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.     

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.     

A Self-Powered,Rechargeable, and Wearable Hydrogel Patch for Wireless Gas Detection with Extraordinary Performance

Flexible gas sensors play an indispensable role in diverse applications spanning from environmental monitoring to portable medical electronics. Full wearable gas monitoring system requires the collaborative support of high-performance sensors and miniaturized circuit module, whereas the realization of low power consumption and sustainable measurement is challenging. Here, a self-powered and reusable all-in-one NO₂ sensor is proposed by structurally and functionally coupling the sensor to the battery, with ultrahigh sensitivity (1.92%/ppb), linearity (R² = 0.999), ultralow theoretical detection limit (0.1 ppb), and humidity immunity. This can be attributed to the regulation of the gas reaction route at the molecular level. The addition of amphiphilic zinc trifluoromethanesulfonate (Zn(OTf)₂) enables the H₂O-poor inner Helmholtz layer to be constructed at the electrode–gel interface, thereby facilitating the direct charge transfer process of NO₂ here. The device is then combined with a well-designed miniaturized low-power circuit module with signal conditioning, processing and wireless transmission functions, which can be used as wearable electronics to realize early and remote warning of gas leakage. This study demonstrates a promising way to design a self-powered, sustainable, and flexible gas sensor with high performance and its corresponding wireless sensing system, providing new insight into the all-in-one system of gas detection.     

A low power wireless node for contact and contactless heart monitoring

Ubiquitous vital signs sensing and processing are promising alternatives to conventional clinical and ambulatory healthcare. Novel sensors, low power solutions for processing and wireless connectivity are creating new opportunities for wearable devices which allow continuous and long term monitoring, while maintaining freedom of movement for the users. This paper presents a low-power embedded platform with novel high sensitivity electric potential dry surface sensors that can be used in either contact or non-contact mode to measure biomedical signals. The proposed low power system is optimized to compute the heart rate and respiratory rate close to the sensors. This approach reduces the amount of data that needs to be transmitted to a host device. It allows also the platform to be autonomous and wearable or even be used in cars for applications such as driver drowsiness detection. Experimental measurements show the acquisition and the processing of data from sensors and the low power consumption achieved with the node in different modes of operation.     

Nanoparticle-Based Strain Gauges: Anisotropic Response Characteristics,Multidirectional Strain Sensing,and Novel Approaches to Healthcare Applications

Directional strain sensing is essential for advanced sensor applications in the field of human-machine interfaces and healthcare. Here, the angle dependent anisotropic strain sensitivity caused by charge carriers percolating through cross-linked gold nanoparticle (GNP) networks is studied and these versatile materials are used for the fabrication of wearable triaxial pulse and gesture sensors. More specifically, the anisotropic response of 1,9-nonanedithiol cross-linked GNP films is separated into geometric and piezoresistive contributions by fitting the measured data with an analytic model. Hereby, piezoresistive coefficients of g₁₁ ∼ 32 and g₁₂ ∼ 21 are extracted, indicating a slightly anisotropic response behavior of the GNP-based material. To use the material for healthcare applications, arrangements of three GNP transducers are patterned lithographically and fully embedded into a highly flexible silicone polymer (Dragon Skin 30). The new encapsulation method ensures good and robust electrical contacts and enables facile handling and protection from external influences. A facile read-out with wireless data transmission using off-the-shelf electrical components underlines the great potential of these devices for applications as skin-wearable healthcare sensors.     

Ultra-Robust and Sensitive Flexible Strain Sensor for Real-Time and Wearable Sign Language Translation

Flexible strain sensors with high sensitivity and high mechanical robustness are highly desirable for their accurate and long-term reliable service in wearable human-machine interfaces. However, the current application of flexible strain sensors has to face a trade-off between high sensitivity and high mechanical robustness. The most representative examples are micro/nano crack-based sensors and serpentine meander-based sensors. The former one typically shows high sensitivity but limited robustness, while the latter is on the contrary. Herein, ultra-robust and sensitive flexible strain sensors are developed by crack-like pathway customization and ingenious modulation of low/high-resistance regions on a serpentine meander structure. The sensors show high cyclic stability (10 000 cycles), strong tolerance to harsh environments, high gauge factor (>1000) comparable with that of the crack-based sensor, and fast response time (<58 ms). Finally, the sensors are integrated into a wearable sign language translation system, which is wireless, low-cost, and lightweight. Recognition rates of over 98% are demonstrated for the translation of 21 sign languages with the assistance of machine learning. This system facilitates achieving barrier-free communication between signers and nonsigners and offers broad application prospects in gesture interaction.     

