Silver Nanoflower Decorated Graphene Oxide Sponges for Highly Sensitive Variable Stiffness Stress Sensors

MXene/PPy@PDMS sponge-based flexible pressure sensor for human posture recognition with the assistance of a convolutional neural network in deep learning

The combination of flexible sensors and deep learning has attracted much attention as an efficient method for the recognition of human postures. In this paper, an in situ polymerized MXene/polypyrrole (PPy) composite is dip-coated on a polydimethylsiloxane (PDMS) sponge to fabricate an MXene/PPy@PDMS (MPP) piezoresistive sensor. The sponge sensor achieves ultrahigh sensitivity (6.8925 kPa-1) at 0-15 kPa, a short response/recovery time (100/110 ms), excellent stability (5000 cycles) and wash resistance. The synergistic effect of PPy and MXene improves the performance of the composite materials and facilitates the transfer of electrons, making the MPP sponge at least five times more sensitive than sponges based on each of the individual single materials. The large-area conductive network allows the MPP sensor to maintain excellent electrical performance over a large-scale pressure range. The MPP sensor can detect a variety of human body activity signals, such as radial artery pulse and different joint movements. The detection and analysis of human motion data, which is assisted by convolutional neural network (CNN) deep learning algorithms, enable the recognition and judgment of 16 types of human postures. The MXene/PPy flexible pressure sensor based on a PDMS sponge has broad application prospects in human motion detection, intelligent sensing and wearable devices.

Transparent Pneumatic Tactile Sensors for Soft Biomedical Robotics

Palpation is a simple but effective method to distinguish tumors from healthy tissues. The development of miniaturized tactile sensors embedded on endoscopic or robotic devices is key to achieving precise palpation diagnosis and subsequent timely treatment. This paper reports on the fabrication and characterization of a novel tactile sensor with mechanical flexibility and optical transparency that can be easily mounted on soft surgical endoscopes and robotics. By utilizing the pneumatic sensing mechanism, the sensor offers a high sensitivity of 1.25 mbar and negligible hysteresis, enabling the detection of phantom tissues with different stiffnesses ranging from 0 to 2.5 MPa. Our configuration, combining pneumatic sensing and hydraulic actuating, also eliminates electrical wiring from the functional elements located at the robot end-effector, thereby enhancing the system safety. The optical transparency path in the sensors together with its mechanical sensing capability open interesting possibilities in the early detection of solid tumor as well as in the development of all-in-one soft surgical robots that can perform visual/mechanical feedback and optical therapy.

Solid-state thermal rectification of bilayers by asymmetric elastic modulus

A highly efficient thermal rectification applicable to large panels still needs to be developed. Here, we experimentally achieve a high thermal rectification efficiency of 33% by carefully engineering elastic modulus asymmetry in a centimeter-scale bilayered silver-graphene oxide sponge. The thermal conduction primarily occurs in the out-of-plane direction, and the forward heat flow direction is from the hard silver to the soft graphene oxide. Surprisingly, the forward heat flow direction is reversed when a silver layer is formed on a harder polystyrene foam. The forward direction is always from the harder side to the softer side, and the asymmetry in elastic modulus is suggested as a possible mechanism based on the one-dimensional Frenkel-Kontorova (FK) model. The finite element analysis indicates that other mechanisms such as temperature-dependent thermal conductivity and radiation asymmetry cannot explain the high rectification efficiency. This scalable work over a wide temperature range may find immediate industrial applications.

