Flexible Electronics Enabled by Fiber Nanocomposites with MXene Nanosheets

Abstract

The incorporation of MXene (Ti3C2Tx) nanosheets into fiber nanocomposites is at the forefront of flexible electronics. This chapter provides an overview of recent progress in preparing MXene fiber nanocomposites, covering methods, interface design, mechanical/electrical properties, and applications. Emphasizing their transformative impact on electrical functionalities, the chapter explores how integrating MXene into fibrous structures has revolutionized material engineering through interfacial interactions. The resulting nanocomposites demonstrate customized mechanical and electronic properties, utilizing MXene nanosheets’ unique attributes to improve their interfacial interactions and expedite charge transport. These versatile fiber nanocomposites enable the creation of innovative devices like flexible electromagnetic interference shielding, thermal management, energy storage, sensors, and so on. With enhanced electronic conductivity and mechanical strength, these fiber nanocomposites pave the way for advancements in flexible electronics. Researchers and practitioners will find this chapter valuable for understanding the current state, challenges, and future directions in fiber nanocomposites for flexible and wearable electronic applications.

1. Introduction

In the dynamic realm of materials science, the integration of two-dimensional (2D) nanosheets into fiber nanocomposites stands as a frontier with significant implications for advancing flexible electronics [1]. The distinctive properties inherent in various 2D nanosheets, including graphene, MXene, transition metal dichalcogenides, and black phosphorus [2, 3, 4, 5], when seamlessly embedded into fibrous structures, provide a transformative platform for engineering materials with heightened functionalities. This burgeoning field is driven by the pursuit of materials capable of harnessing both the exceptional properties of 2D nanosheets and the structural advantages of fibrous architectures simultaneously. The characteristics of 2D nanosheets, combined with the mechanical properties, electrical performance, flexibility, scalability, and lightweight nature of fibers, create a synergistic blend promising to revolutionize wearable electronic applications [6].

MXenes, a novel category of 2D inorganic compounds, exhibit considerable promise for advancing fiber-based devices, thanks to their exceptional mechanical, electrical, and electromagnetic properties. Utilizing MXene nanosheets in the creation of high-performance fiber nanocomposites is facilitated by the presence of diverse surface-terminated moieties (T_x), such as -OH, -O, and -F [7]. These moieties contribute to the enhanced mechanical flexibility, electronic conductivity, and lightweight attributes observed in the resultant fiber nanocomposites. Consequently, this advancement supports the evolution of flexible and wearable electronic devices [8]. The integration of MXene nanosheets with fibrous structures has catalyzed innovations in electronic textiles [9], such as electromagnetic interference (EMI) shielding, human thermal management, sensors [10], and energy storage systems [11].

In this chapter, we delve into the preparation methods, interface design, and mechanical/electrical properties of MXene nanosheet-based fiber nanocomposites for flexible electronics. The focus extends to their applications, aiming to present a comprehensive overview of the current state of the art while addressing both challenges and opportunities on the horizon. Covering fundamental principles guiding material preparation to applications in cutting-edge technologies, this chapter intends to shed light on the dynamic trajectory of this field. Through our exploration, we aspire to contribute to the collective understanding of researchers, engineers, and scientists involved in this intersection of materials science and technology, fostering inspiration for further innovations in fiber nanocomposites.

2. Main text

2.1 Preparation methods

The preparation of fiber nanocomposites, an evolving discipline within materials science, depends on a broad spectrum of preparation approaches to instill customized and improved functionalities. The strategic integration of nanofibers with matrix materials, accomplished through various techniques, is crucial for attaining superior mechanical, electrical, and properties, paving the way for innovative applications. In this context, diverse methodologies, including thermal drawing, wet spinning, coating, electrospinning, and biscrolling [12, 13, 14, 15, 16], present distinct advantages and opportunities for material engineers to finely adjust the attributes of the resulting fiber nanocomposites incorporating MXene nanosheets.

2.1.1 Thermal drawing

The primary and intrinsic method of integrating innovative functionalities into fiber nanocomposites includes introducing novel materials at distinct locations during the preform stage. Following this, the complete assembly undergoes thermal drawing to transform into an elongated MXene fiber nanocomposite, resembling the manufacturing process of traditional silica fibers (Figure 1) [17]. This thermal drawing technique provides a crucial advantage by upholding simplicity and scalability with the encapsulated layer, coupled with numerous inherent attributes throughout the drawing process.

