Highly Ordered Thermoplastic Polyurethane/Aramid Nanofiber Conductive Foams Modulated by Kevlar Polyanion for Piezoresistive Sensing and Electromagnetic Interference Shielding

Porous Spindle-Knot Fiber by Fiber-Microfluidic Phase Separation for Water Collection and Nanopatterning

Porous spindle-knot structures have been found in many creatures, such as spider silk and the root of the soybean plant, which show interesting functions such as droplet collection or biotransformation. However, continuous fabrication of precisely controlled porous spindle-knots presents a big challenge, particularly in striking a balance among good structural controllability, low-cost, and functions. Here, we propose a concept of a fiber-microfluidics phase separation (FMF-PS) strategy to address the above challenge. This FMF-PS combines the advantages of a microchannel regulated Rayleigh instability of polymer solution coated onto a fiber with the nonsolvent-induced phase separation of the polymer solution, which enables continuous and cost-effective production of porous spindle-knot fiber (PSKF) with well-controlled size and porous structures. The critical factors controlling the geometry and the porous structures of the spindle-knot by FMF-PS have been systematically investigated. For applications, the PSKF exhibited faster water droplet nucleation, growth, and maximum water collection capability, compared to the control samples, as revealed by in situ water collection growth curves. Furthermore, high-level fabrics of the PSKFs, including a two-dimensional network and three-dimensional architecture, have been demonstrated for both large-scale water collection and art performance. Finally, the PSKF is demonstrated as a programmable building block for surface nanopatterning.

Oxhide gelatin-regulated aramid nanofiber/liquid metal films with sandwiched structure for electromagnetic interference shielding

Liquid metal (LM) based electromagnetic interference (EMI) shielding materials with high conductivity and continuous deformation capacity are important needs for meeting modern advanced electronic equipment. However, an independent free-standing film with LM is difficult to achieve due to its unique fluidity properties. Here, a simple alternating filtration film-forming method was utilized to orderly construct a sandwiched EMI shielding film with LM stabilized by bio-based oxhide gelatin (gel) as the intermediate conductive layer, and two films of aramid nanofibers/oxhide gel (ANF/gel) as the external insulating protective layers. This design not only prevents LM from being exposed to environmental conditions, but also reduces the risk of chemical corrosion in practical applications. Under optimal LM addition conditions, the sandwiched film (0.3-3 L) exhibited better EMI shielding performance of 50.4 dB in the X-band than the blended film (0.7 dB), as well as excellent mechanical properties (tensile strength of 65.8 MPa, strain 8.6 %). More importantly, the sandwiched film still maintained reliable EMI shielding performance after being experienced largely physical deformation. This study provides a new solution for preparing LM-based EMI shielding composites, and is expected to arouse pursuit of high EMI shielding effects of bio-based gel while also paying attention to their safety.

Ultrafine Aramid Nanofiber-Assisted Large-Area Dense Stacking of MXene Films for Electromagnetic Interference Shielding and Multisource Thermal Conversion

Polymers are often used as adhesives to improve the mechanical properties of flexible electromagnetic interference (EMI) shielding layered films, but the introduction of these insulating adhesives inevitably reduces the EMI performance. Herein, ultrafine aramid nanofibers (UANF) with a diameter of only 2.44 nm were used as the binder to effectively infiltrate and minimize the insulating gaps in MXene films, for balancing the EMI shielding and mechanical properties. Combining the evaporation-induced scalable assembly assisted by blade coating, flexible large-scale MXene/UANF films with highly aligned and compact MXene stacking are successfully fabricated. Compared with the conventional ANF with a larger diameter of 7.05 nm, the UANF-reinforced MXene film exhibits a "brick-mortar" structure with higher orientation and compacter stacking MXene nanosheets, thus showing the higher mechanical properties, electrical conductivity, and EMI shielding performance. By optimizing MXene content, the MXene/UANF film can achieve the optimal tensile strength of 156.9 MPa, a toughness of 2.9 MJ m-3, satisfactory EMI shielding effectiveness (EMI SE) of 40.7 dB, and specific EMI SE (SSE/t) of 22782.4 dB cm2/g). Moreover, the composite film exhibits multisource thermal conversion functions including Joule heating and photothermal conversion. Therefore, the multifunctional MXene/UANF EMI shielding film with flexibility, foldability, and robust mechanical properties shows the practical potential in complex application environments.

