High-performance hydrogel sensors with dual-network structure for wearable devices: Integration of self-healing, antimicrobial, extreme environmental tolerance, and long-term sensing stability

Wearable flexible strain sensors have shown great potential in fields such as medical health, motion monitoring, and human-computer interaction. Developing flexible strain sensors that possess excellent comprehensive performance and a simple manufacturing process remains a great challenge at present. Inspired by the numerous stomata of aquatic plant leaves in nature, here we propose a simple, efficient, and low-cost technique to fabricate flexible strain sensors with hierarchical micropore structures. The hierarchical micropores can effectively control stress distribution and guide crack propagation and termination on heterogeneous film surfaces, resulting in a network-like structure composed of many short, discontinuous crack segments. This feature endows the sensor with excellent comprehensive performances, including high sensitivity (up to 12138), a wide detection range (50%), a low detection limit (0.01%), fast response/recovery (108/97 ms), and outstanding durability (20,000 cycles). Such a high-performance sensor can accurately monitor various human activities such as pulse beating, microexpression, swallowing, joint bending, and vocalization by throat or loudspeaker vibration. Morse code-based information expression and encrypted transmission of complex information have been achieved by using a single or three bidirectionally bendable sensors. This work provides a facile strategy for fabricating high-performance sensors through biomimicry of plant leaves, demonstrating broad application prospects in wearable devices, electronic skins, soft robots, and information interactions.

Developing a high-performance capacitive sensor for diverse application scenarios has posed requirements for the sensor to have high sensitivity, broad detection range, and cost-effectiveness. In this experiment, a flexible pressure sensor with a high sensitivity of 2.08 kPa−1 at pressure lower than 1 kPa, as well as a wide working range of 0–600 kPa and remarkable stability (for at least 4000 cycles), was designed. In the device structure, silver nanowires (Ag NWs)/MXene-composite-coated polydimethylsiloxane (PDMS) and natural bamboo leaves at different growth stages were used as the electrode and the micro-structured dielectric layers, respectively. The rough surface of the composite conductive materials and the hierarchical microstructure of the bamboo leaves ensured a high sensitivity and broad pressure range of the sandwich-structured sensor, and the different sizes of the microstructures yielded adjustable sensitivity of the sensor. Furthermore, the outstanding performance of the proposed device made it possible to detect the actual object load, human physical stimuli, and proximity distance, demonstrating applications of flexible and wearable devices in various fields, such as weight/force tapping, breath/wrist pulse/speech, joint bending, and approach distance.

The development of high-performance, portable and miniaturized gas sensors has aroused increasing interest in the fields of environmental monitoring, security, medical diagnosis, and agriculture. Among different detection tools, metal oxide semiconductor (MOS)-based chemiresistive gas sensors are the most popular choice in commercial applications and have the advantages of high stability, low cost, and high sensitivity. One of the most important ways to further enhance the sensor performance is to construct MOS-based nanoscale heterojunctions (heteronanostructural MOSs) from MOS nanomaterials. However, the sensing mechanism of heteronanostructural MOS-based sensors is different from that of single MOS-based gas sensors in that it is fairly complex. The performance of the sensors is influenced by various parameters, including the physical and chemical properties of the sensing materials (e.g., grain size, density of defects, and oxygen vacancies of materials), working temperatures, and device structures. This review introduces several concepts in the design of high-performance gas sensors by analyzing the sensing mechanism of heteronanostructural MOS-based sensors. In addition, the influence of the geometric device structure determined by the interconnection between the sensing materials and the working electrodes is discussed. To systematically investigate the sensing behavior of the sensor, the general sensing mechanism of three typical types of geometric device structures based on different heteronanostructural materials are introduced and discussed in this review. This review will provide guidelines for readers studying the sensing mechanism of gas sensors and designing high-performance gas sensors in the future.

Real‐time monitoring of respiration is vital for human health, especially for forecasting the sleep‐related diseases. A respiratory monitoring system with high accuracy, wearing comfort, portability, and environmental tolerance, is highly desirable, which however remains a big challenge. Here, a multimodal hydrogel sensor with excellent comprehensive performance is fabricated to monitor respiration and diagnose obstructive sleep apnea syndrome (OSAS). The synthetic cellulose‐based hydrogel exhibits good mechanical properties and extreme temperature tolerance, ascribing to the synergistic effects between chemical cross‐linking and multiple hydrogen bonding within the hydrogel network. The fabricated hydrogel sensor can independently monitor the mechanical variation and the thermal change via output signals of capacitance and resistance, respectively. These extraordinary properties of the hydrogel sensor enable the highly reliable and accurate monitoring of the respiratory events and diagnosis of OSAS. This work provides the new and practical way for real‐time respiratory monitoring and preventing the occurrence of sleep‐related diseases.

Sensitivity, durability, and multifunction are the essential requirements for a high-performance wearable sensor. Here, we report a novel multifunctional sensor with high sensitivity and durability by using a buckled spider silk-like single-walled carbon nanotubes (SSL-SWNTs) film as the conducting network and a crack-shaped Au film as the sensitive transducer. Its high sensitivity is inspired by the crack-shaped structure of the spider's slit organs, while the high durability is inspired by the mechanical robustness of the spider silk. Similar to the spider's slit organs that can detect slight vibrations, our sensor also exhibits a high sensitivity especially to tiny strain. The proposed quantum tunneling model is consistent with experimental data. In addition, this sensor also responds sensitively to temperature with the sensitivity of 1.2%/°C. Because of the hierarchical structure like spider silk, this sensor possesses combined superiority of fast response (<60 ms) and high durability (>10 000 cycles). We also fabricate a wearable device for monitoring various human physiological signals. It is expect that this high-performance sensor will have wide potential applications in intelligent devices, fatigue detection, body monitoring, and human-machine interfacing.

