Wearable health monitoring systems have gained considerable interest in recent years owing to their tremendous promise for personal portable health watching and remote medical practices. The sensors with excellent flexibility and stretchability are crucial components that can provide health monitoring systems with the capability of continuously tracking physiological signals of human body without conspicuous uncomfortableness and invasiveness. The signals acquired by these sensors, such as body motion, heart rate, breath, skin temperature and metabolism parameter, are closely associated with personal health conditions. This review attempts to summarize the recent progress in flexible and stretchable sensors, concerning the detected health indicators, sensing mechanisms, functional materials, fabrication strategies, basic and desired features. The potential challenges and future perspectives of wearable health monitoring system are also briefly discussed.

Flexible, Stretchable Sensors for Wearable Health Monitoring: Sensing Mechanisms, Materials, Fabrication Strategies and Features

The recent surge in using wearable personalized devices has made it increasingly important to have flexible textile-based sensor alternatives that can be comfortably worn and can sense a wide range of body strains. Typically fabricated from rigid materials such as metals or semiconductors, conventional strain sensors can only withstand small strains and result in bulky, inflexible, and hard-to-wear devices. Textile strain sensors offer a new generation of devices that combine strain sensing functionality with wearability and high stretchability. In this review, we discuss recent exciting advances in the fabrication, performance enhancement, and applications of wearable textile strain sensors. We describe conventional and novel approaches to achieve textile strain sensors such as coating, conducting elastomeric fiber spinning, wrapping, coiling, coaxial fiber processing, and knitting. We also discuss how important performance parameters such as electrical conductivity, mechanical properties, sensitivity, sensing range, and stability are influenced by fabrication strategies to illustrate their effects on the sensing mechanism of textile sensors. We summarize the potential applications of textile sensors in structural health monitoring, wearable body movement measurements, data gloves, and entertainment. Finally, we present the challenges and opportunities that exist to date in order to provide meaningful guidelines and directions for future research.

Textile Strain Sensors: A Review of the Fabrication Technologies, Performance Evaluation and Applications

Recent advances in the design and implementation of wearable resistive, capacitive, and optical strain sensors are summarized herein. Wearable and stretchable strain sensors have received extensive research interest due to their applications in personalized healthcare, human motion detection, human–machine interfaces, soft robotics, and beyond. The disconnection of overlapped nanomaterials, reversible opening/closing of microcracks in sensing films, and alteration of the tunneling resistance have been successfully adopted to develop high‐performance resistive‐type sensors. On the other hand, the sensing behavior of capacitive‐type and optical strain sensors is largely governed by their geometrical changes under stretching/releasing cycles. The sensor design parameters, including stretchability, sensitivity, linearity, hysteresis, and dynamic durability, are comprehensively discussed. Finally, the promising applications of wearable strain sensors are highlighted in detail. Although considerable progress has been made so far, wearable strain sensors are still in their prototype stage, and several challenges in the manufacturing of integrated and multifunctional strain sensors should be yet tackled.

Wearable and Stretchable Strain Sensors: Materials, Sensing Mechanisms, and Applications

Wearable electronics have revolutionized the way physiological parameters are sensed, detected, and monitored. In recent years, advances in flexible and stretchable hybrid electronics have created emergent properties that enhance the compliance of devices to our skin. With their unobtrusive attributes, skin conformable sensors enable applications toward real-time disease diagnosis and continuous healthcare monitoring. Herein, critical perspectives of flexible hybrid electronics toward the future of digital health monitoring are provided, emphasizing its role in physiological sensing. In particular, the strategies within the sensor composition to render flexibility and stretchability while maintaining excellent sensing performance are considered. Next, novel approaches to the functionalization of the sensor for physical or biochemical stimuli are extensively covered. Subsequently, wearable sensors measuring physical parameters such as strain, pressure, temperature, as well as biological changes in metabolites and electrolytes are reported. Finally, their implications toward early disease detection and monitoring are discussed, concluding with a future perspective into the challenges and opportunities in emerging wearable sensor designs for the next few years.

Flexible Hybrid Sensors for Health Monitoring: Materials and Mechanisms to Render Wearability.