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.     

Muscle-Inspired MXene Conductive Hydrogels with Anisotropy and Low-Temperature Tolerance for Wearable Flexible Sensors and Arrays

Conductive hydrogels as flexible electronic devices, not only have unique attractions but also meet the basic need of mechanical flexibility and intelligent sensing. How to endow anisotropy and a wide application temperature range for traditional homogeneous conductive hydrogels and flexible sensors is still a challenge. Herein, a directional freezing method is used to prepare anisotropic MXene conductive hydrogels that are inspired by ordered structures of muscles. Due to the anisotropy of MXene conductive hydrogels, the mechanical properties and electrical conductivity are enhanced in specific directions. The hydrogels have a wide temperature resistance range of −36 to 25 °C through solvent substitution. Thus, the muscle-inspired MXene conductive hydrogels with anisotropy and low-temperature resistance can be used as wearable flexible sensors. The sensing signals are further displayed on the mobile phone as images through wireless technology, and images will change with the collected signals to achieve motion detection. Multiple flexible sensors are also assembled into a 3D sensor array for detecting the magnitude and spatial distribution of forces or strains. The MXene conductive hydrogels with ordered orientation and anisotropy are promising for flexible sensors, which have broad application prospects in human–machine interface compatibility and medical monitoring.     

Low-Intensity Sensitive and High Stability Flexible Heart Sound Sensor Enabled by Hybrid Near-Field/Far-Field Electrospinning

Low-cost and wearable heart sound sensors can facilitate early detection and monitoring of cardiovascular and respiratory diseases. Such sensors are currently limited by either the complexity of fabrication processes or low sensitivity and reliability for weak signal detection. Here, a hybrid near-field/far-field electrospinning approach is demonstrated that enables low-cost fabrication and optimization of triboelectric heterostructures for heart sound sensing. Specifically, by combining the far-field produced highly polarized and porous polyvinylidene difluoride network for triboelectric electrification and near-field patterned polyurethane grid spacers for vibration enhancement and charge trapping, the greatly improved sensor output at the heart sound frequency (50–150 Hz) and intensity (<80 dB) range, demonstrating record high sensitivity of 7027 mV Pa⁻¹ and low detection limit of 47 dB. The sensor exhibits excellent stability under both sudden physical disturbance and long-term cycling stress, showing no degradation during 7 h of continuous operation. It is demonstrated that the sensor can be integrated with apparel and capture high-quality heart sound signals at all five diagnostic auscultation points and distinguish characteristic heart sound patterns caused by different cardiovascular diseases.     

Light‐Boosting Highly Sensitive Pressure Sensors Based on Bioinspired Multiscale Surface Structures

Pressure sensors have attracted tremendous attention because of their potential applications in the fields of health monitoring, human–machine interfaces, artificial intelligence, and so on. Improving pressure‐sensing performances, especially the sensitivity and the detection limit, is of great importance to expand the related applications, however it is still an enormous challenge so far. Herein, highly sensitive piezoresistive pressure sensors are reported with novel light‐boosting sensing performances. Rose petal–templated positive multiscale millimeter/micro/nanostructures combined with surface wrinkling nanopatterns endow the assembled pressure sensors with outstanding pressure sensing performance, e.g. an ultrahigh sensitivity (70 KPa^?1, <0.5 KPa), an ultralow detection limit (0.88 Pa), a wide pressure detect ion range (from 0.88 Pa to 32 KPa), and a fast response time (30 ms). Remarkably, simple light illumination further enhances the sensitivity to 120 KPa^?1 (<0.5 KPa) and lowers the detection limit to 0.41 Pa. Furthermore, the flexible light illumination offers unprecedented capabilities to spatiotemporally control any target in multiplexed pressure sensors for optically enhanced/tailorable sensing performances. This light‐control strategy coupled with the introduction of bioinspired multiscale structures is expected to help design next generation advanced wearable electronic devices for unprecedented smart applications.     

