3D‐Structured Stretchable Strain Sensors for Out‐of‐Plane Force Detection

Stretchable strain sensors, as the soft mechanical interface, provide the key mechanical information of the systems for healthcare monitoring, rehabilitation assistance, soft exoskeletal devices, and soft robotics. Stretchable strain sensors based on 2D flat film have been widely developed to monitor the in‐plane force applied within the plane where the sensor is placed. However, to comprehensively obtain the mechanical feedback, the capability to detect the out‐of‐plane force, caused by the interaction outside of the plane where the senor is located, is needed. Herein, a 3D‐structured stretchable strain sensor is reported to monitor the out‐of‐plane force by employing 3D printing in conjunction with out‐of‐plane capillary force‐assisted self‐pinning of carbon nanotubes. The 3D‐structured sensor possesses large stretchability, multistrain detection, and strain‐direction recognition by one single sensor. It is demonstrated that out‐of‐plane forces induced by the air/fluid flow are reliably monitored and intricate flow details are clearly recorded. The development opens up for the exploration of next‐generation 3D stretchable sensors for electronic skin and soft robotics.     

A Flexible Stretchable Hydrogel Electrolyte for Healable All‐in‐One Configured Supercapacitors

The development of integrated high‐performance supercapacitors with all‐in‐one configuration, excellent flexibility and autonomously intrinsic self‐healability, and without the extra healable film layers, is still tremendously challenging. Compared to the sandwich‐like laminated structures of supercapacitors with augmented interfacial contact resistance, the flexible healable integrated supercapacitor with all‐in‐one structure could theoretically improve their interfacial contact resistance and energy densities, simplify the tedious device assembly process, prolong the lifetime, and avoid the displacement and delamination of multilayered configurations under deformations. Herein, a flexible healable all‐in‐one configured supercapacitor with excellent flexibility and reliable self‐healing ability by avoiding the extra healable film substrates and the postassembled sandwich‐like laminated structures is developed. The healable all‐in‐one configured supercapacitor is prepared from in situ polymerization and deposition of nanocomposites electrode materials onto the two‐sided faces of the self‐healing hydrogel electrolyte separator. The self‐healing hydrogel film is obtained from the physically crosslinked hydrogel with enormous hydrogen bonds, which can endow the healable capability through dynamic hydrogen bonding. The assembled all‐in‐one configured supercapacitor exhibits enhanced capacitive performance, good cycling stability, reliable self‐healing capability, and excellent flexibility. It holds broad prospects for obtaining various flexible healable all‐in‐one configured supercapacitors for working as portable energy storage devices in wearable electronics.     

Cryo‐Transferred Ultrathin and Stretchable Epidermal Electrodes

A simple cryo‐transfer method to fabricate ultrathin, stretchable, and conformal epidermal electrodes based on a combination of silver nanowires (AgNWs) network and elastomeric polymers is developed. This method can temporarily enable the soft elastomers with much higher elastic modulus and dimensional contraction through exploiting their glass‐transition behaviors. During this process, a much higher Von Mises stress can be loaded on AgNWs than usual, and the generated strong grip force can facilitate the complete transfer of AgNWs. Afterward, the thawed AgNWs and elastomer composites quickly recover to their soft state at room temperature. The obtained ultrathin and soft electrode with a thickness of 8.4 µm and transmittance of 90.8% at a sheet resistance of 13.2 Ω sq^?1 can tolerate a stretching strain of 70% and 50 000 repeated bending cycles, which meets rigorous requirements of epidermal applications. The as‐prepared epidermal electrodes are effective and comfortable for electrophysiological signal monitoring, and while showing excellent performance exceeding the commercialized gel electrodes.     

Advances in Rational Design and Materials of High‐Performance Stretchable Electromechanical Sensors

Stretchable and wearable sensor technology has attracted significant interests and created high technological impact on portable healthcare and smart human–machine interfaces. Wearable electromechanical systems are an important part of this technology that has recently witnessed tremendous progress toward high‐performance devices for commercialization. Over the past few years, great attention has been paid to simultaneously enhance the sensitivity and stretchability of the electromechanical sensors toward high sensitivity, ultra‐stretchability, low power consumption or self‐power functionalities, miniaturisation as well as simplicity in design and fabrication. This work presents state‐of‐the‐art advanced materials and rational designs of electromechanical sensors for wearable applications. Advances in various sensing concepts and structural designs for intrinsic stretchable conductive materials as well as advanced rational platforms are discussed. In addition, the practical applications and challenges in the development of stretchable electromechanical sensors are briefly mentioned and highlighted.     

