Carbon-Nanotube Fibers for Wearable Devices and Smart Textiles

The early warning research on nursing care of stroke patients with intelligent wearable devices under COVID-19

Stroke patients under the background of the new crown epidemic need to be home-based care. However, traditional nursing methods cannot take care of the patients' lives in all aspects. Based on this, based on machine learning algorithms, our work combines regression models and SVM to build a smart wearable device system and builds a system prediction module to predict patient care needs. The node is used to collect human body motion and physiological parameter information and transmit data wirelessly. The software is used to quickly process and analyze the various motion and physiological parameters of the patient and save the analysis and processing structure in the database. By comparing the results of nursing intervention experiments, we can see that the smart wearable device designed in this paper has a certain effect in stroke care.

Wicking-Driven Evaporation Self-Assembly of Carbon Nanotubes on Fabrics: Generating Controlled Orientational Structures

Nanostructures with orientational order exhibit excellent electrical and optical properties; however, their construction on complex fabrics is challenging. Here, we demonstrate the potential of wicking-driven evaporation self-assembly in the oriented arrangement of carbon nanotubes (CNTs) on fabrics. The solution-evaporation self-assembly in combination with the fabric wicking effect leads to convective flows along the fibers, which makes it possible to prepare orientational nanostructures over large fabric surface areas. The orientation of CNTs is controlled by the fluid drag force from the convective flow during drying, thus the interaction between the CNT and the solution is crucial. We show that the nanostructures of CNTs on fibers depend, for example, on the evaporation temperature, component concentration, and solution pH. Weakening the viscous connection of the fluid with CNTs can lead to an interesting eddy nanostructure of CNTs. The electrical conductivity of the assembled fabrics increases strongly with the degree of orientation and the assembly cycles of CNTs. In this work, the large-scale orientational order of nanomaterial achieved by wicking-driven evaporation self-assembly offers a new strategy for constructing three-dimensional oriented conductive networks in electronic textiles.

Polycarbazole and Its Derivatives: Synthesis and Applications. A Review of the Last 10 Years

Polycarbazole and its derivatives have been extensively used for the last three decades, although the interest in these materials briefly decreased. However, the increasing demand for conductive polymers for several applications such as light emitting diodes (OLEDs), capacitators or memory devices, among others, has renewed the interest in carbazole-based materials. In this review, the synthetic routes used for the development of carbazole-based polymers have been summarized, reviewing the main synthetic methodologies, namely chemical and electrochemical polymerization. In addition, the applications reported in the last decade for carbazole derivatives are analysed. The emergence of flexible and wearable electronic devices as a part of the internet of the things could be an important driving force to renew the interest on carbazole-based materials, being conductive polymers capable to respond adequately to requirement of these devices.

Homogeneity Permitted Robust Connection for Additive Manufacturing Stretchable Electronics

Interface is the Achilles' heel in flexible electronics, where slipping, delamination, or cracking resulting from the intrinsic mechanical property mismatch of different materials might fail the device or cause degradation. Robust connections with strong interfacial strength and low interfacial impedance are highly desired in all kinds of flexible devices, especially when they require stretchability. Here, a new strategy of employing homogeneity is proposed to facilely achieve robust connection without introducing a third-party interlayer. The key to this strategy is implementing different functions in the same component materials, and in this work, this strategy is demonstrated by exploring the uniformity of carbon nanotube (CNT) in thermoplastic polyurethane (TPU) elastomer. Tuning the uniformity of CNT in TPU elastomer, two kinds of CNT-TPU composites (CTCs) are fabricated. They present contrasting properties of conductivity (281 folds), Young's modulus (7.1 folds), and strain sensitivity (15.7 folds) with the same weight fraction of CNT (15 wt %), and thus they can respectively act as strain sensor and interconnector in all-CNT-based stretchable strain monitoring electronics. The interlocking effect caused by spontaneous entanglement of surface polymer chains and the reconstruction of percolation network at the interface during drying confer the connection with robustness. The practical value of this strategy has been exemplified by the fabrication of a robust epidermis device via additive manufacturing. This epidermis device is patterned in a fractal motif for conformal contact and can sensitively monitor body motions and radial artery pulse.

Carbon Nanotube Reinforced Strong Carbon Matrix Composites

As an excellent candidate for lightweight structural materials and nonmetal electrical conductors, carbon nanotube reinforced carbon matrix (CNT/C) composites have potential use in technologies employed in aerospace, military, and defense endeavors, where the combinations of light weight, high strength, and excellent conductivity are required. Both polymer infiltration pyrolysis (PIP) and chemical vapor infiltration (CVI) methods have been widely studied for CNT/C composite fabrications with diverse focuses and various modifications. Progress has been reported to optimize the performance of CNT/C composites from broad aspects, including matrix densification, CNT alignment, microstructure control, and interface engineering, etc. Recent approaches, such as using resistance heating for PIP or CVI, contribute to the development of CNT/C composites. To deliver a timely and up-to-date overview of CNT/C composites, we have reviewed the most recent trends in fabrication processes, summarized the mechanical reinforcement mechanism, and discussed the electrical and thermal properties, as well as relevant case studies for high-temperature applications. Conclusions and perspectives addressing future routes for performance optimization are also presented. Hence, this review serves as a rundown of recent advances in CNT/C composites and will be a valuable resource to aid future developments in this field.