Ultrastrong MXene film induced by sequential bridging with liquid metal

Assembling titanium carbide (Ti3C2Tx) MXene nanosheets into macroscopic films presents challenges, including voids, low orientation degree, and weak interfacial interactions, which reduce mechanical performance. We demonstrate an ultrastrong macroscopic MXene film using liquid metal (LM) and bacterial cellulose (BC) to sequentially bridge MXene nanosheets (an LBM film), achieving a tensile strength of 908.4 megapascals. A layer-by-layer approach using repeated cycles of blade coating improves the orientation degree to 0.935 in the LBM film, while a LM with good deformability reduces voids into porosity of 5.4%. The interfacial interactions are enhanced by the hydrogen bonding from BC and the coordination bonding with LM, which improves the stress-transfer efficiency. Sequential bridging provides an avenue for assembling other two-dimensional nanosheets into high-performance materials.

Recent Advances in Flexible Pressure Sensors Based on MXene Materials

In the past decade, with the rapid development of wearable electronics, medical health monitoring, the Internet of Things, and flexible intelligent robots, flexible pressure sensors have received unprecedented attention. As a very important kind of electronic component for information transmission and collection, flexible pressure sensors have gained a wide application prospect in the fields of aerospace, biomedical and health monitoring, electronic skin, and human-machine interface. In recent years, MXene has attracted extensive attention because of its unique 2D layered structure, high conductivity, rich surface terminal groups, and hydrophilicity, which has brought a new breakthrough for flexible sensing. Thus, it has become a revolutionary pressure-sensitive material with great potential. In this work, the recent advances of MXene-based flexible pressure sensors are reviewed from the aspects of sensing type, sensing mechanism, material selection, structural design, preparation strategy, and sensing application. The methods and strategies to improve the performance of MXene-based flexible pressure sensors are analyzed in details. Finally, the opportunities and challenges faced by MXene-based flexible pressure sensors are discussed. This review will bring the research and development of MXene-based flexible sensors to a new high level, promoting the wider research exploitation and practical application of MXene materials in flexible pressure sensors.

Flexible Piezoresistive Sensors Based on PPy Granule-Anchored Multilayer Fibrous Membranes with a Wide Operating Range and High Sensitivity

The employment of flexible piezoresistive sensors has sparked growing interest within the realm of wearable electronic devices, specifically in the fields of health detection and e-skin. Nevertheless, the advancement of piezoresistive sensors has been impeded by their limited sensitivity and restricted operating ranges. Consequently, it is imperative to fabricate sensors with heightened sensitivity and expanded operating ranges through the utilization of the appropriate methodologies. In this paper, piezoresistive sensors were fabricated utilizing electrospun polyvinylidene fluoride/polyacrylonitrile/polyethylene-polypropylene glycol multilayer fibrous membranes anchored with polypyrrole granules as the sensing layer, while electrospun thermoplastic polyurethane (TPU) fibers were employed as the flexible substrate. The sensitivity of the sensor is investigated by varying the fiber diameter of the sensing layer. The experimental findings reveal that a concentration of 14 wt % in the spinning solution exhibits high sensitivity (996.7 kPa-1) within a wide working range (0-10 kPa). This is attributed to the favorable diameter of the fibers prepared at this concentration, which facilitates the uniform in situ growth of pyrrole. The highly deformable TPU flexible fibers and multilayer sensing layer structure enable different linear responses across a broad pressure range (0-1 MPa). Furthermore, the sensor demonstrates good cyclic stability and can detect human movements under different pressures. These results suggest that the piezoresistive sensor with a wide operating range and high sensitivity has significant potential for future health monitoring and artificial intelligence applications.