Wearable Sensors-Enabled Human–Machine Interaction Systems: From Design to Application

In comparison to traditional bulky and rigid electronic devices, the human–machine interaction (HMI) system with flexible and wearable components is an inevitable future trend. To achieve effective, intuitive, and seamless manipulation of high-performance wearable HMI systems, it is important to develop effective strategies for designing material microstructures on flexible sensors and electric devices with excellent mechanical flexibility and stretchability. The real-time acquisition of human physiology and surrounding signals through accurate and flexible sensors is the basis of wearable HMIs. Herein, the construction of a wearable HMI system that utilizes sensors, communication modes, and actuators is reviewed. The mechanisms and strategies for designing various flexible sensors based on different mechanisms are analyzed and discussed. The functional mechanism, material selection, and novel design strategies of each part are summarized in detail. The different communication modes in interactive systems and the manufacturing technology of soft machines are also introduced. Additionally, the most advanced applications of wearable HMI systems in intelligent identification and security, interactive controls for robots, augmented reality, and virtual reality have been highlighted. The review concludes with an overview of the remaining key challenges and several ideas regarding the further improvement of wearable HMI systems.     

Flexible Battery-Free Wireless Sensor Array Based on Functional Gradient-Structured Wood for Pressure and Temperature Monitoring

Approximately 4.5% to 7.0% of hospitalized patients suffer from pressure ulcers. Mitigating risks for pressure ulcers through sensors remains a challenge and a high requirement. A simple, low-cost, battery-free, multi-parameter passive wireless flexible sensor (MPWFS) for all-around pressure and temperature monitoring to prevent pressure ulcers is developed. The pressure sensing unit is fabricated with functional gradient-structured balsa wood and has high sensitivity of 0.34 kPa⁻¹ with a wide detection range of 0–20 kPa. The temperature sensing unit, which is 0.4 mm × 0.2 mm, is embedded in the surface of the pressure sensing unit, enabling temperature monitoring with a resolution of 0.1 °C. The flexible Radio Frequency energy-harvesting unit, data acquisition, and processing, as well as Bluetooth-Low-Energy wireless transmission, are integrated within a 20 mm × 20 mm unit. It acquires continuous temperature and pressure data without a battery at any position more than 1 m away from the power transmitter. Moreover, the combination of the sensor array design with a mobile terminal provides the MPWFS's various benefits, including tracking changes in the supine posture, warning about pressure ulcers, and monitoring falls out of bed. This study presents a new method for long-term bedridden patient care.     

Piezoionic SnSe Nanosheets-Double Network Hydrogel for Self-Powered Strain Sensing and Energy Harvesting

Flexible strain sensors have enormous potential in wearable devices, robots, and health monitoring equipment. However, the poor stretchability of strain sensors based on semiconductors and the low sensitivity of resistance change-based hydrogel strain sensors hinders the comprehensive application. Herein, a flexible piezoionic SnSe-hydrogel composite with an optimized structure and improved performance is designed. The piezoionic output rises nonlinearly as the applied force increases, with the piezoionic coefficient up to 1780 nV Pa⁻¹ and −7.21 nA Pa⁻¹. The composite can realize the continuous positioning in 1D space based on the piezoionic effect. It also demonstrates self-powered characteristics, an ultrafast response speed of 6–8 ms, and a high gauge factor of 95.89. The sensor is exemplified to monitor fist clenching and finger bending, which has the potential to discriminate different joint movements. Meanwhile, the device can light up a light–emitting diode under pressure and bending. The as-prepared piezoionic SnSe-hydrogel device, having both high stretchability and sensitivity, may shed light on developing high-performance flexible strain sensors and generators.     

Emerging Technologies of Flexible Pressure Sensors: Materials,Modeling, Devices,and Manufacturing

As an important branch of wearable electronics, flexible pressure sensors have attracted extensive research owing to their wide range of applications, such as human–machine interfaces and health monitoring. To fulfill the requirements for different applications, new material design and device fabrication strategies have been developed in order to manipulate the mechanical and electrical properties and enhance device performance. In this paper, the important progresses in flexible pressure sensor development over recent years are selectively reviewed from a material and application perspective. First, an overview of the fundamental working mechanism and the systematic design approach is presented. Particularly, how the theoretical modeling has been used as an auxiliary tool to achieve better sensing performance is discussed. A number of applications, including human–machine interfaces, electronic skin and health monitoring, and certain application‐driven functions, e.g., pressure distribution visualization and direction‐sensitive force detection, are highlighted. Lastly, various advanced manufacturing methods used for realizing large‐scale fabrication are introduced.     

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.     

