Protein-Inspired Self-Healable Ti3C2 MXenes/Rubber-Based Supramolecular Elastomer for Intelligent Sensing

Multi-Interface Engineering of MXenes for Self-Powered Wearable Devices

Self-powered wearable devices with integrated energy supply module and sensitive sensors have significantly blossomed for continuous monitoring of human activity and the surrounding environment in healthcare sectors. The emerging of MXene-based materials has brought research upsurge in the fields of energy and electronics, owing to their excellent electrochemical performance, large surface area, superior mechanical performance, and tunable interfacial properties, where their performance can be further boosted via multi-interface engineering. Herein, a comprehensive review of recent progress in MXenes for self-powered wearable devices is discussed from the aspects of multi-interface engineering. The fundamental properties of MXenes including electronic, mechanical, optical, and thermal characteristics are discussed in detail. Different from previous review works on MXenes, multi-interface engineering of MXenes from termination regulation to surface modification and their impact on the performance of materials and energy storage/conversion devices are summarized. Based on the interfacial manipulation strategies, potential applications of MXene-based self-powered wearable devices are outlined. Finally, proposals and perspectives are provided on the current challenges and future directions in MXene-based self-powered wearable devices.

Recent Advancement in Emerging MXene-Based Photocatalytic Membrane for Revolutionizing Wastewater Treatment

MXene-based photocatalytic membranes provide significant benefits for wastewater treatment by effectively combining membrane separation and photocatalytic degradation processes. MXene represents a pioneering 2D photocatalyst with a variable elemental composition, substantial surface area, abundant surface terminations, and exceptional photoelectric performance, offering significant advantages in producing high-performance photocatalytic membranes. In this review, an in-depth overview of the latest scientific progress in MXene-based photocatalytic membranes is provided. Initially, a brief introduction to the structure and photocatalytic capabilities of MXene is provided, highlighting their pivotal role in promoting the photocatalytic process. Subsequently, in pursuit of the optimal MXene-based photocatalytic membrane, critical factors such as the morphology, hydrophilicity, and stability of MXenes are meticulously taken into account. Various preparation strategies for MXene-based photocatalytic membranes, including blending, vacuum filtration, and dip coating, are also discussed. Furthermore, the application and mechanism of MXene-based photocatalytic membranes in micropollutant removal, oil-water separation, and antibacterial are examined. Lastly, the challenges in the development and practical application of MXene-based photocatalytic membranes, as well as their future research direction are delineated.

Soft-Hard Janus Nanoparticles Triggered Hierarchical Conductors with Large Stretchability, High Sensitivity, and Superior Mechanical Properties

There is a long-standing conflict between the large stretchability and high sensitivity for strain sensors, a strategy of decoupling the mechanical/electrical module by constructing the hierarchical conductor has been developed in this study. The hierarchical conductor, consisting of a mechanically stretchable layer, a conductive network layer, and a strongly bonded interface, can be produced in a simple one-step process with the aid of soft-hard Janus nanoparticles (JNPs). The introduction of JNPs in the stretchable layer can evenly distribute stress and dissipate energy due to forming the rigid-flexible homogeneous networks. Specifically, JNPs can drive graphene nanosheets (GNS) to fold or curl, creating the unique JNPs-GNS building block that can further construct the conductive network. Due to its excellent deformability to hinder crack propagation, the flexible conductive network could be stretched continuously and the local conductive pathways could be reconstructed. Consequently, the hierarchical conductor could detect both subtle strain of 0-2% and large strain of up to 370%, with a gauge factor (GF) from 66.37 to 971.70, demonstrating outstanding stretchability and sensitivity. And it also owns large tensile strength (5.28 MPa) and high deformation stability. This hierarchical design will give graphene-based sensors a major boost in emerging applications.

Recent progress in emerging two-dimensional organic-inorganic van der Waals heterojunctions

Two-dimensional (2D) materials have attracted significant attention in recent decades due to their exceptional optoelectronic properties. Among them, to meet the growing demand for multifunctional applications, 2D organic-inorganic van der Waals (vdW) heterojunctions have become increasingly popular in the development of optoelectronic devices. These heterojunctions demonstrate impressive capability to synergistically combine the favourable characteristics of organic and inorganic materials, thereby offering a wide range of advantages. Also, they enable the creation of innovative device structures and introduce novel functionalities in existing 2D materials, avoiding the need for lattice matching in different material systems. Presently, researchers are actively working on improving the performance of devices based on 2D organic-inorganic vdW heterojunctions by focusing on enhancing the quality of 2D materials, precise stacking methods, energy band regulation, and material selection. Therefore, this review presents a thorough examination of the emerging 2D organic-inorganic vdW heterojunctions, including their classification, fabrication, and corresponding devices. Additionally, this review offers profound and comprehensive insight into the challenges in this field to inspire future research directions. It is expected to propel researchers to harness the extraordinary capabilities of 2D organic-inorganic vdW heterojunctions for a wider range of applications by further advancing the understanding of their fundamental properties, expanding the range of available materials, and exploring novel device architectures. The ongoing research and development in this field hold potential to unlock captivating advancements and foster practical applications across diverse industries.