Programmed Ultrafast Scan Welding of Cu Nanowire Networks with a Pulsed Ultraviolet Laser Beam for Transparent Conductive Electrodes and Flexible Circuits

Metal nanowires (NWs) have shown superior advances for the next-generation transparent conducting (TC) materials. Most concerns were focused on uniform conductive films; however, fabrication of a programmed circuit is still lacking. Here, we demonstrate a programmable ultrafast welding method by pulsed laser beam scanning under ambient conditions to achieve a Cu NW pattern-free TC circuit as well as various size films. High-aspect ratio Cu NWs (> 3000) are synthesized through an oleylamine-mediated solution system. Pulsed ultraviolet laser irradiation together with a programmed moving station is set up for the welding of Cu NW networks. Finite element simulations reveal that the transient heating by efficient absorption of UV light (∼ 250 nm) could remove the organic residues on the surface and realize local welding of interlaced NW junctions. With only 10 ms pulsed irradiation, high optoelectronic performance (33 ohm/sq. at 87% transmittance at 550 nm) and excellent stability of the Cu NW TC film have been achieved. The line-by-line and selected route scanning modes could rapidly make large area TC films and directly write flexible circuits. Moreover, completely transparent micron-size UV and blue LED chips are fabricated and successfully lit with bright emission. This method opens up a future way of circuit and device fabrication by direct one-step laser writing.

Flexible and Stretchable Temperature Sensors Fabricated Using Solution-Processable Conductive Polymer Composites

Accurate monitoring of physiological temperatures is important for the diagnosis and tracking of various medical conditions. This work presents the design, fabrication, and characterization of temperature sensors using conductive polymer composites (CPCs) patterned on both flexible and stretchable substrates through both drop coating and direct ink writing (DIW). These composites were formed using a high melting point biopolymer polyhydroxybutyrate (PHB) as the matrix and the graphenic nanomaterial reduced graphene oxide (rGO) as the nanofiller (from 3 to 12 wt%), resulting in a material that exhibits a temperature-dependent resistivity. At room temperature the composites exhibited electrical percolation behavior. Around the percolation threshold, both the carrier concentration and mobility were found to increase sharply. Sensors were fabricated by drop-coating PHB-rGO composites onto ink-jet printed silver electrodes. The temperature coefficient of resistance was determined to be 0.018 /°C for pressed rGO powders and 0.008 /°C for the 3 wt% samples (the highest responsivity of all composites). Composites were found to have good selectivity to temperature with respect to pressure and moisture. Thermal mapping was demonstrated using 6 × 7 arrays of sensing elements. Stretchable devices with a meandering pattern were fabricated using DIW, demonstrating the potential for these materials in healthcare monitoring devices.

Multi-Modal Contractive Forces of Wools as Actuator

Wool has a long history of use in textiles throughout human civilization. Many smart functions such as reversible shape changes to various stimuli have been demonstrated in the last few years. However, the force-related characteristics are still imperfectly recognized, although they are expected to be used as actuators due to their biological origins and broad applications. Herein, we investigated the feasibility of wools in performing actuating ability through its intrinsic structures and fabrication methods. The diverse modes of contractive forces were obtained in wool materials including platform-like, double-peak, and slope-like shapes, where a molecular model was also presented to trace the origins of stress evolution. After that, a polymeric blend was created to modify the wool materials and a dissimilar performance of stress production was achieved, a square stress mode with stable manner and maintenance, for broad applications in a more efficient way. It is believed that these actuating properties extracted from natural hairs have a large potential in current smart applications and lay down new inspiration in designing actuators.

Shampoo assisted aligning of carbon nanotubes toward strong, stiff and conductive fibers

High alignment and densification of carbon nanotubes (CNTs) are of key importance for strengthening CNT fibers, whereas direct stretching has a very limited effect when CNTs are highly entangled. We report that by lubricating CNT surfaces with viscous alcohols, the relative motion between CNTs improves because of the reduced sliding energy barrier; thus non-stretched regions are effectively eliminated. Owing to the very efficient optimization of the assembled structure, the stretched CNT fibers exhibited an average tensile strength of 2.33 GPa (1.82 N per tex) and modulus of 70.1 GPa (54.8 N per tex). Other fundamental properties, such as electrical and thermal conductivities, were also remarkably improved. Such a strategy can be readily used for manufacturing high-performance CNT assemblies and composites.

Perovskite-Carbon Nanotube Light-Emitting Fibers

Active fibers with electro-optic functionalities are promising building blocks for the emerging and rapidly growing field of fiber and textile electronics. Yet, there remains significant challenges that require improved understanding of the principles of active fiber assembly to enable the development of fiber-shaped devices characterized by having a small diameter, being lightweight, and having high mechanical strength. To this end, the current frameworks are insufficient, and new designs and fabrication approaches are essential to accommodate this unconventional form factor. Here, we present a first demonstration of a pathway that effectively integrates the foundational components meeting such requirements, with the use of a flexible and robust conductive core carbon nanotube fiber and an organic-inorganic emissive composite layer as the two critical elements. We introduce an active fiber design that can be realized through an all solution-processed approach. We have implemented this technique to demonstrate a three-layered light-emitting fiber with a coaxially coated design.

Single-Layer Pressure Textile Sensors with Woven Conductive Yarn Circuit

Today, e-textiles have become a fundamental trend in wearable devices. Fabric pressure sensors, as a part of e-textiles, have also received much interest from many researchers all over the world. However, most of the pressure sensors are made of electronic fibers and composed of many layers, including an intermediate layer for sensing the pressure. This paper proposes the model of a single layer pressure sensor with electrodes and conductive fibers intertwined. The plan dimensions of the fabricated sensors are 14 x 14 mm, and the thickness is 0.4 mm. The whole area of the sensor is the pressure-sensitive point. As expected, results demonstrate an electrical resistance change from 283 Ω at the unload pressure to 158 Ω at the load pressure. Besides, sensors have a fast response time (50 ms) and small hysteresis (5.5%). The hysteresis will increase according to the pressure and loading distance, but the change of sensor loading distance is very small. Moreover, the single-layer pressure sensors also show high durability under many working cycles (20,000 cycles) or washing times (50 times). The single-layer pressure sensor is very thin and more flexible than the multi-layer pressure sensor. The structure of this sensor is also expected to bring great benefits to wearable technology in the future.

