Polymer removal and dispersion exchange of (10,5) chiral carbon nanotubes with enhanced 1.5 µm photoluminescence.

Singe-chirality single-walled carbon nanotubes (SWCNTs) produced by selective polymer extraction have been actively investigated for their semiconductor applications. However, to fulfil the needs of biocompatible applications, the organic solvents in polymer-sorted SWCNTs impose a limitation. In this study, we developed a novel strategy for organic-to-aqueous phase exchange, which involves thoroughly removing polymers from the sorted SWCNTs, followed by surfactant covering and redispersing of the cleaned SWCNTs in water. Importantly, the obtained aqueous system allows us to perform sp3 functionalization of the SWCNTs, leading to a strong photoluminescence emission at 1550 nm from the defect sites of (10,5) SWCNTs. These functionalized SWCNTs as infrared light emitters show considerable potential for bioimaging applications. This exchange-and-functionalization strategy opens the door for future biocompatible applications of polymer-sorted SWCNTs.

Iterative Strategy for Sorting Single-Chirality Single-Walled Carbon Nanotubes from Aqueous to Organic Systems.

Significant advancements in electronic devices and integrated circuits have been facilitated by semiconducting single-walled carbon nanotubes (SWCNTs) sorted by conjugated polymers (CPs). However, the variety of CPs with single-chirality selectivity is limited, and the sorting results are strongly dependent on the chiral distribution of the starting materials. To address this, we develop an iterative strategy to achieve single-chirality SWCNT separation from aqueous to organic systems, based on a multistep tandem extraction technique that allows a gentle and nondestructive separation of surfactants from SWCNTs, ensuring an efficient system transfer. In parallel, we refined the iterative sorting process between CPs. Employing two starting materials with narrow diameter distributions, using three CPs, we successfully sorted out five single-chirality SWCNTs of the (9,5), (8,6), (10,5), (8,7), and (11,3) species in organic systems. This strategy bridges the gap between aqueous and organic separation systems, achieving efficient complementarity between them.

Binary solvent-exchange-induced self-assembly of silk fibroin birefringent fibers for optical applications.

To generate birefringence in artificial materials has attracted increasing attention in terms of their potential for applications in sensor, tissue engineering and optical devices. Silk materials with patterned structures presented unique optical features, however, effectively fabricating of structural anisotropy in silk materials to directly tailor their birefringence is still challenging. Silk fibroin birefringent fibers (SBFs) with tunable birefringence were obtained in this study via a strategy that combined injection technique and binary solvent-exchange-induced self-assembly (BSEISA). The structural deformation of these SBFs that introduced by external stimulus such as tensile and solvent swelling was critical to their birefringence. As a result, pink, yellow, green, cyan, and purple were successfully achieved in the interference color of the SBFs with an exchanging solvent of 25, 55, 75, 90 wt% ethanol aqueous solution, and methanol respectively. Moreover, we respectively exchanged these SBFs against with Congo red (SBF-CR), methyl orange (SBF-MO), methylene blue (SBF-MB) and rhodamine B (SBF-RhB) solutions to produce fibers with diversity in their birefringent performance. Two types of patterns were designed and thereafter constructed by (1) SBF\SBF-CR\SBF-RhB, and (2) SBF\SBF-MB\SBF-CR. Interestingly, the patterns both displayed a letter of "A" in natural light, while displayed different letters of (1) "H" and (2) "U" in polarized light. This study demonstrated that these SBFs with unique optical and birefringent performances are anticipated to act as sensors and code labels for optical applications.

Biomimetic Spun Silk Ionotronic Fibers for Intelligent Discrimination of Motions and Tactile Stimuli.

Innovation in the ionotronics field has significantly accelerated the development of ultraflexible devices and machines. However, it is still challenging to develop efficient ionotronic-based fibers with necessary stretchability, resilience, and conductivity due to inherent conflict in producing spinning dopes with both high polymer and ion concentrations and low viscosities. Inspired by the liquid crystalline spinning of animal silk, this study circumvents the inherent tradeoff in other spinning methods by dry spinning a nematic silk microfibril dope solution. The liquid crystalline texture allows the spinning dope to flow through the spinneret and form free-standing fibers under minimal external forces. The resultant silk-sourced ionotronic fibers (SSIFs) are highly stretchable, tough, resilient, and fatigue-resistant. These mechanical advantages ensure a rapid and recoverable electromechanical response of SSIFs to kinematic deformations. Further, the incorporation of SSIFs into core-shell triboelectric nanogenerator fibers provides outstanding stable and sensitive triboelectric response to precisely and sensitively perceive small pressures. Moreover, by implementing a combination of machine learning and Internet of Things techniques, the SSIFs can sort objects made of different materials. With these structural, processing, performance, and functional merits, the SSIFs prepared herein are expected to be applied in human-machine interfaces.