MXene-Based Elastomer Mimetic Stretchable Sensors: Design, Properties, and Applications

Flexible sensors based on MXene-polymer composites are highly prospective for next-generation wearable electronics used in human-machine interfaces. One of the motivating factors behind the progress of flexible sensors is the steady arrival of new conductive materials. MXenes, a new family of 2D nanomaterials, have been drawing attention since the last decade due to their high electronic conductivity, processability, mechanical robustness and chemical tunability. In this review, we encompass the fabrication of MXene-based polymeric nanocomposites, their structure-property relationship, and applications in the flexible sensor domain. Moreover, our discussion is not only limited to sensor design, their mechanism, and various modes of sensing platform, but also their future perspective and market throughout the world. With our article, we intend to fortify the bond between flexible matrices and MXenes thus promoting the swift advancement of flexible MXene-sensors for wearable technologies.

Nacre-Inspired MXene Nanocomposite-based Strain Sensor with Ultrahigh Sensitivity in a Small Strain Range for Parkinson's Disease Diagnosis

Nowadays, chronic diseases are the primary threat to public health and are getting younger. By taking the advantages of continuousness, convenience, and real-time response, wearable strain sensors have been given great attention to diagnose chronic diseases via analyzing the patient's health state. However, most physiological signals, such as limb tremor of Parkinson's disease, microexpression, and slight joint movement, are tiny and difficult to be detected. Therefore, the development of strain sensors characterized with ultrahigh sensitivity in a small strain range (ε < 10%) is urgent. Inspired by nacre's hierarchical structure, we have fabricated nacre-mimetic nanocomposites with "brick-and-mortar" architecture by employing polyacrylamide (PAM) and Ti3C2Tx MXene nanosheets through a layer-by-layer (LBL) spin-coating technique. The resultant nanocomposite-based strain sensor exhibits ultrahigh sensitivity in a small strain range (GF = 296.8, ε < 10%), attributed to the bioinspired hierarchical structure and hydrogen bond-enhanced interfacial interactions. In addition, a high reliability, broad working sensing range (453%), short response time (183 ms), skin-like tensile stress (7.2 MPa), and excellent durability (2000 cycles) are also achieved. Due to the ultrahigh sensitivity within a small strain, the reported strain sensor can accurately diagnose and distinguish Parkinson's disease symptoms, including thumb pill-rolling tremor, masked face (microexpression), intermittent shaking of the head, and limb cogwheel motion. This work provides new insights to design strain sensors with high sensitivity for monitoring tiny signals and for disease diagnosis.

Are MXenes suitable for soft multifunctional composites?

MXenes are a family of two-dimensional (2D) nanomaterials known for their high electrical and thermal conductivity, as well as high aspect ratios. Recent research has focused on dispersing MXenes within compliant polymer matrices, aiming to create flexible and stretchable composites that harness MXenes' exceptional conductivity and aspect ratios. Experimental findings demonstrate the potential of MXene polymer composites (MXPCs) as flexible electrical, thermal conductors, and high dielectric materials, with promising applications in soft matter engineered systems. However, the 2D structure of MXene inclusions and their relatively large elastic modulus can impart increased stiffness to the polymer matrix, posing limitations on the mechanical flexibility of these functional materials. Here, we introduce a modeling platform to predict the mechanics and functionality of MXene elastomer composites and assess their suitability as soft multifunctional materials. Our investigation primarily focuses on understanding the influence of MXenes' size, layered structure, and percolation arrangements on the effective properties of the resulting composites. Through our model, we successfully determined the elastic modulus, thermal conductivity, and dielectric constant of MXene elastomer composites, and our results exhibit strong agreement with those obtained through finite element analysis. By utilizing this framework, we can theoretically identify the necessary microstructures of MXenes and guide the experiments, enabling the creation of MXPCs with the desired synergistic mechanical and functional properties.

Multifunctional Flexible Sensor Based on PU-TA@MXene Janus Architecture for Selective Direction Recognition

Multifunctional selectivity and mechanical properties are always a focus of attention in the field of flexible sensors. In particular, the construction of biomimetic architecture for sensing materials can endow the fabricated sensors with intrinsic response features and extra-derived functions. Here, inspired by the asymmetric structural features of human skin, a novel tannic acid (TA)-modified MXene-polyurethane film with a bionic Janus architecture is proposed, which is prepared by gravity-driven self-assembly for the gradient dispersion of 2D TA@MXene nanosheets into a PU network. This obtained film reveals strong mechanical properties of a superior elongation at a break of 2056.67% and an ultimate tensile strength of 50.78 MPa with self-healing performance. Moreover, the Janus architecture can lead to a selective multifunctional response of flexible sensors to directional bending, pressure, and stretching. Combined with a machine learning module, the sensor is endowed with high recognition rates for force detection (96.1%). Meanwhile, direction identification in rescue operations and human movement monitoring can be realized by this sensor. This work offers essential research value and practical significance for the material structures, mechanical properties, and application platforms of flexible sensors.

Role of MXenes in advancing soft robotics

MXenes with their unique electronic, optical, chemical, and mechanical properties have shown great promise in soft robotics. MXene-based soft actuators have been designed to display ultrafast actuations and recovery speeds as well as angle-independent structural colors in response to vapor. Several studies have developed soft actuators by combining MXenes with other materials to mimic the movement of natural organisms. Thus, MXene-based soft actuators have the potential to revolutionize the field of soft robotics and flexible electronics (e.g., wearable devices and artificial muscles). MXene-based artificial muscles have been explored for use in kinetic soft robotics as actuators in microsystems requiring exceptional compliance. MXene-based sensors and actuators have already been developed for human-like sensors and photodetection. However, there are still challenges that need to be addressed in such applications, such as the design of stretchable and compliant robotic skins with a high-level functional integration for soft robotics. The integration of various devices, such as power sources, sensors, and actuators, into soft robotics is another crucial challenge. Despite the excellent stretchability and tensile strength of MXene-based composites, there is a vital need to develop their mechanical and electrochemical features and grant them multi-functionalities. Herein, recent developments pertaining to the applications of MXenes and their composites in soft robotics are discussed with a focus on the important challenges and future perspectives.