Figure 1.

(a) Schematic depiction of the fabrication process for MXene fiber nanocomposites, incorporating a protective polymer layer through direct thermal drawing. (b) MXene fiber nanocomposites wound onto a roll. (c) Scanning electron microscopy (SEM) image showing the structure of the MXene fiber nanocomposites [17].

The preform is transformed into a macroscopic hollow tube through a straightforward and cost-effective process, employing equipment capable of controlling features size. Additionally, achieving nanoscale structures in the plastic deformation process along the entire fiber length is crucial for efficient operation, generally leading to a reduction of crossing defects. By scaling down in line with the law of mass conservation, the preform’s volume is concurrently distributed into thin fiber. This approach facilitates the coverage of macroscopic surfaces. As a result, thousands of meters of fiber with an outer diameter from ~50 to 1000 μm can be drawn during a thermal drawing process within a few hours, resulting in a greater functional surface area. Fibers containing polymers and MXene nanosheets demonstrate elevated mechanical and conductive properties, rendering them suitable for integration into surfaces, textiles, and flexible electronic devices. Moreover, this preparation method can simultaneously process diverse microstructures in a single step, catering to various applications.

The initial phase involves crafting a macroscopic preform in the thermal drawing process, utilizing a combination of various polymers and MXene nanosheets [18]. Diverse materials are introduced into the preform using distinct techniques to create a sophisticated structure. For example, the rolling approach is employed for hollow cylindrical fibers based on thin film, the consolidation approach involves assembling materials in a vacuum oven or using a hot press for solid fibers, the stack-and-draw method, and the extrusion approach [19, 20, 21, 22, 23]. During thermal drawing, this resultant solid preform evolves into a slender fiber while maintaining the microscopic cross-sectional structure along the entire length. This transformation is contingent upon the chosen thermal drawing parameters and materials.

Beyond employing a preform with uniform material for scalability and simplicity in the thermal drawing process, this method may require simultaneous co-thermal drawing of materials with distinct thermo-mechanical properties. The interplay of interfacial interactions between inner materials and outer encapsulated layer, becomes crucial in determining achievable microstructures and their geometries due to the generation of surface area in the thermal drawing procedure. To counteract these effects, the law of viscosity engineering is commonly utilized. Consequently, when constructing a multi-material fiber system, various parameters such as glass-transition temperature, thermal expansion, chemical reactions, crystallization, and mixing must be taken into account. These factors impose specific requirements, leading to a restricted selection of multi-material combinations that can be concurrently implemented within the fibers.

The polymer cladding, making up the majority of the preform and ensuring assembly cohesion, necessitates processing at a relatively high viscosity. This viscosity range strikes a balance: It is low enough to facilitate plastic deformation into fibers without fracturing, yet high enough to maintain the mechanical integrity of the desired structure. Beyond temperature-dependent viscosity, evaluating cladding materials includes considering their susceptibility to degradation at elevated temperatures. As both viscosity and interfacial interactions are material-specific and temperature-dependent, the temperature profile within the preform becomes crucial in the drawing process. Preventing significant contrasts in thermal expansion coefficients is essential to mitigate excessive residual stresses in the resulting fibers, especially for diverse polymer cladding. Material consolidation in a confined space and elevated temperature can potentially lead to species diffusion and chemical interactions among different compounds. While often undesirable, these reactions may be intentional and regulated. Ultimately, the interplay between interfacial interactions and viscosity determines the final feature sizes in various material geometries of preforms targeted within the fiber.

2.1.2 Wet spinning

Wet spinning has become a compelling method for crafting nanocomposite fibers with two-dimensional materials, providing a versatile approach to leverage the distinctive properties of these materials in the development of advanced fibrous structures (Figure 2) [18]. In the case of MXene nanosheets, the wet-spinning process entails dispersing these nanoscale building blocks in a liquid medium to create a stable spinning solution. Several key considerations for wet spinning include size, solvent, rheological properties, coagulation bath, and more.

Figure 2.