Facile Fabrication of a Highly Sensitive and Robust Flexible Pressure Sensor with Batten Microstructures

As the foremost component of wearable devices, flexible pressure sensors require high sensitivity, wide operating ranges, and great stability. In this paper, a pressure sensor comprising a regular batten microstructure active layer is presented. First, the influences of the dimensional parameters of the microstructures on the performances of the sensors were investigated by the mechanical finite element method (FEM). Then, parameters were optimized and determined based on the results of this investigation. Next, active layers were prepared by molding multiwalled carbon nanotube/polyurethane (MWCNT/PU) conductive composite using a printed circuit board template. Finally, a resistive flexible pressure sensor was fabricated by combining an active layer and an interdigital electrode. With advantages in terms of the structure and materials, the sensor exhibited a sensitivity of up to 46.66 kPa⁻¹ in the range of 0–1.5 kPa and up to 6.67 kPa⁻¹ in the range of 1.5–7.5 kPa. The results of the experiments show that the designed flexible pressure sensor can accurately measure small pressures and realize real-time human physiological monitoring. Furthermore, the preparation method has the advantages of a low cost, simple design, and high consistency. Thus, it has potential to promote the development of flexible sensors, wearable devices, and other related devices.

Xylem-Inspired Polyimide/MXene Aerogels with Radial Lamellar Architectures for Highly Sensitive Strain Detection and Efficient Solar Steam Generation

Polyimide aerogels with mechanical robustness, great compressibility, excellent antifatigue properties, and intriguing functionality have captured enormous attention in diverse applications. Here, enlightened by the xylem parenchyma of dicotyledonous stems, a radially architectured polyimide/MXene composite aerogel (RPIMX) with reversible compressibility is developed by combining the interfacial enhancing strategy and radial ice-templating method. The strong interaction between MXene flakes and polymer can glue the MXene to form continuous lamellae, the ice crystals grow preferentially along the radial temperature gradient can effectively constrain the lamellae to create a biomimetic radial lamellar architecture. As a result, the nature-inspired RPIMX composite aerogel with centrosymmetric lamellar structure and oriented channels manifests excellent mechanical strength, electrical conductivity, and water transporting capability along the longitudinal direction, endowing itself with intriguing applications for accurate human motion monitoring and efficient photothermal evaporation. These exciting properties make the biomimetic RPIMX aerogels promising candidates for flexible piezoresistive sensors and photothermal evaporators.

Synthesis of silver leaves and their potential application for analysis and degradation of phenolic pollutants

A one-pot bottom-up synthesis method was used to synthesise multi-level leaf-like nano-silver (silver leaf) by simply mixing AgNO3 , L-ascorbic acid, Sodium sodium citrate, and polyvinylpyrrolidone (PVP) in the ethanol-water mixed solvents. Scanning electron microscopy (SEM) characterisations show that the silver leaves have tertiary structures and their sizes are controllable. In addition, silver leaves exhibit excellent Raman enhancement effect (SERS) and chemical catalytic activities for phenolic molecules. Interestingly, the SERS and catalytic activities increase as the size of the silver leaves decrease within a certain range, but when the size is too small, both of these performances weaken. The nanometre size and interstitial structure have a common amplification effect and influence on these activities. The present work not only showed a new method for the synthesis of silver leaves but also could be generalised to find other metallic leaves that could be used as promising heterogeneous catalysts for various reactions. The production of such small-sized silver leaves will facilitate the analysis of phenolic pollutants through Raman enhancement and treat these pollutants through catalytic degradation.

Recent Development of Flexible Tactile Sensors and Their Applications

With the rapid development of society in recent decades, the wearable sensor has attracted attention for motion-based health care and artificial applications. However, there are still many limitations to applying them in real life, particularly the inconvenience that comes from their large size and non-flexible systems. To solve these problems, flexible small-sized sensors that use body motion as a stimulus are studied to directly collect more accurate and diverse signals. In particular, tactile sensors are applied directly on the skin and provide input signals of motion change for the flexible reading device. This review provides information about different types of tactile sensors and their working mechanisms that are piezoresistive, piezocapacitive, piezoelectric, and triboelectric. Moreover, this review presents not only the applications of the tactile sensor in motion sensing and health care monitoring, but also their contributions in the field of artificial intelligence in recent years. Other applications, such as human behavior studies, are also suggested.