Effective Surface Modification of 2D MXene toward Thermal Energy Conversion and Management

Thermal energy management is a crucial aspect of many research developments, such as hybrid and soft electronics, aerospace, and electric vehicles. The selection of materials is of critical importance in these applications to manage thermal energy effectively. From this perspective, MXene, a new type of 2D material, has attracted considerable attention in thermal energy management, including thermal conduction and conversion, owing to its unique electrical and thermal properties. However, tailored surface modification of 2D MXenes is required to meet the application requirements or overcome specific limitations. Herein, a comprehensive review of surface modification of 2D MXenes for thermal energy management is discussed. First, this work discusses the current progress in the surface modification of 2D MXenes, including termination with functional groups, small-molecule organic compound functionalization, and polymer modification and composites. Subsequently, an in situ analysis of surface-modified 2D MXenes is presented. This is followed by an overview of the recent progress in the thermal energy management of 2D MXenes and their composites, such as Joule heating, heat dissipation, thermoelectric energy conversion, and photothermal conversion. Finally, some challenges facing the application of 2D MXenes are discussed, and an outlook on surface-modified 2D MXenes is provided.

Biomimetic Flexible Sensors and Their Applications in Human Health Detection

Bionic flexible sensors are a new type of biosensor with high sensitivity, selectivity, stability, and reliability to achieve detection in complex natural and physiological environments. They provide efficient, energy-saving and convenient applications in medical monitoring and diagnosis, environmental monitoring, and detection and identification. Combining sensor devices with flexible substrates to imitate flexible structures in living organisms, thus enabling the detection of various physiological signals, has become a hot topic of interest. In the field of human health detection, the application of bionic flexible sensors is flourishing and will evolve into patient-centric diagnosis and treatment in the future of healthcare. In this review, we provide an up-to-date overview of bionic flexible devices for human health detection applications and a comprehensive summary of the research progress and potential of flexible sensors. First, we evaluate the working mechanisms of different classes of bionic flexible sensors, describing the selection and fabrication of bionic flexible materials and their excellent electrochemical properties; then, we introduce some interesting applications for monitoring physical, electrophysiological, chemical, and biological signals according to more segmented health fields (e.g., medical diagnosis, rehabilitation assistance, and sports monitoring). We conclude with a summary of the advantages of current results and the challenges and possible future developments.

Recent Progress in Emerging Novel MXenes Based Materials and their Fascinating Sensing Applications

Early transition metals based 2D carbides, nitrides and carbonitrides nanomaterials are known as MXenes, a novel and extensive new class of 2D materials family. Since the first accidently synthesis based discovery of Ti₃ C₂ in 2011, more than 50 additional compositions have been experimentally reported, including at least eight distinct synthesis methods and also more than 100 stoichiometries are theoretically studied. Due to its distinctive surface chemistry, graphene like shape, metallic conductivity, high hydrophilicity, outstanding mechanical and thermal properties, redox capacity and affordable with mass-produced nature, this diverse MXenes are of tremendous scientific and technological significance. In this review, first we'll come across the MXene based nanomaterials possible synthesis methods, their advantages, limitations and future suggestions, new chemistry related to their selected properties and potential sensing applications, which will help us to explain why this family is growing very fast as compared to other 2D families. Secondly, problems that help to further improve commercialization of the MXene nanomaterials based sensors are examined, and many advances in the commercializing of the MXene nanomaterials based sensors are proposed. At the end, we'll go through the current challenges, limitations and future suggestions.

A review on accelerated development of skin-like MXene electrodes: from experimental to machine learning

Foreshadowing future needs has catapulted the progress of skin-like electronic devices for human-machine interactions. These devices possess human skin-like properties such as stretchability, self-healability, transparency, biocompatibility, and wearability. This review highlights the recent progress in a promising material, MXenes, to realize soft, deformable, skin-like electrodes. Various structural designs, fabrication strategies, and rational guidelines adopted to realize MXene-based skin-like electrodes are outlined. We explicitly discussed machine learning-based material informatics to understand and predict the properties of MXenes. Finally, an outlook on the existing challenges and the future roadmap to realize soft skin-like MXene electrodes to facilitate technological advances in the next-generation human-machine interactions has been described.

Emerging MXene-Based Flexible Tactile Sensors for Health Monitoring and Haptic Perception

Due to their potential applications in physiological monitoring, diagnosis, human prosthetics, haptic perception, and human-machine interaction, flexible tactile sensors have attracted wide research interest in recent years. Thanks to the advances in material engineering, high performance flexible tactile sensors have been obtained. Among the representative pressure sensing materials, 2D layered nanomaterials have many properties that are superior to those of bulk nanomaterials and are more suitable for high performance flexible sensors. As a class of 2D inorganic compounds in materials science, MXene has excellent electrical, mechanical, and biological compatibility. MXene-based composites have proven to be promising candidates for flexible tactile sensors due to their excellent stretchability and metallic conductivity. Therefore, great efforts have been devoted to the development of MXene-based composites for flexible sensor applications. In this paper, the controllable preparation and characterization of MXene are introduced. Then, the recent progresses on fabrication strategies, operating mechanisms, and device performance of MXene composite-based flexible tactile sensors, including flexible piezoresistive sensors, capacitive sensors, piezoelectric sensors, triboelectric sensors are reviewed. After that, the applications of MXene material-based flexible electronics in human motion monitoring, healthcare, prosthetics, and artificial intelligence are discussed. Finally, the challenges and perspectives for MXene-based tactile sensors are summarized.

