Stretchable and Temperature‐Sensitive Polymer Optical Fibers for Wearable Health Monitoring

Stretchable physical sensors that can detect and quantify human physiological signals such as temperature, are essential to the realization of healthcare devices for biomedical monitoring and human–machine interfaces. Despite recent achievements in stretchable electronic sensors using various conductive materials and structures, the design of stretchable sensors in optics remains a considerable challenge. Here, an optical strategy for the design of stretchable temperature sensors, which can maintain stable performance even under a strain deformation up to 80%, is reported. The optical temperature sensor is fabricated by the incorporation of thermal‐sensitive upconversion nanoparticles (UCNPs) in stretchable polymer‐based optical fibers (SPOFs). The SPOFs are made from stretchable elastomers and constructed in a step‐index core/cladding structure for effective light confinements. The UCNPs, incorporated in the SPOFs, provide thermal‐sensitive upconversion emissions at dual wavelengths for ratiometric temperature sensing by near‐infrared excitation, while the SPOFs endow the sensor with skin‐like mechanical compliance and excellent light‐guiding characteristics for laser delivery and emission collection. The broad applications of the proposed sensor in real‐time monitoring of the temperature and thermal activities of the human body, providing optical alternatives for wearable health monitoring, are demonstrated.     

Bio-Inspired Artificial Perceptual Devices for Neuromorphic Computing and Gesture Recognition

Artificial perception technologies capable of sensing and feeling mechanical stimuli like human skins are critical enablers for electronic skins (E-Skins) needed to achieve artificial intelligence. However, most of the reported electronic skin systems lack the capability to process and interpret the sensor data. Herein, a new design of artificial perceptual system integrating ZnO-based synaptic devices with Pt/carbon nanofibers-based strain sensors for stimuli detection and information processing is presented. Benefiting from the controllable ion migration after indium doping, the device can emulate various essential functions, such as short-term/long-term plasticity, paired-pulse facilitation, excitatory post-synaptic current, and synaptic plasticity depending on the number, frequency, amplitude, and width of the applied pulses. The Pt/carbon nanofibers-based strain sensors can detect subtle human motion and convert mechanical stimuli into electrical signals, which are further processed by the ZnO devices. By attaching the integrated devices to finger joints, it is demonstrated that they can recognize handwriting and gestures with a high accuracy. This work offers new insights in designing artificial synapses and sensors to process and recognize information for neuromorphic computing and artificial intelligence applications.     

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.     

Field‐Structured Chemiresistors

A significantly improved material is developed for application to chemiresistors, which are resistance‐based sensors for volatile organic compounds. This material is a polymer composite containing Au‐coated magnetic particles organized into electrically conducting pathways by magnetic fields. This improved material overcomes the various problems inherent to conventional carbon‐black chemiresistors, while achieving an unprecedented response magnitude. When exposed to chemical vapors, the polymer swells only slightly, yet this is amplified into large, reversible resistance changes, as much as (1 × 10¹¹)% at a swelling of only 1.5%. These conductor–insulator transitions occur over such a narrow range of analyte vapor concentration that these devices can be described as chemical switches. The sensitivity and response range of these sensors can be tailored over a wide range by controlling the stress within the composite, including through the application of a magnetic field. Such tailorable sensors can be used to create sensor arrays that can accurately determine analyte concentration over a broad concentration range, or can be used to create logic circuits that signal a particular chemical environment.     

Wireless Monitoring of Small Strains in Intelligent Robots via a Joule Heating Effect in Stretchable Graphene–Polymer Nanocomposites

Flexible strain sensors are an important component for future intelligent robotics. However, the majority of current strain sensors must be electrically connected to a corresponding monitoring system via conducting wires, which increases system complexity and restricts the working environment for monitoring strains. Here, stretchable graphene–polymer nanocomposites that act as strain sensors using a Joule heating effect are reported. When the resistance of the sensor changes in response to a strain, the resulting change in temperature is wirelessly detected in an intelligent robot. By engineering and optimizing the surface structure of graphene–polymer nanocomposites, the fabricated strain sensors exhibit excellent stability when subjected to periodic temperature signals over 400 cycles while being periodically strained and deliver a high strain sensitivity of 7.03 × 10^?4 °C^?1 %^?1 for strain levels of 0% to 30%. As a wearable electronic device, the approach provides the capability to wirelessly monitor small strains for intelligent robots at a high strain resolution of ≈0.1%. Moreover, when the strain sensing system operates as a multichannel structure, it allows precise strain detection simultaneously, or in sequence, for each finger of an intelligent robot.     

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.     

