompt variations in therapeutic care following infection diagnosis. However, integration of this functionality in therapeutic wound dressings is still challenging. We hypothesised that a theranostic dressing could be realised by integrating a collagen-based wound contact layer with previously demonstrated wound healing capability, and a halochromic dye, i.e. bromothymol blue (BTB), undergoing colour change following infection-associated pH changes (pH: 5-6 ➔ >7). Two different BTB integration strategies, i.e. electrospinning and drop-casting, were pursued to introduce long-lasting visual infection detection capability through retention of BTB within the dressing. Both systems had an average BTB loading efficiency of 99 wt% and displayed a colour change within 1 min of contact with simulated wound fluid. Drop-cast samples retained up to 85 wt% of BTB after 96 h in a near-infected wound environment, in contrast to the fibre-bearing prototypes, which released over 80 wt% of BTB over the same time period. An increase in collagen denaturation temperature (DSC) and red shifts (ATR-FTIR) suggest the formation of secondary interactions between the collagen-based hydrogel and the BTB, which are attributed to count for the long-lasting dye confinement and durable dressing colour change. Given the high L929 fibroblast viability in drop-cast sample extracts (92 %, 7 days), the presented multiscale design is simple, cell- and regulatory-friendly, and compliant with industrial scale-up. This design, therefore, offers a new platform for the development of theranostic dressings enabling accelerated wound healing and prompt infection diagnosis.

Reverse iontophoresis with the development of flexible electronics: A review

Skin-centric diagnosis techniques, such as epidermal physiological parameter monitoring, have developed rapidly in recent years. The analysis of interstitial fluid (ISF), a body liquid with abundant physiological information, is a promising method to obtain health status because ISF is easily assessed by implanted or percutaneous measurements. Reverse iontophoresis extracts ISF by applying an electric field onto the skin, and it is a promising method to noninvasively obtain ISF, which, in turn, enables noninvasive epidermal physiological parameter monitoring. However, the development of reverse iontophoresis was relatively slow around the 2010s due to the rigidity and low biocompatibility of the applied devices. With the rapid development of flexible electronic technology in recent years, new progress has been made in the field of reverse iontophoresis, especially in the field of blood glucose monitoring and drug monitoring. This review summarizes the recent advances and discusses the challenges and opportunities of reverse iontophoresis.

Conformational-switch biosensors as novel tools to support continuous, real-time molecular monitoring in lab-on-a-chip devices

Recent years have seen continued expansion of the functionality of lab on a chip (LOC) devices. Indeed LOCs now provide scientists and developers with useful and versatile platforms across a myriad of chemical and biological applications. The field still fails, however, to integrate an often important element of bench-top analytics: real-time molecular measurements that can be used to "guide" a chemical response. Here we describe the analytical techniques that could provide LOCs with such real-time molecular monitoring capabilities. It appears to us that, among the approaches that are general (i.e., that are independent of the reactive or optical properties of their targets), sensing strategies relying on binding-induced conformational change of bioreceptors are most likely to succeed in such applications.

A wearable electronic based on flexible pressure sensor for running motion monitoring

The flexible pressure sensor is expected to be applied in the new generation of sports wearable electronic devices. Developing flexible pressure sensors with a wide linear range and great sensitivity, however, remains a significant barrier. In this work, we propose a hybrid conductive elastomeric film oxide-based material with a concave-shape micro-patterned array (P-HCF) on the surface that sustainably shows the necessary sensing qualities. To enhance sensing range and sensitivity, one-dimensional carbon fibers and two-dimensional MXene are incorporated into the polydimethylsiloxane matrix to form a three-dimensional conductive network. Micro-patterns with a curved shape in P-HCFs can be able to linear sensitivity across the sensing range by controlling the pressure distribution inside the material. Besides, the sensitivity of P-HCF pressure sensor can reach 31.92 kPa^-1, and meanwhile, the linear band of P-HCF pressure sensor can arrive at 24 Pa-720 kPa, which makes it a good choice for sports monitoring. The designed pressure sensor can be used to monitor the foot pressure during running. By analyzing the gait information during running, it can provide data support and strategy improvement for running. This new dual working mode pressure P-HCF sensor will provide a new way for the development of intelligent sports.