High density mechanical energy storage with carbon nanothread bundle

The excellent mechanical properties of carbon nanofibers bring promise for energy-related applications. Through in silico studies and continuum elasticity theory, here we show that the ultra-thin carbon nanothreads-based bundles exhibit a high mechanical energy storage density. Specifically, the gravimetric energy density is found to decrease with the number of filaments, with torsion and tension as the two dominant contributors. Due to the coupled stresses, the nanothread bundle experiences fracture before reaching the elastic limit of any individual deformation mode. Our results show that nanothread bundles have similar mechanical energy storage capacity compared to (10,10) carbon nanotube bundles, but possess their own advantages. For instance, the structure of the nanothread allows us to realize the full mechanical energy storage potential of its bundle structure through pure tension, with a gravimetric energy density of up to 1.76 MJ kg⁻¹, which makes them appealing alternative building blocks for energy storage devices.

Carbon nanothreads are promising for applications in mechanical energy storage and energy harvesting. Here the authors use large-scale molecular dynamics simulations and continuum elasticity theory to explore mechanical energy storage in carbon nanothreads-based bundles.

Invisible Silver Nanomesh Skin Electrode via Mechanical Press Welding

Silver nanowire (AgNW) has been studied as an important material for next-generation wearable devices due to its high flexibility, high electrical conductivity and high optical transmittance. However, the inherently high surface roughness of AgNWs and low adhesion to the substrate still need to be resolved for various device applications. In this study, an embedded two-dimensional (2D) Ag nanomesh was fabricated by mechanical press welding of AgNW networks with a three-dimensional (3D) fabric shape into a nanomesh shape, and by embedding the Ag nanomesh in a flexible substrate. The effect of the embedded AgNWs on the physical and electrical properties of a flexible transparent electrode was investigated. By forming embedded nanomesh-type AgNWs from AgNW networks, improvements in physical and electrical properties, such as a 43% decrease in haziness, 63% decrease in sheet resistance, and 26% increase in flexibility, as well as improved adhesion to the substrate and low surface roughness, were observed.

Recent Progress of Fiber Shaped Lighting Devices for Smart Display Applications-A Fibertronic Perspective

Advances in material science and nanotechnology have fostered the miniaturization of devices. Over the past two decades, the form-factor of these devices has evolved from 3D rigid, volumetric devices through 2D film-based flexible electronics, finally to 1D fiber electronics (fibertronics). In this regard, fibertronic strategies toward wearable applications (e.g., electronic textiles (e-textiles)) have attracted considerable attention thanks to their capability to impart various functions into textiles with retaining textiles' intrinsic properties as well as imperceptible irritation by foreign matters. In recent years, extensive research has been carried out to develop various functional devices in the fiber form. Among various features, lighting and display features are the highly desirable functions in wearable electronics. This article discusses the recent progress of materials, architectural designs, and new fabrication technologies of fiber-shaped lighting devices and the current challenges corresponding to each device's operating mechanism. Moreover, opportunities and applications that the revolutionary convergence between the state-of-the-art fibertronic technology and age-long textile industry will bring in the future are also discussed.

Understanding the Mechanical and Conductive Properties of Carbon Nanotube Fibers for Smart Electronics

The development of fiber-based smart electronics has provoked increasing demand for high-performance and multifunctional fiber materials. Carbon nanotube (CNT) fibers, the 1D macroassembly of CNTs, have extensively been utilized to construct wearable electronics due to their unique integration of high porosity/surface area, desirable mechanical/physical properties, and extraordinary structural flexibility, as well as their novel corrosion/oxidation resistivity. To take full advantage of CNT fibers, it is essential to understand their mechanical and conductive properties. Herein, the recent progress regarding the intrinsic structure-property relationship of CNT fibers, as well as the strategies of enhancing their mechanical and conductive properties are briefly summarized, providing helpful guidance for scouting ideally structured CNT fibers for specific flexible electronic applications.

Recent Advances in Fiber Supercapacitors: Materials, Device Configurations, and Applications

Fiber supercapacitors (SCs), with their small size and weight, excellent flexibility and deformability, and high capacitance and power density, are recognized as one of the most robust power supplies available for wearable electronics. They can be woven into breathable textiles or integrated into different functional materials to fit curved surfaces for use in day-to-day life. A comprehensive review on recent important development and progress in fiber SCs is provided, with respect to the active electrode materials, device configurations, functions, integrations. Active electrode materials based on different electrochemical mechanisms and intended to improve performance including carbon-based materials, metal oxides, and hybrid composites, are first summarized. The three main types of fiber SCs, namely parallel, twist, and coaxial structures, are then discussed, followed by the exploration of some functions including stretchability and self-healing. Miniaturized integration of fiber SCs to obtain flexible energy fibers and integrated sensing systems is also discussed. Finally, a short conclusion is made, combining with comments on the current challenges and potential solutions in this field.

Recent Advances in 1D Stretchable Electrodes and Devices for Textile and Wearable Electronics: Materials, Fabrications, and Applications

Research on wearable electronic devices that can be directly integrated into daily textiles or clothes has been explosively grown holding great potential for various practical wearable applications. These wearable electronic devices strongly demand 1D electronic devices that are light-weight, weavable, highly flexible, stretchable, and adaptable to comport to frequent deformations during usage in daily life. To this end, the development of 1D electrodes with high stretchability and electrical performance is fundamentally essential. Herein, the recent process of 1D stretchable electrodes for wearable and textile electronics is described, focusing on representative conductive materials, fabrication techniques for 1D stretchable electrodes with high performance, and designs and applications of various 1D stretchable electronic devices. To conclude, discussions are presented regarding limitations and perspectives of current materials and devices in terms of performance and scientific understanding that should be considered for further advances.