A Highly Stretchable ZnO@Fiber‐Based Multifunctional Nanosensor for Strain/Temperature/UV Detection

Stretchable and multifunctional sensors can be applied in multifunctional sensing devices, safety forewarning equipment, and multiparametric sensing platforms. However, a stretchable and multifunctional sensor was hard to fabricate until now. Herein, a scalable and efficient fabrication strategy is adopted to yield a sensor consisting of ZnO nanowires and polyurethane fibers. The device integrates high stretchability (tolerable strain up to 150%) with three different sensing capabilities, i.e., strain, temperature, and UV. Typically achieved specifications for strain detection are a fast response time of 38 ms, a gauge factor of 15.2, and a high stability of >10 000 cyclic loading tests. Temperature is detected with a high temperature sensitivity of 39.3% °C^?1, while UV monitoring features a large ON/OFF ratio of 158.2. With its fiber geometry, mechanical flexibility, and high stretchability, the sensor holds tremendous prospect for multiparametric sensing platforms, including wearable devices.     

Response Regulation for Epidermal Fabric Strain Sensors via Mechanical Strategy

Advances in fabric strain sensors have established a route to comfortable-to-wear flexible electronics with particularly remarkable permeability and low modulus due to their porous fabric microstructure. A key challenge that remains unsolved is to regulate the sensor response via on-demand design for a variety of application scenarios to sufficiently exploit the highest possible sensitivity. While recent reports have described a variety of options in varying the material and orientation of the overall fiber mat, the development of approaches where multiple sensors with different responses can be integrated on a single substrate without affecting macroscopic mechanical properties remains an area of continued interest. Herein, a simple mechanical strategy is reported, which plates the patterned functional material on the fabric mat at a pre-stretched state in the prescribed direction, and control of direction and prestrain forms either sensors with different responses or strain-insensitive interconnects. A systematic study has revealed the underlying mechanism of this strategy, which can serve as a guideline for the on-demand design and fabrication of fabric strain sensors. Demonstration applications in motion monitoring bandages and gesture recognition gloves illustrate capabilities in functional epidermal sensing devices.     

Stretchable Thermoplastic Elastomer Optical Fibers for Sensing of Extreme Deformations

The design of advanced materials with coupled optical and mechanical properties is an important challenge in materials science; especially, the implementation of soft materials in optics has recently gained significant interest. Soft optical systems are particularly versatile in sensing, where large and repeated deformations require dynamically responsive materials. Here, stretchable step‐index optical fibers, which are capable of reversibly sustaining strains of up to 300% while guiding light, are demonstrated. A continuous and scalable melt‐flow process is used to coextrude two thermoplastic elastomers, thereby forming the fibers' high index core‐low index cladding structure. Deformation of the fibers through stretching, bending, and indentation induces detectable, predictable, reversible, and wavelength‐dependent changes in light transmission. Quantitative knowledge about the coupling of the fibers' mechanical and optical properties forms the basis for the design of fiber‐based sensors that are capable of reliably assessing extreme mechanical stimuli. The fibers utility in sensing scenarios is demonstrated in a knee brace for continuous knee motion tracking, a glove for control of a virtual hand model, and a tennis racket capable of locating ball impacts. Such devices can greatly improve quantitative assessment of human motion in rehabilitation, sports, and anywhere else where large deformations need to be monitored reliably.     

Fatigue-Resistant Conducting Polymer Hydrogels as Strain Sensor for Underwater Robotics

Conducting polymer hydrogels are widely used as strain sensors in light of their distinct skin-like softness, strain sensitivity, and environmental adaptiveness in the fields of wearable devices, soft robots, and human-machine interface. However, the mechanical and electrical properties of existing conducting polymer hydrogels, especially fatigue-resistance and sensing robustness during long-term application, are unsatisfactory, which severely hamper their practical utilities. Herein, a strategy to fabricate conducting polymer hydrogels with anisotropic structures and mechanics is presented through a combined freeze-casting and salting-out process. The as-fabricated conducting polymer hydrogels exhibit high fatigue threshold (>300 J m⁻²), low Young's modulus (≈100 kPa), as well as long-term strain sensing robustness (over 10 000 cycles). Such superior performance enables their application as strain sensors to monitor the real-time movement of underwater robotics. The design and fabrication strategy for conducting polymer hydrogels reported in this study may open up an enticing avenue for functional soft materials in soft electronics and robotics.     

Conducting‐Polymer Nanomaterials for High‐Performance Sensor Applications: Issues and Challenges

Owing to their promising applications in electronic and optoelectronic devices, conducting polymers have been continuously studied during the past few decades. Nevertheless, only limited progress had been made in conducting‐polymer‐based sensors until nanostructured conducting polymers were demonstrated for high‐performance signal transducers. Significant advances in the synthesis of conducting‐polymer nanomaterials have been recently reported, with enhanced sensitivity relative to their bulk counterparts. Today, conducting‐polymer nanomaterials rival metal and inorganic semiconductor nanomaterials in sensing capability. However, there are still several technological challenges to be solved for practical sensor applications of conducting‐polymer nanomaterials. Here, the key issues on conducting‐polymer nanomaterials in the development of state‐of‐the‐art sensors are discussed. Furthermore, a perspective on next‐generation sensor technology from a materials point of view is also given.     