Preparation of high electromagnetic interference shielding and humidity responsivity composite film through modified Ti3C2Tx MXene cross-linking with sodium alginate

This paper established a new kind of L-citrulline-modified MXene cross-linked sodium alginate composite film through solution blending and casting film methods. The L-citrulline-modified MXene cross-linked sodium alginate composite film exhibited high electromagnetic interference shielding efficiency of 70 dB and high tensile strength of 7.9 MPa, which were much higher than the sodium alginate film without L-citrulline-modified MXene. In addition, the L-citrulline-modified MXene cross-linked sodium alginate film appeared humidity responsibility in a water vapor environment, the weight, thickness, and current appeared to increase trend and the resistance appeared to decrease trend after it absorbed water, and these parameters recovered to their original values after drying.

Ru(II) Complex Grafted Ti3C2Tx MXene Nano Sheet with Photothermal/Photodynamic Synergistic Antibacterial Activity

For a long time, the emergence of microbial drug resistance due to the abuse of antibiotics has greatly reduced the therapeutic effect of many existing antibiotics. This makes the development of new antimicrobial materials urgent. Light-assisted antimicrobial therapy is an alternative to antibiotic therapy due to its high antimicrobial efficiency and non-resistance. Here, we develop a nanocomposite material (Ru@MXene) which is based on Ru(bpy)(dcb)²⁺ connected to MXene nanosheets by ester bonding as a photothermal/photodynamic synergistic antibacterial material. The obtained Ru@MXene nanocomposites exhibit a strengthened antimicrobial capacity compared to Ru or MXene alone, which can be attributed to the higher reactive oxygen species (ROS) yield and the thermal effect. Once exposed to a xenon lamp, Ru@MXene promptly achieved almost 100% bactericidal activity against Escherichia coli (200 μg/mL) and Staphylococcus aureus (100 μg/mL). This is ascribed to its synergistic photothermal therapy (PTT) and photodynamic therapy (PDT) capabilities. Consequently, the innovative Ru@MXene can be a prospective non-drug antimicrobial therapy that avoids antibiotic resistance in practice. Notably, this high-efficiency PTT/PDT synergistic antimicrobial material by bonding Ru complexes to MXene is the first such reported model. However, the toxic effects of Ru@MXene materials need to be studied to evaluate them for further medical applications.

Facile Approach to Fabricate Oriented Porous PDMS Composites for Movements Monitoring and Identifying Motion Patterns

The facile and rapid fabrication of oriented porous polymers is crucial for flexible pressure sensors. Herein, a pressure sensor is developed based on oriented porous polydimethylsiloxane (PDMS) composites for detecting human motion and identifying joint motion patterns. The oriented porous PDMS composite is first constructed through thiol-ene click chemistry and directional freezing within only 30 min, then fabricated by interfacial in situ polymerization of dopamine and pyrrole to generate robust interfaces. As a result, the as-prepared oriented porous PDMS composite is assembled into a pressure sensor that shows potential applications in pressure and human motion detection. Interestingly, a sensor assembled by orthogonally stacking the PDMS composites can be used for joint motion pattern recognition with potential monitoring of football motion due to their directional structures. This facile strategy coupled with the oriented porous structure is expected to help design advanced wearable electronic devices.

Self-Assembly of Glutamic Acid and Serine on Au(111)

Amino acids provide novel and superior performance for two-dimensional materials and bio-based devices. The interaction and adsorption of amino acid molecules on substrates have thus attracted extensive research for exploring the driving forces involved in the formation of nanostructures. Nevertheless, the interactions within amino acid molecules on inert surfaces have not been fully understood. Herein, from the interplay of high-resolution scanning tunneling microscopy imaging and density functional theory calculations, we show the self-assembled structures of Glu and Ser molecules on Au(111), which are dominated by intermolecular hydrogen bonds, and further investigate their most stable structural models at the atomic scale. This study would be of fundamental importance in understanding the formation processes of biologically relevant nanostructures and provide possibilities for chemical modification.

Ultra-sensitive, Multi-directional Flexible Strain Sensors Based on an MXene Film with Periodic Wrinkles

The fast-growing motion capturing/monitoring technique has raised a great demand for flexible strain sensors. For capturing complex motions (e.g., facial motion), both the strain amplitude and direction should be accurately detected. Although some reported sensors based on anisotropic conductive networks are proved to show accurate localization of strain directions, it is still a great challenge to achieve both high sensitivity and a high sensing range in these designs. Here, a self-assembled Ti₃C₂T_x MXene film with parallel and periodic wrinkles is fabricated on a stretchable poly(dimethylsiloxane) substrate for constructing multi-directional strain sensors. During stretching, relative slip and crack will occur between the stacked MXene nanosheets, which contribute to high structural sensitivity in the MXene film. Meanwhile, the wrinkled structure contributes to high stretchability. As a result, the sensor based on the film with one-dimensional periodic wrinkles shows a large sensing range (>50%) and a gauge factor of 45. Furthermore, the sensor can accurately detect both the strain amplitude and direction by using the MXene film with two-dimensional wrinkles. It shows distinguishable electrical responses when detecting different-amplitude human/robot motions such as joint bending and walking. Additionally, the directions in complex human motions (e.g., facial motion) can also be well-tracked. This work provides an effective strategy to detect multi-directional motions.

