Paper, as a flexible, low-cost, lightweight, tailorable, environmental-friendly, degradable, and renewable material, is emerging in electronic devices. Especially, many kinds of paper-based (PB) ...sensors have been reported for wearable applications in recent years. Among them, the PB gas, humidity, and strain sensors are widely studied for monitoring gas, humidity, and strain from the human body and the environment. However, gas, humidity, and strain often coexist and interact, and the paper itself is hydrophilic and flexible, resulting in that it is still challenging to develop high-performance PB sensors specialized for gas, humidity, and strain detections. Therefore, it is necessary to summarize and discuss them systematically. In this review, we focus on summarizing the state-of-art studies of the PB gas, humidity, and strain sensors. Specifically, the fabrications (electrodes and sensing materials) and applications of PB gas, humidity, and strain sensors are summarized and discussed. The current challenges and the potential trends of PB sensors for gas, humidity, and strain detections are also outlined. This review not only can help readers to understand the development status of the PB gas, humidity, and strain sensors but also is helpful for readers to find out and solve the problems in this field through comparative reading.
•The strain sensing mechanisms and performance of graphene-based sensors.•The design and fabrication strategies of four graphene-based strain sensors.•The applications of graphene-based flexible ...strain sensors in numerous fields.•Existing challenges and future opportunities for graphene-based sensors.
Flexible and wearable electronics have recently gained considerable research interest due to their potential applications in personal healthcare, electronic skins, and human–machine interfaces. In particular, strain sensors that can efficiently transmit external stimuli into electrical signals are essential for wearable electronics. Two-dimensional carbon-based materials such as graphene are potentially versatile platforms for the above applications, mainly attributed to their combined properties of excellent flexibility, thermal and electrical conductivity, and mechanical strength. Although there are numerous reports devoted to the design, fabrication and application of graphene-based strain sensors, a comprehensive overview dedicated on attributes of graphene-based strain sensors that can be systematically correlated with their mechanisms, fabrication strategies and applications is urgently required in the field. Specially this review is aimed to explore the following topics, i.e., (i) the strain sensing mechanisms and key performance parameters of graphene-based sensors; (ii) the recent progress of major graphene-based sensors including those of film-based, fiber-based, foam-based and hydrogel-based; (iii) applications of graphene-based sensors for human motion sensing, health indicators, electronic skins and human machine interfaces; and finally (iv) challenges and future directions for the design of graphene-based sensors.
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•A kind of LM complex was designed, which initiated the polymerization of vinyl monomers, allowing LMCH hydrogels to polymerize without the need for an initiator.•In the structural ...design strategy of LM complexes, MWCNT-80 acts as a bridge connecting individual LM droplets, solving the problem that independent LM nanodroplets cannot form a conductive pathway.•The hydrogel has excellent adhesion on various interfaces, which enables the hydrogel to fit perfectly with the interface and ensure the accuracy and stability of sensing.•Strain sensors and self-powered sensors based on LMCH hydrogels are ultimately used for joint rehabilitation training.
Liquid metal (LM) can serve as functional fillers, initiators, and cross-linkers for hydrogels. However, the high surface energy of LM makes it difficult to disperse uniformly and stably in the hydrogels. In this work, multi-walled carbon nanotubes (MWCNT-80) modified with the surfactant Tween 80, along with hyaluronic acid (HA), were employed to collectively stabilize LM to form LM complex for initiating the polymerization of vinyl monomers. The LM complex was used as a conductive filler, which not only enhanced the mechanical properties of the hydrogel, but also initiated free radical polymerization of the monomer. The prepared liquid metal conductive hydrogel (LMCH) has excellent stretchability (1192 %), adhesion (15.3 kPa) and conductivity (0.78 S/m). This LMCH hydrogel was further applied in the construction of multifunctional sensors. On one hand, strain sensors constructed with this hydrogel could monitor extensive joint range movements in real-time. On the other hand, a flexible triboelectric nanogenerator (L-TENG) constructed with the hydrogel as an electrode was employed for self-powered sensing. When integrated with a Microcontroller Unit (MCU), the joint training status of individuals can be transmitted in real-time to a mobile phone, allowing for the quantification and monitoring of human activities. This study presents a novel approach to the development of hydrogels for multifunctional sensors, and the prepared hydrogel holds significant potential in the fields of rehabilitation training and human–machine interaction.