Programmable Macroporous Photonic Crystals Enabled by Swelling‐Induced All‐Room‐Temperature Shape Memory Effects

This study reports unconventional, all‐room‐temperature shape memory (SM) effects using templated macroporous shape memory polymer (SMP) photonic crystals comprising a glassy copolymer with high‐glass transition temperature. “Cold” programming of permanent periodic structures into temporary disordered configurations can be achieved by slowly evaporating various swelling solvents (e.g., ethanol) imbibed in the interconnecting macropores. The deformed macropores can be instantaneously recovered to the permanent geometry by exposing it to vapors and liquids of swelling solvents. By contrast, nonswelling solvents (e.g., hexane) cannot trigger “cold” programming and SM recovery. Extensive experimental and theoretical investigations reveal that the dynamics of swelling‐induced plasticizing effects caused by fast diffusion of solvent molecules into the walls of macropores with nanoscopic thickness dominate both “cold” programming and recovery processes. Importantly, the striking color changes associated with the reversible SM transitions enable novel chromogenic sensors for selectively detecting trace amounts of swelling analytes mixed in nonswelling solvents. Using ethanol–hexane solutions as proof‐of‐concept mixtures, the ethanol detection limit of 150 ppm has been demonstrated. Besides reusable sensors, which can find important applications in environmental monitoring and petroleum process/product control, the programmable SMP photonic crystals possessing high mechanical strengths and all‐room‐temperature processability can provide vast opportunities in developing reconfigurable/rewritable nanooptical devices.     

Large‐Area Compliant,Low‐Cost,and Versatile Pressure‐Sensing Platform Based on Microcrack‐Designed Carbon Black@Polyurethane Sponge for Human–Machine Interfacing

It is a challenge to manufacture pressure‐sensing materials that possess flexibility, high sensitivity, large‐area compliance, and capability to detect both tiny and large motions for the development of artificial intelligence products. Herein, a very simple and low‐cost approach is proposed to fabricate versatile pressure sensors based on microcrack‐designed carbon black (CB)@polyurethane (PU) sponges via natural polymer‐mediated water‐based layer‐by‐layer assembly. These sensors are capable of satisfying the requirements of ultrasmall as well as large motion monitoring. The versatility of these sensors benefits from two aspects: microcrack junction sensing mechanism for tiny motion detecting (91 Pa pressure, 0.2% strain) inspired by the spider sensory system and compressive contact of CB@PU conductive backbones for large motion monitoring (16.4 kPa pressure, 60% strain). Furthermore, these sensors exhibit excellent flexibility, fast response times (<20 ms), as well as good reproducibility over 50 000 cycles. This study also demonstrates the versatility of these sensors for various applications, ranging from speech recognition, health monitoring, bodily motion detection to artificial electronic skin. The desirable comprehensive performance of our sensors, which is comparable to the recently reported pressure‐sensing devices, together with their significant advantages of low‐cost, easy fabrication, especially versatility, makes them attractive in the future of artificial intelligence.     

Multiaxial and Transparent Strain Sensors Based on Synergetically Reinforced and Orthogonally Cracked Hetero‐Nanocrystal Solids

Wearable strain sensors are widely researched as core components in electronic skin. However, their limited capability of detecting only a single axial strain, and their low sensitivity, stability, opacity, and high production costs hinder their use in advanced applications. Herein, multiaxially highly sensitive, optically transparent, chemically stable, and solution‐processed strain sensors are demonstrated. Transparent indium tin oxide and zinc oxide nanocrystals serve as metallic and insulating components in a metal–insulator matrix and as active materials for strain gauges. Synergetic sensitivity‐ and stability‐reinforcing agents are developed using a transparent SU‐8 polymer to enhance the sensitivity and encapsulate the devices, elevating the gauge factor up to over 3000 by blocking the reconnection of cracks caused by the Poisson effect. Cross‐shaped patterns with an orthogonal crack strategy are developed to detect a complex multiaxial strain, efficiently distinguishing strains applied in various directions with high sensitivity and selectivity. Finally, all‐transparent wearable strain sensors with Ag nanowire electrodes are fabricated using an all‐solution process, which effectively measure not only the human motion or emotion, but also the multiaxial strains occurring during human motion in real time. The strategies can provide a pathway to realize cost‐effective and high‐performance wearable sensors for advanced applications such as bio‐integrated devices.     

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.     

