Capacitive‐type strain sensors based on hydrogel ionic conductors have undergone rapid development benefited from their robust structure, drift‐free sensing, higher sensitivity, and precision. ...However, the unsatisfactory electro‐mechanical stability of the conventional hydrogel conductors, which are normally vulnerable to large deformation and severe mechanical impacts, remains a challenge. In addition, there is not enough research regarding the adhesiveness and mechanical properties of the dielectric layer, which is also critical for the mechanical adaptability of the whole device. Here, a dynamically super‐tough capacitive‐type strain sensor based on energy‐dissipative dual‐crosslinked hydrogel conductors and an organogel dielectric with high adhesive strength is developed. Combining with the mechanical advantages of the hydro/organo‐gels, the capacitive strain sensor exhibits high stretchability and superior linear dependence of sensitivity with a gauge factor of ≈0.8% at 100% strain. Moreover, the sensor displayed ultrastability against various severe mechanical stimuli that can even survive unprecedentedly from extremely catastrophic car run‐over by 20 times. With these synergistic mechanical advantages, the capacitive strain sensor is successfully applied as a highly‐reliable wearable sensing system to monitor diverse faint physiological signals and large‐range human motions.
A super‐tough capacitive strain sensor is developed based on energy‐dissipative dual‐crosslinked hydrogel conductors, which display excellent stretchability, broad‐range sensing, and pronounced operational stability for monitoring faint physiological signals and large‐range human motions.
Abstract A high sensitivity and large stretchability are desirable for strain sensors in wearable applications. However, these two performance indicators are contradictory, since the former requires ...a conspicuous structural change under a tiny strain, whereas the latter demands morphological integrity upon a large deformation. Developing strain sensors with both a high sensitivity (gauge factor (GF) > 100) and a broad strain range (>50%) is a considerable challenge. Herein, a unique Ti 3 C 2 T x MXene nanoparticle–nanosheet hybrid network is constructed. The migration of nanoparticles leads to a large resistance variation while the wrapping of nanosheet bridges the detached nanoparticles to maintain the connectivity of the conductive pathways in a large strain region. The synergetic motion of nanoparticles and nanosheets endows the hybrid network with splendid electrical–mechanical performance, which is reflected in its high sensitivity (GF > 178.4) over the entire broad range (53%), the super low detection limit (0.025%), and a good cycling durability (over 5000 cycles). Such high performance endows the strain sensor with the capability for full‐range human motion detection.
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•A flexible and sensitive strain sensor was fabricated using biomass Juncus effusus.•Strong flexibility, high sensitivity, and multifunctionality were achieved ...simultaneously.•Geometric changes and cracks propagation of microfibrils contributed to the performance.•Good repellency against mechanical deformations and chemicals were obtained.•Morse code communication and real-time monitoring of human activities were achieved.
Flexible strain sensors have received extensive attention owing to their booming development. However, achieving high sensitivity and stretchability simultaneously for strain sensors is still a challenge. Herein, we report a fiber-based stretchable strain sensor with a unique three-dimensional (3D) reticular structure for human motion monitoring. The biomass Juncus effusus (JE) fiber was functionalized with carbon nanotubes (CNTs) and Ecoflex (EF) using a facile dip-coating-drying approach, and a syringe extraction method, respectively. The resulted JE/CNTs/EF fiber strain sensors exhibited advanced performance of strong flexibility, high sensitivity, and multifunctionality, which displayed a superior sensitivity (gauge factor of 24.95–76.79) and high stretchability (600%). Moreover, good repellency to chemicals and stable responses to various stimuli frequencies over 270 cycles were obtained for the as-prepared sensors. The microfibrils of the JE fiber formed a complicated 3D reticular structure acting as the conductive bridges when stretching. Consequently, the fiber-based JE/CNTs/EF strain sensor can not only accurately detect a full range of body motions and subtle signals, but also can be a promising candidate as a Morse code generator for cryptographic communication.
Highly stretchable fiber strain sensors are one of the most important components for various applications in wearable electronics, electronic textiles, and biomedical electronics. Herein, we present ...a facile approach for fabricating highly stretchable and sensitive fiber strain sensors by embedding Ag nanoparticles into a stretchable fiber with a multifilament structure. The multifilament structure and Ag-rich shells of the fiber strain sensor enable the sensor to simultaneously achieve both a high sensitivity and largely wide sensing range despite its simple fabrication process and components. The fiber strain sensor simultaneously exhibits ultrahigh gauge factors (∼9.3 × 105 and ∼659 in the first stretching and subsequent stretching, respectively), a very broad strain-sensing range (450 and 200% for the first and subsequent stretching, respectively), and high durability for more than 10 000 stretching cycles. The fiber strain sensors can also be readily integrated into a glove to control a hand robot and effectively applied to monitor the large volume expansion of a balloon and a pig bladder for an artificial bladder system, thereby demonstrating the potential of the fiber strain sensors as candidates for electronic textiles, wearable electronics, and biomedical engineering.