(a) Fabrication process of MXene fiber nanocomposites via continuous wet spinning followed by thermal drawing. (b) Photographic depiction of MXene fiber nanocomposites alongside an SEM image [18].

MXene nanosheets, with their nanoscale thickness, exhibit varying lateral flake sizes determined by synthesis methods and post-processing processes. Controlling the lateral size is crucial for both the material’s processability into intricate structures and the properties of the resulting fiber nanocomposites. Larger flakes contribute not only to fiber fabrication but also enhance the mechanical properties and conductivity of the resulting fiber nanocomposites. This enhancement is typically attributed to high alignment and a reduction of defects, including voids and wrinkles. For instance, MXene nanosheets with sizes up to 10 μm for the nanoscale building blocks, when used in wet spinning, significantly promote the mechanical and conductive properties with few defects within the fiber nanocomposites [24]. The choice of solvent plays a significant role in fiber spinning, influencing the viscosity of the spinning solution, stability, spinning method, and choice of coagulating bath, thereby causing fluctuations in the performance of fiber nanocomposites.

Certain MXene nanosheets are easily processed using aqueous dispersions due to their surface terminations with water-soluble polymers (e.g., polyvinyl alcohol, sodium alginate, cellulose, etc.) [9]. Additionally, the dispersibility of MXene nanosheets in organic solvents has been explored through straightforward solvent exchange procedures involving repeated centrifugation. In various fiber preparation methods, the rheological properties of the spinning solution play a significant role in determining the ability to produce continuous fibers or achieve uniformity. The study of the rheology of MXene nanosheets in the spinning solution reveals that the spinning process relies on viscosity, loss moduli (G′), storage moduli (G″), and the ratio of G′/G″ [25]. It is typically essential to ensure good flowability with high viscosity and shear-thinning behavior under shear stress. Furthermore, structural integrity must be maintained with a G′/G″ ratio higher than 1 after the removal of shear force, whether in aqueous or organic solvents. Moreover, adjusting the concentration of the spinning solution from 10 to 160 mg mL⁻¹ allows for the tuning of these parameters, enabling a broad spectrum of wet spinning fabrication methods. To initiate the wet-spinning procedure, a carefully tailored solution containing MXene nanosheets is extruded through a spinneret into a coagulation bath. This coagulation bath induces rapid solvent exchange with a non-solvent, accelerating the precipitation of two-dimensional materials and the formation of continuous fibers. The choice of solvents and coagulation bath conditions is critical in governing the kinetics of the process and influencing the ultimate morphology and properties of the fibers [9].

In the spinning process, the efficacy is intricately linked to both the extrusion rate and collection speed. The extrusion rate, representing the speed at which the spinning solution is forced through the spinneret, is a critical parameter influencing wet spinning. Optimal extrusion rates are crucial for achieving uniform fiber morphology and controlling the dimensions of resulting MXene fiber nanocomposites [24, 25]. Deviations in extrusion rates can lead to variations in fiber diameter, alignment, and overall structural integrity. Simultaneously, the collection speed, denoting the rate at which the freshly spun fibers are gathered or coagulated, plays a pivotal role in determining the final properties of wet-spun fibers. The interplay between extrusion rate and collection speed influences the stretching and alignment of fibers during the coagulation process. Controlling the collection speed is essential for ensuring proper coagulation and preventing defects such as irregularities, buckling, or coalescence in wet-spun fibers. A delicate balance between extrusion rate and collection speed is imperative to attain desired fiber characteristics, including mechanical properties, conductivity, surface morphology, and porosity. Researchers continue to explore and optimize the interdependence of these parameters in wet spinning to enhance the reproducibility and scalability of the process for various applications, ranging from textiles to biomedical materials. Understanding the impact of extrusion rate and collection speed on wet spinning is fundamental for advancing the precision and efficiency of this fiber formation method.

2.1.3 Coating

Coating-based techniques have become a versatile approach for crafting fibers with MXene nanosheets, providing a distinctive pathway to leverage the exceptional properties of these nanomaterials in creating advanced fibrous structures (Figure 3). This process entails applying a uniform layer of MXene nanosheets onto a substrate, utilizing their surface functional groups. This is typically done on a pre-existing fiber or a template that aids in the formation of the final fibrous structure, employing methods such as drop-casting, dip coating, and spray [26].

Figure 3.