Polyimide Nanofiber-Reinforced Ti3C2Tx Aerogel with "Lamella-Pillar" Microporosity for High-Performance Piezoresistive Strain Sensing and Electromagnetic Wave Absorption

Assembling two-dimensional MXenes into 3D macroscopic structures is an applicable method to give full play to its excellent electrical and mechanical properties toward multi-functionality. Considering the weak interfacial interaction and poor gelation ability of MXenes, short polyimide nanofibers (PINFs) are utilized as cross-linking and supporting building blocks in this work to construct a lightweight, robust, and elastic PINF/Ti₃C₂T_x MXene composite aerogel (PINF/MA) via a simple synergistic assembly strategy. Taking advantage of its unique 3D "lamella-pillar" microporous architecture, the designed PINF/MA composite aerogel exhibits excellent piezoresistive sensing performance in terms of a wide pressure range of 0-8 kPa (50% strain), a high piezoresistive sensitivity of 22.32 kPa^-1, an ultra-low detection limit of 0.1% strain, and great compression/rebound stability (signal remained stable after 1500 cycles). These remarkable piezoresistive sensing properties enable the PINF/MA with intriguing capability to detect small and large human activities in real time (wrist and finger bending, pulse, and vocal cord vibration). More interestingly, the parallelly aligned leaf vein-like lamellae also empower the PINF/MA with prominent wave absorption performance [RL_min is -40.45 dB at 15.19 GHz, with an effective absorption bandwidth of 5.66 GHz (12.34-18 GHz)], making the multi-functional PINF/MA composite aerogels promising candidates for wearable strain sensors and microwave absorbers.

Micro-Nano Processing of Active Layers in Flexible Tactile Sensors via Template Methods: A Review

Template methods are regarded as an important method for micro-nano processing in the active layer of flexible tactile sensors. These template methods use physical/chemical processes to introduce micro-nano structures on the active layer, which improves many properties including sensitivity, response/recovery time, and detection limit. However, since the processing process and applicable conditions of the template method have not yet formed a perfect system, the development and commercialization of flexible tactile sensors based on the template method are still at a relatively slow stage. Despite the above obstacles, advances in microelectronics, materials science, nanoscience, and other disciplines have laid the foundation for various template methods, enabling the continuous development of flexible tactile sensors. Therefore, a comprehensive and systematic review of flexible tactile sensors based on the template method is needed to further promote progress in this field. Here, the unique advantages and shortcomings of various template methods are summarized in detail and discuss the research progress and challenges in this field. It is believed that this review will have a significant impact on many fields of flexible electronics, which is beneficial to promote the cross-integration of multiple fields and accelerate the development of flexible electronic devices.

Benzofurazan derivatives modified graphene oxide nanocomposite: Physico-chemical characterization and interaction with bacterial and tumoral cells

Two novel graphene oxide-benzofuran derivatives composites were obtained through the covalent immobilization of [4-hydrazinyl-7nitrobenz-[2,1,3-d]-oxadiazole (NBDH) and respectively, N1-(7-nitrobenzo[c][1,2,5]oxadiazol-4-yl)benzene-1,2-diamine (NBD-PD), on graphene oxide. This covalent functionalization was achieved by activating the carboxylic groups on the surface of graphene oxide by the reaction with thionyl chloride followed by coupling with the amino group of benzofurazane derivatives to obtain the NBD derivatives grafted on graphene oxide. The formation of new materials was check by Raman spectroscopy, fluorescence, infrared spectroscopy and X-ray photoelectron spectroscopy, thermal analysis, scanning electron microscopy, and elemental mapping. The antimicrobial effect of the new composites was evaluated on Staphylococcus aureus, Escherichia coli and Pseudomonas aeruginosa, both on planktonic and adherent biofilm populations. The cytotoxic effects of the materials on human colon cancer HCT-116 cell line and the normal human fibroblast BJ cell line were evaluated by investigating cell viability and membrane integrity. Apoptosis and colony forming ability of tumor cells were also investigated following exposure to new materials. The biological results of this study have shown that the new materials have potential in combating biofilm formation and also, the tested materials induced cytotoxicity in human colon cancer HCT-116 cell line with limited effects on normal BJ fibroblasts, suggesting their antitumor potential.

