Engineering Materials for Electrochemical Sweat Sensing

Human sweat contains vast physiological information, which has been a promising resource for on-body and real-time health monitoring. Wearable sweat sensors have recently attracted an ever-increasing interest due to their promising capabilities for continuously tracking changes in health status. However, the commercialization of sweat sensors is seriously hindered by drawbacks of materials including high manufacturing and consumables costs, complex integration technology, as well as limited electrochemical signal transduction. In this review, sweat sensing principles are elaborately interpreted, and the latest advances in functional materials for biomarkers sensing in sweat are systematically summarized. Subsequently, the complex structure–activity relationships between various functional materials and sensing capabilities are further elucidated by coupling chemical structures, geometrics, electrochemical properties, and approaches for materials manufacturing. Furthermore, the integration of each component into sensing device for sweat detection and analysis is also discussed. Finally, challenges and opportunities for wearable sweat sensors are delineated in the development of future personalized and predictive healthcare.     

Wearable and Highly Sensitive Graphene Strain Sensors for Human Motion Monitoring

Sensing strain of soft materials in small scale has attracted increasing attention. In this work, graphene woven fabrics (GWFs) are explored for highly sensitive sensing. A flexible and wearable strain sensor is assembled by adhering the GWFs on polymer and medical tape composite film. The sensor exhibits the following features: ultra‐light, relatively good sensitivity, high reversibility, superior physical robustness, easy fabrication, ease to follow human skin deformation, and so on. Some weak human motions are chosen to test the notable resistance change, including hand clenching, phonation, expression change, blink, breath, and pulse. Because of the distinctive features of high sensitivity and reversible extensibility, the GWFs based piezoresistive sensors have wide potential applications in fields of the displays, robotics, fatigue detection, body monitoring, and so forth.     

On‐Body Bioelectronics: Wearable Biofuel Cells for Bioenergy Harvesting and Self‐Powered Biosensing

The growing power demands of wearable electronic devices have stimulated the development of on‐body energy‐harvesting strategies. This article reviews the recent progress on rapidly emerging wearable biofuel cells (BFCs), along with related challenges and prospects. Advanced on‐body BFCs in various wearable platforms, e.g., textiles, patches, temporary tattoo, or contact lenses, enable attractive advantages for bioenergy harnessing and self‐powered biosensing. These noninvasive BFCs open up unique opportunities for utilizing bioenergy or monitoring biomarkers present in biofluids, e.g., sweat, saliva, interstitial fluid, and tears, toward new biomedical, fitness, or defense applications. However, the realization of effective wearable BFC requires high‐quality enzyme‐electronic interface with efficient enzymatic and electrochemical processes and mechanical flexibility. Understanding the kinetics and mechanisms involved in the electron transfer process, as well as enzyme immobilization techniques, is essential for efficient and stable bioenergy harvesting under diverse mechanical strains and changing operational conditions expected in different biofluids and in a variety of outdoor activities. These key challenges of wearable BFCs are discussed along with potential solutions and future prospects. Understanding these obstacles and opportunities is crucial for transforming traditional bench‐top BFCs to effective and successful wearable BFCs.     

Flexible Piezoresistive Sensor Patch Enabling Ultralow Power Cuffless Blood Pressure Measurement

Noninvasive and real‐time cuffless blood pressure (BP) measurement realizes the idea of unobtrusive and continuous BP monitoring which is essential for diagnosis and prevention of cardiovascular diseases associated with hypertension. In this paper, a wearable sensor patch system that integrates flexible piezoresistive sensor (FPS) and epidermal electrocardiogram (ECG) sensors for cuffless BP measurement is presented. By developing parametric models on the FPS sensing mechanism and optimizing operational conditions, a highly stable epidermal pulse monitoring method is established and beat‐to‐beat BP measurement from the ECG and epidermal pulse signals is demonstrated. In particular, this study highlights the compromise between sensor sensitivity and signal stability. As compared with the current optical‐based cuffless BP measurement devices, the sensing patch requires much lower power consumption (3 nW) and is capable of detecting subtle physiological signal variations, e.g., pre and postexercises, thus providing a promising solution for low‐power, real‐time, and home‐based BP monitoring.     