MXene-based wireless facemask enabled wearable breath acetone detection for lipid metabolic monitoring

Breath acetone (BrAC) detection presents a promising scheme for noninvasive monitoring of metabolic health due to its close correlation to diets and exercise-regulated lipolysis. Herein, we report a Ti₃C₂T_x MXene-based wireless facemask for on-body BrAC detection and real-time tracking of lipid metabolism, where Ti₃C₂T_x MXene serves as a versatile nanoplatform for not only acetone detection but also breath interference filtration. The incorporation of in situ grown TiO₂ and short peptides with Ti₃C₂T_x MXene further improves the acetone sensitivity and selectivity, while TiO₂-MXene interfaces facilitate light-assisted response calibration. To further realize wearable breath monitoring, a miniaturized flexible detection tag has been integrated with a commercially available facemask, which enables facile BrAC detection and wireless data transmission. Through the hierarchically designed filtration-detection-calibration-transmission system, we realize BrAC detection down to 0.31 ppm (part per million) in breath. On-body breath tests validate the facemask in dynamically monitoring of lipid metabolism, which could guide dieter, athletes, and fitness enthusiasts to arrange diets and exercise activities. The proposed wearable platform opens up new possibility toward the practice of breath analysis as well as daily lipid metabolic management.

Recent Progress of Tactile and Force Sensors for Human-Machine Interaction

Human-Machine Interface (HMI) plays a key role in the interaction between people and machines, which allows people to easily and intuitively control the machine and immersively experience the virtual world of the meta-universe by virtual reality/augmented reality (VR/AR) technology. Currently, wearable skin-integrated tactile and force sensors are widely used in immersive human-machine interactions due to their ultra-thin, ultra-soft, conformal characteristics. In this paper, the recent progress of tactile and force sensors used in HMI are reviewed, including piezoresistive, capacitive, piezoelectric, triboelectric, and other sensors. Then, this paper discusses how to improve the performance of tactile and force sensors for HMI. Next, this paper summarizes the HMI for dexterous robotic manipulation and VR/AR applications. Finally, this paper summarizes and proposes the future development trend of HMI.

A review related to MXene preparation and its sensor arrays of electronic skins

MXenes have been flourishing over the last decade as a high-performance 2D material, which combines the advantages of high electrical conductivity, photothermal conversion, and easy dispersion. They have been used to create soft, highly conductive, self-healing, and tactile-simulating electronic skins (E-skins). However, these E-skins remain generally limited to one or two functions with a complex preparation process. Next-generation E-skins necessitate not only large-scale fabrication using simple and fast methods but also the integration of multiple sensing functions and signal analysis components in order to provide functionality that was not unattainable in the past. Starting with the synthesis of pure MXenes, we walk through the steps of designing MXene sensors, integrating electronic skin arrays, and determining the function of MXene-based electronic skins. We also summarise the problems with existing MXene-based E-skins and possible futuristic directions.

MXene-Based Nanomaterials for Multifunctional Applications

MXene is becoming a "rising star" material due to its versatility for a wide portfolio of applications, including electrochemical energy storage devices, electrocatalysis, sensors, biomedical applications, membranes, flexible and wearable devices, etc. As these applications promote increased interest in MXene research, summarizing the latest findings on this family of materials will help inform the scientific community. In this review, we first discuss the rapid evolutionary change in MXenes from the first reported M₂XT_x structure to the last reported M₅X₄T_x structure. The use of systematically modified synthesis routes, such as foreign atom intercalation, tuning precursor chemistry, etc., will be further discussed in the next section. Then, we review the applications of MXenes and their composites/hybrids for rapidly growing applications such as batteries, supercapacitors, electrocatalysts, sensors, biomedical, electromagnetic interference shielding, membranes, and flexible and wearable devices. More importantly, we notice that its excellent metallic conductivity with its hydrophilic nature distinguishes MXene from other materials, and its properties and applications can be further modified by surface functionalization. MXene composites/hybrids outperform pristine MXenes in many applications. In addition, a summary of the latest findings using MXene-based materials to overcome application-specific drawbacks is provided in the last few sections. We hope that the information provided in this review will help integrate lab-scale findings into commercially viable products.

Mechanoluminescent Device: In Situ Renewable Carbazole Derivatives Sandwiched by Self-Healing Disulfide-Containing Polyurethane for Mechanical Signals Detection

Mechanoluminescent (ML) materials can emit visible light by utilizing mechanical energy, which shows unique advantages in visual mechanical sensing, displays, and biomechanical monitoring due to the correlation between force stimulation and luminescence intensity. Most organic ML materials exhibit luminescence intensity attenuation, disappearing completely with force stimulation and failing to recover. Here, organic luminogens (Cz-alkyl₆) can be synthesized by introducing a soft alkyl chain into the carbazole, which exhibits ML emission with self-assembly units. Furthermore, organic luminogens can be generated repeatedly by simply recrystallizing the fracture crystal in situ after a short thermal treatment (70 °C) within 14 s. More importantly, the quantitative correlation between force pressure and ML intensity has been established by a sandwich-type ML device based on a novel carbazole derivative (Cz-alkyl₆). The ML device presents a capacity for detecting mechanical signals up to 13 N according to its ML intensity (≤275 a.u.), exhibiting potential application value in engineering damage detection, anticounterfeiting, and advanced visual mechanical sensing.