A Mediator‐Free Electroenzymatic Sensing Methodology to Mitigate Ionic and Electroactive Interferents' Effects for Reliable Wearable Metabolite and Nutrient Monitoring

Wearable electroenzymatic sensors enable monitoring of clinically informative biomolecules in epidermally retrievable biofluids. Conventional wearable enzymatic sensors utilize Prussian Blue (a redox mediator) to achieve selectivity against electroactive interferents. However, the use of Prussian Blue presents fundamental challenges including: 1) the susceptibility of the sensor response to dynamic concentration variation of ionic species and 2) the poor operational stability due to the degradation of its framework. As an alternative wearable electroenzymatic sensor development methodology to bypass the aforementioned limitations, a mediator‐free sensing interface is devised, comprising of a coupled platinum nanoparticle/multiwall carbon nanotube layer and a permselective membrane. The interface is adapted to develop sensors targeting glucose, lactate, and choline (as examples of informative metabolites and nutrients), showing high degrees of sensitivity, selectivity (against a wide panel of naturally present and diverse interfering species), stability (<6.5% signal drift over 20 h operation), and reliability of sensing operation in sweat samples. By integration within a readout board, a wireless sample‐to‐answer system is realized for on‐body sweat biomarker analysis. This methodology can be adapted to target a wide panel of biomarkers in various biofluids, introducing a new sensor development direction for personal health monitoring.     

Flexible Transparent Reduced Graphene Oxide Sensor Coupled with Organic Dye Molecules for Rapid Dual‐Mode Ammonia Gas Detection

Flexible chemical sensors utilizing chemically sensitive nanomaterials are of great interest for wearable sensing applications. However, obtaining high performance flexible chemical sensors with high sensitivity, fast response, transparency, stability, and workability at ambient conditions is still challenging. Herein, a newly designed flexible and transparent chemical sensor of reduced graphene oxide (R‐GO) coupled with organic dye molecules (bromophenol blue) is introduced. This device has promising properties such as high mechanical flexibility (>5000 bending cycles with a bending radius of 0.95 cm) and optical transparency (>60% in the visible region). Furthermore, stacking the water‐trapping dye layer on R‐GO enables a higher response as well as workability in a large relative humidity range (up to 80%), and dual‐mode detection capabilities of colorimetric and electrical sensing for NH₃ gas (5–40 ppm). These advantageous attributes of the flexible and transparent R‐GO sensor coupled with organic dye molecules provide great potential for real‐time monitoring of toxic gas/vapor in future practical chemical sensing at room conditions in wearable electronics.     

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.     

Percolating Network of Ultrathin Gold Nanowires and Silver Nanowires toward “Invisible” Wearable Sensors for Detecting Emotional Expression and Apexcardiogram

2 nm thin gold nanowires (AuNWs) have extremely high aspect ratio (≈10 000) and are nanoscale soft building blocks; this is different from conventional silver nanowires (AgNWs), which are more rigid. Here, highly sensitive, stretchable, patchable, and transparent strain sensors are fabricated based on the hybrid films of soft/hard networks. They are mechanically stretchable, optically transparent, and electrically conductive and are fabricated using a simple and cost‐effective solution process. The combination of soft and more rigid nanowires enables their use as high‐performance strain sensors with the maximum gauge factor (GF) of ≈236 at low strain (<5%), the highest stretchability of up to 70% strain, and the optical transparency is from 58.7% to 66.7% depending on the amount of the AuNW component. The sensors can detect strain as low as 0.05% and are energy efficient to operate at a voltage as low as 0.1 V. These attributes are difficult to achieve with a single component of either AuNWs or AgNWs. The outstanding sensing performance indicates their potential applications as “invisible” wearable sensors for biometric information collection, as demonstrated in applications for detecting facial expressions, respiration, and apexcardiogram.     

The Era of Digital Health: A Review of Portable and Wearable Affinity Biosensors

Digital health facilitated by wearable/portable electronics and big data analytics holds great potential in empowering patients with real‐time diagnostics tools and information. The detection of a majority of biomarkers at trace levels in body fluids using mobile health (mHealth) devices requires bioaffinity sensors that rely on “bioreceptors” for specific recognition. Portable point‐of‐care testing (POCT) bioaffinity sensors have demonstrated their broad utility for diverse applications ranging from health monitoring to disease diagnosis and management. In addition, flexible and stretchable electronics‐enabled wearable platforms have emerged in the past decade as an interesting approach in the ambulatory collection of real‐time data. Herein, the technological advancements of mHealth bioaffinity sensors evolved from laboratory assays to portable POCT devices, and to wearable electronics, are synthesized. The involved recognition events in the mHealth affinity biosensors enabled by bioreceptors (e.g., antibodies, DNAs, aptamers, and molecularly imprinted polymers) are discussed along with their transduction mechanisms (e.g., electrochemical and optical) and system‐level integration technologies. Finally, an outlook of the field is provided and key technological bottlenecks to overcome identified, in order to achieve a new sensing paradigm in wearable bioaffinity platforms.