(a) Schematic process of the preparation for asymmetric supercapacitor through the coating method. (b) SEM image depicting MXene-coated fibers. (c) SEM image showing Ru₂O-coated fibers [26].

This dispersion can take the form of a liquid solution, similar to wet spinning. The subsequent steps involve drying or curing the coated substrate to eliminate residual solvents and enhance the adhesion of the two-dimensional materials onto the substrate. The coating process offers a high degree of control over the thickness and distribution of MXene nanosheets on the substrate, facilitating the creation of functionalized fibers with tailored properties. The resulting fibers demonstrate enhanced mechanical strength, electrical conductivity, and other distinctive characteristics attributed to the inherent properties of the coated MXene nanosheets. Therefore, the versatility of coating methods renders them suitable for various applications, including the development of smart textiles and flexible electronics. By precisely adjusting coating parameters (such as flake size, solvent, rheological properties, and concentrations), researchers can optimize the structural and functional attributes of the fibers, paving the way for the integration of two-dimensional materials into a wide range of innovative and practical applications.

2.1.4 Electrospinning

Electrospinning has proven highly effective in fabricating fibers incorporating MXene nanosheets within a polymer or polymer-based composite. This method provides a versatile approach to leverage the unique properties of MXene nanomaterials in the development of advanced fibrous structures (Figure 4). In electrospinning, an electrostatic field is employed to draw a fine jet of polymer solution or melt, which includes MXene nanosheets, from a spinneret toward a grounded collector [27].

Figure 4.

(a) Preparation of MXene/carbon nanofibers via electrospinning from solutions containing MXene and polyacrylonitrile. (b) SEM image. (c) Transmission Electron Microscope (TEM) image displaying MXene fibers produced by electrospinning [27].

In the electrospinning process, a solution is prepared containing dispersed nanosheets, typically involving the dissolution of polymers and MXene nanosheets in a suitable solvent. The solution is then electrostatically charged by applying a high voltage to the spinneret. The repulsion of charges within the solution results in the formation of a fine jet, which elongates and eventually solidifies into continuous fibers during its trajectory to the grounded collector. The collector, which can be a rotating drum, a flat plate, or a conductive surface, plays a crucial role in the deposition and alignment of the electrospun fibers.

The electrospinning method provides precise control over the morphology within the fibers, leveraging the high surface area and aspect ratio of these materials to form nanostructured fibers with unique properties. These properties include enhanced mechanical strength, electrical conductivity, and surface reactivity. This technique is particularly advantageous for fabricating short fiber nanocomposites, where MXene nanosheets are homogeneously dispersed within a polymeric matrix. The resulting fibers have applications in diverse fields, including sensors, energy storage devices, and advanced textiles. This showcases the potential of electrospinning as a powerful tool for incorporating the extraordinary properties of MXene nanosheets into functional and tailored fibrous materials.

2.1.5 Biscrolling

Biscrolling, a novel technique in fiber preparation, involves the controlled rolling of MXene nanosheets into fiber structures, leading to the creation of unique and functional fibers. This method stands out for its versatility in utilizing a diverse range of materials, including carbon nanotubes and MXene [9, 28]. The core concept of biscrolling lies in the controlled interaction between layers, resulting in the formation of seamless and well-defined fibers. This technique provides precise control over the diameter, length, and morphology of the resulting fiber nanocomposites, making it an attractive approach for tailored applications in electronics, sensors, and composite materials. The inherent flexibility of biscrolled fibers is a notable advantage, allowing them to conform to various shapes and surfaces. One of the key benefits of biscrolling is its capacity to incorporate different functional elements during the fabrication process (Figure 5). This includes the integration of nanoparticles, polymers, or other materials, enhancing the overall properties of the resulting fibers. Additionally, biscrolled fibers exhibit intriguing mechanical, electrical, and thermal properties, making them promising candidates for advanced materials and device applications.

Figure 5.

(a) The process of fabrication of MXene/CNT fibers via biscrolling. (b–d) SEM images of the obtained MXene fibers [28].