Spun Carbon Nanotube Fibres and Films as an Alternative to Printed Electronic Components

Current studies of carbon nanotubes have enabled both new electronic applications and improvements to the performance of existing ones. Manufacturing of macroscopic electronic components with this material generally involves the use of printed electronic methods, which must use carbon nanotube (CNT) powders. However, in recent years, it has been shown that the use of ready-made self-standing macroscopic CNT assemblies could have considerable potential in the future development of electronic components. Two examples of these are spun carbon nanotube fibers and CNT films. The following paper considers whether these spun materials may replace printed electronic CNT elements in all applications. To enable the investigation of this question some practical experiments were undertaken. They included the formation of smart textile elements, flexible and transparent components, and structural electronic devices. By taking this approach it has been possible to show that CNT fibres and films are highly versatile materials that may improve the electrical and mechanical performance of many currently produced printed electronic elements. Additionally, the use of these spun materials may enable many new applications and functionalities particularly in the area of e-textiles. However, as with every new technology, it has its limitations, and these are also considered.

Synergetic Effect of Porous Elastomer and Percolation of Carbon Nanotube Filler toward High Performance Capacitive Pressure Sensors

Wearable pressure sensors have been attracting great attention for a variety of practical applications, including electronic skin, smart textiles, and healthcare devices. However, it is still challenging to realize wearable pressure sensors with sufficient sensitivity and low hysteresis under small mechanical stimuli. Herein, we introduce simple, cost-effective, and sensitive capacitive pressure sensor based on porous Ecoflex-multiwalled carbon nanotube composite (PEMC) structures, which leads to enhancing the sensitivity (6.42 and 1.72 kPa^-1 in a range of 0-2 and 2-10 kPa, respectively) due to a synergetic effect of the porous elastomer and percolation of carbon nanotube fillers. The PEMC structure shows excellent mechanical deformability and compliance for an effective integration with practical wearable devices. Also, the PEMC-based pressure sensor shows not only the long-term stability, low-hysteresis, and fast response under dynamic loading but also the high robustness against temperature and humidity changes. Finally, we demonstrate a prosthetic robot finger integrated with a PEMC-based pressure sensor and an actuator as well as a healthcare wristband capable of continuously monitoring blood pressure and heart rate.

Ultrastable and High-Performance Silk Energy Harvesting Textiles

Energy harvesting textiles (EHTs) have attracted much attention in wearable electronics and the internet-of-things for real-time mechanical energy harvesting associated with human activities. However, to satisfy practical application requirements, especially the demand for long-term use, it is challenging to construct an energy harvesting textile with elegant trade-off between mechanical and triboelectric performance. In this study, an energy harvesting textile was constructed using natural silk inspired hierarchical structural designs combined with rational material screening; this design strategy provides multiscale opportunities to optimize the mechanical and triboelectric performance of the final textile system. The resulting EHTs with traditional advantages of textiles showed good mechanical properties (tensile strength of 237 ± 13 MPa and toughness of 4.5 ± 0.4 MJ m-3 for single yarns), high power output (3.5 mW m-2), and excellent structural stability (99% conductivity maintained after 2.3 million multi-type cyclic deformations without severe change in appearance), exhibiting broad application prospects in integrated intelligent clothing, energy harvesting, and human-interactive interfaces.

Scalable Fabrication of Highly Flexible Porous Polymer-Based Capacitive Humidity Sensor Using Convergence Fiber Drawing

In this study, we fabricated a highly flexible fiber-based capacitive humidity sensor using a scalable convergence fiber drawing approach. The sensor’s sensing layer is made of porous polyetherimide (PEI) with its porosity produced in situ during fiber drawing, whereas its electrodes are made of copper wires. The porosity induces capillary condensation starting at a low relative humidity (RH) level (here, 70%), resulting in a significant increase in the response of the sensor at RH levels ranging from 70% to 80%. The proposed humidity sensor shows a good sensitivity of 0.39 pF/% RH in the range of 70%–80% RH, a maximum hysteresis of 9.08% RH at 70% RH, a small temperature dependence, and a good stability over a 48 h period. This work demonstrates the first fiber-based humidity sensor fabricated using convergence fiber drawing.

Vertically aligned laser sliced MWCNTs

Applications of multi-walled carbon nanotubes (MWCNTs) benefit from the availability of specific lengths of the material while keeping the outer walls pristine, for example, for applications requiring vertically aligned tubes. To this end, a simple and effective continuous flow 'top down' process to control the length of sliced MWCNTs has been developed using a vortex fluidic device (VFD) coupled with a 1064 nm pulse laser, with the process in the absence of chemicals and any auxiliary substances. Three different length distributions of the sliced MWCNTs, centered at 75 ± 2.1 nm, 300 ± 1.8 nm and 550 ± 1.4 nm, have been generated with the length depending on the VFD operating parameters and laser energy, with the processing resulting in a decrease in side wall defects of the material. We also show the ability to vertically self assemble short MWCNTs on a silicon substrate with control of the surface density coverage using a simple dipping and rinsing method.

Building Better Batteries in the Solid State: A Review

Most of the current commercialized lithium batteries employ liquid electrolytes, despite their vulnerability to battery fire hazards, because they avoid the formation of dendrites on the anode side, which is commonly encountered in solid-state batteries. In a review two years ago, we focused on the challenges and issues facing lithium metal for solid-state rechargeable batteries, pointed to the progress made in addressing this drawback, and concluded that a situation could be envisioned where solid-state batteries would again win over liquid batteries for different applications in the near future. However, an additional drawback of solid-state batteries is the lower ionic conductivity of the electrolyte. Therefore, extensive research efforts have been invested in the last few years to overcome this problem, the reward of which has been significant progress. It is the purpose of this review to report these recent works and the state of the art on solid electrolytes. In addition to solid electrolytes stricto sensu, there are other electrolytes that are mainly solids, but with some added liquid. In some cases, the amount of liquid added is only on the microliter scale; the addition of liquid is aimed at only improving the contact between a solid-state electrolyte and an electrode, for instance. In some other cases, the amount of liquid is larger, as in the case of gel polymers. It is also an acceptable solution if the amount of liquid is small enough to maintain the safety of the cell; such cases are also considered in this review. Different chemistries are examined, including not only Li-air, Li-O2, and Li-S, but also sodium-ion batteries, which are also subject to intensive research. The challenges toward commercialization are also considered.

