Mechanically Robust and Elastic Graphene/Aramid Nanofiber/Polyaniline Nanotube Aerogels for Pressure Sensors

Advances in Ice-Templated Graphene Aerogels: Fabrication, Properties, and Applications

Graphene has been one of the most widely explored two-dimensional (2D) assemblies due to its outstanding mechanical, electrical, and thermal properties resulting from its unique characteristics of high anisotropy and strong carbon-carbon bonds. Aerogels, characterized by their ultralow density and ultrahigh specific surface area, stand out as leading porous materials. Therefore, the integration of graphene and aerogels would boost the development of multifunctional porous materials. Among the various methods for the fabrication of aerogels, ice-templating has received significant interest due to its ecofriendly nature as a physical process, its broad applicability across material systems, and its proficiency in constructing abundant structures for multifunctionalities. Consequently, ice-templating has become a prevalent technique for the efficient assembly of graphene nanosheets into aerogels with the inherited properties of graphene, the multifunctionality derived from diverse constituents, and the well-controlled architecture. In this review, we systematically summarize the development and progress of ice-templated graphene-based aerogels. Initially, we introduce the fabrication process of these aerogels, elaborating each step from precursor preparation to freezing, drying, and post-treatment. Subsequently, we demonstrate the multifunctional applications of ice-templated graphene aerogels with various macroarchitectures and microstructures. Finally, this review concludes with a straightforward summary, highlighting the challenges and opportunities associated with the ice-templated fabrication of graphene-based aerogels. This systematic review of graphene aerogels aims to offer new insights into the design and ice-templated fabrication of innovative aerogels with multiscale architecture and multifunctionalities, which are crucial for a variety of engineering applications.

Challenges and Opportunities of Uranium Extraction From Seawater: a Systematic Roadmap From Laboratory to Industry

Uranium extraction from seawater (UES) is crucial for ensuring the sustainable development of nuclear power and has seen significant advancements in recent years. However, natural seawater is a highly complex biogeochemical system, characterized by an extremely low uranium (U) concentration (≈3.3 µg L-1), abundant competitive ions, and significant marine biological pollution, making UES a formidable challenge. This review addresses the challenges encountered in UES and explores potential methods for enhancing the industrial UES system, including membrane separation, electrochemistry, photocatalysis, and biosorption. Additionally, several representative marine tests are summarized and restrictive factors of large-scale UES are analyzed. Finally, the further development of UES from laboratory to industry applications is promoted, with a focus on technological innovation. The goal is to stimulate innovative ideas and provide fresh insights for the future development of the UES system, bridging the gap between laboratory research and industrial implementation.

Highly compressible lamellar graphene/cellulose/sodium alginate aerogel via bidirectional freeze-drying for flexible pressure sensor

Graphene exhibits exceptional electrical properties, and aerogels made from it demonstrate high sensitivity when used in sensors. However, traditional graphene aerogels have poor biocompatibility and sustainability, posing potential environmental and health risks. Moreover, the stacking of their internal structures results in low compressive strength and fatigue resistance, which limits their further applications. In this study, green and sustainable cellulose nanofibers/sodium alginate/reduced graphene oxide aerogels (BCSRA) were synthesized, featuring a well-defined lamellar structure and a fiber cross-linked network, employing techniques such as bidirectional freeze-drying, ionic cross-linking, and thermal annealing. The unique internal architecture of BCSRA results in a compressive strength of up to 64.55 kPa at 70 % compression deformation and an extremely low density of merely 7.21 mg/cm3. Furthermore, BCSRA displays outstanding fatigue resistance, maintaining 82.17 % of its stress after 100 compression cycles at 70 % compressive strain and 86.99 % after 1000 cycles at 50 % compressive strain. Remarkably, when utilized as a flexible pressure sensor, BCSRA achieves a sensitivity of 5.71 kPa-1 and endures over 2200 cycles at 40 % compression, all while ensuring consistent electrical signal output. These properties underscore its significant potential for application in wearable flexible pressure sensors, capable of detecting various human movements.

