Flexible sensors have wide applications in various fields such as biomedicine, environmental monitoring and smart wearable devices, as they can adapt to diverse complex environments and curved surfaces. This study aimed to develop resilient and flexible oxygen sensors based on fluorescence quenching. A flexible oxygen sensing component was prepared, comprising aluminum silicate fibers as the support, polydimethysiloxane (PDMS) as the matrix, and platinum tetrakis pentafluorophenyl porphyrin (PtTFPP) as the oxygen probe. The component exhibited superhydrophobicity with a water contact angle of 152°, which was beneficial for maintaining integrity in humid atmospheres and aqueous solutions. It showed the fluorescence quenching effect towards gaseous oxygen and dissolved oxygen in water, which could be well fitted by the Stern-Volmer equation with K_SV constants of 0.020 h·Pa^-1 for the gaseous oxygen and 2.94 L·mmol^-1 for the dissolved oxygen. The component also demonstrated good reversibility and fast response in rapidly altered atmosphere, with a response time of 0.9 s from nitrogen switching to oxygen and a recovery time of 2.7 s from oxygen switching to nitrogen. Additionally, the PtTFPP-PDMS component displayed remarkable stability concerning its relative fluorescence intensity and water contact angle even after exposure to 100 ℃ steam for 15 h, soaking in pH 1-10 aqueous solutions, and enduring 400 bending cycles. The aluminum silicate fiber-supported PtTFPP-PDMS film developed in this study exhibited excellent fluorescent oxygen sensing properties and stability, making it a promising candidate for oxygen sensors, and suitable for determination of gaseous and dissolved oxygen in challenging environments.

Flexible thermoelectric devices, capable of generating electricity from the slight temperature difference between the human body and the environment, demonstrate significant potential for continuous power supply in wearable devices. However, the poor thermoelectric performance still limits their widespread application. This study reports a method for fabricating high-performance flexible thermoelectric thin films using inkjet printing. AgCuTe nanowires prepared by a chemical transfer method were dispersed in ethanol to form the ink with no significant sedimentation, which could be stably and continuously sprayed to print p-type thermoelectric films on polyimide substrates. Dense thermoelectric films were then obtained through thermal treatment by a spark plasma sintering furnace, and the effect of sintering temperature on thermoelectric properties was studied. The results showed that the film sintered at a pressure of 10 MPa and a temperature of 400 ℃ for 15 min possessed a room temperature power factor of 432 µW·m^-1·K^-2, which is 182% higher than that of inkjet-printed p-type Bi₂Te₃ films (a room temperature power factor of 153 µW·m^-1·K^-2) reported in literature. This advancement further expands the application of inkjet printing in the field of flexible thermoelectrics and provides more possibilities for the fabrication of a new generation of high-performance flexible thermoelectric devices.

Selenium (Se) is considered as a new generation energy storage material of lithium-selenium (Li-Se) battery due to its high volume specific capacity (3253 mAh·cm^-3) and high electronic conductivity (1×10^-5 S·m^-1). To address the problems of volume expansion, fast capacity decay, and low utilization of active materials during its charging-discharging process, a ZIF-L derived nitrogen-doped nanosheets/selenium (Se@NC/CC) flexible self-supported composite electrode is designed in this study for lithium-selenium battery by growing two-dimensional zinc-based metallic organic framework (ZIF-L) on carbon cloth (CC). The rich microporous structure in the nitrogen-doped carbon nanosheets can effectively alleviate the volume expansion during the reaction process, and the doping of heteroatoms N helps to adsorb Li₂Se and reduce the loss of active substances. In particular, it is found that there is strong chemical bonding between Se and C in the Se@NC/CC electrode, which also helps to reduce the loss of active materials and improve the performance. Electrochemical results show that the initial discharge specific capacity of the Se@NC/CC electrode is 574 mAh·g^-1 at a current density of 0.5C (1.0C=675 mAh·g^-1), demonstrating a high initial discharge specific capacity. At a current density of 2.0C, the initial discharge specific capacity is 453.3 mAh·g^-1, which maintains at 406.2 mAh·g^-1 after cycling for 500 cycles and it also displays excellent rate performance compared to literature. Such a flexible self-supported selenium cathode designed in this study provides a new research route on the design of selenium host materials for advanced alkali metal-selenium batteries.