Flexible strain sensors have received widespread attention because of their great potential in many fields. Carbon nanotubes (CNTs) have been used as conductive materials for flexible strain sensors ...due to their excellent electrical and mechanical properties, and the fabricated flexible strain sensors have excellent sensing performance. This paper systematically summarizes the advances in flexible resistance‐type strain sensors based on CNTs. The strain sensing mechanisms are introduced, including crack extension, tunneling effect, and disconnection of overlapping materials. The performance parameters of the sensors, including sensitivity, stretchability, linearity, hysteresis, dynamic durability, and transparency, are discussed comprehensively. The coating methods, 3D printing techniques, chemical vapor deposition, transfer methods, and spinning processes used to fabricate CNT strain sensors are highlighted. The effect of isolated and porous internal conductive structures, folded and microcracked surface structures, films and fabrics macroscopic structures on sensor performance were systematically analyzed. The applications of the sensors in medical health, motion monitoring, gesture recognition, human–computer interaction, and soft robotics are provided in detail. Finally, the future challenges of CNT flexible strain sensors are summarized and the outlook is presented. Although CNT strain sensors have made great progress so far, there are still many problems that need researchers’ attention and solutions.
Carbon nanotubes CNT flexible strain sensors play an important role in health care, motion monitoring, gesture recognition, human–computer interaction, aerospace monitoring and food safety and so on. It can effectively improve the sensitivity, stretchability, dynamic durability, hysteresis, response time, and linearity of the sensor by tuning the conductive network structure of the CNT strain sensor.
Biomaterials capable of transmitting signals over longer distances than those in rigid electronics can open new opportunities for humanity by mimicking the way tissues propagate information. For ...seamless mirroring of the human body, they also have to display conformability to its curvilinear architecture, as well as, reproducing native‐like mechanical and electrical properties combined with the ability to self‐heal on demand like native organs and tissues. Along these lines, a multifunctional composite is developed by mixing silk fibroin and reduced graphene oxide. The material is coined “CareGum” and capitalizes on a phenolic glue to facilitate sacrificial and hierarchical hydrogen bonds. The hierarchal bonding scheme gives rise to high mechanical toughness, record‐breaking elongation capacity of ≈25 000%, excellent conformability to arbitrary and complex surfaces, 3D printability, a tenfold increase in electrical conductivity, and a fourfold increase in Young's modulus compared to its pristine counterpart. By taking advantage of these unique properties, a durable and self‐healing bionic glove is developed for hand gesture sensing and sign translation. Indeed, CareGum is a new advanced material with promising applications in fields like cyborganics, bionics, soft robotics, human–machine interfaces, 3D‐printed electronics, and flexible bioelectronics.
Biomaterials that transmit electricity like the human body can open new opportunities. For seamless mirroring of our anatomy, they also need to conform to its curvilinear architecture, and display self‐healing ability like native organs and tissues. To address this, a conductive, adhesive, reconfigurable, and viscoelastic gum (CareGum), with promising applications in cyborganics, bionics, and soft robotics, is developed.
Flexible sensors have shown great potential in remote health monitoring, body movements track, electronic skin, human-machine interfaces, and soft robotics. Hydrogels possess exceptional ...stretchability, flexibility and biocompatibility that render them appealing candidates for wearable flexible sensors. Among them, considerable efforts have been devoted to developing conductive hydrogels to achieve multifunctional wearable sensing through using functional groups/additives/nanofillers to modify the hydrogel network in recent years. This review summarizes recent advances of applications of hydrogels in flexible wearable sensors, such as sweat sampling and flexible electrodes, strain/pressure sensors and touch panels, focuses on the multifunctional conductive hydrogels-based flexible wearable sensors with self-healing, self-adhesion, or anti-freezing capabilities. A brief introduction to representative synthesis methods and strategies of conductive hydrogels is also presented. In the end, we also provide a personal perspective on the future development, and address the remaining challenges in the commercialization of conductive hydrogels-based multifunctional flexible wearable sensors.