2.2 Interfacial interactions design

The development of fiber nanocomposites using MXene nanosheets, featuring abundant oxygen-containing functional groups [29], employs a sophisticated strategy to customize material properties for optimal performance. Through leveraging diverse interfacial interactions like hydrogen bonds, ionic bonds, covalent bonds, and synergistic interfacial bonds [30], the resulting composites showcase exceptional characteristics. The strategic application of these interactions facilitates the production of materials exhibiting superior mechanical strength, enhanced electrical properties, and improved structural integrity. Furthermore, the integration of multiple interfacial interactions collectively contributes to the overall outstanding performance of the fiber nanocomposites.

2.2.1 Hydrogen bonds

The advancements in designing and fabricating fiber nanocomposites highlight the growing significance of interface interactions. A crucial aspect of this paradigm is the utilization of hydrogen bonding, a widely prevalent molecular interaction that can be deliberately designed between MXene nanosheets and polymers featuring abundant oxygen-containing functional groups. The specificity and strength of hydrogen bonds facilitate controlled assembly at the nanoscale, influencing the overall properties of the resulting fiber nanocomposites. A comprehensive understanding of the principles governing hydrogen bonding becomes imperative for tailoring these interactions to achieve the desired material characteristics.

Various fabrication techniques capitalize on hydrogen bonding at interfaces, including thermal drawing [31], wet spinning, coating, electrospinning, and solution blending. These methodologies are pivotal for integrating nanoparticles, polymers, and other additives into MXene fiber nanocomposites, each offering unique advantages in scalability, precision, and versatility. Hydrogen bonding interface interactions play a significant role in shaping the mechanical, thermal, and electrical properties of fiber nanocomposites. By carefully controlling bonding dynamics, researchers can attain enhanced structural integrity, thermal stability, and electrical conductivity, effectively tailoring the material for specific applications.

2.2.2 Ionic bonds

Ionic bonding, characterized by the transfer of electrons between atoms, is the fundamental principle propelling this innovative approach. The electrostatic interactions among ions provide precise control over the assembly of nanoscale constituents within the fiber matrix, shaping the overall properties of the resulting nanocomposites. A nuanced comprehension of ionic bonding principles is essential for tailoring these interactions to achieve the desired material characteristics. For MXene nanosheets featuring abundant negative oxygen-containing functional groups, the preparation of fiber nanocomposites becomes easily achievable with ions such as Ca²⁺, Mg²⁺, NH⁴⁺, and others, utilizing ionic bonds [26, 31]. As an illustration, MXene fibers can be synthesized through ionic bonding between MXene nanosheets and Mg²⁺. This bond imparts not only high tensile strength but also enhances conductivity [32]. Additionally, MXene fiber nanocomposites can be fabricated by forming ionic bonds with NH⁴⁺, resulting in high Young’s modulus and electrical conductivity, achieved through reduced porosity and enhanced alignment of MXene nanosheets [24]. Consequently, the impact of ionic bonding interface interactions on the mechanical, thermal, and electrical properties of MXene fiber nanocomposites is significant. Through careful control of the strength and specificity of ionic bonds, researchers can attain customized enhancements in structural integrity, thermal stability, and electrical conductivity. This enables tailored adjustments based on specific application requirements.

2.2.3 Covalent bonds

The formation of a covalent bond typically occurs between MXene nanosheets featuring abundant oxygen-containing functional groups and certain organic derivatives with -OH and -CHO functional groups or N,N′-carbonyldiimidazole [33]. In a recent attempt, the α-cyclodextrin rings of polyrotaxane were utilized to crosslink with MXene nanosheets, fabricating MXene fiber nanocomposites through covalent bonds with the aid of N,N′-carbonyldiimidazole via wet spinning [33]. Consequently, the resulting MXene fiber exhibited a high toughness of 60 MJ m⁻³ and a remarkable electrical conductivity of approximately 1100 S cm⁻¹. Moreover, through covalent bonds formed with glutaraldehyde, robust inner MXene fiber nanocomposites were successfully prepared via wet spinning. The covalent bond imparted a high tensile strength of approximately 565.2 MPa and an electrical conductivity of around 8110.4 S cm⁻¹ to the MXene fiber nanocomposites. Hence, covalent bonds play a pivotal role in profoundly influencing the mechanical properties of fiber nanocomposites based on 2D materials [18].