Multifunctional Wet-Spun Filaments through Robust Nanocellulose Networks Wrapping to Single-Walled Carbon Nanotubes

Cellulose nanofibrils (CNFs) and single-walled carbon nanotubes (SWNTs) hold potential for fabricating multifunctional composites with remarkable performance. However, it is technically tough to fabricate materials by CNFs and SWNTs with their intact properties, mainly because of the weakly synergistic interaction. Hence, constructing sturdy interfaces and sequential connectivity not only can enhance mechanical strength but also are capable of improving the electrical conductivity. In that way, we report CNF/SWNT filaments composed of axially oriented building blocks with robust CNF networks wrapping to SWNTs. The composite filaments obtained through the combination of three-mill-roll and wet-spinning strategy display high strength up to ∼472.17 MPa and a strain of ∼11.77%, exceeding most results of CNF/SWNT composites investigated in the previous literature. Meanwhile, the filaments possess an electrical conductivity of ∼86.43 S/cm, which is also positively dependent on temperature changes. The multifunctional filaments are further manufactured as a strain sensor to measure mass variation and survey muscular movements, leading to becoming optimistic incentives in the fields of portable gauge measuring and wearable bioelectronic therapeutics.

Microfluidics-Based Biomaterials and Biodevices

The rapid development of microfluidics technology has promoted new innovations in materials science, particularly by interacting with biological systems, based on precise manipulation of fluids and cells within microscale confinements. This article reviews the latest advances in microfluidics-based biomaterials and biodevices, highlighting some burgeoning areas such as functional biomaterials, cell manipulations, and flexible biodevices. These areas are interconnected not only in their basic principles, in that they all employ microfluidics to control the makeup and morphology of materials, but also unify at the ultimate goals in human healthcare. The challenges and future development trends in biological application are also presented.

One-Step Synthesis of Monodispersed Mesoporous Carbon Nanospheres for High-Performance Flexible Quasi-Solid-State Micro-Supercapacitors

Cost-effective synthesis of carbon nanospheres with a desirable mesoporous network for diversified energy storage applications remains a challenge. Herein, a direct templating strategy is developed to fabricate monodispersed N-doped mesoporous carbon nanospheres (NMCSs) with an average particle size of 100 nm, a pore diameter of 4 nm, and a specific area of 1093 m² g^-1 . Hexadecyl trimethyl ammonium bromide and tetraethyl orthosilicate not only play key roles in the evolution of mesopores but also guide the assembly of phenolic resins to generate carbon nanospheres. Benefiting from the high surface area and optimum mesopore structure, NMCSs deliver a large specific capacitance up to 433 F g^-1 in 1 m H₂ SO₄ . The NMCS electrodes-based symmetric sandwich supercapacitor has an output voltage of 1.4 V in polyvinyl alcohol/H₂ SO₄ gel electrolyte and delivers an energy density of 10.9 Wh kg^-1 at a power density of 14014.5 W kg^-1 . Notably, NMCSs can be directly applied through the mask-assisted casting technique by a doctor blade to fabricate micro-supercapacitors. The micro-supercapacitors exhibit excellent mechanical flexibility, long-term stability, and reliable power output.

Microfluidic spinning of editable polychromatic fibers

Chromatic fibers that change color in response to external stimuli are expected to be widely used in various applications such as anti-counterfeiting, military camouflage, and wearable displays. Advanced chromatic fibers with polychromatic and editable color properties behavior are strongly desired for practical applications but have not yet been realized using existing spinning technologies. Here, we present the low-cost, continuous microfluidic spinning of editable polychromatic polylactide (PLA) fibers. The structure and performance of the polychromatic PLA fibers were precisely controlled by adjusting the parameters used in microfluidic spinning. The structure of the as-spun products evolved through three different stages based on the editable encapsulation of functional materials into the PLA matrix. Fibers with versatile performance were achieved. A beaded polychromatic PLA fiber showed the possibility to delivery coded information through its editable chromatic behavior. A core-shell polychromatic PLA fiber showed good mechanical properties and knittability, which make it promising to fabricate smart color-changing textiles.

Carbon nanotube fibers with enhanced longitudinal carrier mobility for high-performance all-carbon thermoelectric generators

With the increase in practical interest in flexible thermoelectric (TE) generators, the demand for high-performance alternatives to brittle TE materials is growing. Herein, we have demonstrated wet-spun CNT fibers with high TE performance by systematically controlling the longitudinal carrier mobility without a significant change in the carrier concentration. The carrier mobility optimized by CNT alignment increases the electrical conductivity without decreasing the thermopower, thus improving the power factor. On further adjusting the charge carriers via mild annealing, the CNT fibers exhibit a high power factor of 432 μW m-1 K-2. Based on the excellent TE performance and shape advantages for modular design of the CNT fiber, the all-carbon based flexible TE generator without an additional metal electrode has been fabricated. The flexible TE generator based on 40 pairs of p- and n-type CNT fibers shows the maximum power density of 15.4 and 259 μW g-1 at temperature differences (ΔT) of 5 and 20 K, respectively, currently one of the highest values reported for TE generators based on flexible materials. The strategy proposed here can improve the performance of flexible TE fibers by optimizing the carrier mobility without a change in the carrier concentration, and shows great potential for flexible TE generators.