Porous Organic Field‐Effect Transistors for Enhanced Chemical Sensing Performances

The thin‐film structures of chemical sensors based on conventional organic field‐effect transistors (OFETs) can limit the sensitivity of the devices toward chemical vapors, because charge carriers in OFETs are usually concentrated within a few molecular layers at the bottom of the organic semiconductor (OSC) film near the dielectric/semiconductor interface. Chemical vapor molecules have to diffuse through the OSC films before they can interact with charge carriers in the OFET conduction channel. It has been demonstrated that OFET ammonia sensors with porous OSC films can be fabricated by a simple vacuum freeze‐drying template method. The resulted devices can have ammonia sensitivity not only much higher than the pristine OFETs with thin‐film structure but also better than any previously reported OFET sensors, to the best of our knowledge. The porous OFETs show a relative sensitivity as high as 340% ppm^?1 upon exposure to 10 parts per billion (ppb) NH₃. In addition, the devices also exhibit decent selectivity and stability. This general and simple strategy can be applied to a wide range of OFET chemical sensors to improve the device sensitivity.     

Bubble‐Decorated Honeycomb‐Like Graphene Film as Ultrahigh Sensitivity Pressure Sensors

Recently, macroporous graphene monoliths (MGMs), with ultralow density and good electrical conductivity, have been considered as excellent pressure sensors due to their excellent elasticity with a rapid rate of recovery. However, MGMs can only exhibit good sensitivity when the strain is higher than 20%, which is undesirable for touch‐type pressure sensors, such as artificial skin. Here, an innovative method for the fabrication of freestanding flexible graphene film with bubbles decorated on honeycomb‐like network is demonstrated. Due to the switching effect depended on “point‐to‐point” and “point‐to‐face” contact modes, the graphene pressure sensor has an ultrahigh sensitivity of 161.6 kPa^?1 at a strain less than 4%, several hundred times higher than most previously reported pressure sensors. Moreover, the graphene pressure sensor can monitor human motions such as finger bending and pulse with a very low operating voltage of 10 mV, which is sufficiently low to allow for powering by energy‐harvesting devices, such as triboelectric generators. Therefore, the high sensitivity, low operating voltage, long cycling life, and large‐scale fabrication of the pressure sensors make it a promising candidate for manufacturing low‐cost artificial skin.     

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.     

Multifunctional Ionic Skin with Sensing,UV-Filtering,Water-Retaining,and Anti-Freezing Capabilities

Conductive polymer hydrogels are receiving considerable attention in applications such as soft robots and human-machine interfaces. Herein, a transparent and highly ionically conductive hydrogel that integrates sensing, UV-filtering, water-retaining, and anti-freezing performances is achieved by the organic combination of tannic acid-coated hydroxyapatite nanowires (TA@HAP NWs), polyvinyl alcohol (PVA) chains, ethylene glycol (EG), and metal ions. The highly ionic conductivity of the hydrogel enables tensile strain, pressure, and temperature sensing capabilities. In particular, in terms of the hydrogel strain sensors based on ionic conduction, it has high sensitivity (GF = 2.84) within a wide strain range (350%), high linearity (R² = 0.99003), fast response (≈50 ms) and excellent cycle stability. In addition, the incorporated TA@HAP NWs act as a nano-reinforced filler to improve the mechanical properties and confer a UV-shielding ability upon the hydrogel due to its size effect and the characteristics of absorbing ultraviolet light waves, which can reflect and absorb short ultraviolet rays and transmit visible light. Meanwhile, owing to the water-locking effect between EG and water molecules, the hydrogel exhibits freezing resistance at low temperatures and moisture retention at high temperatures. This biocompatible and multifunctional conductive hydrogel provides new ideas for the design of novel ionic skin devices.     

Enhanced Field Ionization Enabled by Metal Induced Surface States on Semiconductor Nanotips

Recent progress in the realization of material structures with quantum confinement and high surface to volume ratio in nanoscale interwoven metal and semiconductor building blocks offers a strong potential to build highly functional nanodevices. Ultra‐sharp tips with distinct material dependent properties of metal and semiconductor exhibit important functionalities in devices including gas ionization sensors, field emission devices, and ion‐mobility spectrometry. Herein, a dramatically enhanced field ionization process and a device based on charged particle beams for which the geometrical and surface properties of the constituent semiconductor nanotips are engineered with controlled introduction of metallic impurities to realize close to three orders of magnitude reduction in the ionization electric‐field strength are described. Experimentally observed low voltage field ionization phenomenon is explained using the geometrical field enhancement, surface states induced by controlled introduction of metallic impurities, and polarizabilities of gas particles at the nanotips. The nanotips are employed to design field ionization gas sensors whose nanoscale pristine semiconductor tips are controllably decorated with atomic metal impurities to boost the electron tunneling properties under extremely low bias voltages. These devices also outperform their solid‐state macroscopic counterparts in terms of simplicity of their construction and higher selectivity.