Ultrahigh compressibility and superior elasticity carbon framework derived from shaddock peel for high-performance pressure sensing

Shaddock peel, a crop by-product mainly composed of cellulose, hemicellulose, lignin, and pectin, was developed as a flexible sensitive material for detecting environmental external pressure. Firstly, a natural carbon framework (C-SPF) with high conductivity was prepared using hydrothermal treatment followed by carbonization. Then, the PDMS elastomer was coated on the C-SPF instead of dense filling to convert the brittle C-SPF into elastic porous materials (M-SPF). Benefiting from the large deformation space of the porous framework and the stable interactions between PDMS and C-SPF, M-SPF exhibited ultrahigh coercibility (up to 99.0% strain) and high elasticity (99.4% height retention for 10 000 cycles at 50.0% strain). The M-SPF-based pressure sensor also exhibited a quick response (loading and unloading times were 20 ms and 30 ms), high sensitivity (63.4 kPa⁻¹), wide working range (from 0 to 800 kPa), and stable stress-electric current response (10 000 cycles). These advantages open a door to a variety of applications, such as flexible wearable devices, which demonstrated human physiological signal monitoring. The low cost, simple design and portable use of piezoresistive sensors highlight the potential application of the crop by-product shaddock peel as a high-value material.

The piezoresistive sensor constructed by a PDMS modified Shaddock peel 3D carbon skeleton has an excellent sensing performance, which has promising potential in the field of human health detection.

Polyimide-Based Foams: Fabrication and Multifunctional Applications

Because of their unique three-dimensional cellular structure and intrinsic properties, polyimide foam materials have bright prospects for development in multiple functional equipment, which arouses extensive concern. In this Spotlight on Applications, several typical fabrication methods of polyimide foams and the related synthesis mechanism have been systematically described. The advantages and disadvantages of the preparation methods have been compared with each other. Representative functions and the corresponding mechanism models have been concluded, which involve thermal, mechanical, sensing, electromagnetic, environmental, and electrical fields. In the end, the severe tasks and challenges of polyimide foam materials have been summarized, and their promising future development is worth expecting.

CB Nanoparticles Optimized 3D Wearable Graphene Multifunctional Piezoresistive Sensor Framed by Loofah Sponge

Three-dimensional (3D) wearable piezoresistive sensors with excellent performance are urgently needed in many emerging fields. Herein, a hybrid piezoresistive sensor with 3D structure, which is framed by loofah sponge and coated with reduced graphene oxide modified with carbon black nanoparticles (rGO-CB@LS), was obtained via a facile solvothermal method. The ingenious use of loofah sponge (LS) provides a 3D highly ordered structure with excellent flexibility for the hybrid sensor, which assists the sensor free from the dependence on an organic substrate and eliminates the pollution to the environment. While the addition of carbon black (CB) nanoparticles can reduce the contact resistance between rGO sheets, improve the conductivity and sensitivity effectively, and shorten the response/recovery time of the sensor. An ultralight piezoresistive sensor, which is low cost and environmentally friendly, was obtained under the synergy of LS and rGO-CB, accompanied by high sensitivity and good stability. This novel sensor also exhibits excellent performance in detecting tiny and big human activities, demonstrating its great potential for a new generation of 3D wearable sensors.