EXPERIMENTAL INVESTIGATION OF THE MECHANISMS AND PERFORMANCE OF ACTIVE AUXETIC AND SHEARING TEXTILES

Anisotropic textiles are commonly used in wearable applications to achieve varied bi-axial stress-strain behavior around the body. Auxetic textiles, specifically those that exhibit a negative Poisson's ratio (v), likewise exhibit intriguing behavior such as volume increase in response to impact or variable air permeability. Active textiles are traditional textile structures that integrate smart materials, such as shape memory alloys, shape memory polymers, or carbon nanotubes, to enable spatial actuation behavior, such as contraction for on-body compression or corrugation for haptic feedback. This research is a first experimental investigation into active auxetic and shearing textile structures. These textile structures leverage the bending- and torsional-deformations of the fibers/filaments within traditional textile structures as well as the shape memory effect of shape memory alloys to achieve novel, spatial performance. Five textile structures were fabricated from shape memory alloy wire deformed into needle lace and weft knit textile structures. All active structures exhibited anisotropic behavior and four of the five structures exhibited auxetic behavior upon free recovery, contracting in both x- and y-axes upon actuation (v = -0.3 to -1.5). One structure exhibited novel shearing behavior, with a mean free angle recovery of 7°. Temperature-controlled biaxial tensile testing was conducted to experimentally investigate actuation behavior and anisotropy of the designed structures. The presented design and performance of these active auxetic, anisotropic, and shearing textiles inspire new capabilities for applications, such as smart wearables, soft robotics, reconfigurable aerospace structures, and medical devices.

Multicolored Photonic Crystal Carbon Fiber Yarns and Fabrics with Mechanical Robustness for Thermal Management

Multicolored photonic crystal carbon fiber (CF) yarns and fabrics with mechanical robustness in a full spectrum are reported. By facilely controlling the thickness of the periodic layer, a series of photonic CF yarns and fabrics with vivid structural colors ranging from purple, green, yellow, orange, to red are obtained. Interestingly, the prepared multicolored CF yarns show anisotropic optical reflection properties because of their unique axisymmetric geometry, while the plain-woven fabrics exhibit vivid colors even under ambient scattering light. Most importantly, they can withstand cyclical mechanical rubbing, laundering, and accelerated light aging, indicating great potential for practical uses. Finally, considering such impressive characteristics as well as reflection in the visible and near-infrared regions, the above photonic crystal microstructure is further used as a new material for the application of outdoor reflective cooling of the textile surface, demonstrating a superior temperature reduction up to ∼12 °C with respect to the control sample.

2D Crystal-Based Fibers: Status and Challenges

2D crystals are emerging new materials in multidisciplinary fields including condensed state physics, electronics, energy, environmental engineering, and biomedicine. To employ 2D crystals for practical applications, these nanoscale crystals need to be processed into macroscale materials, such as suspensions, fibers, films, and 3D macrostructures. Among these macromaterials, fibers are flexible, knittable, and easy to use, which can fully reflect the advantages of the structure and properties of 2D crystals. Therefore, the fabrication and application of 2D crystal-based fibers is of great importance for expanding the impact of 2D crystals. In this Review, 2D crystals that are successfully prepared are overviewed based on their composition of elements. Subsequently, methods for preparing 2D crystals, 2D crystals dispersions, and 2D crystal-based fibers are systematically introduced. Then, the applications of 2D crystal-based fibers, such as flexible electronic devices, high-efficiency catalysis, and adsorption, are also discussed. Finally, the status-of-quo, perspectives, and future challenges of 2D crystal-based fibers are summarized. This Review provides directions and guidelines for developing new 2D crystal-based fibers and exploring their potentials in the fields of smart wearable devices.

Carbon nanotube synthesis and spinning as macroscopic fibers assisted by the ceramic reactor tube

Macroscopic fibers of carbon nanotubes (CNT) have emerged as an ideal architecture to exploit the exceptional properties of CNT building blocks in applications ranging from energy storage to reinforcement in structural composites. Controlled synthesis and scalability are amongst the most pressing challenges to further materialize the potential of CNT fibers. This work shows that under floating catalyst chemical vapor conditions in the direct spinning method, used both in research and industry, the ceramic reactor tube plays an unsuspected active role in CNT growth, leading for example to doubling of reaction yield when mullite (Al_4+2xSi_2−2xO_10−x(x ≈ 0:4)) is used instead of alumina (Al₂O₃), but without affecting CNT morphology in terms of number of layers, purity or degree of graphitization. This behaviour is confirmed for different carbon sources and when growing either predominantly single-walled or multi-walled CNTs by adjusting promotor concentration. Analysis of large Si-based impurities occasionally found in CNT fiber fabric samples, attributed to reactor tube fragments that end up trapped in the porous fibers, indicate that the role of the reactor tube is in catalyzing the thermal decomposition of hydrocarbons, which subsequently react with floating Fe catalyst nanoparticles and produce extrusion of the CNTs and formation of an aerogel. Reactor gas analysis confirms that extensive thermal decomposition of the carbon source occurs in the absence of Fe catalyst particles, and that the concentration of different carbon species (e.g. carbon dioxide and ethylene) is sensitive to the reactor tube type. These finding open new avenues for controlled synthesis of CNT fibers by decoupling precursor decomposition from CNT extrusion at the catalyst particle.