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•The cracking behavior of a double-network hydrogel has been studied.•Excellent sensitivity (GF = 2.2) and high linear working range (0–250 % strain) are obtained.•Crack from network ...ruptures boosts sensor sensitivity.•The crack hydrogel shows outstanding performance for detecting both large and small strain of human body.
Recently, owing to their high sensitivity, spider-inspired crack strain sensors have received considerable attention. However, their short linear range leads to severe limitations in their application in sports health monitoring. Herein, we fabricated a cracked hydrogel strain sensor with a wide linear range using κ-carrageenan and polyacrylamide. This crack hydrogel strain sensor has a wide linear range (ε = 250 %) and excellent linearity (R2 ≈ 0.999) while ensuring high sensitivity (gauge factor GF = 2.2). Moreover, this sensor is sensitive to small vibration signals. We compared it with a traditional hydrogel strain sensor and analyzed how crack appeared. Finally, we presented the application of this sensor in human health monitoring and detection of multiaxial strains.
•A single PAC hydrogel can generate an open-circuit voltage of 0.92 V.•PAC possesses adequate mechanical properties, exceptional conductivity, antibacterial effectiveness, and frost resistance ...simultaneously.•When functioning as a strain sensor, PAC is capable of producing both resistance and current as output signals.•By stacking in series, the PAC battery pack offers impressive output current and voltage.
Self-powered hydrogels have gained attention for addressing energy needs of small flexible devices, but creating ones with high voltage and current remains challenging. Here, we report a novel polyacrylamide hydrogel loaded with copper sulfate and ammonium chloride (PAC), which not only exhibits good mechanical properties (maximum strain of 390 %), high conductivity (conductivity of 2.37 S/m), but also possesses excellent antibacterial and antifreeze properties. PAC demonstrates remarkable stimulus responsiveness. When using resistance variation as the output signal, its GF value reaches as high as 5.3 within the strain range of 35 % to 280 %. Additionally, PAC can utilize current variation as an output signal, providing options for various sensor applications. On the other hand, PAC hydrogels can achieve self-powering capabilities through zinc electrodes, with an open-circuit voltage of 0.92 V. By connecting PAC hydrogels in parallel or series, higher output voltage and current can be achieved, providing a strong guarantee for powering small devices. In a stacked series configuration, 8 sets of PACs can generate an output voltage of 7.16 V and an output current of 52.6 mA. In sum, PAC is a high-performance self-powered hydrogel with tremendous potential in the fields of flexible sensing and energy supply.
In today’s world, the progress of wearable tools has gained increasing momentum. Notably, the demand for stretchable strain sensors has considerably increased owing to various potential and emerging ...applications like human motion monitoring, soft robotics, prosthetics, and electronic skin. Hydrogels possess excellent biocompatibility, flexibility, and stretchability that render them ideal candidates for flexible/wearable substrates. Among them, enormous efforts were focused on the progress of polyvinyl alcohol (PVA) hydrogels to realize multifunctional wearable sensing through using additives/nanofillers/functional groups to modify the hydrogel network. Herein, this review offers an up-to-date and comprehensive summary of the research progress of PVA hydrogel-based wearable sensors in view of their properties, strain sensory efficiency, and potential applications, followed by specifically highlighting their probes using metallic/non-metallic, liquid metal (LM), 2D materials, bio-nanomaterials, and polymer nanofillers. Indeed, flexible electrodes and strain/pressure sensing performance of designed PVA hydrogels for their effective sensing are described. The representative cases are carefully selected and discussed regarding the construction, merits and demerits, respectively. Finally, the necessity and requirements for future advances of conductive and stretchable hydrogels engaged in the wearable strain sensors are also presented, followed by opportunities and challenges.
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•Focuse on nanomaterial based PVA nanocomposite hydrogels for biomedical sensing applications.•The numerous utilization of PVA hydrogel-based flexible/wearable nanoprobes in different fields.•Discusse on the construction, merits and demerits of conductive and stretchable PVA hydrogels
•The LM/CNTs hydrogel achieves high conductivity and excellent transparency.•This strain sensor shows the lowest detection limit (0.01 mm, 0.25 % stretch).•The strain sensor implants on the heart of ...the endangered Chinese sturgeon.•The LM/CNTs hydrogel broadens the preparation strategies for sensing materials.