High-Compressive, Elastic, and Wearable Cellulose Nanofiber-Based Carbon Aerogels for Efficient Electromagnetic Interference Shielding

Developing excellent electromagnetic interference (EMI) shielding materials with robust EMI shielding efficiency (SE), high mechanical performance, and multifunctionality is imperative. Carbon materials are well recognized as promising alternatives for high-performance EMI shielding, but their high brittleness greatly hampers their applications. In this work, a cellulose nanofiber/reduced graphene oxide-glucose carbon aerogel (C-CNFs/rGO-glu) with high compression, elasticity, and excellent EMI shielding performance was fabricated by directional freeze-drying followed by carbonization. Specifically, the height and stress retention are 88% and 90.9%, respectively, after 100 cycles of compression release at a high strain of 70%. The electromagnetic shielding effectiveness of the aerogels reached 67.5 dB and presented an absorption-dominant shielding mechanism with a 97.5% absorption loss ratio. Further, the carbon aerogel could capture subtle electrical signals to monitor different human behaviors and showed excellent heat insulation and infrared stealth performance.

Multifunctional black phosphorus pressure sensors with bending angle monitoring and direction recognition characteristics

Flexible pressure sensors, an important class of intelligent sensing devices, are widely explored in body-motion and medical health monitoring, artificial intelligence and human-machine interaction. As a unique layered nanomaterial, black phosphorus (BP) has excellent electrical, mechanical, and flexible characteristics, which make it a promising candidate for fabricating high-performance pressure sensors. Herein, hierarchically structured BP-based pressure sensors were constructed. The sensors exhibit high sensitivity, stability and a wide sensing range and respond to various human motions including finger pressure, swallowing, and wrist bending. The sensors can identify different handwriting processes with featured signals. In particular, benefiting from the unique structure of loose-dense layers, the sensors show a distinctive response to bending angles and directions, revealing a characteristic of direction recognition. This feature facilitates the sensors to monitor human motions. The sensors have been successfully powered by a home-made Cu2ZnSn(S,Se)4 thin-film solar cell, which demonstrates the sustainability, flexibility and low power consumption of integrated devices. This work offers a strategy to construct hierarchically structured pressure/strain sensors with direction recognition and provides further insights into manufacturing portable sensing devices for realistic and innovative applications.

Scalable Production of Highly Conductive 2D NbSe2 Monolayers with Superior Electromagnetic Interference Shielding Performance

Thin, flexible, and electrically conductive films are in demand for electromagnetic interference (EMI) shielding. Two-dimensional NbSe2 monolayers have an electrical conductivity comparable to those of metals (106-107 S m-1) but are challenging for high-quality and scalable production. Here, we show that electrochemical exfoliation of flake NbSe2 powder produces monolayers on a large scale (tens of grams), at a high yield (>75%, monolayer), and with a large average lateral size (>20 μm). The as-exfoliated NbSe2 monolayer flakes are easily dispersed in diverse organic solvents and solution-processed into various macroscopic structures (e.g., free-standing films, coatings, patterns, etc.). Thermal annealing of the free-standing NbSe2 films reduces the interlayer distance of restacked NbSe2 from 1.18 to 0.65 nm and consequently enhances the electrical conductivity to 1.16 × 106 S m-1, which is superior to those of MXenes and reduced graphene oxide. The optimized NbSe2 film shows an EMI shielding effectiveness (SE) of 65 dB at a thickness of 5 μm (>110 dB for a 48-μm-thick film), among the highest in materials of similar thicknesses. Moreover, a laminate of two layers of the NbSe2 film (2 μm thick) with an insulating interlayer shows a high SE of 85 dB, surpassing that of the 20-μm-thick NbSe2 film (83 dB). A two-layer theoretical model is proposed, and it agrees with the experimental EMI SE of the laminated NbSe2 films. The ability to produce NbSe2 monolayers on a tens of grams scale will enable their diverse applications beyond EMI shielding.