Biocompatible Carbon Nanotube-Based Hybrid Microfiber for Implantable Electrochemical Actuator and Flexible Electronic Applications

Biocompatible, electrically conductive microfibers with superior mechanical properties have received a great attention due to their potential applications in various biomedical applications such as implantable medical devices, biosensors, artificial muscles, and microactuators. Here, we developed an electrically conductive and mechanically stable carbon nanotube-based microactuator with a low degradability that makes it usable for an implantable device in the body or biological environments. The microfiber was composed of hyaluronic acid (HA) hydrogel and single-wall carbon nanotubes (SWCNTs) (HA/SWCNT). HA hydrogel acts as biosurfactant and ion-conducting binder to improve the dispersion of SWCNTs resulting in enhanced electrical and mechanical properties of the hybrid microfiber. In addition, HA was crosslinked to prevent the leaking of the nanotubes from the composite. Crosslinking of HA hydrogel significantly enhances Young's modulus, the failure strain, the toughness, the stability of the electrical conductivity, and the resistance to biodegradation and creep of hybrid microfibers. The obtained crosslinked HA/SWCNT hybrid microfibers show an excellent capacitance and actuation behavior under mechanical loading with a low potential of ±1 V in a biological environment. Furthermore, the HA/SWCNT microfibers exhibit an excellent in vitro viability. Finally, the biocompatibility is shown through the resolution of an early inflammatory response in less than 3 weeks after the implantation of the microfibers in the subcutaneous tissue of mice.

Flexible and High-Voltage Coaxial-Fiber Aqueous Rechargeable Zinc-Ion Battery

Extensive efforts have been devoted to construct a fiber-shaped energy-storage device to fulfill the increasing demand for power consumption of textile-based wearable electronics. Despite the myriad of available material selections and device architectures, it is still fundamentally challenging to develop eco-friendly fiber-shaped aqueous rechargeable batteries (FARBs) on a single-fiber architecture with high energy density and long-term stability. Here, we demonstrate flexible and high-voltage coaxial-fiber aqueous rechargeable zinc-ion batteries (CARZIBs). By utilizing a novel spherical zinc hexacyanoferrate with prominent electrochemical performance as cathode material, the assembled CARZIB offers a large capacity of 100.2 mAh cm^-3 and a high energy density of 195.39 mWh cm^-3, outperforming the state-of-the-art FARBs. Moreover, the resulting CARZIB delivers outstanding flexibility with the capacity retention of 93.2% after bending 3000 times. Last, high operating voltage and output current are achieved by the serial and parallel connection of CARZIBs woven into the flexible textile to power high-energy-consuming devices. Thus, this work provides proof-of-concept design for next-generation wearable energy-storage devices.

Flexible Organic Thermoelectric Materials and Devices for Wearable Green Energy Harvesting

In the past few decades, organic thermoelectric materials/devices, which can exhibit remarkable potential in green energy conversion, have drawn great attention and interest due to their easy processing, light weight, intrinsically low thermal conductivity, and mechanical flexibility. Compared to traditional batteries, thermoelectric materials have high prospects as alternative power generators for harvesting green energy. Although crystalline inorganic semiconductors have dominated the fields of thermoelectric materials up to now, their practical applications are limited by their intrinsic fragility and high toxicity. The integration of organic polymers with inorganic nanoparticles has been widely employed to tailor the thermoelectric performance of polymers, which not only can combine the advantages of both components but also display interesting transport phenomena between organic polymers and inorganic nanoparticles. In this review, parameters affecting the thermoelectric properties of materials were briefly introduced. Some recently developed n-type and p-type thermoelectric films and related devices were illustrated along with their thermoelectric performance, methods of preparation, and future applications. This review will help beginners to quickly understand and master basic knowledge of thermoelectric materials, thus inspiring them to design and develop more efficient thermoelectric devices.

Wet-Spinning Assembly of Continuous, Highly Stable Hyaluronic/Multiwalled Carbon Nanotube Hybrid Microfibers

Effective multiwalled carbon nanotube (MWCNT) fiber manufacturing methods have received a substantial amount of attention due to the low cost and excellent properties of MWCNTs. Here, we fabricated hybrid microfibers composed of hyaluronic acid (HA) and multiwalled carbon nanotubes (MWCNTs) by a wet-spinning method. HA acts as a biosurfactant and an ionic crosslinker, which improves the dispersion of MWCNTs and helps MWCNT to assemble into microfibers. The effects of HA concentration, dispersion time, injection speed, and MWCNT concentration on the formation, mechanical behavior, and conductivity of the HA/MWCNT hybrid microfibers were comprehensively investigated through SEM, UV-Vis spectroscopy, tensile testing, and conductivity testing. The obtained HA/MWCNT hybrid microfibers presented excellent tensile properties in regard to Young’s modulus (9.04 ± 1.13 GPa) and tensile strength (130.25 ± 10.78 MPa), and excellent flexibility and stability due to the superior mechanical and electrical properties of MWCNTs. This work presents an effective and easy-to-handle preparation method for high-performance MWCNT hybrid microfibers assembly, and the obtained HA/MWCNT hybrid microfibers have promising applications in the fields of energy storage, sensors, micro devices, intelligent materials, and high-performance fiber-reinforced composites.

Artificial muscle with reversible and controllable deformation based on stiffness-variable carbon nanotube spring-like nanocomposite yarn

Carbon nanotube yarn actuators are in great demand for flexible devices or intelligent applications. Artificial muscles based on carbon nanotube yarn have achieved great progress over past decades. However, uncontrollable, small deformations and relatively slow deformation recovery are still great challenges for carbon nanotube yarn artificial muscles. Here we propose an artificial muscle based on a stiffness-variable carbon nanotube spring-like nanocomposite yarn. This nanocomposite yarn can be fabricated as artificial muscles by directly inflating epoxy resin on spring-like carbon nanotube yarn, and it shows a rapid response, and reversible and controllable deformation. The driving mechanism of the nanocomposite yarn artificial muscle is based on the change in the resin modulus controlled by Joule heat. This novel nanocomposite yarn artificial muscle can work at low voltages (≤0.8 V), and the whole reversible driving process is completed within 5 seconds (the deformation recovery process is about 2 seconds). The strain of the nanocomposite yarn artificial muscle is controlled by applied voltages, and the maximum strain can reach more than 12%. The novel nanocomposite yarn artificial muscle can produce output forces more than 20 times higher than human skeletal muscle. This CNT nanocomposite yarn artificial muscle with a spiral structure shows potential applications for actuators, sensors and micro robots.