Cytoskeleton-inspired hydrogel ionotronics for tactile perception and electroluminescent display in complex mechanical environments.

The emerging applications of hydrogel ionotronics (HIs) in devices and machines require them to maintain their robustness under complex mechanical environments. Nevertheless, existing HIs still suffer from various mechanical limitations, such as the lack of balance between softness, strength, toughness, and fatigue fracture under cyclic loads. Inspired by the structure of the cytoskeleton, this study develops a sustainable HI supported by a double filamentous network. This cytoskeleton-like structure can enhance the strength of the HI by 26 times and its toughness by 3 times. It also enables HI to tolerate extreme mechanical stimuli, such as severe deformation, long-term cyclic loading, and high-frequency shearing and shocking. The advantages of these structurally- and mechanically-optimized HI devices in tactile perception and electroluminescent display, i.e., two practical applications where complex mechanical stimuli need to be sustained, are demonstrated. The findings reported in this study can inspire the design of human skin-like robust and anti-fatigue-fracture HI devices for long-term stable use.

Mechanically and electrically biocompatible hydrogel ionotronic fibers for fabricating structurally stable implants and enabling noncontact physioelectrical modulation.

Narrowing the mechanical and electrical mismatch between tissue and implantable microelectronics is essential for reducing immune responses and modulating physioelectrical signals. Nevertheless, the design of such implantable microelectronics remains a challenge due to the limited availability of suitable materials. Here, the fabrication of an electrically and mechanically biocompatible alginate hydrogel ionotronic fiber (AHIF) is reported, which is constructed by combing ionic chelation-assisted wet-spinning and mechanical training. The synergistic effects of these two processes allow the alginate to form a highly-oriented nanofibril and molecular network, with a hierarchical structure highly similar to that of natural fibers. These favourable structural features endow AHIF with tissue-mimicking mechanical characteristics, such as self-stiffening and soft tissue-like mechanical properties. In addition, tissue-like chemical components, i.e., biomacromolecules, Ca2+ ions, and water, endow AHIF with properties including biocompatibility and tissue-matching conductivity. These advantages bring light to the application of AHIFs in electrically-conductive implantable devices. As a prototype, an AHIF is designed to perform physioelectrical modulation through noncontact electromagnetic induction. Through experimental and machine learning optimizations, physioelectrical-like signals generated by the AHIF are used to identify the geometry and tension state of the implanted device in the body. Such an intelligent AHIF system has promising application prospects in bioelectronics, IntelliSense, and human-machine interactions.

Electroassisted Core-Spun Triboelectric Nanogenerator Fabrics for IntelliSense and Artificial Intelligence Perception.

IntelliSense fabrics that can sense transient mechanical stimuli are widely anticipated in flexible and wearable electronics. However, most IntelliSense fabrics developed so far are only sensitive to quasi-static forces, such as stretching, bending, or twisting. In this work, a sheath-core triboelectric nanogenerator (SC-TENG) yarn was developed via a rational design, electroassisted core spinning technique, that consisted of a rough nanoscale dielectric surface and mechanically strong and electrically conductive core yarns. The resulting system was used to sense and distinguish the instantaneous mechanical stimuli generated by different materials. To further improve the sensing accuracy, a machine learning model, based on a classification coding and recurrent neural network, was built to predict the type of contact materials from the peak profiles of output voltages. With these experimental and algorithmic optimizations, we finally used SC-TENG yarn to identify the type of materials in real-time. Moreover, by applying Internet of Things techniques, we investigated that SC-TENG yarn could be integrated into an IntelliSense system to recognize and control various electronic and electrical systems, demonstrating promising applications in wearable energy supply, IntelliSense fabrics, and human-machine interactions.

Fire-Resistant and Hierarchically Structured Elastic Ceramic Nanofibrous Aerogels for Efficient Low-Frequency Noise Reduction.