Soft machines are a promising design paradigm for human-centric devices1,2 and systems required to interact gently with their environment3,4. To enable soft machines to respond intelligently to their surroundings, compliant sensory feedback mechanisms are needed. Specifically, soft alternatives to strain gauges—with high resolution at low strain (less than 5 per cent)—could unlock promising new capabilities in soft systems. However, currently available sensing mechanisms typically possess either high strain sensitivity or high mechanical resilience, but not both. The scarcity of resilient and compliant ultra-sensitive sensing mechanisms has confined their operation to laboratory settings, inhibiting their widespread deployment. Here we present a versatile and compliant transduction mechanism for high-sensitivity strain detection with high mechanical resilience, based on strain-mediated contact in anisotropically resistive structures (SCARS). The mechanism relies upon changes in Ohmic contact between stiff, micro-structured, anisotropically conductive meanders encapsulated by stretchable films. The mechanism achieves high sensitivity, with gauge factors greater than 85,000, while being adaptable for use with high-strength conductors, thus producing sensors resilient to adverse loading conditions. The sensing mechanism also exhibits high linearity, as well as insensitivity to bending and twisting deformations—features that are important for soft device applications. To demonstrate the potential impact of our technology, we construct a sensor-integrated, lightweight, textile-based arm sleeve that can recognize gestures without encumbering the hand. We demonstrate predictive tracking and classification of discrete gestures and continuous hand motions via detection of small muscle movements in the arm. The sleeve demonstration shows the potential of the SCARS technology for the development of unobtrusive, wearable biomechanical feedback systems and human–computer interfaces. Strain gauges with both high sensitivity and high mechanical resilience, based on strain-mediated contact in anisotropically resistive structures, are demonstrated within a sensor-integrated, textile-based sleeve that can recognize human hand motions via muscle deformations.

Ultra-sensitive and resilient compliant strain gauges for soft machines.

The recent booming development of wearable electronics urgently calls for high-performance flexible strain sensors. To date, it is still a challenge to manufacture flexible strain sensors with superb sensitivity and a large workable strain range simultaneously. Herein, a facile, quick, cost-effective, and scalable strategy is adopted to fabricate novel strain sensors based on reduced graphene oxide woven fabrics (GWF). By pyrolyzing commercial cotton bandages coated with graphene oxide (GO) sheets in an ethanol flame, the reduction of GO and the pyrolysis of the cotton bandage template can be synchronously completed in tens of seconds. Due to the unique hierarchical structure of the GWF, the strain sensor based on GWF exhibits large stretchability (57% strain) with high sensitivity, inconspicuous drift, and durability. The GWF strain sensor is successfully used to monitor full-range (both subtle and vigorous) human activities or physical vibrational signals of the local environment. The present work offers an effective strategy to rapidly prepare low-cost flexible strain sensors with potential applications in the fields of wearable electronics, artificial intelligence devices, and so forth.

Highly Stretchable, Ultrasensitive, and Wearable Strain Sensors Based on Facilely Prepared Reduced Graphene Oxide Woven Fabrics in an Ethanol Flame

Enhanced photovoltaic-pyroelectric coupled effect of BiFeO3/Au/ZnO heterostructures

Excellent piezo-photocatalytic performance of Bi4Ti3O12 nanoplates synthesized by molten-salt method