2.2.4 Synergistic interfacial bonds

Beyond individual interfacial interactions such as hydrogen bonds, ionic bonds, and covalent bonds, the enhancement of fiber nanocomposites can be achieved by incorporating combinations of two or more of these interactions, resulting in a superior level of performance. An illustration of this is the fabrication of ultra-tough MXene fibers with a protective polymer layer of cyclic olefin copolymer through thermal drawing, utilizing both hydrogen bonds and tablet interlocks [17]. The resulting fiber nanocomposites exhibit significantly improved toughness and tensile strength, reaching up to 125.12 MJ m⁻³ and 707.73 MPa, respectively, owing to interfacial interactions that promote high orientation and low porosity. Additionally, the electrical conductivity and electromagnetic shielding performance of woven textiles based on MXene fibers are enhanced. Therefore, the properties of MXene fiber nanocomposites are indeed elevated through the synergistic effects of interfacial bonds.

2.3 Mechanical/electrical properties

The mechanical and electrical characteristics of fiber nanocomposites utilizing MXene nanosheets play a pivotal role in determining their applicability across diverse industries. The integration of nanosheets with matrix materials has been crucial in achieving outstanding mechanical strength, toughness, and flexibility in these nanocomposites. Through detailed engineering of interfaces, including hydrogen bonds, ionic bonds, covalent bonds, and synergistic interfacial bonds, using various preparation approaches, the resulting fiber nanocomposites demonstrate improved mechanical performance, featuring remarkable tensile strength and toughness. Simultaneously, the inclusion of MXene nanosheets contributes to exceptional electrical conductivity. These unique electrical properties render these nanocomposites highly suitable for applications in electronic textiles, conductive films, and flexible electronics.

2.3.1 Mechanical property

The mechanical properties of individual MXene nanosheets far exceed those of the corresponding fiber nanocomposites. Single MXene layer nanosheets, for instance, exhibit a Young’s modulus of approximately 330 GPa and a tensile strength of around 17.3 GPa [34]. However, the current MXene fiber nanocomposites display suboptimal tensile strength and Young’s modulus due to the widely varying mechanical characteristics influenced by factors like interface, structure, and fabrication approaches. To enhance the mechanical properties of fiber nanocomposites derived from MXene nanosheets, it becomes essential to introduce interfacial interactions through diverse preparation approaches. For instance, the incorporation of hydrogen bonds resulted in MXene-coated cotton yarns with an elevated tensile strength of 468 MPa [35]. This improvement is attributed to the inherent strength of the fiber substrate, boasting a tensile strength of approximately 334 MPa, complemented by the reinforcing effect of MXene with a weight percentage of 79%.

Nevertheless, composite fibers in fiber nanocomposites have demonstrated improved tensile strength, a consequence of both the inherent mechanical properties of the host nanosheets and the interfacial interactions between additives and nanosheets. For instance, pure MXene fibers consistently exhibit poor tensile strength ranging from 10 to 100 MPa, depending on various spinning coagulations [36]. However, the introduction of Mg²⁺ into the interlayers via ionic bonds significantly enhances the tensile strength of resulting MXene fibers, reaching 118 MPa, a substantial improvement over pure MXene fibers [30]. Simultaneously, through crosslinking with MXene nanosheets via covalent and coordination bonds at the interface, MXene fiber nanocomposites exhibit a high toughness of 60 MJ m⁻³ and a remarkable tensile strength of approximately 502 MPa [33].

Furthermore, the mechanical properties of fiber materials are significantly influenced by the orientation of nanosheets and the porosity of the structure. The orientation of nanosheets within the fiber matrix plays a crucial role in determining the load-bearing capabilities and overall structural integrity of the composite material. Well-aligned MXene nanosheets contribute to enhanced tensile strength and Young’s modulus along the preferred direction of alignment, reinforcing the fiber material. Conversely, random or misaligned orientations may lead to less effective load transfer and weaker mechanical performance.