Flexible Sensors—From Materials to Applications

Flexible sensors have the potential to be seamlessly applied to soft and irregularly shaped surfaces such as the human skin or textile fabrics. This benefits conformability dependant applications including smart tattoos, artificial skins and soft robotics. Consequently, materials and structures for innovative flexible sensors, as well as their integration into systems, continue to be in the spotlight of research. This review outlines the current state of flexible sensor technologies and the impact of material developments on this field. Special attention is given to strain, temperature, chemical, light and electropotential sensors, as well as their respective applications.

Mechanical properties of carbon nanotube fibers at extreme temperatures

Carbon nanotube (CNT) fibers are strong, flexible, and multifunctional, which makes them promising candidates for use at extreme temperatures. However, the current reported mechanical properties of CNT fibers were commonly obtained at room temperature. Here, we report the measurement of the mechanical properties of CNT fibers at temperatures ranging from -196 °C to 2400 °C. Compared with the room temperature strength and modulus, CNT fibers tested at 1000 °C and 2400 °C retained 82% and 54% of the strength, and 71% and 50% of the modulus, respectively, while 68% and 220% increases in the strength and modulus, respectively, were observed for CNT fibers tested at -196 °C. We attributed the decay in the mechanical properties at high temperatures to the weakening of individual nanotubes and intertube interactions, and the strength enhancement at low temperature to the increased activation energy to break the nanotubes. The present study provides the fundamental mechanical properties of CNT fibers at extreme temperatures, which could facilitate the applications of CNT fibers in aeronautics and astronautics where extreme temperature conditions commonly exist.

Stretchable and Highly Sensitive Braided Composite Yarn@Polydopamine@Polypyrrole for Wearable Applications

Flexible wearable devices for various applications have attracted research attention in recent years. To date, it is still a big challenge to fabricate strain sensors with a large workable strain range while maintaining their high sensitivity. Herein, we report the fabrication of highly sensitive wearable strain sensors from braided composite yarns (BYs) by in situ polymerization of polypyrrole (PPy) on the surface of yarns after polydopamine templating (BYs-PDA). The electromechanical performance and strain sensing properties of the fabricated braided composite yarn@polydopamine@polypyrrole (BYs-PDA-PPy) were investigated. Because of the unique braided structure of BYs, the BYs-PDA-PPy strain sensors exhibit fascinating performance, including a large workable strain range (up to 105% strain), high sensitivity (gauge factor of 51.2 in strain of 0%-40% and of 27.6 in strain of 40%-105%), long-term stability and great electrical heating performance. Furthermore, the BYs-PDA-PPy sensors can be used in real-time monitoring subtle and large human motions. The BYs-PDA-PPy strain sensors can also be woven into fabrics for large area electric heating. These results demonstrate the potential of BYs-PDA-PPy in wearable electronics.

Highly Conductive Ti3 C2 Tx MXene Hybrid Fibers for Flexible and Elastic Fiber-Shaped Supercapacitors

Fiber-shaped supercapacitors (FSCs) are promising energy storage solutions for powering miniaturized or wearable electronics. However, the scalable fabrication of fiber electrodes with high electrical conductivity and excellent energy storage performance for use in FSCs remains a challenge. Here, an easily scalable one-step wet-spinning approach is reported to fabricate highly conductive fibers using hybrid formulations of Ti₃ C₂ T_x MXene nanosheets and poly(3,4-ethylenedioxythiophene):polystyrene sulfonate. This approach produces fibers with a record conductivity of ≈1489 S cm^-1 , which is about five times higher than other reported Ti₃ C₂ T_x MXene-based fibers (up to ≈290 S cm^-1 ). The hybrid fiber at ≈70 wt% MXene shows a high volumetric capacitance (≈614.5 F cm^-3 at 5 mV s^-1 ) and an excellent rate performance (≈375.2 F cm^-3 at 1000 mV s^-1 ). When assembled into a free-standing FSC, the energy and power densities of the device reach ≈7.13 Wh cm^-3 and ≈8249 mW cm^-3 , respectively. The excellent strength and flexibility of the hybrid fibers allow them to be wrapped on a silicone elastomer fiber to achieve an elastic FSC with 96% capacitance retention when cyclically stretched to 100% strain. This work demonstrates the potential of MXene-based fiber electrodes and their scalable production for fiber-based energy storage applications.

3D-Printed Graphene/Polydimethylsiloxane Composites for Stretchable and Strain-Insensitive Temperature Sensors

Materials possessing exceptional temperature sensitivity and high stretchability are of importance for real-time temperature monitoring on three-dimensional components with complex geometries, when operating under various external deformation modes. Herein, we develop a stretchable temperature sensor consisting of cellular graphene/polydimethylsiloxane composite. The first of its kind, graphene-based polymer composites with desired microstructures are produced through a direct 3D ink-writing technique. The resultant composites possess long-range-ordered and precisely controlled cellular structure. Temperature-sensing properties of three cellular structures, including grid, triangular, and hexagonal porous structures are studied. It is found that all three cellular composites present more stable sensitivities than solid composites under external strains because of the fine porous structure that can effectively share the external strain, and the composites with a grid structure delivered particularly a stable sensing performance, showing only ∼15% sensitivity decrease at a large tensile strain of 20%. Taking full advantage of the composites with a grid structure in terms of sensitivity, durability, and stability, practical applications of the composite are demonstrated to monitor the cooling process of a heated tube and measure skin temperature accompanying an arbitrary wristwork.