A Systematic Review on the Advanced Techniques of Wearable Point-of-Care Devices and Their Futuristic Applications

Personalized point-of-care testing (POCT) devices, such as wearable sensors, enable quick access to health monitoring without the use of complex instruments. Wearable sensors are gaining popularity owing to their ability to offer regular and continuous monitoring of physiological data by dynamic, non-invasive assessments of biomarkers in biofluids such as tear, sweat, interstitial fluid and saliva. Current advancements have concentrated on the development of optical and electrochemical wearable sensors as well as advances in non-invasive measurements of biomarkers such as metabolites, hormones and microbes. For enhanced wearability and ease of operation, microfluidic sampling, multiple sensing, and portable systems have been incorporated with materials that are flexible. Although wearable sensors show promise and improved dependability, they still require more knowledge about interaction between the target sample concentrations in blood and non-invasive biofluids. In this review, we have described the importance of wearable sensors for POCT, their design and types of these devices. Following which, we emphasize on the current breakthroughs in the application of wearable sensors in the realm of wearable integrated POCT devices. Lastly, we discuss the present obstacles and forthcoming potentials including the use of Internet of Things (IoT) for offering self-healthcare using wearable POCT.

UV-VIS-NIR broadband flexible photodetector based on layered lead-free organic-inorganic hybrid perovskite

The flexible photodetector is viewed as a research hotspot for numerous advanced optoelectronic applications. Recent progress has manifested that lead-free layered organic-inorganic hybrid perovskites (OIHPs) are highly attractive to engineering flexible photodetectors due to the effective overlapping of several unique properties, including efficient optoelectronic characteristics, exceptional structural flexibility, and the absence of Pb toxicity to humans and the environment. The narrow spectral response of most flexible photodetectors with lead-free perovskites is still a big challenge to practical applications. In this work, we demonstrate the flexible photodetector based on a novel (to our knowledge) narrow-bandgap OIHP of (BA)₂(MA)Sn₂I₇, with achieving a broadband response across an ultraviolet-visible-near infrared (UV-VIS-NIR) region as 365-1064 nm. The high responsivities of 28.4 and 2.0 × 10^-2 A/W are obtained at 365 and 1064 nm, respectively, corresponding to detectives of 2.3 × 10¹⁰ and 1.8 × 10⁷ Jones. This device also shows remarkable photocurrent stability after 1000 bending cycles. Our work indicates the huge application prospect of Sn-based lead-free perovskites in high-performance and eco-friendly flexible devices.

Flexible Sweat Sensors: From Films to Textiles

Flexible sweat sensors have found widespread potential applications for long-term wear and tracking and real-time monitoring of human health. However, the main substrate currently used in common flexible sweat sensors is thin film, which has disadvantages such as poor air permeability and the need for additional wearables. In this Review, the recent progress of sweat sensors has been systematically summarized by the types of monitoring methods of sweat sensors. In addition, this Review introduces and compares the performance of sweat sensors based on thin film and textile substrates such as fiber/yarn. Finally, opportunities and suggestions for the development of flexible sweat sensors are presented by summarizing the integration methods of sensors and human body monitoring sites.

Epidermal Wearable Biosensors for Monitoring Biomarkers of Chronic Disease in Sweat

Biological information detection technology is mainly used for the detection of physiological and biochemical parameters closely related to human tissues and organ lesions, such as biomarkers. This technology has important value in the clinical diagnosis and treatment of chronic diseases in their early stages. Wearable biosensors can be integrated with the Internet of Things and Big Data to realize the detection, transmission, storage, and comprehensive analysis of human physiological and biochemical information. This technology has extremely wide applications and considerable market prospects in frontier fields including personal health monitoring, chronic disease diagnosis and management, and home medical care. In this review, we systematically summarized the sweat biomarkers, introduced the sweat extraction and collection methods, and discussed the application and development of epidermal wearable biosensors for monitoring biomarkers in sweat in preclinical research in recent years. In addition, the current challenges and development prospects in this field were discussed.