Nacre-Inspired Aramid Nanofibers/Basalt Fibers Composite Paper with Excellent Flame Retardance and Thermal Stability by Constructing an Organic-Inorganic Fiber Alternating Layered Structure

The flame-retardant paper has gradually evolved into a necessary material in various industries as a result of the rising importance of fire safety, energy efficiency, and environmental preservation. Traditional cellulose paper requires the addition of a large amount of flame retardants to achieve flame retardancy, which poses a serious threat to mechanical quality and the environment. Therefore, there is an urgent need to develop inorganic fiber flame-retardant paper with good flexibility, high thermal stability, and inherent flame retardancy. Herein, inspired by the "brick-and-mortar" layered structure of nature nacre, we developed a layered composite paper with a unique alternating arrangement of organic-inorganic fibers by synergistically integrating environmentally sustainable basalt fiber (BF) and high-performance aramid nanofibers (ANFs) through a vacuum-assisted filtration process. The as-prepared ANFs/BF composite paper exhibited low thermal conductivity (0.024 W m-1 K-1), high tensile strength (54.22 MPa), and excellent flexibility. Thanks to its excellent thermal stability, the mechanical strength remains at a high level (92%) after heat treatment at 300 °C for 60 min. Furthermore, the peak heat release rate and smoke generation of ANFs/BF composite paper decreased by 44.6 and 95.3%, respectively. Therefore, the composite paper is promising for applications as a protective layer in flexible electronic devices, cables, and fire-retardant and high-temperature fields.

Robust, ultrathin and flexible electromagnetic interference shielding paper designed with all-polysaccharide hydrogel and MXene

An effective strategy was demonstrated to design an electromagnetic interference (EMI) shielding paper via a facile surface treatment on paper. TEMPO-oxidized cellulose nanofibers (TOCN) were first integrated with Ti3C2Tx MXene, and subsequently cast onto a filter paper with cationic guar gum (CGG) in a sequential way. TOCN and CGG generated a self-assembling hydrogel and formed a MXene-containing hydrogel film on top of the filter paper. The hydrogel film enhanced the tensile strength (9.49 MPa) of composite paper, and resulted in a 17 % increase as compared to the control. The composite paper containing 80 mg MXene (namely, 2.07 mg·cm-2) showed a conductivity of 3843 S·m-1 and EMI shielding effectiveness (EMI SE) of 49.37 dB. Furthermore, the 2-layer assembled TC-M 80 hydrogel composite paper achieved an EMI SE of 73.99 dB. Importantly, this composite paper showed higher EMI SE and lower thickness than a lot of reported materials. The presence of TOCN and CGG also protected MXene against several solvents and the incorporation of polydimethylsiloxane (PDMS) further improved the durability of the composite paper. This work provides a novel and simple strategy to design robust, ultrathin and flexible EMI shielding materials, and it might also inspire other work in paper-based functional materials.

Metal-Organic Gel Leading to Customized Magnetic-Coupling Engineering in Carbon Aerogels for Excellent Radar Stealth and Thermal Insulation Performances

Metal-organic gel (MOG) derived composites are promising multi-functional materials due to their alterable composition, identifiable chemical homogeneity, tunable shape, and porous structure. Herein, stable metal-organic hydrogels are prepared by regulating the complexation effect, solution polarity and curing speed. Meanwhile, collagen peptide is used to facilitate the fabrication of a porous aerogel with excellent physical properties as well as the homogeneous dispersion of magnetic particles during calcination. Subsequently, two kinds of heterometallic magnetic coupling systems are obtained through the application of Kirkendall effect. FeCo/nitrogen-doped carbon (NC) aerogel demonstrates an ultra-strong microwave absorption of - 85 dB at an ultra-low loading of 5%. After reducing the time taken by atom shifting, a FeCo/Fe3O4/NC aerogel containing virus-shaped particles is obtained, which achieves an ultra-broad absorption of 7.44 GHz at an ultra-thin thickness of 1.59 mm due to the coupling effect offered by dual-soft-magnetic particles. Furthermore, both aerogels show excellent thermal insulation property, and their outstanding radar stealth performances in J-20 aircraft are confirmed by computer simulation technology. The formation mechanism of MOG is also discussed along with the thermal insulation and electromagnetic wave absorption mechanism of the aerogels, which will enable the development and application of novel and lightweight stealth coatings.