An Integrated Wearable Sweat Sensing Patch for Passive Continuous Analysis of Stress Biomarkers at Rest

Real-time monitoring of mental stress biomarkers in sweat provides the possibility to evaluate mental status in a precise manner. In general, wearable sweat sensors suffer from inconvenient sweat collection, low levels of diagnostic biomarkers in sweat, sophisticated signal processing, and challenges with data visualization. To overcome these challenges, herein an integrated wearable sweat-sensing patch for continuous analysis of stress biomarkers (cortisol, Mg²⁺, and pH) at rest is demonstrated. The sweat sensing patch comprised a microfluidic chip, a highly sensitive sensing platform, an on-site signal processing circuitry (SPCs), and a smartphone installed with a home-developed display software. The sweat collection at rest is realized using a microfluidic chip without perspiration assistance. A ternary composite electrode is designed to obtain good conductivity, high surface area, and massive reactive sites, thereby yielding excellent electrochemical performances and high sensitivity to trace stress biomarkers. The on-site SPC has the function of signal transduction, conditioning, processing, and wireless transmission. The detection results can be displayed on a smartphone through the software. This work represents a significant scientific and technological advancement toward indexing mental stress status and can be used as an innovative tool for psychological diagnosis.     

Wireless,Flexible, Ionic,Perspiration-Rate Sensor System with Long-Time and High Sweat Volume Functions Toward Early-Stage,Real-Time Detection of Dehydration

Flexible sensors that can be attached to the body to collect vital data wirelessly enable real-time, early-stage diagnosis for human health management. Wearable sweat sensors have received considerable attention for real-time physiological monitoring. Unlike conventional methods that require blood-drawing in a clinic, sweat analyses may enable noninvasive tracking of health conditions for early-stage diagnosis. Even though a variety of studies to monitor metabolites and other substances have been conducted, automatic, continuous, long-term, simultaneous monitoring of perspiration rate and electrolytes, which are important parameters in dehydration, has yet to be achieved because of challenges related to sensor design. Here a wireless, wearable, integrated, microfluidic sensor system that can continuously measure these parameters in real-time for prolonged periods are presented. The proposed sensors are systematically characterized, and machine learning is used to predict device tilt angle to calibrate sensor output signals. Using the sensor design to form a water droplet in a fluidic channel, high-volume perspiration rate is continuously monitored for more than 7000 s (total sweat volume >170 µL). By testing 10 subjects, physiological responses to ingestion of a sports drink are confirmed by measuring perspiration rhythm changes extracted from real-time, continuous sweat impedance and rate.     

Stretchable,Skin‐Mountable,and Wearable Strain Sensors and Their Potential Applications: A Review

There is a growing demand for flexible and soft electronic devices. In particular, stretchable, skin‐mountable, and wearable strain sensors are needed for several potential applications including personalized health‐monitoring, human motion detection, human‐machine interfaces, soft robotics, and so forth. This Feature Article presents recent advancements in the development of flexible and stretchable strain sensors. The article shows that highly stretchable strain sensors are successfully being developed by new mechanisms such as disconnection between overlapped nanomaterials, crack propagation in thin films, and tunneling effect, different from traditional strain sensing mechanisms. Strain sensing performances of recently reported strain sensors are comprehensively studied and discussed, showing that appropriate choice of composite structures as well as suitable interaction between functional nanomaterials and polymers are essential for the high performance strain sensing. Next, simulation results of piezoresistivity of stretchable strain sensors by computational models are reported. Finally, potential applications of flexible strain sensors are described. This survey reveals that flexible, skin‐mountable, and wearable strain sensors have potential in diverse applications while several grand challenges have to be still overcome.     