Cortisol Immunosensors: A Literature Review

Cortisol is a steroid hormone that is involved in a broad range of physiological processes in human/animal organisms. Cortisol levels in biological samples are a valuable biomarker, e.g., of stress and stress-related diseases; thus, cortisol determination in biological fluids, such as serum, saliva and urine, is of great clinical value. Although cortisol analysis can be performed with chromatography-based analytical techniques, such as liquid chromatography-tandem mass spectrometry (LC-MS/MS), conventional immunoassays (radioimmunoassays (RIAs), enzyme-linked immunosorbent assays (ELISAs), etc.) are considered the "gold standard" analytical methodology for cortisol, due to their high sensitivity along with a series of practical advantages, such as low-cost instrumentation, an assay protocol that is fast and easy to perform, and high sample throughput. Especially in recent decades, research efforts have focused on the replacement of conventional immunoassays by cortisol immunosensors, which may offer further improvements in the field, such as real-time analysis at the point of care (e.g., continuous cortisol monitoring in sweat through wearable electrochemical sensors). In this review, most of the reported cortisol immunosensors, mainly electrochemical and also optical ones, are presented, focusing on their immunosensing/detection principles. Future prospects are also briefly discussed.

Plasmonic Bridge Sensor Enabled by Carbon Nanotubes and Au-Ag Nano-Rambutan for Multifunctional Detection of Biomechanics and Bio/Chemical Molecules

Wearable, noninvasive, and simultaneous sensing of subtle strains and eccrine molecules on human body is essential for future health monitoring and personalized medicine. However, there is a huge chasm between biomechanics and bio/chemical molecule detections. Here, a wearable plasmonic bridge sensor with multiple abilities to monitor subtle strains and molecules is developed. Hollow Au-Ag nano-rambutans and carbon nanotubes (CNTs) are adsorbed in the nonwoven fabrics (NWFs) conjointly, where the gap between the conducting network of CNTs is bridged by the Au-Ag nano-rambutans during the subtle strain sensing, and the detection sensitivity for stress is improved at least 1 order of magnitude compared to that with the only CNTs. In order to acquire the accurate human action recognition, a machine learning algorithm (support vector machines) based on output biomechanics data is designed. The average accuracy of our plasmonic bridge sensor reaches 89.0% for human action recognition. Moreover, due to the hollow structure and high nanoroughness, the single Au-Ag nano-rambutan particle has strong localized surface plasmon resonance effect and high surface-enhanced Raman scattering (SERS) activity. Based on their unique SERS spectra introduced by the hollow Au-Ag nano-rambutan adsorbed in the NWFs, noninvasive extraction and "fingerprint" recognition of bio/chemical molecules could be realized during the wearable sensing. In sum, the NWFs/CNTs/Au-Ag sensor bridges the barrier between the bodily strain detection and molecule recognition during the wearable sensing. Such integrated and multifunctional sensing strategy for universal biomechanics and bio/chemical molecules means to assess human health to be of importance.

Advances in wearable electrochemical antibody-based sensors for cortisol sensing

Cortisol is a crucial hormone involving many physiological processes. Hence, cortisol detection is essential. This review highlights the key progress made on wearable electrochemical sensors using antibodies. It covers the design, principle, and electroanalytical methodology for detecting cortisol noninvasively. This article also analyzes and collects the analytical performances of electrochemical cortisol sensors. The development of these sensors continues to face challenges such as biofouling, sample management, sensitivity, flexibility, stability, and recognition layer performance. It is also necessary to develop a sensitive electrode and material. This article also presents potential strategies for designing antibody electrodes and provides examples of sensing systems. Additionally, it discusses the challenges in translating research into practical applications.