Innovations Progress in Emerging Multifunctional Aramid Nanofiber Aerogels

Aerogels have attracted great attention in the academic and industrial fields due to their intrinsic low density, high porosity, and high specific surface area. As an ideal nanobuilding block for advanced aerogels, the research community has shown a strong interest in aramid nanofiber (ANF) aerogel materials and their applications in frontier fields. However, there is a lack of a comprehensive review of the basic development and practical applications of pure ANF aerogels regarding these latest achievements in the past decade. This review aims to fill this gap by comprehensively exploring the preparation, properties, application progress, challenges, and prospects of ANF aerogels. We begin with a summary of the rheological principles of ANF dispersions, different preparation strategies, and drying methods of ANF aerogels. Then, the unique properties of ANF aerogels of various morphologies and outstanding advantages based on these properties are critically reviewed. Besides, the progress of multifunctional applications in the fields of flexible thermal insulation, environmental protection, energy storage, and other fields is summarized. Finally, we explore the main challenges and prospects of ANF aerogels to give insights for further development promotion from lab-scale to industry-scale.

Biomimetic Honeycomb-like Ti3C2Tx MXene/Bacterial Cellulose Aerogel-Based Flexible Pressure Sensor for the Human-Computer Interface

The pursuit of efficient and accurate human-computer interface design urgently requires high-performance sensors with pressure sensitivity, a wide detection range, and excellent cycling stability. Herein, a biomimetic honeycomb-like Ti3C2Tx MXene/bacterial cellulose (BC) aerogel with a negative Poisson's ratio (ν = -0.14) synthesized from the bidirectional freeze-drying method is used as the active material for a flexible pressure sensor, which exhibits high sensitivity (20.14 kPa-1), fast response time (100 ms), excellent mechanical durability (5000 cycles), and a low detection limit (responding to a grain of rice weighing about 0.022 g). Moreover, when assembled into the sandwich-structured bending sensor with the aerogel layer at just 0.8 mm in thickness, the aerogel-based device has a wide angular detection range (2.7-156.3°), high sensitivity (0.47 deg-1), and good robustness, proving outstanding electromechanical performance. Significantly, a smart glove consisting of five bending sensors fixed to the proximal knuckles and a flexible circuit board as the signal processing unit was designed for the identification of the shape, demonstrating its promising applications in the field of human-computer interaction.

Wide-temperature-range pressure sensing by an aramid nanofibers/reduced graphene oxide flakes composite aerogel

Aerogel-based conductive materials have emerged as a major candidate for piezoresistive pressure sensors due to their excellent mechanical and electrical performance besides light-weighted and low-cost characteristics, showing great potential for applications in electronic skins, biomedicine, robot controlling and intelligent recognition. However, it remains a grand challenge for these piezoresistive sensors to achieve a high sensitivity across a wide working temperature range. Herein, we report a highly flexible and ultra-light composite aerogel consisting of aramid nanofibers (ANFs) and reduced graphene oxide flakes (rGOFs) for application as a high-performance pressure sensing material in a wide temperature range. By controlling the orientations of pores in the composite framework, the aerogel promotes pressure transfer by aligning its conductive channels. As a result, the ANFs/rGOFs aerogel-based piezoresistive sensor exhibits a high sensitivity of up to 7.10 kPa-1, an excellent stability over 12,000 cycles, and an ultra-wide working temperature range from -196 to 200 °C. It is anticipated that the ANFs/rGOFs composite aerogel can be used as reliable sensing materials in extreme environments.

Highly Flexible and Compressible 3D Interconnected Graphene Foam for Sensitive Pressure Detection

A flexible pressure sensor, capable of effectively detecting forces exerted on soft or deformable surfaces, has demonstrated broad application in diverse fields, including human motion tracking, health monitoring, electronic skin, and artificial intelligence systems. However, the design of convenient sensors with high sensitivity and excellent stability is still a great challenge. Herein, we present a multi-scale 3D graphene pressure sensor composed of two types of 3D graphene foam. The sensor exhibits a high sensitivity of 0.42 kPa-1 within the low-pressure range of 0-390 Pa and 0.012 kPa-1 within the higher-pressure range of 0.4 to 42 kPa, a rapid response time of 62 ms, and exceptional repeatability and stability exceeding 10,000 cycles. These characteristics empower the sensor to realize the sensation of a drop of water, the speed of airflow, and human movements.