MXene Functionalized,Highly Breathable and Sensitive Pressure Sensors with Multi-Layered Porous Structure

Breathable, flexible, and highly sensitive pressure sensors have drawn increasing attention due to their potential in wearable electronics for body-motion monitoring, human-machine interfaces, etc. However, current pressure sensors are usually assembled with polymer substrates or encapsulation layers, thus causing discomfort during wearing (i.e., low air/vapor permeability, mechanical mismatch) and restricting their applications. A breathable and flexible pressure sensor is reported with nonwoven fabrics as both the electrode (printed with MXene interdigitated electrode) and sensing (coated with MXene/silver nanowires) layers via a scalable screen-printing approach. Benefiting from the multi-layered porous structure, the sensor demonstrates good air permeability with high sensitivity (770.86–1434.89 kPa⁻¹), a wide sensing range (0–100 kPa), fast response/recovery time (70/81 ms), and low detection limit (≈1 Pa). Particularly, this sensor can detect full-scale human motion (i.e., small-scale pulse beating and large-scale walking/running) with high sensitivity, excellent cycling stability, and puncture resistance. Additionally, the sensing layer of the pressure sensor also displays superior sensitivity to humidity changes, which is verified by successfully monitoring human breathing and spoken words while wearing a sensor-embedded mask. Given the outstanding features, this breathable sensor shows promise in the wearable electronic field for body health monitoring, sports activity detection, and disease diagnosis.     

Fluid-Dynamics-Processed Highly Stretchable,Conductive, and Printable Graphene Inks for Real-Time Monitoring Sweat during Stretching Exercise

With the development of wearable electronics, the use of engineered functional inks with printing technologies has attracted attention owing to its potential for applications in low-cost, high-throughput, and high-performance devices. However, the improvement in conductivity and stretchability in the mass production of inks is still a challenge for practical use in wearable applications. Herein, a scalable and efficient fluid dynamics process that produces highly stretchable, conductive, and printable inks containing a high concentration of graphene is reported. The resulting inks, in which the uniform incorporation of exfoliated graphene flakes into a viscoelastic thermoplastic polyurethane is employed, facilitated the screen-printing process, resulting in high conductivity and excellent electromechanical stability. The electrochemical analysis of a stretchable sodium ion sensor based on a serpentine-structured pattern results in excellent electrochemical sensing performance even under strong fatigue tests performed by repeated stretching (300% strain) and release cycles. To demonstrate the practical use of the proposed stretchable conductor, on-body tests are carried out in real-time to monitor the sweat produced by a volunteer during simultaneous physical stretching and stationary cycling. These functional graphene inks have attractive performance and offer exciting potential for a wide range of flexible and wearable electronic applications.     

A Soft Resistive Acoustic Sensor Based on Suspended Standing Nanowire Membranes with Point Crack Design

An artificial basilar membrane (ABM) is an acoustic transducer that mimics the mechanical frequency selectivity of the real basilar membrane, which has the potential to revolutionize current cochlear implant technology. While such ABMs can be potentially realized using piezoelectric, triboelectric, and capacitive transduction methods, it remains notoriously difficult to achieve resistive ABM due to the poor frequency discrimination of resistive‐type materials. Here, a point crack technology on noncracking vertically aligned gold nanowire (V‐AuNW) films is reported, which allows for designing soft acoustic sensors with electric signals in good agreement with vibrometer output—a capability not achieved with corresponding bulk cracking system. The strategy can lead to soft microphones for music recognition comparable to the conventional microphone. Moreover, a soft resistive ABM is demonstrated by integrating eight nanowire‐based sensor strips on a soft trapezoid frame. The wearable ABM exhibits high‐frequency selectivity in the range of 319–1951 Hz and high sensitivity of 0.48–4.26 Pa^?1. The simple yet efficient fabrication in conjunction with programmable crack design indicates the promise of the methodology for a wide range of applications in future wearable voice recognition devices, cochlea implants, and human–machine interfaces.     