Electrochemical Cell-Free Biosensors for Antibody Detection

We report here the development of an electrochemical cell-free biosensor for antibody detection directly in complex sample matrices with high sensitivity and specificity that is particularly suitable for point-of-care applications. The approach is based on the use of programmable antigen-conjugated gene circuits that, upon recognition of a specific target antibody, trigger the cell-free transcription of an RNA sequence that can be consequently detected using a redox-modified probe strand immobilized to a disposable electrode. The platform couples the features of high sensitivity and specificity of cell-free systems and the strength of cost-effectiveness and possible miniaturization provided by the electrochemical detection. We demonstrate the sensitive, specific, selective, and multiplexed detection of three different antibodies, including the clinically-relevant Anti-HA antibody.

Soft Fiber Electronics Based on Semiconducting Polymer

Fibers, originating from nature and mastered by human, have woven their way throughout the entire history of human civilization. Recent developments in semiconducting polymer materials have further endowed fibers and textiles with various electronic functions, which are attractive in applications such as information interfacing, personalized medicine, and clean energy. Owing to their ability to be easily integrated into daily life, soft fiber electronics based on semiconducting polymers have gained popularity recently for wearable and implantable applications. Herein, we present a review of the previous and current progress in semiconducting polymer-based fiber electronics, particularly focusing on smart-wearable and implantable areas. First, we provide a brief overview of semiconducting polymers from the viewpoint of materials based on the basic concepts and functionality requirements of different devices. Then we analyze the existing applications and associated devices such as information interfaces, healthcare and medicine, and energy conversion and storage. The working principle and performance of semiconducting polymer-based fiber devices are summarized. Furthermore, we focus on the fabrication techniques of fiber devices. Based on the continuous fabrication of one-dimensional fiber and yarn, we introduce two- and three-dimensional fabric fabricating methods. Finally, we review challenges and relevant perspectives and potential solutions to address the related problems.

Electrochemical-based remote biomarker monitoring: Toward Internet of Wearable Things in telemedicine

Internet of Wearable Things (IoWT) will be a major breakthrough for remote medical monitoring. In this scenario, wearable biomarker sensors have been developing not only to diagnose point-of-care (POC) of diseases, but also to continuously manage them. On-body tracking of biomarkers in biofluids is regarded as a proper substitution of conventional biomarker sensors for dynamic sampling and analyzing due to their high sensitivity, conformability, and affordability, creating ever-rising the market demand for them. In a wireless body area network (WBAN), data is captured from all sensors on the body to a smartphone/laptop, and sent the sensed data to a cloud for storing, processing, and retrieving, and ultimately displayed the data on custom applications (Apps). Wearable IoT biomarker sensors are used for early diseases diagnosis and continuous monitoring in developing countries in which people hardly access to healthcare systems. In this review, we aim to highlight a wide range of wearable electrochemical biomarker sensors, accompanied by microfluidics for continuous sampling, which will pave the way toward developing wearable IoT biomarker sensors to track health status. The current challenges and future perspective in skin-conformal biomarker sensors will be discussing their potential applicability for IoWT in cloud-based telemedicine.

A Comprehensive Study: Traditional and Cutting-Edge Analytical Techniques for the Biomarker Based Detection of the Micronutrients & POC Sensing Directions for Next-Generation Diagnostic