Traffic noise has been regarded as one of the most annoying pollutions that induce severe hazards to human health, both physiological and psychological. The commonly used fibrous noise absorption materials are limited by their large density, poor sound absorption ability at low frequencies, and unsatisfactory fire-resistant ability. Here, we develop hierarchically structured elastic ceramic electrospun nanofibrous aerogels, which possess lightweight properties (density of 13.29 mg cm-3) and superior low-frequency sound absorption ability (NRC value of 0.59). Specifically, the obtained ceramic electrospun nanofibrous aerogel is nonflammable on exposure to fire and can be compressed and quickly recover to its original height without any visible damage. Moreover, the resultant aerogels could be facilely and efficiently manufactured into designed shapes on a large scale, demonstrating their potential for industrialization. The successful design of such ceramic-based bulk materials may provide new insights for the further development of the next-generation high-efficiency sound-absorbing products.

Flexible ceramic nanofibrous sponges with hierarchically entangled graphene networks enable noise absorption.

Traffic noise pollution has posed a huge burden to the global economy, ecological environment and human health. However, most present traffic noise reduction materials suffer from a narrow absorbing band, large weight and poor temperature resistance. Here, we demonstrate a facile strategy to create flexible ceramic nanofibrous sponges (FCNSs) with hierarchically entangled graphene networks, which integrate unique hierarchical structures of opened cells, closed-cell walls and entangled networks. Under the precondition of independent of chemical crosslinking, high enhancement in buckling and compression performances of FCNSs is achieved by forming hierarchically entangled structures in all three-dimensional space. Moreover, the FCNSs show enhanced broadband noise absorption performance (noise reduction coefficient of 0.56 in 63-6300 Hz) and lightweight feature (9.3 mg cm-3), together with robust temperature-invariant stability from -100 to 500 °C. This strategy paves the way for the design of advanced fibrous materials for highly efficient noise absorption.

Moderate conformational transition promotes the formation of a self-reinforced highly oriented silk fibroin network structure.

A highly oriented molecular network structure (HOMNS) is a common and favorable design in natural and regenerated silks to achieve self-reinforcement of the material. However, the fundamental issues related to the formation of the HOMNS in silk fibroin materials and its influence on mechanical performance have not yet been addressed. By combining experimental characterization and molecular dynamics simulation, this work revealed that moderate conformational transition of silk fibroin promoted the formation of a low-density crosslinking molecular network among proteins. Such a molecular network is beneficial to further rearrangement of amorphous proteins in subsequent processing to form HOMNS. Here, a structure was confirmed that can strengthen the materials several times compared with the same material without HOMNS. These investigations improved the in-depth understanding of the fundamental questions related to the silk fibroin assembly, revealed their crucial structural remodeling, and paved the way for new fabrication strategies of mechanical-enhanced silk fibroin materials.

Spinning Methods Used for Construction of One- and Two-Dimensional Fibrous Protein Materials.

Natural silk protein fibers have shown a great attraction to the researchers due to the extraordinary mechanical property, biocompatibility, and functional diversity. Unfortunately, the low yield and unevenness have hampered the scale use of the natural silk fibers. Herein, the appearance of the bioinspired artificial spinning strategy offers an effective way to fabricate silk fibers with controllable structures and functionality. This chapter describes an experimental method to prepare silk protein fibers on a large scale and summarizes the method to investigate the effects of the structure-property relationship of the recombinant protein fibers.

Synchrotron FTIR Microspectroscopy Methods to Understand the Conformation of Single Animal Silk Fibers.

Animal silks have received extensive attention in these years due to their unique mechanical properties. The study of the structure-property relationship of animal silks is not only critical for the understanding of the design secrets of natural materials but also can inspire the engineering material designs. Fourier transform infrared spectroscopy (FTIR) has been used to study the secondary structure of animal silk, which is considered to be critical to the mechanical properties of animal silk. However, most of these characterizations are conducted on silk fiber bundles. In this respect, synchrotron FTIR microspectroscopy (S-micro FTIR) has unique advantages in characterizing single animal silks, as S-micro FTIR has significant advantages in ultrahigh brightness and high spatial resolution to characterize samples with small size. Here, we will introduce the methods for using synchrotron FTIR microspectroscopy to analyze the conformation and orientation of single animal silk fibers, which would be an efficient method to elucidate the "structure-property" relationship within animal silks.

Structural Characterization of Silk Fibers by Wide-Angle X-Ray Scattering.