Metal and Metal Oxide Nanoparticle Incorporation in Polyurethane Foams: A Solution for Future Antimicrobial Materials?

With the technological developments witnessed in recent decades, nanotechnology and nanomaterials have found uses in several common applications and products we encounter daily. On the other hand, polyurethane (PU) foams represent an extremely versatile material, being widely recognized for their extensive application possibilities and possessing a multitude of fundamental attributes that enhance their broad usability across various application fields. By combining the versatility of PU with the antimicrobial properties of nanoparticles, this emerging field holds promise for addressing the urgent need for effective antimicrobial materials in various applications. In this comprehensive review, we explore the synthesis methods, properties and applications of these nanocomposite materials, shedding light on their potential role in safeguarding public health and environmental sustainability. The main focus is on PU foams containing metal and metal oxide nanoparticles, but a brief presentation of the progress documented in the last few years regarding other antimicrobial nanomaterials incorporated into such foams is also given within this review in order to obtain a larger image of the possibilities to develop improved PU foams.

Preparation and Mechanical Properties of UV-Curable Epoxy Acrylate/Modified Aramid Nanofiber Nanocomposite Films

In order to enhance the mechanical properties of UV-curable epoxy acrylate (EA)-based coatings, 3-(trimethoxysilyl)propyl methacrylate modified aramid nanofibers (T-ANFs) were synthesized and used as nanofillers to prepare EA/T-ANF nanocomposite films. The morphology of T-ANFs was characterized by transmission electron microscopy. The chemical structure of T-ANFs was analyzed via infrared spectroscopy, confirming successful grafting of methyl methacryloyloxy groups onto the surface of aramid nanofibers (ANFs). Real-time infrared spectroscopy was employed to investigate the influence of ANFs and T-ANFs on the photopolymerization kinetics of the EA film. The results revealed that the addition of ANFs and T-ANFs led to a decrease in the photopolymerization rate during the initial stage but had little impact on the final double bond conversion, with all samples exhibiting a conversion rate of over 83%. The incorporation of ANFs improved the tensile strength of the films while significantly reducing their Young's modulus. In contrast, the addition of T-ANFs led to a substantial increase in both tensile stress and Young's modulus of the films. For instance, the tensile strength and Young's modulus of the 0.1 wt% of T-ANF film increased by 52.7% and 41.6%, respectively, compared to the pure EA film. To further study the dispersion morphology and reinforcement mechanism, the cross-sectional morphology of the films was characterized by scanning electron microscopy.

Flexible MXene/Nanocellulose Composite Aerogel Film with Cellular Structure for Electromagnetic Interference Shielding and Photothermal Conversion

With the rapid development of wearable devices and integrated systems, protection against electromagnetic waves is an issue. For solving the problems of poor flexibility and a tendency to corrode traditional electromagnetic interference (EMI) shielding materials, two-dimensional (2D) nanomaterial MXene was employed to manufacture next-generation EMI shielding materials. Vacuum-assisted filtration combined with the liquid nitrogen prefreezing strategy was adopted to prepare flexible MXene/cellulose nanofibers (CNFs) composite aerogel film with unique cellular structure. Here, CNFs were employed as the reinforcement, and such a cellular structure design can effectively improve the shielding effectiveness (SE). In particular, the composite shows an outstanding EMI SE of 54 dB. Furthermore, the MXene/CNFs composite aerogel film exhibited prominent and steady photothermal conversion ability, which could obtain the maximum equilibrium temperature of 89.4 °C under an 808 nm NIR laser. Thus, our flexible composite aerogel film with appealing cellular construction holds great promise for wearable EMI shielding materials and heating applications in a cold and complex practical environment.