Large‐Area Ultrathin Graphene Films by Single‐Step Marangoni Self‐Assembly for Highly Sensitive Strain Sensing Application

Promoted by the demand for wearable devices, graphene has been proved to be a promising material for potential applications in flexible and highly sensitive strain sensors. However, low sensitivity and complex processing of graphene retard the development toward the practical applications. Here, an environment‐friendly and cost‐effective method to fabricate large‐area ultrathin graphene films is proposed for highly sensitive flexible strain sensor. The assembled graphene films are derived rapidly at the liquid/air interface by Marangoni effect and the area can be scaled up. These graphene‐based strain sensors exhibit extremely high sensitivity with gauge factor of 1037 at 2% strain, which represents the highest value for graphene platelets at this small deformation so far. This simple fabrication for strain sensors with highly sensitive performance of strain sensor makes it a novel approach to applications in electronic skin, wearable sensors, and health monitoring platforms.     

A Robust Wearable Point-of-Care CNT-Based Strain Sensor for Wirelessly Monitoring Throat-Related Illnesses

Point-of-care testing (POC) has the ability to detect chronic and infectious diseases early or at the time of occurrence and provide a state-of-the-art personalized healthcare system. Recently, wearable and flexible sensors have been employed to analyze sweat, glucose, blood, and human skin conditions. However, a flexible sensing system that allows for the real-time monitoring of throat-related illnesses, such as salivary parotid gland swelling caused by flu and mumps, is necessary. Here, for the first time, a wearable, highly flexible, and stretchable piezoresistive sensing patch based on carbon nanotubes (CNTs) is reported, which can record muscle expansion or relaxation in real-time, and thus act as a next-generation POC sensor. The patch offers an excellent gauge factor for in-plane stretching and spatial expansion with low hysteresis. The actual extent of muscle expansion is calculated and the gauge factor for applications entailing volumetric deformations is redefined. Additionally, a bluetooth-low-energy system that tracks muscle activity in real-time and transmits the output signals wirelessly to a smartphone app is utilized. Numerical calculations verify that the low stress and strain lead to excellent mechanical reliability and repeatability. Finally, a dummy muscle is inflated using a pneumatic-based actuator to demonstrate the application of the affixed wearable next-generation POC sensor.     

Development of Film Sensors Based on Conjugated Polymers for Copper (II) Ion Detection

A fluorescent film sensor was prepared by chemical modification of a polyfluorene derivative on a glass‐plate surface. X‐ray photoelectron spectroscopy and ellipsometry measurements demonstrate the covalent attachment of the polyfluorene derivative to the glass‐plate surface. The sensor was used to detect Cu²⁺ ions in aqueous solution by a mechanism exploiting fluorescence quenching of conjugated polymers. Among the tested metal ions, the film sensor presents good selectivity towards Cu²⁺ ions. Further experiments show that the sensing process is reversible. Moreover, sensory microarrays based on conjugated polymers targeting Cu²⁺ ions are constructed, which display similar sensing performance to that of the film sensor. The structural motif in which conjugated polymers are covalently confined to a solid substrate surface offers several attractive advantages for sensing applications. First, in comparison with film sensors in which small fluorescent molecules are employed as sensing elements, the sensitivity of our new film sensor is enhanced due to the signal‐amplifying effect of the conjugated polymers. Second, the film sensors or microarrays can be used in aqueous environments, which is crucial for their potential use in a wide range of real‐world systems. Since the sensing process is reversible, the sensing materials can be reused. Third, unlike physically coated polymer chains, the covalent attachment of the grafted chains onto a material surface precludes desorption and imparts long‐term stability of the polymer chains.     

Highly Aligned,Anisotropic Carbon Nanofiber Films for Multidirectional Strain Sensors with Exceptional Selectivity

Realization of sensing multidirectional strains is essential to understanding the nature of complex motions. Traditional uniaxial strain sensors lack the capability to detect motions working in different directions, limiting their applications in unconventional sensing technology areas, like sophisticated human–machine interface and real‐time monitoring of dynamic body movements. Herein, a stretchable multidirectional strain sensor is developed using highly aligned, anisotropic carbon nanofiber (ACNF) films via a facile, low‐cost, and scalable electrospinning approach. The fabricated strain sensor exhibits semitransparency, good stretchability of over 30%, outstanding durability for over 2500 cycles, and remarkable anisotropic strain sensing performance with maximum gauge factors of 180 and 0.3 for loads applied parallel and perpendicular to fiber alignment, respectively. Cross‐plied ACNF strain sensors are fabricated by orthogonally stacking two single‐layer ACNFs, which present a unique capability to distinguish the directions and magnitudes of strains with a remarkable selectivity of 3.84, highest among all stretchable multidirectional strain sensors reported so far. Their unconventional applications are demonstrated by detecting multi‐degrees‐of‐freedom synovial joint movements of the human body and monitoring wrist movements for systematic improvement of golf performance. The potential applications of novel multidirectional sensors reported here may shed new light into future development of next‐generation soft, flexible electronics.     