As one of the most advanced techniques to gain insight into the structure of the materials, wide-angle X-ray scattering (WAXS) records the scattering information at wide angles which typically larger than 5° (2θ), where contains abundant and detailed atomic-scale structure information of the matter. To improve the intensity and time-resolution, the WAXS can be further coupled with a synchrotron light source. The resultant technique, that is, Synchrotron WAXS can reach and even surpass the spatial and time resolution of 0.1 nm and microsecond scale, respectively, thus is very suitable for characterization of animal silks both statically and quasi-dynamically. This chapter would show methods to understand the structure-property relationship of animal silks by WAXS.

Structural Characterization of Silk Fibers by Small-Angle X-Ray Scattering.

Nature is rich in all kinds of unbelievably designed microstructures, which endows the natural materials with various fantastic performances. Unveiling the mystery of the sophisticated configurations and the relationship between those microstructures and the corresponding functions is helpful for the manufacture of artificial functional materials. Small-angle X-ray scattering (SAXS) is an advanced tool to gain the microstructural features of materials within a spatial scale much larger than the atomic scale (1000 nm), which can be carried out along with WAXS to conduct more systematic investigation over different kinds of materials. With the help of SAXS/WAXS, one may generate an insightful understanding of the mechanisms of structure-property evolution (which is efficient guidance for the artificial material designs). This chapter will introduce the mathematics and the methodologies used by SAXS when investigating the microstructure of natural materials.

Electro-Blown Spun Silk/Graphene Nanoionotronic Skin for Multifunctional Fire Protection and Alarm.

Artificial protective skins are widely used in artificial intelligence robots, such as humanoid robots, mobile manipulation robots, and automatic probe robots, but their safety in use, especially flame retardancy, is rarely considered. As many artificial skins are designed for use in flammable or even explosive environments, flammability is a significant concern. Herein, a flame-retardant silk/graphene nanoionotronic (SGNI) skin is developed by using a rationally designed high-throughput electro-blown spinning technique, with a more efficient production efficiency than electrospinning. These flame retardant SGNI skins combine the advantages of nanofibrous and ionotronic materials, and they are sustainable, conductive, highly porous, mechanically robust, highly stretchable, self-adhesive, and humidity- and temperature-sensitive. These merits support the assembly of SGNI skins into a fire alarm system, with real-time alarm (response in 2 s) to mobile phones, clouds, and a central control system. The concept that combines a flame retardant and fire alarm material into an intelligent skin may provide potential solutions toward the design of protective skins for robotics and human-machine interactions.

Mechanical Training-Driven Structural Remodeling: A Rational Route for Outstanding Highly Hydrated Silk Materials.

Highly hydrated silk materials (HHSMs) have been the focus of extensive research due to their usefulness in tissue engineering, regenerative medicine, and soft devices, among other fields. However, HHSMs have weak mechanical properties that limit their practical applications. Inspired by the mechanical training-driven structural remodeling strategy (MTDSRS) in biological tissues, herein, engineered MTDSRS is developed for self-reinforcement of HHSMs to improve their inherent mechanical properties and broaden potential utility. The MTDSRS consists of repetitive mechanical training and solvent-induced conformation transitions. Solvent-induced conformation transition enables the formation of ß-sheet physical crosslinks among the proteins, while the repetitive mechanical loading allows the rearrangement of physically crosslinked proteins along the loading direction. Such synergistic effects produce strong and stiff mechanically trained-HHSMs (MT-HHSMs). The fracture strength and Young's modulus of the resultant MT-HHSMs (water content of 43 ± 4%) reach 4.7 ± 0.9 and 21.3 ± 2.1 MPa, respectively, which are 8-fold stronger and 13-fold stiffer than those of the as-prepared HHSMs, as well as superior to most previously reported HHSMs with comparable water content. In addition, the animal silk-like highly oriented molecular crosslinking network structure also provides MT-HHSMs with fascinating physical and functional features, such as stress-birefringence responsibility, humidity-induced actuation, and repeatable self-folding deformation.

Self-adhesive and contractile silk fibroin/graphene nano-ionotronic skin for strain sensing of irregular surfaces.

Nanofiber-based artificial skin has shown promise for application in flexible wearable electronics due to its favorable breathability and comfortable wearability. However, the electrospinning method commonly used for nanofiber preparation suffers from poor spinning performance when used for ionotronic solutions. Moreover, the resulting membrane usually lacks self-adhesive and self-adapting properties when it is attached to an irregular subject, which greatly hinders its practical usage. Herein, a self-adhesive and contractile silk fibroin/graphene nano-ionotronic skin was successfully prepared using a high-yield electro-blowing technique. The electro-blowing technique was able to effectively overcome the instability of the spinning jet and raise the feed rate to at least 5 ml h-1. The high Ca2+content provided the fabricated nano-ionotronic skin with humidity-induced stretchability and robusticity. More importantly, the ionotronic skin also possessed a self-adhesive property and was able to contract to adapt to irregular surfaces. Additionally, an analytical piezoresistive model was successfully built to predict the response of the sensors to stress. Furthermore, due to its stable conductivity, sensitivity, and self-adapting property, the obtained nano-ionotronic skin can be used for body monitoring, for example, for bending of the arm and hand gestures. The design and manufacture concept proposed in this work might inspire the development of high-yield ionotronic nanofibers and the design of self-adapting artificial skin.