High-performance flexible pressure sensors are crucial for achieving precise tactile sensing and play an indispensable role in human motion detection and human-machine interaction. In this study, a new low-cost flexible capacitive pressure sensor (CPS) is designed using the bionic microstructure of a grasshopper leg with polydimethylsiloxane (PDMS) as the dielectric layer. Through finite element simulation and structural optimization, the CPS can achieve high sensitivity (0.925 kPa<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{-{1}}$ </tex-math></inline-formula>), a wide pressure sensing range (5 Pa–388 kPa), fast response time (30 ms), excellent consistency across sensor batches, and outstanding stability. Additionally, the study demonstrates the CPS’s capabilities in intelligent robots manipulator operations, human hand grasping objects with tactile feedback, human motion posture detection, and information transfer of Morse code in practical applications. Given the outstanding performance of the CPS, it is poised to be a preferred choice for future wearable devices and human-machine interaction.

Ultrafast self-healing zwitterionic hydrogels reinforced by carboxymethyl chitosan-oxidized hyaluronic acid and graphene oxide toward high-performance strain sensors

As a flexible artificial material, the conductive hydrogel has broad application prospects in flexible wearable electronics, soft robotics, and biomedical monitoring. However, traditional hydrogels still face many challenges, such as long-term stability, availability in extreme environments, and long-lasting adhesion to the skin surface under sweaty or humid conditions. To circumvent the above issues, one kind of ionic conductive hydrogel was prepared by a simple one-pot method that dissolved chitosan (CS), 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS), tannic acid (TA), and 2-methoxy-ethyl acrylate (MEA) into dimethyl sulfoxide (DMSO)/H2O solvent. The resulting hydrogel showed excellent tensile properties (1440%), extreme environmental tolerance (-40-60 °C), adhesion (72 KPa at porcine skin), ionic conductivity (0.87 S m-1), and high-efficiency antibacterial property. Furthermore, the produced organohydrogel strain sensor exhibited high strain sensitivity (GF = 4.07), excellent signal sensing capabilities (human joint movement, microexpression, and sound signals), and long-term cyclic stability (400 cycles). Looking beyond, this work provides a simple and promising strategy for using hydrogel sensors in extreme environments for e-skin, health monitoring, and wearable electronic devices.

Programmable three-dimensional advanced materials based on nanostructures as building blocks for flexible sensors

A high-performance hydrogen gas sensor has been developed that utilizes a proton affinity of 1,4-diketo-3,6-bis- (-pyridyl)-pyrrolo-[3,4-c]-pyrrole (DPPP) known as a red pigment. We found that the N atom of the pyridyl ring of the DPPP can easily be protonated by protons dissociated from to induce a remarkable change in electrical conductivity by several orders of magnitude. The sensor operates in two steps: the first step is the dissociation of by means of a sputtered Pd-layer, followed by capturing protons by the N atom of the pyridyl ring (proton acceptor). The device structure is: . The appealing feature of the device is the reversible operation at room temperature as characterized by a change in electrical resistivity by two orders of magnitude even under 0.05% . The material is quite stable and the device is simple and compact.

Flexible and wearable sensors have drawn extensive concern due to their wide potential applications in wearable electronics and intelligent robots. Flexible sensors with high sensitivity, good flexibility, and excellent stability are highly desirable for monitoring human biomedical signals, movements and the environment. The active materials and the device structures are the keys to achieve high performance. Carbon nanomaterials, including carbon nanotubes (CNTs), graphene, carbon black and carbon nanofibers, are one of the most commonly used active materials for the fabrication of high-performance flexible sensors due to their superior properties. Especially, CNTs and graphene can be assembled into various multi-scaled macroscopic structures, including one dimensional fibers, two dimensional films and three dimensional architectures, endowing the facile design of flexible sensors for wide practical applications. In addition, the hybrid structured carbon materials derived from natural bio-materials also showed a bright prospect for applications in flexible sensors. This review provides a comprehensive presentation of flexible and wearable sensors based on the above various carbon materials. Following a brief introduction of flexible sensors and carbon materials, the fundamentals of typical flexible sensors, such as strain sensors, pressure sensors, temperature sensors and humidity sensors, are presented. Then, the latest progress of flexible sensors based on carbon materials, including the fabrication processes, performance and applications, are summarized. Finally, the remaining major challenges of carbon-based flexible electronics are discussed and the future research directions are proposed.

High-performance integrated field-effect transistor-based sensors

The advent of the 5G era means that the concepts of robot, VR/AR, UAV, smart home, smart healthcare based on IoT (Internet of Things) have gradually entered human life. Since then, intelligent life has become the dominant direction of social development. Humidity sensors, as humidity detection tools, not only convey the comfort of human living environment, but also display great significance in the fields of meteorology, medicine, agriculture and industry. Graphene-based materials exhibit tremendous potential in humidity sensing owing to their ultra-high specific surface area and excellent electron mobility under room temperature for application in humidity sensing. This review begins with the introduction of examples of various synthesis strategies of graphene, followed by the device structure and working mechanism of graphene-based humidity sensor. In addition, several different structural design methods of graphene are summarized, demonstrating the structural design of graphene can not only optimize the performance of graphene, but also bring significant advantages in humidity sensing. Finally, key challenges hindering the further development and practical application of high-performance graphene-based humidity sensors are discussed, followed by presenting the future perspectives.

Anti-bacterial silk-based hydrogels for multifunctional electrical skin with mechanical-thermal dual sensitive integration

Multi-component collaborative design yields robust hydrogel sensors with superior environmental adaptability for machine learning-assisted gesture recognition.