Self-Healable Elastomeric Network with Dynamic Disulfide, Imine, and Hydrogen Bonds for Flexible Strain Sensor

Self-healable and stretchable elastomeric material is essential for the development of flexible electronics devices to ensure their stable performance. In this study, a strain sensor (PIH₂ T₁ -tri/CNT-3) composed of self-repairable crosslinked elastomer substrate (PIH₂ T₁ -tri, containing multiple reversible repairing sites such as disulfide, imine, and hydrogen bonds) and conductive layer (carbon nanotube, CNT) was prepared. The PIH₂ T₁ -tri elastomer had excellent self-healing ability (healing efficiency=91 %). It exhibited good mechanical integrity in terms of elongation at break (672 %), tensile strength (1.41 MPa). The Young's modulus (0.39 MPa) was close to that of human skin. The PIH₂ T₁ -tri/CNT-3 sensor also demonstrated an effective self-healing function for electrical conduction and sensing property. Meanwhile, it had high sensitivity (gauge factor (GF)=24.1), short response time (120 ms), and long-term durability (4000 cycles). This study offers a novel self-healable elastomer platform with carbon based conductive components to develop flexible strain sensors towards high performance soft electronics.

Increase in Properties and Self-Healing Ability of Conductive Butyl Rubber/Epoxidized Natural Rubber Composites by Using Bis(triethoxysilylpropyl)tetrasulfide Coupling Agent

Flexible self-healing composite was fabricated based on blending the bromobutyl rubber (BIIR) and epoxide natural rubber (ENR) filled with hybrid fillers of carbon nanotubes (CNT) and carbon black (CB). To achieve self-recoverability, modification of BIIR was carried out through butyl imidazole (IM), and the healing capability was then activated by the addition of bis(triethoxysilylpropyl)tetrasulfide (TESPT), which resulted in good dispersion of CNT/CB in BIIR/ENR blends. The silanization of TESPT and CNT/CB hybrid filler surfaces was confirmed by attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy. Adding CNT/CB and incorporating TESPT into the composites effectively improved the curing and mechanical properties of the blends in terms of estimated crosslink density and tensile modulus. Further, the self-healing propagation rate was enhanced by the thermal conductivity of fillers and the ion-dipole intermolecular forces between the rubber chains, leading to the highest abrasion resistance and electrical conductivity. Using an environmentally friendly process, the recyclability of the self-healing composites was improved by the re-compression of the samples. With this, the constant conductivity relating to the rearrangement of the CNT/CB network is examined related to the usability of the composites at 0 and 60 °C. The conductive composites filled with a TESPT silane coupling agent present an opportunity for vehicle tires and other self-repairing applications.

Flexible Torsional Photoactuators Based on MXene-Carbon Nanotube-Paraffin Wax Films

A flexible actuator, which can convert external stimuli to mechanical motion, is an essential component of every soft robot and determines its performance. As a novel two-dimensional material, MXene has been used to fabricate flexible actuators due to its excellent physical properties. Although MXene-based actuators exhibit excellent actuation performance, their bending deformation is solely due to the in-plane isotropy of the MXene film, and an MXene torsional actuator has not been reported. This study presents a flexible torsional actuator based on an MXene-carbon nanotube (CNT)-paraffin wax (PW) film. In this actuator, the MXene thin film acts as a light absorption layer with wavelength selectivity, superaligned CNT provides structural anisotropy for the composite film, and PW acts as the active layer. The chirality and helical structure of the actuator could be tuned by the orientation of the CNT film. Such an actuator delivers excellent actuation performance, including high work density (∼1.2 J/cm³), low triggering power (77 mW/cm²), high rotational speed (320°/s), long lifetime (30,000 cycles), and wavelength selectivity. Inspired by vines, we used the torsional actuator as a spiral grabber, which lifted an object that weighs 20 times more than the actuator. The high-performance torsional actuator would be potentially used as a noncontact sensor, rotary motor, and grabbing tool in the soft robot system.

3D printing of flexible strain sensor based on MWCNTs/flexible resin composite

The flexible strain sensor is an indispensable part in flexible integrated electronic systems and an important intermediate in external mechanical signal acquisition. The 3D printing technology provides a fast and cheap way to manufacture flexible strain sensors. In this paper, a MWCNTs/flexible resin composite for photocuring 3D printing was prepared using mechanical mixing method. The composite has a low percolation threshold (1.2%ωt). Based on the composite material, a flexible strain sensor with high performance was fabricated using digital light processing technology. The sensor has a GF of 8.98 under strain conditions ranging between 0% and 40% and a high elongation at break (48%). The sensor presents mechanical hysteresis under cyclic loading. With the increase of the strain amplitude, the mechanical hysteresis becomes more obvious. At the same time, the resistance response signal of the sensor shows double peaks during the unloading process, which is caused by the competition of disconnection and reconstruction of conductive network in the composite material. The test results show that the sensor has different response signals to different types of loads. Finally, its practicability is verified by applying it to balloon pressure detection.