DOI: 10.1002/aelm.201800412 electronic equipment.[7] Among them, the memory chip, as an important component, should also be transparent and flexible to accommodate the development. As the promising candidate for the nextgeneration nonvolatile memory (NVM), the resistive-switching random access memory (ReRAM) offers many technological advantages, such as simple device configuration, fast operation speed, high endurance, excellent scalability, small size, and low-power consumption.[8–11] Hence, attractions on flexible and transparent ReRAM are increasing in recent years.[12–16] In order to achieve high-performance transparent and flexible characteristics of ReRAM, the functional layers are commonly organic materials or binary oxides that are prepared near room temperature.[4,17–21] These functional materials are compatible well with the transparent and flexible polymer-based substrates (e.g., polyimide (PI), polyethylene terephthalate (PET), polydimethylsiloxane (PDMS), and polyphthalamide (PPA)) and transparent conducting films (indium–tin oxides (ITO), Al-doped ZnO (AZO), carbon nanotube (CNT), and some organic conducting polymers), which cannot endure safely at >500 °C and oxygen ambience. Previously, Lee and co-workers used polyimide:6-phenyl-C61 butyric acid methyl ester to fabricate the ReRAM, which has the high/low resistance ratio of over 106 and can be maintained over 104 s without notable substantial fluctuation under bending and stretching cycling.[22] Nagareddy et al. employed hybrid graphene oxide–titanium oxide to fabricate the ReRAM, which has ultrafast programming speed (60 ns).[21] In spite of the extensive studies, it is still a big challenge to directly prepare the transparent, flexible and high-quality memories based on perovskite oxide films (POFs), which generally need to be prepared at oxygen ambience and >500 °C. Many POFs (e.g., BaTiO3, SrTiO3, and SrZrO3) simultaneously exhibit extraordinary resistive-switching characteristic, favorable thermal stability, outstanding radioresistance, and transparency.[23–25] In order to achieve transparent and flexible ReRAMs, researchers usually grew POFs on rigid substrates (i.e., Si or SrTiO3), peeled off the POFs from substrates, and then transferred them onto flexible organic ones.[3,26] The significant advantage of the “grow-transfer” method is that the transferred layer remains the same in composition and structure.[27,28] A rapid surge in the research of lightweight, invisible, and flexible electronics is occurring with the arrival of Internet of Things (IoT). However, multifunctional perovskite oxide electronics are commonly hard and should be synthesized at high temperature and oxygen ambience, where most transparent conductive films will become brittle or highly resistive. Thus, the realization of transparent and flexible nonvolatile perovskite oxide resistive memory remains a big challenge. Here, a transparent, flexible, nonvolatile, and all-inorganic memory with the mica substrate is prepared: the 2.7 wt% Ag-doped indium–tin oxide (Ag–ITO) film as bottom electrodes, the BaTi0.95Co0.05O3−δ (BTCO) film as resistive-switching functional layer, and the Ag/ITO films as top electrodes. The Ag–ITO/BTCO/Ag/ITO heterosturcture shows a unipolar resistive-switching behavior, and its high/low resistance ratio is up to 5 × 103 under a low operating voltage (<2.8 V) with fast response speed (≈50 ns). Either highor low-resistance states remain stable even after the 14 400 cycles’ dynamic bending test with the minimum radius of 3 mm. Additionally, the bipolar resistive-switching characteristic is observed in the ≈85 nm diameter region. As a result, such resistive memory shows potential to be used in many transparent and flexible devices, such as electronic skins and flexible displays.

An All-Inorganic, Transparent, Flexible, and Nonvolatile Resistive Memory

Ferroelectric polarization allows a depolarization electric field to separate electron-hole pairs excited by lights, and thus, the photovoltaic properties of ABO3-type films on a hard SrTiO3 or Si substrate have been extensively studied recently. However, there are a few reports on the photocapacitance and photoimpedance of these oxide films, especially on flexible substrates. In this work, the strong photovoltaic, giant photocapacitance, and photoimpedance effects were observed in the flexible (111) BiFeO3 films with natural downward polarization. The flexible mica/SrRuO3/BiFeO3/Au cells show a maximum photovoltaic efficiency at 150 °C, a 95.5% decrease in photoimpedance effects, and a 316% increase in photocapacitance effects under 405-nm-wavelength light illumination at 25 °C. Most importantly, these properties do not show obvious degradation when the device is bent to 3 mm radius for 104 times. This work is of vital importance for us to develop new flexible photoelectronic devices.

Zhongshuai Xie

Papers

Highly Stretchable, Ultrasensitive, and Wearable Strain Sensors Based on Facilely Prepared Reduced Graphene Oxide Woven Fabrics in an Ethanol Flame

Enhanced photovoltaic-pyroelectric coupled effect of BiFeO3/Au/ZnO heterostructures

Excellent piezo-photocatalytic performance of Bi4Ti3O12 nanoplates synthesized by molten-salt method

An All-Inorganic, Transparent, Flexible, and Nonvolatile Resistive Memory

Photovoltaic, photo-impedance, and photo-capacitance effects of the flexible (111) BiFeO3 film