In addition, porosity introduces voids or empty spaces within the material, impacting its density and mechanical strength. Higher porosity generally results in reduced density and, consequently, weaker mechanical properties, including tensile strength, toughness, and modulus. Therefore, it is imperative to promote the alignment of MXene nanosheets in the fiber nanocomposite and reduce porosity to achieve high mechanical properties. The alignment of MXene nanosheets in MXene fibers is indeed promoted with the increasing drawing ratio from 1 to 3 through wet spinning, resulting in an increase in tensile strength from ~304 to ~343 MPa and an increase in Young’s modulus up to ~122 GPa. Simultaneously, the porosity of MXene fibers decreases from 21.59 to 6.13% [24]. Moreover, introducing interfacial interactions into the interlayers between MXene nanosheets significantly improves orientation with low porosity. The resulting MXene fibers with a compact structure exhibit ultra-toughness of ~66.7 MJ m⁻³ and excellent tensile strength of ~585.5 MPa [18].

2.3.2 Electrical property

Due to interfacial interactions among MXene nanosheets, coupled with high orientation and low porosity, fiber nanocomposites demonstrate exceptional electrical properties along the axial direction. MXene nanosheets themselves exhibit notable electrical conductivity exceeding ~10⁵ S cm⁻¹. Consequently, the resulting MXene fibers achieve a conductivity of up to ~7700 S cm⁻¹ through wet spinning [36]. Furthermore, induced by thermal drawing stresses, MXene fibers also exhibit superior electrical conductivity (~1.2 × 10⁴ S cm⁻¹) via thermal drawing [17]. Therefore, fiber nanocomposites incorporating 2D nanosheets with high electrical conductivity are positioned to play a crucial role in prospective applications such as wearable electronics and textiles.

2.4 Applications

By incorporating improved mechanical and electrical properties, both pure and hybrid fiber nanocomposites utilizing MXene nanosheets are applied across diverse domains. These applications span flexible electromagnetic interference (EMI) shielding textiles, human thermal management textiles, energy storage systems, sensors, energy harvesters, and more. The cutting-edge uses predominantly revolve around flexible and wearable electronics, capitalizing on the inherent tensile strength, toughness, electrical conductivity, and flexibility of MXene fiber nanocomposites.

2.4.1 Flexible electromagnetic interference shielding

Due to the widespread use of telecommunication and portable electronic devices, concerns related to electromagnetic interference (EMI) become prominent, posing potential risks to human health, data misinterpretation, data loss, and even system failure. This is especially critical when these devices are in close proximity and subject to strong electromagnetic induction effects. Leveraging the high electrical conductivity and remarkable mechanical properties of textiles prepared from MXene fiber nanocomposites proves effective in achieving superior EMI shielding performance as shown in Figure 6 [17]. For instance, MXene fiber nanocomposites, featuring an encapsulated layer of cyclic olefin copolymer, are fabricated through a thermal drawing process. These textiles exhibit high total EMI shielding efficiency (EMI SE) of ~81.5 dB within the frequency range of 2.6 to 18.0 GHz. Moreover, the woven textiles demonstrate EMI SE retention exceeding 99.0% at 8.0 GHz after 2 × 10⁴ cycles of folding [17]. Additionally, textiles derived from MXene fibers display remarkable durability and stability, retaining high EMI SE under various extreme environments. Consequently, MXene fibers, with their high electrical conductivity and desired mechanical properties, play a crucial role in EMI protection for human beings.

Figure 6.

(a) Large-scale textile with MXene fiber nanocomposites and cotton yarn. (b) EMI SE of the textiles with various weft densities of MXene fibers nanocomposites at the range of frequency from 2.6 to 18.0 GHz. (c) EMI SE of the textiles obtained from the fiber nanocomposites at different draw-down ratios. (d) EMI SE of the textiles measured at the frequency of 8.0 GHz. (e) The mechanism of EMI shielding employed by the textiles. (f) Assessment of the durability and stability of the resulting textiles under various extreme environmental conditions [17].

2.4.2 Flexible thermal management

The flexible and wearable textiles for human thermal management are urgently needed for human protection in daily life. MXene fibers nanocomposites with high conductivity usually perform the high performance of thermal management. Recently, the MXene fiber nanocomposites with the protective layer of polycarbonate were successfully fabricated via wet spinning and thermal drawing. As a result in Figure 7, the textiles based on the MXene fiber nanocomposites can generate the heat with the temperature increasing up to ~130°C when applying different voltages at a broad range of 2 to 8 V [18]. Moreover, because of the high tensile strength and toughness of fibers nanocomposites, the obtained textiles can remain stable under flatting, bending, twisting, pressing, and even washing.