Hierarchically maze-like structured nanofiber aerogels for effective low-frequency sound absorption.

Noise has been regarded as an environmental pollutant that greatly affects people's physical and psychiatric health. Fibrous sound absorption materials are widely used to release the annoyance that brought by noise pollution, however, the fibrous materials are limited by poor sound absorption ability in low-frequency, heavyweight, and excessive thickness. Herein, composite nanofiber aerogels are designed with a hierarchical maze-like microstructure, which is fabricated by interweaving the cellulose nanocrystal lamellas with polyacrylonitrile electrospun nanofiber networks through the freeze-casting technique. The designed maze-like structure shows obvious enhancement in the low-frequency sound absorption band compared to the fiber aerogels made by the network structure. Moreover, through carefully regulating the maze structure, composite nanofiber aerogels with excellent sound absorption performance (with an NRC of 0.58) and lightweight property (11.05 mg cm-3) can be fabricated. In addition to the superior sound absorption ability, the hierarchical nature of the maze-like structure also guarantees the nanofiber aerogels with robust mechanical properties, which can be tailored to various shaped objects on a large scale. These favorable characters present that the composite nanofiber aerogels potential choice for sound absorption in the fields of vehicles, buildings, and indoor reverberation.

Mechanism of Mechanical Training-Induced Self-Reinforced Viscoelastic Behavior of Highly Hydrated Silk Materials.

Mechanical training is an operation where a sample is cyclically stretched in a solvent. It is accepted as an effective strategy to strengthen and stiffen the highly hydrated silk materials (HHSMs). However, the detailed reinforcement mechanism of the process still remains to be understood. Herein, this process is studied by the integration of experimental characterization and theoretical analysis. The results from time-resolved Fourier transform infrared spectroscopy and real-time birefringent characterization reveal that the silk proteins rapidly formed a molecular cross-linking network (MCN) during the mechanical training. The cross-links were the ß-sheet nanocrystals generated from the conformation transition of silk proteins. With the progress in mechanical training, these MCNs gradually remodeled to a highly oriented molecular network structure. The final structure of the silk proteins in HHSMs is highly similar to the structural organization of silk proteins in the natural animal silk. The training process significantly improved the mechanical strength and modulus of the material. With regards to the dynamic behavior of conformation transition and MCN orientation, the structural evaluation of silk proteins during mechanical training was divided into three distinct stages, namely, the MCN-forming stage, MCN-orienting stage, and oriented-MCN stage. Such division is in complete agreement with the three-stage viscoelastic behavior observed in the cyclic loading and unloading tests. Hence, a five-parameter viscoelastic model has been established to elucidate the structure-property relationship of these three stages. This work improves in-depth understanding of the fundamental issues related to structure-property relationships of HHSMs and thus provides inspiration and guidance in the design of soft silk functional materials.

Stretchable, tough and elastic nanofibrous hydrogels with dermis-mimicking network structure.

Nanofiber-structured hydrogels with robust mechanical properties are promising candidates for the development of multifunctional materials in advanced fields. However, creating such materials that combine the virtues of high elongation, robust strength, and good elasticity remains an enormous challenge. Here, we demonstrate a nature-inspired methodology to fabricate dermis-mimicking network structured electrospun nanofibrous hydrogels with robust mechanical properties by combining the advantages of sustainable plant-based zein and elastic waterborne polyurethane (WPU). The reversible hydrogen bonding and strong covalent bonding between zein and WPU molecules are constructed in the double-network (DN) structured nanofibrous hydrogels (NFHs) with tunable stretchability and strength. The resulting NFHs exhibit the integrated characteristics of a stretch of 683%, a fracture strength of 6.5 MPa, a toughness of 20.7 MJ m-3, and complete recovery from large deformation. This nature-inspired structural design strategy may pave the way for designing mechanically robust nanofibrous hydrogels in structurally adaptive and scalable form.