Boosted π-Li Cation Effect in the Stabilized Small Organic Molecule Electrode via Hydrogen Bonding with MXene

The high solubility of the small organic molecule materials in organic electrolytes hinders their development in rechargeable batteries. Hence, this work designs an ultrarobust hydrogen-bonded organic-inorganic hybrid material: the small organic unit of the 1,3,6,8-tetrakis (p-benzoic acid) pyrene (TBAP) molecule connected with the hydroxylated Ti₃C₂T_x MXene through hydrogen bonds between the terminal groups of -COOH and -OH. The robust and elastic hydrogen bonds can empower the TBAP, despite being a low-molecule organic chemical, with unusually low solubility in organic electrolytes and thermal stability. The alkali-treated Ti₃C₂T_x MXene provides a hydroxyl-rich conductive network, and the small organic molecule of TBAP reduces the restacking of MXene layers. Therefore, the combination of these two materials complements each other well, and this organic-inorganic TBAP@D-Ti₃C₂T_x electrode delivers large reversible capacities and long cyclic life. Notably, with the assistance of the in situ FT-IR characterization of the electrode within the fully lithiated (0.005 V) and the delithiated (3.0 V) states, it is revealed that a powerful π-Li cation effect mainly governs the lithium-storage mechanism with the highly activated benzene ring and each C6 aromatic ring, which can reversibly accept six Li-ions to form a 1:1 Li/C complex.

Morphological Engineering of Sensing Materials for Flexible Pressure Sensors and Artificial Intelligence Applications

Various morphological structures in pressure sensors with the resulting advanced sensing properties are reviewed comprehensively. Relevant manufacturing techniques and intelligent applications of pressure sensors are summarized in a complete and interesting way. Future challenges and perspectives of flexible pressure sensors are critically discussed.

Gelation-Assisted Assembly of Large-Area, Highly Aligned, and Environmentally Stable MXene Films with an Excellent Trade-Off between Mechanical and Electrical Properties

Layered MXene films have shown enormous potential for wide applications due to their high electrical conductivity and unique laminated microstructure. However, the intrinsic susceptibility to oxidation and the mechanical fragility of MXene films are the two major bottlenecks that prevent their widespread industrial applications. Here, a facile yet efficient assembly strategy is proposed to address these issues by increasing the alignment and compactness of MXene layers as well as strengthening the interlayer interactions. This method involves the gelation of MXene flakes with a multifunctional inorganic "mortar" polymer (ammonium polyphosphate, APP) followed by quasi-solid-state assembly enabled by a mechanical rolling process, by which the 3D gel network is transformed into 2D freestanding MXene films with unprecedented flake alignment and compactness. Besides, due to the multiple molecular-level interactions (hydrogen bonding, coordination bonding, and electrostatic force) between APP and MXene flakes, the resultant MXene-APP film (MAF) displays high mechanical strength (286.4 ± 20.3 MPa) and excellent electrical conductivity (8012.4 ± 325.6 S cm^-1 ), along with remarkable environmental stability. As an application demonstration, MAF exhibits outstanding electromagnetic interference shielding effectiveness with long-term durability, highlighting the great potential of this gelation-assisted assembly strategy in fabricating large-area, high-performance MXene films for diverse real-world applications.

Recyclable, Self-Healing Solid Polymer Electrolytes by Soy Protein-Based Dynamic Network

Compared to traditional organic liquid electrolytes, which often present leakage, flammability, and chemical stability problems, solid polymer electrolytes (SPEs) are widely regarded as one of the most promising candidates for the development of safer lithium-ion batteries. Vitrimers are a new class of polymer materials consisting of dynamic covalent networks that can change their topology by thermally activated bond-exchange reactions. Herein, the recyclable and self-healing solid polymer electrolytes (SPEs) with a soy protein isolate (SPI)-based imine bond dynamic network are reported. This malleable covalent cross-linked network polymer can be reshaped and recycled at high temperature (100 °C) or only with water at ambient temperature (25 °C), which may realize the green processing of energy materials. The introduction of bis(trifluoromethane) sulfonimide lithium (LiTFSI) significantly reinforces the conductivity of the dynamic network to a maximum of 3.3 × 10-4 S cm-1 . This simple and applicable method establishes new principles for designing scalable and flexible strategies for fabricating polymer electrolytes.

A novel electrochemical sandwich-like immunosensor based on carboxyl Ti3C2Tx MXene and rhodamine b/gold/reduced graphene oxide for Listeria monocytogenes

Listeria monocytogenes (LM) is one of the most common food-borne pathogens and can induce a series of diseases with a high mortality rate to humans; hence, it is very necessary to develop a highly sensitive method for LM detection. Based on this need, a new sandwich-like electrochemical immunosensing platform was developed herein by preparing carboxyl Ti₃C₂T_x MXene (C-Ti₃C₂T_x MXene) as the sensing platform and rhodamine b/gold/reduced graphene oxide (RhB/Au/RGO) as the signal amplifier. The high conductivity and large surface area of C-Ti₃C₂T_x MXene make it a desirable nanomaterial to fix the primary antibody of LM (PAb), while the prepared Au/RGO/RhB nanohybrid is dedicated to assembling the secondary antibody (SAb) of LM, offering an amplified response signal. Through the use of RhB molecule as the signal probe, the experiments showed that the peak currents of RhB increase along with an increase in the concentration of LM from 10 to 10⁵ CFU mL^-1, and an extremely low limit of detection (2 CFU mL^-1) was obtained on the basis of the proposed immunosensing platform after optimizing various conditions. Hence, it is confirmed that the developed sandwich-like immunosensor based on C-Ti₃C₂T_x MXene and RhB/Au/Gr has great application in the detection of LM and other analytes.