As a kind of important functional material, flexible piezoelectric materials can realize the effective conversion between mechanical energy and electrical energy, with the advantages of good toughness, high plasticity and light weight. Therefore, they can be attached to the human body to obtain human or environment information in real time, which is widely used in the fields of motion detection, health monitoring, and human-computer interaction. Due to high requirements of various three-dimensional (3D) structures of the flexible piezoelectric materials, additive manufacturing has been extensively utilized to fabricate different kinds of piezoelectric materials. This technology is expected to break the bottleneck of traditional processing of piezoelectric material by improving the structural design freedom and the performance of flexible piezoelectric materials, and provides enormous potential and opportunities for the application of flexible piezoelectric materials. Based on the introduction of the classification and features of flexible piezoelectric materials, this paper explained the main additive manufacturing technologies, including fused deposition modeling, direct ink writing, selective laser sintering, electric-assisted direct writing, stereolithography, and inkjet printing that commonly used in processing these materials. Then, various structural designs, such as multi-layer structure, porous structure, and interdigital structure for the flexible piezoelectric materials produced by different additive manufacturing approaches were reviewed. Moreover, the applications of additive manufactured flexible piezoelectric materials in energy harvesting, piezoelectric sensing, human-computer interaction, and bioengineering were introduced. Finally, the challenges faced by additive manufacturing on processing flexible piezoelectric materials and the development trends in the future were summarized and prospected.

Recently, perovskite solar cells have developed marvelously of which power conversion efficiency (PCE) achieved 26.1%, but the mechanical bending and environmental stability of flexible perovskite solar cells (F-PSCs) have remained major obstacles to their commercialization. In this study, the quality and crystallization of perovskite thin films were enhanced by adding agarose (AG). The interaction mechanism, PCE, mechanical bending and environmental stability of the assembled F-PSCs were investigated. It was found that the perovskite films modified by the optimal concentration of AG (3 mmol/L) exhibited denser and smoother morphology, higher crystallinity and absorbance, the lowest defect state density, and lower charge transfer resistance of 2191 Ω. Based on the optimal photoelectric properties, PCE increased from 15.17% to 17.30%. TiO₂ nanoparticles (0.75 mmol/L) were further introduced to form a synergistic interaction with AG (3 mmol/L), which provided a rigid backbone structure, and thus enhanced the mechanical and environmental stability of perovskite layers. After 1500 cycles of bending (3 mm in radius), the AG/TiO₂ co-modified F-PSCs maintained 84.73% of initial PCE, much higher than the blank device (9.32%). After 49 d in the air, the optimal F-PSCs still maintained 83.27% of initial PCE, superior than the blank device (62.21%). This work provides possibility for preparing highly efficient and stable F-PSCs.

Developing novel low-dimensional materials for terahertz electromagnetic shielding and absorbing applications represents a critical research frontier. Their unique electrical, mechanical, and electromagnetic responses hold great potential in enabling more efficient solutions for electromagnetic shielding and absorbing. Two-dimensional transition metal carbides, nitrides, and carbonitride MXenes have already demonstrated excellent electromagnetic shielding and absorbing performance in the low-frequency spectrum. MXenes possess high conductivity, low density, and high flexibility, which are advantageous for future portability and integration of terahertz devices and systems. However, practical implementation of MXene-based terahertz electromagnetic shielding and absorption materials faces challenges in adhesion stability, environmental resilience, and high-temperature tolerance, hindering their suitability for aerospace and future next generation communication applications. Moreover, in terahertz frequency band, lacking more comprehensive and reliable electromagnetic scattering and absorbing measurement methods limits the development of THz shielding and absorbing materials. Extensive research efforts have targeted on these limitations, exploring fundamental architectural and theoretical aspects of prevalent electromagnetic materials. This review specifically highlights the terahertz electromagnetic shielding and absorption characteristics inherent in various MXenes and their compositions, such as Ti₃C₂T_x, Mo₂Ti₂C₃T_x, Mo₂TiC₂T_x, Nb₄C₃T_x, and Nb₂CT_x. Additionally, this review envisages the forthcoming challenges and prospects of MXenes as a pivotal electromagnetic shielding and absorbing material within the terahertz frequency band.