Research Progress of Flexible Piezoresistive Sensors Based on Polymer Porous Materials

Flexible piezoresistive sensors are in high demand in areas such as wearable devices, electronic skin, and human-machine interfaces due to their advantageous features, including low power consumption, excellent bending stability, broad testing pressure range, and simple manufacturing technology. With the advancement of intelligent technology, higher requirements for the sensitivity, accuracy, response time, measurement range, and weather resistance of piezoresistive sensors are emerging. Due to the designability of polymer porous materials and conductive phases, and with more multivariate combinations, it is possible to achieve higher sensitivity and lower detection limits, which are more promising than traditional flexible sensor materials. Based on this, this work reviews recent advancements in research on flexible pressure sensors utilizing polymer porous materials. Furthermore, this review examines sensor performance optimization and development from the perspectives of three-dimensional porous flexible substrate regulation, sensing material selection and composite technology, and substrate and sensing material structure design.

Nanofiber-Reinforced MXene/rGO Composite Aerogel for a High-Performance Piezoresistive Sensor and an All-Solid-State Supercapacitor Electrode Material

The assembly of two-dimensional (2D) nanomaterials into a three-dimensional (3D) aerogel can effectively prevent the problem of restacking. Here, nanofiber-reinforced MXene/reduced graphene oxide (rGO) conductive aerogel is prepared via the hydrothermal reduction of GO using pyrrole and in situ composite with MXene. Combined with low-content 2D conductive nanosheets (MXene and rGO) as "brick", conductive polypyrrole as "mortar", and one-dimensional (1D) nanofiber as "rebar", a strong interfacial cross-linking of MXene and rGO nanosheets is realized through covalent and noncovalent bonds to synergistically improve its mechanical performance. Based on the prepared MXene/rGO aerogel, a high-performance piezoresistive sensor with a sensitivity of up to 20.80 kPa-1 in a wide pressure range of 15.6 kPa is obtained, and it can withstand more than 5000 cyclic compressions. Besides, the sensor shows a stable output and can be applied to monitor various human motion signals. In addition, an all-solid-state supercapacitor electrode is also fabricated, which shows a high area-specific capacitance of up to 274 mF/cm2 at a current density of 1 mA/cm2.

High-Performance Graphene Flexible Sensors for Pulse Monitoring and Human-Machine Interaction

Flexible sensors are of great interest due to their potential applications in human physiological signal monitoring, wearable devices, and healthcare. However, sensor devices employed for cardiovascular testing are normally bulky and expensive, which hamper wearability and point-of-care use. Herein, we report a simple method for preparing multifunctional flexible sensors using hydrazine hydrate (N2H4·H2O) as the reducing agent, graphene as the active material, and polyethylene (PE) tape as the encapsulation material. The flexible sensor produced with this method has a low detection limit of 100 mg, a fast response and recovery time of 40 and 20 ms, and shows no performance degradation even after up to 30,000 motion cycles. The sensors we have developed are capable of monitoring the pulse with relative accuracy, which presents an opportunity to replace bulky devices and normalize cardiovascular testing in the future. In order to further broaden the application field, the sensor is installed as a sensor array to recognize objects of different weights and shapes, showing that the sensor has excellent application potential in wearable artificial intelligence.

MXene-Based Chemo-Sensors and Other Sensing Devices

MXenes have received worldwide attention across various scientific and technological fields since the first report of the synthesis of Ti3C2 nanostructures in 2011. The unique characteristics of MXenes, such as superior mechanical strength and flexibility, liquid-phase processability, tunable surface functionality, high electrical conductivity, and the ability to customize their properties, have led to the widespread development and exploration of their applications in energy storage, electronics, biomedicine, catalysis, and environmental technologies. The significant growth in publications related to MXenes over the past decade highlights the extensive research interest in this material. One area that has a great potential for improvement through the integration of MXenes is sensor design. Strain sensors, temperature sensors, pressure sensors, biosensors (both optical and electrochemical), gas sensors, and environmental pollution sensors targeted at volatile organic compounds (VOCs) could all gain numerous improvements from the inclusion of MXenes. This report delves into the current research landscape, exploring the advancements in MXene-based chemo-sensor technologies and examining potential future applications across diverse sensor types.