Biocompatible Soft Fluidic Strain and Force Sensors for Wearable Devices

Fluidic soft sensors have been widely used in wearable devices for human motion capturing. However, thus far, the biocompatibility of the conductive liquid, the linearity of the sensing signal, and the hysteresis between the loading and release processes have limited the sensing quality as well as the applications of these sensors. In this paper, silicone based strain and force sensors composed of a novel biocompatible conductive liquid (potassium iodide and glycerol solution) are introduced. The strain sensors exhibit negligible hysteresis up to 5 Hz, with a gauge factor of 2.2 at 1 Hz. The force sensors feature a novel multifunctional layered structure, with microcylinder‐filled channels to achieve high linearity, low hysteresis (5.3% hysteresis at 1 Hz), and good sensitivity (100% resistance increase at a 5 N load). The sensors' gauge factors are stable at various temperatures and humidity levels. These biocompatible, low hysteresis, and high linearity sensors are promising for safe and reliable diagnostic devices, wearable motion capture, and compliant human–computer interfaces.     

Fully Flexible Electromagnetic Vibration Sensors with Annular Field Confinement Origami Magnetic Membranes

Sensing of mechanical motion based on flexible electromagnetic sensors is challenging due to the complexity of obtaining flexible magnetic membranes with confined and enhanced magnetic fields. A fully flexible electromechanical system (MEMS) sensor is developed to conduct wearable monitoring of mechanical displacement with excellent adaptability to complex surface morphology through a suspended flexible magnet enclosed within a novel setup formed by a multi‐layer flexible coil and annular origami magnetic membranes. The annular membranes not only regulate the overall distribution of the magnetic field and enhance the overall magnetism by 291%, but also greatly increase the range of the magnetic field to cover the entire region of the coil. The sensor offers a broad frequency response ranging from 1 Hz to 10 kHz and a sensitivity of 0.59 mV µm^?1 at 1.7 kHz. The fully flexible format of the sensor enables various applications demonstrated by biophysical sensing, motion detection, voice recognition, and machine diagnostics through direct attachment on soft and curvilinear surfaces. Similar sensors can combine multiple sensing and energy harvesting modalities to achieve battery‐less and self‐sustainable operation, and can be deployed in large numbers to conduct distributed sensing for machine status assessment, health monitoring, rehabilitation, and speech aid.     

Microchannel‐Confined MXene Based Flexible Piezoresistive Multifunctional Micro‐Force Sensor

Multifunctional micro‐force sensing in one device is an urgent need for the higher integration of the smaller flexible electronic device toward wearable health‐monitoring equipment, intelligent robotics, and efficient human–machine interface. Herein, a novel microchannel‐confined MXene‐based flexible piezoresistive sensor is demonstrated to simultaneously achieve multi‐types micro‐force sensing of pressure, sound, and acceleration. Benefiting from the synergistically confined effect of the fingerprint‐microstructured channel and the accordion‐microstructured MXene materials, the as‐designed sensor remarkably endows a low detection limit of 9 Pa, a high sensitivity of 99.5 kPa^?1, and a fast response time of 4 ms, as well as non‐attenuating durability over 10 000 cycles. Moreover, the fabricated sensor is multifunctionally capable of sensing sounds, micromotion, and acceleration in one device. Evidently, such a multifunctional sensing characteristic can highlight the bright prospect of the microchannel‐confined MXene‐based micro‐force sensor for the higher integration of flexible electronics.     