•We briefly introduce the synthesis methods and strategies of conductive hydrogels.•Flexible hydrogel-based wearable sensors can achieve real-time monitoring physiological signals.•Such sensors exhibit self-healing, self-adhesive or anti-freezing properties.•We provide a personal perspective into the challenges and future commercialization.
Ionic conducting eutectogels have attracted enormous attention as an alternative to the conventional temperature‐intolerant hydrogels and costly ionic liquid gels in constructing flexible electronic ...devices. However, current eutectogels prepared via cross‐linked polymer or low‐molecular‐weight gelators suffer from limited stretchability and insufficient surface‐adaptive adhesion. Herein, a low‐molecular‐weight supramolecular network is introduced into a covalent polymer network in a eutectogel architecture, and a novel supramolecular‐polymer double‐network (SP‐DN) strategy is demonstrated to fabricate conductive SP‐DN eutectogels with high stretchability (>4000% elongation) and toughness (≈800 J m−2), as well as self‐healing, self‐adhesive and anti‐freezing/anti‐drying characteristics. These unique features lead to the successful realization of SP‐DN eutectogels in wearable self‐adhesive strain sensors, which can conformally deform with the skin and dynamically monitor body movements with high sensitivity and long‐term stability over a wide temperature range (−40 to 60 °C). Furthermore, the strain sensors can accurately detect body movements along two opposite directions (bend up or bend down), which are rarely reported in the literature. Distinct from the widely explored polymer double‐network (P‐DN) hydrogels, the developed SP‐DN eutectogel platform is capable of well‐regulating molecular‐scale noncovalent and covalent interactions, providing a paradigm for the creation of smart soft materials with versatile performance and high environmental adaptability.
A novel low‐molecular‐weight supramolecular‐polymer double‐network (SP‐DN) strategy is developed to fabricate conductive SP‐DN eutectogels with high stretchability and excellent toughness, as well as self‐healing, self‐adhesive, and anti‐freezing/anti‐drying characteristics. These unique features allow for the successful application of SP‐DN eutectogels for self‐adhesive and bidirectional sensors with high sensitivity and long‐term stability over a wide temperature range (−40 to 60 °C).
Eutectogels based on natural polymers have attracted significant attention as an alternative to easily dehydrated hydrogels and expensive ionogels in the development of flexible strain sensors. The ...feasibility of employing eutectogels derived from pure natural polymers could be greatly enhanced if their mechanical properties satisfy the requirements of applications. Herein, alginate eutectogels (AEs) with high mechanical properties (tensile strain 217 % and strength 2.26 MPa at fracture), and excellent transparency (over 90 %) are acquired via CaCl2 inducing ionic crosslinking and subsequent deep eutectic solvents (DESs, composed of glycerol and choline chloride) initiating physical crosslinking with a universal solvent- replacement strategy. Among them, sodium alginate, a natural polysaccharide polymer, is selected as representative supporting scaffolds and forms water-insoluble alginate hydrogels (AHs) in CaCl2 coagulation bath. The exchange of DESs with water of AHs not only restrengthens the polymer network by physical crosslinking, but also endows the obtained AEs with long-term solvent retention and high temperature resistance. In addition, the AEs not only have high reliability but also exhibit better linear sensitivity in a wide strain range (0–200 %). In particular, the AEs display multiple sensitivity to stretching, bending, and human motions, demonstrating feasibility as sensitive strain sensors.
•Alginate eutectogel (AE) prepared by a universal solvent-replacement strategy.•AEs have high mechanical properties and excellent transparency.•AEs have long-term solvent retention and high temperature resistance.•AEs based strain sensors display high reliability and multiple sensitivity.
With recent advancements in novel composite nanomaterials and microstructures, wearable electronic devices, particularly flexible and stretchable strain sensors, are receiving significant attention. ...This article reviews recent developments in composite-based flexible and stretchable strain sensors for wearable applications, such as those in healthcare and human motion detection, sports and physical training, soft robotics, and smart textiles. Material compositions and structures are categorically discussed based on their respective sensing mechanisms and novel structural interfaces. Four major categories of composites are reviewed in detail: carbon materials, nanowires (NWs) and nanoparticles (NPs), liquids, and newly emerging bio-hybrid nanocomposites. Parametric evaluations are conducted on the performance characteristics of these stretchable sensors, including those related to their respective composite interfaces. Potential applications of these high-performance strain sensors are discussed, along with the key technological challenges and future trends for improving sensor fabrication and performance.