Hierarchical 3D All‐Carbon Composite Structure Modified with N‐Doped Graphene Quantum Dots for High‐Performance Flexible Supercapacitors

Flexible supercapacitors have shown enormous potential for portable electronic devices. Herein, hierarchical 3D all‐carbon electrode materials are prepared by assembling N‐doped graphene quantum dots (N‐GQDs) on carbonized MOF materials (cZIF‐8) interweaved with carbon nanotubes (CNTs) for flexible all‐solid‐state supercapacitors. In this ternary electrode, cZIF‐8 provides a large accessible surface area, CNTs act as the electrical conductive network, and N‐GQDs serve as highly pseudocapactive materials. Due to the synergistic effect and hierarchical assembly of these components, N‐GQD@cZIF‐8/CNT electrodes exhibit a high specific capacitance of 540 F g^?1 at 0.5 A g^?1 in a 1 m H₂SO₄ electrolyte and excellent cycle stability with 90.9% capacity retention over 8000 cycles. The assembled supercapacitor possesses an energy density of 18.75 Wh kg^?1 with a power density of 108.7 W kg^?1. Meanwhile, three supercapacitors connected in series can power light‐emitting diodes for 20 min. All‐solid‐state N‐GQD@cZIF‐8/CNT flexible supercapacitor exhibits an energy density of 14 Wh kg^?1 with a power density of 89.3 W kg^?1, while the capacitance retention after 5000 cycles reaches 82%. This work provides an effective way to construct novel electrode materials with high energy storage density as well as good cycling performance and power density for high‐performance energy storage devices via the rational design.     

Large‐Area All‐Textile Pressure Sensors for Monitoring Human Motion and Physiological Signals

Wearable pressure sensors, which can perceive and respond to environmental stimuli, are essential components of smart textiles. Here, large‐area all‐textile‐based pressure‐sensor arrays are successfully realized on common fabric substrates. The textile sensor unit achieves high sensitivity (14.4 kPa^?1), low detection limit (2 Pa), fast response (≈24 ms), low power consumption (<6 µW), and mechanical stability under harsh deformations. Thanks to these merits, the textile sensor is demonstrated to be able to recognize finger movement, hand gestures, acoustic vibrations, and real‐time pulse wave. Furthermore, large‐area sensor arrays are successfully fabricated on one textile substrate to spatially map tactile stimuli and can be directly incorporated into a fabric garment for stylish designs without sacrifice of comfort, suggesting great potential in smart textiles or wearable electronics.     

All‐Carbon Pressure Sensors with High Performance and Excellent Chemical Resistance

An all‐carbon pressure sensor is designed and fabricated based on reduced graphene oxide (rGO) nanomaterials. By sandwiching one layer of superelastic rGO aerogel between two freestanding high‐conductive rGO thin papers, the sensor works based on the contact resistance at the aerogel–paper interfaces, getting rid of the alien materials such as polymers and metals adopted in traditional sensors. Without the limitation of alien materials, the all‐carbon sensors demonstrate an ultrawide detecting range (0.72 Pa–130 kPa), low energy consumption (≈0.58 µW), ultrahigh sensitivity (349–253 kPa^?1) at low‐pressure regime (<1.4 Pa), fast response time (8 ms at 1 kPa), high stability (10 000 unloading–loading cycles between 0 and 1 kPa), light weight (<10 mg), easily scalable fabrication process, and excellent chemical stability. These merits enable them to detect real‐time human physiological signals and monitor the weights of various droplets of not only water but also hazardous chemical reagents including strong acid, strong alkali, and organic solvents. This shows their great potential applications in real‐time health monitoring, sport performance detecting, harsh environment‐related robotics and industry, and so forth.     