Figure 7.

The curves of temperature-time of MXene fiber nanocomposites under a 6 V DC voltage (a) and a range of 2 to 8 V (b). (c) Photographs depicting fibers with various letter shapes under different DC voltages ranging from 2 to 8 V. (d) Temperature-time curves of individual fibers at bending angles ranging from 0 to 180°. (e) Evaluation of temperature performance retention. (f) Assessment of temperature performance retention for a single MXene fiber nanocomposite after 1.1 × 10⁵ bending cycles across angles from 0 to 180° [18].

2.4.3 Flexible energy storage

Supercapacitors demonstrate significant potential as energy storage devices for applications in compact, flexible electronics, featuring notable characteristics such as elevated capacitance, rapid charge-discharge rates, and dependable cycling stability. As science and technology progress rapidly, supercapacitors are required to embody qualities such as lightweight design, portability, and ease of wear for optimal performance. To achieve this performance, the resulting fibers can serve directly as electrodes for wearable supercapacitors. The MXene/CNT fiber, prepared by biscrolling with an impressive MXene loading of approximately 98 wt%, attained exceptional specific volumetric capacitance of ~1100 F cm⁻³, gravimetric capacitance of 428 F g⁻¹, and linear capacitance values of 118 mF cm⁻¹ in a 3 M H₂SO₄ electrolyte [28].

2.4.4 Flexible sensors

Benefiting from the outstanding electrical conductivity and exceptional flexibility inherent in fiber nanocomposites derived from MXene nanosheets, these materials are well-suited for applications in wearable sensors, encompassing wearable strain sensors, pressure sensors, and gas sensors [37]. Utilizing the dip coating and drying method, a cotton textile with an MXene coating was fabricated [37]. Leveraging the undulating network structure of the textile substrate and MXene’s exceptional electrical properties, the resulting sensor displayed notable sensitivity, registering ~12,000 kPa⁻¹ for pressures in the range of 29 to 40 kPa. It demonstrated a rapid response time of 26 ms and remarkable stability across 5600 cycles. Additionally, the MXene-textile sensor facilitated real-time monitoring of human activities, including wrist pulse, pronunciation, and finger movements like touching and bending. These tests underscore the potential of textile sensors based on MXene nanosheets for integration into wearable and flexible electronics.

2.4.5 Flexible energy harvesters

Energy harvesters serve a crucial role as wearable devices for converting various forms of energy, extracting electrical power from sources such as mechanical strain energy (e.g., walking, bending, and running), thermal energy, sound, and light. Typically, these devices undergo conversion by responding to specific stimuli. The scalable fabrication of MXene/polyvinylidene fluoride (PVDF) fibers through thermal drawing facilitates the creation of triboelectric nanogenerators [31]. The synergistic interaction between the surface termination groups of MXene and the polar PVDF polymer yields a 53% improvement in open-circuit voltage and a 58% improvement in short-circuit current. Furthermore, the fabric demonstrates a power density of 40.8 mW m⁻² at the matching load of 8 MΩ, maintaining stable performance throughout 1.2 × 10⁴ cycles.

3. Conclusion

This chapter explores how integrating MXene nanosheets into fibers enhances mechanical strength, toughness, electrical conductivity, and flexibility through various preparation methods, creating opportunities for innovation across multiple domains. The successful development of MXene fiber nanocomposites highlights their potential applications in flexible electromagnetic interference shielding, flexible thermal management, flexible energy storage, flexible sensors, and flexible energy harvesters. The enhanced mechanical and electrical performance of these nanocomposites facilitates their seamless integration into flexible and wearable electronics. Already, 2D nanosheet-based fiber nanocomposites have demonstrated their versatility in diverse applications. As research in this field progresses, we anticipate further breakthroughs, pushing the limits of what can be achieved with these groundbreaking materials. The collaboration between materials science, nanotechnology, and engineering is set to unlock new possibilities, establishing these nanocomposites as crucial components in the next generation of advanced materials.

Acknowledgments

This work was partly supported by the National Natural Science Foundation of China (NSFC) (52203078), the National Postdoctoral Program for Innovative Talents (BX2021025), and the Postdoctoral Science Foundation (2021 M690005).

Conflict of interest

The authors declare no conflict of interest.