Flexible Piezoresistive Sensor with the Microarray Structure Based on Self-Assembly of Multi-Walled Carbon Nanotubes

High-performance flexible pressure sensors have great application prospects in numerous fields, including the robot skin, intelligent prosthetic hands and wearable devices. In the present study, a novel type of flexible piezoresistive sensor is presented. The proposed sensor has remarkable superiorities, including high sensitivity, high repeatability, a simple manufacturing procedure and low initial cost. In this sensor, multi-walled carbon nanotubes were assembled onto a polydimethylsiloxane film with a pyramidal microarray structure through a layer-by-layer self-assembly system. It was found that when the applied external pressure deformed the pyramid microarray structure on the surface of the polydimethylsiloxane film, the resistance of the sensor varied linearly as the pressure changed. Tests that were performed on sensor samples with different self-assembled layers showed that the pressure sensitivity of the sensor could reach - 2.65     kPa - 1 , which ensured the high dynamic response ability and the high stability of the sensor. Moreover, it was proven that the sensor could be applied as a strain sensor under the tensile force to reflect the stretching extent or the bending object. Finally, a flexible pressure sensor was installed on five fingers and the back of the middle finger of a glove. The obtained results from grabbing different weights and different shapes of objects showed that the flexible pressure sensor not only reflected the change in the finger tactility during the grasping process, but also reflected the bending degree of fingers, which had a significant practical prospect.

Highly Compressible and Robust Polyimide/Carbon Nanotube Composite Aerogel for High-Performance Wearable Pressure Sensor

Wearable pressure sensors are in great demand with the rapid development of intelligent electronic devices. However, it is still a huge challenge to obtain high-performance pressure sensors with high sensitivity, wide response range, and low detection limit simultaneously. Here, a polyimide (PI)/carbon nanotube (CNT) composite aerogel with the merits of superelastic, high porosity, robust, and high-temperature resistance was successfully prepared through the freeze drying plus thermal imidization process. Benefiting from the strong chemical interactions between PI and CNT and stable electrical property, the composite aerogel exhibits versatile and superior brilliant sensing performance, which includes wide sensing range (80% strain, 61 kPa), ultrahigh sensitivity (11.28 kPa^-1), ultralow detection limit (0.1% strain, <10 Pa), fast response time (50 ms) and recovery time (70 ms), remarkable long-term stability (1000 cycles), and exceptional detection ability toward different deformations (compression, distortion, and bending). Furthermore, the composite aerogel also shows stable sensing performance after annealing under different high temperatures and good thermal insulation property, making it workable in various harsh environments. As a result, the composite aerogel is suitable for the full-range human motion detection (including airflow, pulse, vocal cord vibration, and human movement) and precise detection of the pressure distribution when it is assembled into E-skin, demonstrating its great potential to serve as a high-performance wearable pressure sensor.

Three-Dimensional Graphene Composite Containing Graphene-SiO₂ Nanoballs and Its Potential Application in Stress Sensors

Combining functional nanomaterials composite with three-dimensional graphene (3DG) is a promising strategy for improving the properties of stress sensors. However, it is difficult to realize stress sensors with both a wide measurement range and a high sensitivity. In this paper, graphene-SiO₂ balls (GSB) were composed into 3DG in order to solve this problem. In detail, the GSB were prepared by chemical vapor deposition (CVD) method, and then were dispersed with graphene oxide (GO) solution to synthesize GSB-combined 3DG composite foam (GSBF) through one-step hydrothermal reduction self-assembly method. The prepared GSBF owes excellent mechanical (95% recoverable strain) and electrical conductivity (0.458 S/cm). Furthermore, it exhibits a broad sensing range (0⁻10 kPa) and ultrahigh sensitivity (0.14 kPa^-1). In addition, the water droplet experiment demonstrates that GSBF is a competitive candidate of high-performance materials for stress sensors.

Significantly enhanced phonon mean free path and thermal conductivity by percolation of silver nanoflowers