In recent years, pressure sensors have been widely applied in the fields of smart wearable textile, health detection, and electronic skin. The emergence of the two-dimensional nanomaterial MXene has brought a brand-new breakthrough for pressure sensing. Ti₃C₂T_x is the most popular studied MXene in the field of pressure sensing and shows good mechanical, electrical properties, excellent hydrophilicity, and extensive modifiability, enabling it an ideal material for pressure sensing. Hence, researchers have conducted a lot of explorations and studies on design and application of MXene in pressure sensors in recent years. Herein, the preparation technologies and antioxidant methods are summarized. Design of MXene-based microstructures is also introduced, including aerogels/porous structural materials, hydrogels, flexible substrates, and films, which are beneficial to improve the response range, sensitivity, and flexibility of pressure sensors, and promote the rapid development of pressure sensors. The mechanisms of MXene pressure sensors are further broached, including piezoresistive, capacitive, piezoelectric, triboelectric, battery typed and nanofluidic. MXene has been applied in a wide range of sensors for various mechanisms due to its excellent characteristics. Finally, the chance and challenge in the synthesis, properties, and pressure sensing performance of MXene materials are prospected.

Two-dimensional (2D) transition metal carbides/nitrides (MXenes), since their discovery in 2011, have attracted great attention in the fields of energy storage, catalysis, sensors, electromagnetic interference shielding and microwave absorption and so on, owing to their special 2D layered structure, excellent electrical conductivity and electrochemical properties. In recent years, with the deep understanding of MXenes, the research on the realm related to optoelectronic properties has been widely concerned. Unlike other application fields, optoelectronic devices based on MXenes focus on extending semiconductor properties, including tunable band gap of the MXenes via design of the surface functional groups and layer control, etc., so as to achieve their transformation from metal to semiconductor properties. This paper revolves around the photoelectric properties of MXenes, mainly introduce its application in flexible optoelectronic devices, and systematically describe their current status and trend in transparent electronic devices, photodetectors, image sensors, transistors, and artificial neural vision network systems. The challenges and future development prospects of MXenes-based flexible optoelectronic devices are also proposed.

The gas diffusion layer (GDL) is a critical component of proton exchange membrane fuel cells (PEMFCs) and accounts for 40%-50% of the fuel cell membrane's cost. Developing a low-cost and high-performance GDL is imperative to advance the commercialization of PEMFCs. In this study, we generated a flexible carbon cloth with high electrical conductivity and porosity from cellulose cloth at a low temperature (1500 ℃). The carbon cloth is composed of micron-sized carbon fibers with a porosity of up to 76.93%. Through catalytic graphitization of iron-based compounds, massive carbon nanotube clusters were formed in situ on the surface of carbon fibers, which effectively enhanced the electrical conductivity of the carbon cloth. The in-plane resistance was as low as 34 mΩ·cm while the through-plane resistance was 2.8 mΩ·cm under a pressure of 2 MPa, meeting the performance standard of commercial carbon cloth. Furthermore, the PEMFC with the prepared carbon cloth as GDLs exhibits a power density of 0.4 W·cm^-2 at current density of 0.7 A·cm^-2, exceeding the device with commercial carbon cloth (0.34 W·cm^-2 at 0.7 A·cm^-2). This study demonstrates that the prepared biomass-derived carbon cloth with low-cost and high- performance holds great potential for advanced GDLs for PEMFCs.

Wearable instruments are functional devices that can be worn on human body, sensing, transmitting and processing body or environmental information in real time, and show broad application prospects in medical health, especially artificial intelligence, sports and entertainment. With the development of wearable instruments, various flexible sensors have emerged. Flexible mechanical sensors based on piezoelectric effect have attracted much attention because of their advantages of wide sensing frequency, fast response, good linearity, and self-power supply. However, traditional piezoelectric materials are mostly brittle ceramics and crystalline materials, which limit their application in flexible devices. With the deepening of research, more and more flexible piezoelectric materials and piezoelectric composites continue to emerge, injecting new development vitality into flexible wearable mechanical devices. This article mainly summarizes the cutting-edge progress of flexible wearable piezoelectric devices, including piezoelectric principle, preparation and performance improvement methods of flexible piezoelectric materials. In addition, the main application directions of flexible wearable piezoelectric devices, including medical health and human-computer interaction, as well as the challenges and opportunities encountered, are summarized.