Recent Advances in Electronic Skins with Multiple-Stimuli-Responsive and Self-Healing Abilities

Wearable electronic skin (e-skin) has provided a revolutionized way to intelligently sense environmental stimuli, which shows prospective applications in health monitoring, artificial intelligence and prosthetics fields. Drawn inspiration from biological skins, developing e-skin with multiple stimuli perception and self-healing abilities not only enrich their bionic multifunctionality, but also greatly improve their sensory performance and functional stability. In this review, we highlight recent important developments in the material structure design strategy to imitate the fascinating functionalities of biological skins, including molecular synthesis, physical structure design, and special biomimicry engineering. Moreover, their specific structure-property relationships, multifunctional application, and existing challenges are also critically analyzed with representative examples. Furthermore, a summary and perspective on future directions and challenges of biomimetic electronic skins regarding function construction will be briefly discussed. We believe that this review will provide valuable guidance for readers to fabricate superior e-skin materials or devices with skin-like multifunctionalities and disparate characteristics.

UV illumination-enhanced ultrasensitive ammonia gas sensor based on (001)TiO2/MXene heterostructure for food spoilage detection

Ammonia has been used as an important marker to indicate the extent of food spoilage. However, current gas sensors for ammonia suffer from either insufficient sensitivity and selectivity or unsatisfactory levels of automation, impeding their practical application for on-site and real-time monitoring of food quality. To overcome these limitations, we propose here the design of a sensing material by in-situ growing (001)TiO₂ onto a two-dimensional transition-metal carbide (Ti₃C₂T_x, MXene). In this design, TiO₂ with a highly active (001) crystal plane provides efficient photogeneration under UV irradiation, while Ti₃C₂T_x can store holes through Schottky junction formed at the interface with TiO₂, which greatly promotes the separation of electron-hole pairs, thereby enhancing ammonia sensing performance. By further introducing UV light for electron excitation, the (001)TiO₂/Ti₃C₂T_x based sensor shows 34 times higher sensitivity for ammonia (30 ppm) than that of Ti₃C₂T_x. The density functional theory further revealed that the (001) plane of TiO₂ and Ti₃C₂T_x composite configuration exhibited the highest adsorption affinity towards ammonia. Finally, an integrated circuit alarm system including near-field communication and a micro-controller system was designed to detect the decay process of fresh pork, fish, and shrimp. We believe such a sensing technology holds great promise in food quality monitoring.

Recent Advances in Multiresponsive Flexible Sensors towards E-skin: A Delicate Design for Versatile Sensing

Multiresponsive flexile sensors with strain, temperature, humidity, and other sensing abilities serving as real electronic skin (e-skin) have manifested great application potential in flexible electronics, artificial intelligence (AI), and Internet of Things (IoT). Although numerous flexible sensors with sole sensing function have already been reported since the concept of e-skin, that mimics the sensing features of human skin, was proposed about a decade ago, the ones with more sensing capacities as new emergences are urgently demanded. However, highly integrated and highly sensitive flexible sensors with multiresponsive functions are becoming a big thrust for the detection of human body motions, physiological signals (e.g., skin temperature, blood pressure, electrocardiograms (ECG), electromyograms (EMG), sweat, etc.) and environmental stimuli (e.g., light, magnetic field, volatile organic compounds (VOCs)), which are vital to real-time and all-round human health monitoring and management. Herein, this review summarizes the design, manufacturing, and application of multiresponsive flexible sensors and presents the future challenges of fabricating these sensors for the next-generation e-skin and wearable electronics.

Recent Progress in Ti3C2Tx MXene-Based Flexible Pressure Sensors

The rapid development of consumer electronics, artificial intelligence, and clinical medicine generates an increasing demand for flexible pressure sensors, whose performance depends significantly on sensitive materials with high flexibility and proper conductivity. MXene, a type of 2D nanomaterial, has attracted extensive attention due to its good electrical conductivity, hydrophilicity, and flexibility. The synthesis methods for MXenes make it relatively easy to control their microstructure and surface termination groups. Hence, MXenes can obtain peculiar microstructures and facilely combine with other functional materials, making them promising prospects for use in flexible pressure sensors. In this Review, recent advances in MXenes are summarized, mainly focusing on the synthesis methods and their application in flexible pressure sensors. Finally, the challenges and potential solutions for future development are also discussed.

Covalent Surface Modification of Ti3C2Tx MXene with Chemically Active Polymeric Ligands Producing Highly Conductive and Ordered Microstructure Films

As interest continues to grow in Ti₃C₂T_x and other related MXenes, advancement in methods of manipulation of their surface functional groups beyond synthesis-based surface terminations (T_x: -F, -OH, and ═O) can provide mechanisms to enhance solution processability as well as produce improved solid-state device architectures and coatings. Here, we report a chemically important surface modification approach in which "solvent-like" polymers, polyethylene glycol carboxylic acid (PEG6-COOH), are covalently attached onto MXenes via esterification chemistry. Surface modification of Ti₃C₂T_x with PEG6-COOH with large ligand loading (up to 14% by mass) greatly enhances dispersibility in a wide range of nonpolar organic solvents (e.g., 2.88 mg/mL in chloroform) without oxidation of Ti₃C₂T_x two-dimensional flakes or changes in the structure ordering. Furthermore, cooperative interactions between polymer chains improve the nanoscale assembly of uniform microstructures of stacked MXene-PEG6 flakes into ordered thin films with excellent electrical conductivity (∼16,200 S·cm^-1). Most importantly, our covalent surface modification approach with ω-functionalized PEG6 ligands (ω-PEG6-COOH, where ω: -NH₂, -N₃, -CH═CH₂) allows for control over the degree of functionalization (incorporation of valency) of MXene. We believe that installing valency onto MXenes through short, ion conducting PEG ligands without compromising MXenes' features such as solution processability, structural stability, and electrical conductivity further enhance MXenes surface chemistry tunability and performance and widens their applications.