In the context of the rapid development of artificial intelligence and robotics, their application scenarios are continuously expanding to a variety of complex environments, with increasing attention being paid to the use of flexible sensors in low-temperature environments. In this study, an ionic hydrogel was synthesized using acrylamide (AM), hydroxyethyl cellulose (HEC), and lithium chloride (LiCl) as composites. This hydrogel exhibits high adhesion, excellent sensitivity (gauge factor (GF) = 2.84), rapid response time (100 ms), exceptional stretch ability (> 1776%), high toughness (2.5 MJ/m3), and the ability to maintain detectability at low temperatures (−60 °C). HEC imparts reliable mechanical properties to the sensor through hydrogen bonding interactions of its hydroxyl groups. LiCl ensures that the sensor has outstanding anti-freezing properties, maintains good conductivity and mechanical performance. Used for robotic attitude detection, the sensor demonstrated accurate recognition of various joint movements at both 20 and −20 °C. This technology was extended to industrial operations and maintenance, where a mechanical claw was used to grasp parts at both room temperature and low temperature. A convolutional neural network deep learning algorithm was employed to identify and classify eight types of parts, achieving an impressive recognition accuracy of 98.8%. The polyacrylamide (PAM)/HEC/LiCl hydrogel sensor demonstrates the capability for wide-temperature range detection in flexible robotics, holding significant potential for future applications in human-machine interaction, tactile perception, and related fields.
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Respiratory monitoring is essential for determining the vital characteristics of the human body and diagnosis of sleep-disordered breathing, critical diseases, and neonates. Moreover, micro/nanosensors play an important role in healthcare. In particular, they can detect changes in humidity around the nose and mouth for analysis of respiratory behavior. Herein, a high-performance flexible wearable humidity sensor was developed using a chitosan/conductive carbon black (CS/CB) hybrid film as the sensitive layer for respiratory detection.
The humidity sensor was fabricated by screen-printing a CS/CB film on a flexible poly (ethylene terephthalate) substrate with interdigitated electrodes. Scanning electron microscopy (SEM) was conducted to demonstrate the successful synthesis and combination of the CS/CB hybrid film. The SEM results showed that the CB particles are uniformly embedded in the CS/CB film with porous and rough nanostructures, which can help water molecules diffuse into the sensitive film. The humidity performance of the humidity sensor was tested by switching the device between different RH levels established by several saturated salt solutions (11%–97% RH) at room temperature. The resistance of the CS/CB sensor was measured with a high-precision data logger. The CS/CB humidity sensor had good electrical conductivity, and its resistance was increased with the increase in RH with good linearity. The sensor was experimentally evaluated for its performance parameters, such as sensitivity, response recovery characteristics, and repeatability. The humidity switching test demonstrated that the sensor has a fast response rate and good discrimination to different humidity environments. The sensor under the humidity rise reduction test showed good symmetry and coincidence, indicating that the hysteresis effect can be ignored. The CS/CB humidity sensor demonstrated good repeatability, rapid response/recovery time, and negligible hysteresis. A flexible wearable film-type breath detection platform was developed using a single-chip microcomputer and LabVIEW technologies, which achieved real-time data reading and processing of humidity, communication between the microcontroller and computer terminal, and display of breath waveforms. The detection platform has low production costs and is easy to operate, providing new insights into the practical application of high-performance humidity sensor detection technology. The sensor is placed in the mask for respiratory behavior monitoring, including normal breathing, slow breathing, rapid breathing, stop breathing, Biot’s breathing, wave breathing, tidal breathing, febrile patient breathing, etc. The breathing frequency and intensity can be observed clearly.
The flexible wearable humidity sensor could monitor and analyze human respiratory conditions with high accuracy and reliability. It demonstrated good application in real- time human breath monitoring. The humidity-sensitive mechanism of the sensor was ascribed to the good hydrophilicity and many active sites (hydrophilic groups and vacancies) on the CS/CB nanomaterials to absorb water molecules. In addition, the CS/CB nanomaterial with high surface area had numerous surface-active sites to adsorb water molecules.
The developed CS/CB humidity sensor has great practical potential in wearable applications, especially in humidity sensing detection. It also promotes students’ in-depth understanding of the experiments and applications of flexible wearable sensors, effectively enhancing their motivation and effectiveness of teaching.
In recent years, multi-modal flexible tactile sensors have become an important direction in the development of electronic skin because of their excellent sensitivity, flexibility and wearable properties. In this work, a humidity-pressure multi-modal flexible sensor based on polypyrrole (PPy)/Ti3C2Tx sensitive film packaged with porous polydimethylsiloxane (PDMS) is investigated by combining the sensitive structure generation mechanism of in situ polymerization to achieve the simultaneous detection of humidity and pressure, which has a sensitivity of 89,113.4 Ω/% RH in a large humidity range of 0%–97% RH, and response/recovery time of 2.5/1.9 s. The tactile pressure sensing has a high sensitivity, a fast response of 67/52 ms, and a wide detection limit. The device also has excellent performance in terms of stability and repeatability, making it promising for respiratory pattern and motion detection. This work provides a new solution to address the construction of multi-modal tactile sensors with potential applications in the fields of medical health, epidemic prevention.
Two-dimensional (2D) nanomaterials have been widely used in gas sensing due to their large specific surface area, high surface reactivity, and excellent gas adsorption properties. This paper reviews the typical synthesis methods of various types of 2D nanomaterials and summarizes the recent progress in gas sensors based on 2D materials, such as noble metal nanoparticles (NPs), metal oxides (MOS), conductive polymers, other new 2D materials. The methods of doping, modification, and photoexcitation can effectively improve the gas-sensing properties of 2D materials. The sensitive mechanisms of heterojunction, Schottky junction, and photoexcitation in 2D material sensors are discussed in detail. This paper discusses the application prospects of 2D materials in wearable gas sensors, food safety, and self-powered sensing, and provides ideas for further applications in environmental quality monitoring and disease diagnosis. In addition, the opportunities and challenges for gas sensors based on 2D materials are also discussed.
Humidity sensors are of significance in various fields, such as environmental and food quality monitoring, industrial processing, wearable and flexible electronics, and human health care. High-performance humidity sensors with high sensitivity, rapid response time, and good stability are of paramount importance in humidity sensing. In this paper, diversiform humidity sensors with different sensing mechanisms are summarized, including resistive, impedance, capacitive, quartz crystal microbalance (QCM), surface acoustic wave (SAW), field-effect transistor (FET), and optical fiber humidity sensors. Versatile nanomaterials such as graphene, transition-metal chalcogenide, MXenes, black phosphorus (BP), boron nitride (BN), polymers, and nanofibers were promising building-blocks for constructing humidity sensors. The latest progress in the wearable and flexible humidity sensors, and self-powered humidity sensors was summarized. The diversiform applications of the humidity sensors with great prospects were demonstrated in various fields in terms of human respiratory monitoring, skin dryness diagnosing, fingertip approaching, and non-contact switch. Moreover, the challenges and prospects of nanomaterials-based humidity sensors were discussed.
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