Graphene-based Two-Stage Enhancement Pressure Sensor for Subtle Mechanical Force Monitoring

The development of pressure sensors with high sensitivity and a low detection limit for subtle mechanical force monitoring and the understanding of the sensing mechanism behind subtle mechanical force monitoring are of great significance for intelligent technology. Here, we proposed a graphene-based two-stage enhancement pressure sensor (GTEPS), and we analyzed the difference between subtle mechanical force monitoring and conventional mechanical force monitoring. The GTEPS exhibited a high sensitivity of 62.2 kPa-1 and a low detection limit of 0.1 Pa. Leveraging its excellent performance, the GTEPS was successfully applied in various subtle mechanical force monitoring applications, including acoustic wave detection, voice-print recognition, and pulse wave monitoring. In acoustic wave detection, the GTEPS achieved a 100% recognition accuracy for six words. In voiceprint recognition, the sensor exhibited accurate identification of distinct voiceprints among individuals. Furthermore, in pulse wave monitoring, GTEPS demonstrated effective detection of pulse waves. By combination of the pulse wave signals with electrocardiogram (ECG) signals, it enabled the assessment of blood pressure. These results demonstrate the excellent performance of GTEPS and highlight its great potential for subtle mechanical force monitoring and its various applications. The current results indicate that GTEPS shows great potential for applications in subtle mechanical force monitoring.

Preparation and characterization of highly conductive lignin aerogel based on tunicate nanocellulose framework

The highly conductive and elastic three-dimensional mesh porous material is an ideal platform for the fabrication of high electrical conductivity conductive aerogels. Herein, a multifunctional aerogel that is lightweight, highly conductive and stable sensing properties is reported. Tunicate nanocellulose (TCNCs) with a high aspect ratio, high Young's modulus, high crystallinity, good biocompatibility and biodegradability was used as the basic skeleton to prepare aerogel by freeze-drying technique. Alkali lignin (AL) was used as the raw material, polyethylene glycol diglycidyl ether (PEGDGE) was used as the cross-linking agent, and polyaniline (PANI) was used as the conductive polymer. Preparation of aerogels by freeze-drying technique, in situ synthesis of PANI, and construction of highly conductive aerogel from lignin/TCNCs. The structure, morphology and crystallinity of the aerogel were characterized by FT-IR, SEM, and XRD. The results show that the aerogel has good conductivity (as high as 5.41 S/m) and excellent sensing performance. When the aerogel was assembled as a supercapacitor, the maximum specific capacitance can reach 772 mF/cm² at 1 mA/cm² current density, and maximum power and energy density can reach 59.4 μWh/cm² and 3600 μW/cm², respectively. It is expected the aerogel can be applied in the field of wearable devices and electronic skin.

A Review of Graphene-Based Materials/Polymer Composite Aerogels

The fabrication of composite materials is an effective way to improve the performance of a single material and expand its application range. In recent years, graphene-based materials/polymer composite aerogels have become a hot research field for preparing high-performance composites due to their special synergistic effects in mechanical and functional properties. In this paper, the preparation methods, structures, interactions, properties, and applications of graphene-based materials/polymer composite aerogels are discussed, and their development trend is projected. This paper aims to arouse extensive research interests in multidisciplinary fields and provide guidance for the rational design of advanced aerogel materials, which could then encourage efforts to use these new kinds of advanced materials in basic research and commercial applications.

Silver nanoparticles decorated meta-aramid nanofibrous membrane with advantageous properties for high-performance flexible pressure sensor

Flexible pressure sensors have received tremendous attention for various wearable applications. However, it remains a critical challenge to develop a flexible pressure sensor with excellent sensitivity performances and multiple advantageous properties. Herein, a high-performance flexible piezoresistive pressure sensor PMIA@PDA@Ag was developed, which sensitive component is consisted of Ag nanoparticles decorated polydopamine (PDA)-modified meta-aramid (poly(m-phenylene isophthalamide), PMIA) nanofibrous membrane. The PMIA@PDA@Ag pressure sensor shows excellent mechanical, thermal insulation, antibacterial and breathable properties, as well as remarkable sensing performances including high sensitivity, wide detectable pressure range, rapid response speed and good cyclic durability. In addition, it also shows great sensing performances in monitoring various human behaviors in real-time, including large-scale motions and subtle physiological signals.