Multifunctional and Wearable Patches Based on Flexible Piezoelectric Acoustics for Integrated Sensing,Localization, and Underwater Communication

Flexible and wearable sensors are highly desired for health monitoring, agriculture, sport, and indoor positioning systems applications. However, the currently developed wireless wearable sensors, which are communicated through radio signals, can only provide limited positioning accuracy and are often ineffective in underwater conditions. In this paper, a wireless platform based on flexible piezoelectric acoustics is developed with multiple functions of sensing, communication, and positioning. Under a high frequency (≈13 MHz) stimulation, Lamb waves are generated for respiratory monitoring. Whereas under low-frequency stimulation (≈20 kHz), this device is agitated as a vibrating membrane, which can be implemented for communication and positioning applications. Indoor communication is demonstrated within 2.8 m at 200 bps or 4.2 m at 25 bps. In combination with the sensing function, real-time respiratory monitoring and wireless communication are achieved simultaneously. The distance measurement is achieved based on the phase differences of transmitted and received acoustic signals within a range of 100 cm, with a maximum error of 3 cm. This study offers new insights into the communication and positioning applications using flexible acoustic wave devices, which are promising for wireless and wearable sensor networks.     

Enhanced Sensitivity of Capacitive Pressure and Strain Sensor Based on CaCu3Ti4O12 Wrapped Hybrid Sponge for Wearable Applications

Pressure sensors with highly sensitive and flexible characteristics have extensive applications in wearable electronics, soft robotics, human–machine interface, and more. Herein, an effective strategy is explored to enhance the sensitivity of the capacitive pressure sensor by fabricating a dielectric hybrid sponge consisting of calcium copper titanate (CaCu₃Ti₄O₁₂, CCTO), a giant dielectric permittivity material, in polyurethane (PU). An ultrasoft CCTO@PU hybrid sponge is fabricated via dip‐coating the PU sponge into surface‐modified CCTO nanoparticles using 3‐aminopropyl triethoxysilane. The overall results show that the –NH₂ functionalized CCTO attributes proper adhesion of CCTO with the –OCN group of the PU to enhance interfacial polarization leading to a high dielectric permittivity (167.05) and low loss tangent (0.71) beneficial for flexible pressure sensing applications. Moreover, the as‐prepared CCTO@PU hybrid sponge at 30 wt% CCTO concentration exhibits excellent electromechanical properties with an ultralow compression modulus of 27.83 kPa and a high sensitivity of 0.73 kPa^?1 in a low‐pressure regime (<1.6 kPa). Finally, pressure and strain sensing performance is demonstrated for the detection of human activities by mounting the sensor on various parts of the human body. The work reveals a new opportunity for the facile fabrication of high performance CCTO‐based capacitive sensors with multifunctional properties.     

A Flexible and Regenerative Aptameric Graphene–Nafion Biosensor for Cytokine Storm Biomarker Monitoring in Undiluted Biofluids toward Wearable Applications

Wearable sensors that can conveniently detect cytokine levels in human biofluids are essential for assisting hospitals to maximize the benefits of anti-inflammatory therapies and avoid cytokine storms. Measurement of cytokine levels in biofluids still remains challenging for existing sensors due to high interference from the background. Here, this challenge is overcome through developing a flexible and regenerative aptameric field-effect transistor biosensor, consisting of a graphene–Nafion composite film, for detecting cytokine storm biomarkers in undiluted human biofluids. The composite film enables the minimization of nonspecific adsorption and empowers the renewability to the biosensor. With these capabilities, the device is capable of consistently and sensitively monitoring cytokines (e.g., IFN-γ, an inflammatory and cancer biomarker) in undiluted human sweat with a detection range from 0.015 to 250 nm and limit of detection down to 740 fm . The biosensor is also shown to incur no visible mechanical damage and maintain a consistent sensing response throughout the regenerative (up to 80 cycles) and crumpling (up to 100 cycles) tests. Experimental results demonstrate that the biosensor is expected to offer opportunities for developing wearable biosensing systems for distinguishing acute infectious disease patients and monitoring of patients’ health conditions in daily life.