Healable,Transparent, Room‐Temperature Electronic Sensors Based on Carbon Nanotube Network‐Coated Polyelectrolyte Multilayers

Transparent and conductive film based electronics have attracted substantial research interest in various wearable and integrated display devices in recent years. The breakdown of transparent electronics prompts the development of transparent electronics integrated with healability. A healable transparent chemical gas sensor device is assembled from layer‐by‐layer‐assembled transparent healable polyelectrolyte multilayer films by developing effective methods to cast transparent carbon nanotube (CNT) networks on healable substrates. The healable CNT network‐containing film with transparency and superior network structures on self‐healing substrate is obtained by the lateral movement of the underlying self‐healing layer to bring the separated areas of the CNT layer back into contact. The as‐prepared healable transparent film is assembled into healable transparent chemical gas sensor device for flexible, healable gas sensing at room temperature, due to the 1D confined network structure, relatively high carrier mobility, and large surface‐to‐volume ratio. The healable transparent chemical gas sensor demonstrates excellent sensing performance, robust healability, reliable flexibility, and good transparency, providing promising opportunities for developing flexible, healable transparent optoelectronic devices with the reduced raw material consumption, decreased maintenance costs, improved lifetime, and robust functional reliability.     

Stretchable,Wireless Sensors and Functional Substrates for Epidermal Characterization of Sweat

This paper introduces materials and architectures for ultrathin, stretchable wireless sensors that mount on functional elastomeric substrates for epidermal analysis of biofluids. Measurement of the volume and chemical properties of sweat via dielectric detection and colorimetry demonstrates some capabilities. Here, inductively coupled sensors consisting of LC resonators with capacitive electrodes show systematic responses to sweat collected in microporous substrates. Interrogation occurs through external coils placed in physical proximity to the devices. The substrates allow spontaneous sweat collection through capillary forces, without the need for complex microfluidic handling systems. Furthermore, colorimetric measurement modes are possible in the same system by introducing indicator compounds into the depths of the substrates, for sensing specific components (OH^?, H⁺, Cu⁺, and Fe²⁺) in the sweat. The complete devices offer Young's moduli that are similar to skin, thus allowing highly effective and reliable skin integration without external fixtures. Experimental results demonstrate volumetric measurement of sweat with an accuracy of 0.06 μL/mm² with good stability and low drift. Colorimetric responses to pH and concentrations of various ions provide capabilities relevant to analysis of sweat. Similar materials and device designs can be used in monitoring other body fluids.     

Nanostructured Materials for Room‐Temperature Gas Sensors

Sensor technology has an important effect on many aspects in our society, and has gained much progress, propelled by the development of nanoscience and nanotechnology. Current research efforts are directed toward developing high‐performance gas sensors with low operating temperature at low fabrication costs. A gas sensor working at room temperature is very appealing as it provides very low power consumption and does not require a heater for high‐temperature operation, and hence simplifies the fabrication of sensor devices and reduces the operating cost. Nanostructured materials are at the core of the development of any room‐temperature sensing platform. The most important advances with regard to fundamental research, sensing mechanisms, and application of nanostructured materials for room‐temperature conductometric sensor devices are reviewed here. Particular emphasis is given to the relation between the nanostructure and sensor properties in an attempt to address structure–property correlations. Finally, some future research perspectives and new challenges that the field of room‐temperature sensors will have to address are also discussed.     

Chemical Design of Palladium‐Based Nanoarchitectures for Catalytic Applications

Palladium (Pd) plays an important role in numerous catalytic reactions, such as methanol and ethanol oxidation, oxygen reduction, hydrogenation, coupling reactions, and carbon monoxide oxidation. Creating Pd‐based nanoarchitectures with increased active surface sites, higher density of low‐coordinated atoms, and maximized surface coverage for the reactants is important. To address the limitations of pure Pd, various Pd‐based nanoarchitectures, including alloys, intermetallics, and supported Pd nanomaterials, have been fabricated by combining Pd with other elements with similar or higher catalytic activity for many catalytic reactions. Herein, recent advances in the preparation of Pd‐based nanoarchitectures through solution‐phase chemical reduction and electrochemical deposition methods are summarized. Finally, the trend and future outlook in the development of Pd nanocatalysts toward practical catalytic applications are discussed.