Soft thermal interface materials (TIMs) composed of thermally conductive fillers and polymer matrixes have been widely employed for thermal management in electronic and energy devices. However, the thermal conductivity (κ) of TIMs is significantly smaller than the intrinsic κ of fillers due to the large interfacial thermal contact resistance between fillers. Here we achieve a very efficient thermal percolation network of flower-shaped silver nanoparticles (silver nanoflowers, Ag NFs) in soft polyurethane (PU) matrix TIMs. A record high κ (42.4 W m-1 K-1) is achieved compared with soft isotropic TIMs in the literature. Ag nanoflake-PU and Ag nanosphere-PU TIMs provide significantly smaller κ (7.9 and 15.0 W m-1 K-1) at an identical filler concentration (38 vol%). Surprisingly, the phonon transport of the Ag NF-PU TIM dramatically increases (κlat = 22.2 W m-1 K-1) compared with Ag nanoflake-PU and Ag nanosphere-PU (κlat = 0.2 and 1.2 W m-1 K-1) TIMs. Kinetic theory reveals that the phonon mean free path (39.6 nm) is significantly increased for the Ag NF-PU TIM by the active coalescence of metallic Ag NFs. The hierarchically structured Ag NFs construct an excellent thermal percolation network in soft isotropic TIMs.

Ultrasensitive detection of uric acid in serum of patients with gout by a new assay based on Pt@Ag nanoflowers

A ultrasensitive assay for the determination of uric acid (UA) based on Pt@Ag nanoflowers (Pt@Ag NFs) was constructed. H₂O₂ was formed by the reaction of uricase and UA and produced the hydroxyl radical (˙OH). The system was catalyzed by Pt@Ag NFs to change the color of 3,3′,5,5′-tetramethylbenzidine (TMB) from colorless to blue, and the morphology and chemical properties of Pt@Ag NFs were characterized by transmission electron microscopy and X-ray photoelectron spectroscopy. Under the optimized conditions, a linear relationship between the absorbance and UA concentration was in the range of 0.5–150 μM (R² = 0.995) with a limit of detection of 0.3 μM (S/N = 3). The method can be applied to detection of UA in actual samples with satisfactory results. The proposed assay was successfully applied to the detection of UA in human serum with recoveries over 96.8%. Thus, these results imply that the UA assay provides an effective tool in fast clinical analysis of gout.

A ultrasensitive assay for the determination of uric acid (UA) based on Pt@Ag nanoflowers (Pt@Ag NFs) was constructed.

A Superior Method for Constructing Electrical Percolation Network of Nanocomposite Fibers: In Situ Thermally Reduced Silver Nanoparticles

Nanocomposite fibers, composed of conductive nanoparticles and polymer matrix, are crucial for wearable electronics. However, the nanoparticle mixing approach results in aggregation and dispersion problems. A revolutionary synthesis method by premixing silver precursor ions (silver ammonium acetate) with polyvinyl alcohol is reported here. The solvation of ions-prevented aggregation, and uniformly distributed silver nanoparticles (in situ AgNPs, 77 nm) are formed after thermal reduction (155 °C) without using additional reducing or dispersion agents. The conductive fiber is synthesized by the wet spinning technology. After careful optimization, flower-shaped silver nanoparticles (AgNFs, 350-450 nm) are also employed as cofillers. The addition of in situ AgNPs (9.5 vol%) to AgNFs (30 vol%) increases electrical conductivity by 1434% (2090 to 32 064 S cm^-1 ) through the efficient construction of percolation networks. The in situ AgNPs provide significantly higher conductivity compared with other secondary nanoparticle fillers. The gaseous byproducts dramatically increase flexibility with a moderate compromise in tensile strength (55 MPa). The particle-free ion-level uniform mixing of silver precursors, followed by in situ reduction, would be a fundamental paradigm shift in nanocomposite synthesis.

Rapid and sensitive detection of uranyl ion with citrate-stabilized silver nanoparticles by the surface-enhanced Raman scattering technique

Uranium contamination poses a huge threat to human health due to its widespread use in the nuclear industry and weapons. We proposed a simple and convenient wet-state SERS method for uranyl detection based on the citrate-stabilized silver nanoparticles. The effect of citrate on the detection performance was also discussed. By using the citrate as an internal reference to normalize the peak of uranyl, a quantitative analysis was achieved and a good linear relationship of uranyl concentration from 0.2 to 5 µM with the limit of detection of 60 nM was obtained. With its simplicity, convenience and cost-effectiveness, this method has great potential for the detection of other molecules also.