In recent years, inspired by the unique operation mode of the human brain, emulation of the perception and computing functions of synapses and neurons by artificial neuromorphic devices has attracted more and more attention. So far, many researches have been reported about neuromorphic transistors (NMT), but most devices are fabricated on rigid substrates. The flexible neuromorphic transistors can not only realize signal transmission and training learning at the same time, but also carry out nonlinear spatio-temporal integration and cooperative regulation of multiple signals. It can also closely fit the soft human skin and withstand the high physiological strain of organs and tissues. More importantly, flexible neuromorphic transistors have unique advantages and application potential in detecting low amplitude signals at physiologically relevant time scales in biological environments due to their designable flexibility and excellent biocompatibility. Flexible neuromorphic transistors have been widely used in electronic skin, artificial vision system, intelligent wearable system, and other fields. At present, it is one of the most important tasks to develop low-power consumption, high-density integrated flexible neuromorphic transistors. In this paper, the research progress of NMT based on different flexible substrates is reviewed. In addition, the bright application prospect of flexible neuromorphic transistors is prospected. This review provides a reference for the development and application of flexible neuromorphic transistors in the future.

The exploration of flexible electronic devices with information processing functions of biological neurons is of great significance for the development of intelligent wearable technologies. Due to lack of inherent mechanical flexibility, conventional threshold-switching memristor based on rigid materials that can implement the computing functions of biological neurons is difficult to fulfill the requirements for potential applications in the future. In this work, an intrinsically stretchable threshold-switching memristor was prepared by using silver nanowire-polyurethane composite as the dielectric layer and liquid metal as the electrodes, respectively. Under application of a sweeping voltage, the device exhibited reliable threshold switching characteristics, which was switched from the high resistance state (HRS) to the low resistance state (LRS) during device programming and spontaneously relaxed to the HRS upon voltage application. Further analysis shows that the underlying mechanism can be attributed to the dynamic formation and rupture of discontinuous silver conductive filaments formed between silver nanowires. In the pulse programming mode, memristor device is able to emulate the integration and firing characteristics of biological neurons, suggesting its great potential as an artificial neuron. Moreover, the pulse amplitude and pulse interval modulated neuronal spiking behaviors are successfully replicated using such devices. Under 20% tensile strain, the threshold-switching memristor shows negligible changes in the operating parameters during device switching and neuronal function implementations, suggesting its excellent mechanical flexibility and stability. This work provides important guidelines for the development of high-performance stretchable artificial neuronal devices and next-generation intelligent wearable systems.

Compared with other electric energy storage devices, dielectric capacitors made of dielectric composites have great advantages in fast charging and discharging capacity with high power density. A dilemma of improving the energy density of dielectric composites and synchronous optimizing their breakdown performance is becoming an intriguing research direction. To further adjust the contradiction between dielectric constant and dielectric breakdown performance, here a finite element numerical simulation based on dielectric breakdown model (DBM) was proposed to study the effect of the distribution of inorganic fillers on the electric field and breakdown damage morphology in flexible polydimethylsiloxane(PDMS) based dielectric composite system. The results show that a large dielectric difference is observed between filler and matrix, which indicates that polymer matrix with a large dielectric constant or inorganic filler with a small dielectric constant can realize reducing the size of the high electric field area at the interface and improving the breakdown resistance of the material. This study further reveals that the more dispersed structure of inorganic fillers, the more likely its dendritic damage channels tend to branch, indicating that this situation is conducive to the increase of damage sites of dielectric breakdown dendritic damage channels, the decrease of damage rate, and the improvement of breakdown resistance of materials. All above data demonstrate that this study provides certain guidance for the development of organic-inorganic dielectric composites with both high energy storage and excellent breakdown performance.

Perovskite solar cells (PSCs) with structure of TiO₂/ZrO₂/carbon triple-layer are attractive recently because of their inexpensive raw materials, scalable fabrication process, and outstanding stability. But little progress has been made in the low temperature fabrication of TiO₂/ZrO₂/carbon triple-layer structured PSCs. A major reason is that it is rather difficult to construct the ZrO₂ spacer layer at low temperature. Herein, we report a facile low-temperature spray-coating method to prepare effective ZrO₂ spacer layer in TiO₂/ZrO₂/carbon triple-layer PSCs using urea to tune the porosity. After optimizing the amount of urea and the thickness of zirconia to 1100 nm, power conversion efficiencies (PCE) of 14.7% for a single cell and 10.8% for a module with 5 cells connected in series (5×0.9 cm× 2.5 cm) were achieved. Furthermore, the PSCs could be stable for 200 d at constant temperature (25 ℃) and humidity (40%). With this spray coating method, the zirconia layer on flexible substrate can endure 50 times of bending without any cracking. Compared to the conventional screen-printing method of ZrO₂ spacer layer, the spray-coating alternative developed in this work shows advantages of more convenient to process, preparation under lower temperature, and compatibility to flexible substrate.