Micronutrient deficiency is wide spread and highly affects morbidity, mortality, and well-being of human beings. Micronutrient deficiency gradually manifests into diseases, which effects pathophysiology directly or indirectly. There is an imprecision in the diagnosis of micronutrient deficiency because of two causes; the selection of the standard biomarker and the diagnostic technique used. In appropriate diagnosis could increase the severity of the disorder. Instead of a single a combination of biomarkers can give more stringent results for micronutrient testing. Several traditional analytical techniques are used for diagnosis but HPLC, ELISA & LCMS/MS are most sensitive and reliable methods used by CLSIA-certified labs. However, these techniques require well-equipped, centralized laboratory facilities. The diagnostic era moves toward the Point of Care Testing (POCT), a boon in emerging diagnostics, breaking all paradigms of traditional analytical techniques. POCT led us toward the development of biosensors, which encompasses many techniques like paper-based sensors, microfluidic chip, wearable devices, and smartphone-assisted diagnostics, which become more popular diagnostic tools. This outlook summarizes the micronutrients like vitamins A, B5, B6, B7, B9, B12 C, D, and E and Minerals like iron, calcium, zinc, magnesium, and sodium; along with its biomarkers, analytical techniques, and point of care innovation in micronutrients.

Recent advances in conductive hydrogels: classifications, properties, and applications

Hydrogel-based conductive materials for smart wearable devices have attracted increasing attention due to their excellent flexibility, versatility, and outstanding biocompatibility. This review presents the recent advances in multifunctional conductive hydrogels for electronic devices. First, conductive hydrogels with different components are discussed, including pure single network hydrogels based on conductive polymers, single network hydrogels with additional conductive additives (i.e., nanoparticles, nanowires, and nanosheets), double network hydrogels based on conductive polymers, and double network hydrogels with additional conductive additives. Second, conductive hydrogels with a variety of functionalities, including self-healing, super toughness, self-growing, adhesive, anti-swelling, antibacterial, structural color, hydrophobic, anti-freezing, shape memory and external stimulus responsiveness are introduced in detail. Third, the applications of hydrogels in flexible devices are illustrated (i.e., strain sensors, supercapacitors, touch panels, triboelectric nanogenerator, bioelectronic devices, and robot). Next, the current challenges facing hydrogels are summarized. Finally, an imaginative but reasonable outlook is given, which aims to drive further development in the future.

Functional Fiber Materials to Smart Fiber Devices

The development of fiber materials has accompanied the evolution of human civilization for centuries. Recent advances in materials science and chemistry offered fibers new applications with various functions, including energy harvesting, energy storing, displaying, health monitoring and treating, and computing. The unique one-dimensional shape of fiber devices endows them advantages to work as human-interfaced electronics due to the small size, lightweight, flexibility, and feasibility for integration into large-scale textile systems. In this review, we first present a discussion of the basics of fiber materials and the design principles of fiber devices, followed by a comprehensive analysis on recently developed fiber devices. Finally, we provide the current challenges facing this field and give an outlook on future research directions. With novel fiber devices and new applications continuing to be discovered after two decades of research, we envision that new fiber devices could have an important impact on our life in the near future.

Flexible Textile-Based Sweat Sensors for Wearable Applications

The current physical health care system has gradually evolved into a form of virtual hospitals communicating with sensors, which can not only save time but can also diagnose a patient's physical condition in real time. Textile-based wearable sensors have recently been identified as detection platforms with high potential. They are developed for the real-time noninvasive detection of human physiological information to comprehensively analyze the health status of the human body. Sweat comprises various chemical compositions, which can be used as biomarkers to reflect the relevant information of the human physiology, thus providing references for health conditions. Combined together, textile-based sweat sensors are more flexible and comfortable than other conventional sensors, making them easily integrated into the wearable field. In this short review, the research progress of textile-based flexible sweat sensors was reviewed. Three mechanisms commonly used for textile-based sweat sensors were firstly contrasted with an introduction to their materials and preparation processes. The components of textile-based sweat sensors, which mainly consist of a sweat transportation channel and collector, a signal-selection unit, sensing elements and sensor integration and communication technologies, were reviewed. The applications of textile-based sweat sensors with different mechanisms were also presented. Finally, the existing problems and challenges of sweat sensors were summarized, which may contribute to promote their further development.