Strain sensors are intriguing in advanced electronics and are typically composed of active sensing materials. Good elastic recovery, self-healing, good adhesion, high transparency, and great mechano-response are essential but can be rarely met in one material. Here, we report a wireless strain sensor that exhibits tissue-like mechanical properties (softness), is conformable to soft tissues (heart and tail) and has the lowest limit of detection (0.25 % stretch). After introducing multiwalled carbon nanotubes (CNTs) bridged liquid metal (LM) composites (LM/CNTs), the strain sensor simultaneously achieves excellent transparency and conductivity. This LM/CNTs hydrogel overcomes the limitations of the traditional hydrogel and achieves high conductivity (up to 94 S/m), good stretchability (2,200 %), and a high degree of transparency (93 %), in addition to enough adhesion (20 kPa) and rapid self-healing (20 min). The strain sensor device is composed of intrinsically soft LM/CNTs hydrogel as sensing material, a microcontroller, signal-processing circuits, and Bluetooth transceiver. This device is applied to recording high-quality heartbeat signals and clear in-site observation of cardiac bleeding, which has helped a Chinese sturgeon survive after Caesarean. This approach provides an ideal strategy for protecting and saving endangered animals and broadens the applications of strain sensors.
This review examines the growing field of flexible and stretchable strain sensors utilized in human walking gait analysis, focusing on determining knee bending angle and muscle activities when a ...human subject walks. Recent advancements have enabled these sensors to accurately capture biomechanical parameters while accommodating the body's natural movements. We examine the principles underlying these sensors' design and fabrication techniques, emphasizing their flexibility, stretchability, and biocompatibility for seamless integration into wearable systems, suitable for capturing the movement of the lower limb aimed at walking gait analysis. Through critical evaluation of existing methodologies, we assess the reliability and validity of these sensors in quantifying gait parameters. Furthermore, we discuss this technology's clinical and research implications, highlighting its potential in personalized rehabilitation and sports performance optimization. We provide an overview of flexible strain sensors and their application for analyzing human walking gait, especially for observing walking gait in rehabilitation programs. First, we present the basic working principles of various flexible strain sensors. Second, by the application of flexible strain sensors as walking gait monitoring. Finally, current challenges and future opportunities in this research area are discussed. By synthesizing current knowledge and outlining future directions, this review aims to provide insights for researchers and practitioners seeking to leverage flexible and stretchable strain sensors for comprehensive human gait analysis.
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Transparent and healable ionogels with very high mechanical strength, ionic conductivity, and resilience were fabricated for use as strain sensors with satisfactory reliability. The ionogels were ...fabricated by casting an aqueous solution of poly(vinyl alcohol) (PVA)–poly(vinylpyrrolidone) (PVP) complexes and 1-ethyl-3-methylimidazolium dicyanamide (EMImDCA), followed by evaporation of water at room temperature. The use of EMImDCA endowed the resulting ionogels with ionic conductivity at room temperature as high as 19.7 mS cm–1. Owing to the synergy between the abundant number of hydrogen bonds between PVA and PVP and the crystallized PVA segments that served as nanofillers, the resulting ionogels had good mechanical properties with a tensile stress of 7.7 MPa, a strain of 821%, and good resilience. In addition, the resulting ionogels showed rapid and repeatable sensing signals over a wide strain range (0.1–400%). This enabled them to detect both vigorous muscle movements, such as walking and jumping, and subtle muscle movements, such as pulse. Moreover, owing to the reversibility of hydrogen bonds, physically damaged mechanical properties, conductivity, and sensing ability of the ionogels could be conveniently healed with the assistance of water.
In order to replace nonrenewable resources and decrease electronic waste disposal, there is a rapidly rising demand for the utilization of reproducible and degradable biopolymers in flexible ...electronics. Natural biopolymers have many remarkable characteristics, including light weight, excellent mechanical properties, biocompatibility, non-toxicity, low cost, etc. Thanks to these superior merits, natural functional biopolymers can be designed and optimized for the development of high-performance flexible electronic devices. Herein, we provide an insightful overview of the unique structures, properties and applications of biopolymers for electronic skins (e-skins) and flexible strain sensors. The relationships between properties and sensing performances of biopolymers-based sensors are also investigated. The functional design strategies and fabrication technologies for biopolymers-based flexible sensors are proposed. Furthermore, the research progresses of biopolymers-based sensors with various functions are described in detail. Finally, we provide some useful viewpoints and future prospects of developing biopolymers-based flexible sensors.
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