Liquid Exfoliated Natural Silk Nanofibrils: Applications in Optical and Electrical Devices

Flexible Meso Electronics and Photonics Based on Cocoon Silk and Applications

Flexible electronics, applicable to enlarged health, AI big data medications, etc., have been one of the most important technologies of this century. Due to its particular mechanical properties, biocompatibility, and biodegradability, cocoon silk (or SF, silk fibroin) plays a key role in flexible electronics/photonics. The review begins with an examination of the hierarchical meso network structures of SF materials and introduces the concepts of meso reconstruction, meso doping, and meso hybridization based on the correlation between the structure and performance of silk materials. The SF meso functionalization was developed according to intermolecular nuclear templating. By implementation of the techniques of meso reconstruction and functionalization in the refolding of SF materials, extraordinary performance can be achieved. Relying on this strategy, particularly designed flexible electronic and photonic components can be developed. This review covers the latest ideas and technologies of meso flexible electronics and photonics based on SF materials/meso functionalization. As silk materials are biocompatible and human skin-friendly, SF meso flexible electronic/photonic components can be applied to wearable or implanted devices. These devices are applicable in human physiological signals and activities sensing/monitoring. In the case of human-machine interaction, the devices can be applicable in in-body information transmission, computation, and storage, with the potential for the combination of artificial intelligence and human intelligence.

Versatile Deprotonation-Induced Exfoliation and Functionalization of Biological Nanofibrils for Actuation and Fluorescence

Biological nanofibrils not only are characteristic of many species of biomasses but also serve as a promising type of sustainable nanomaterials for various applications. However, their production has long relied on an invasive and energy-consuming mechanical shear. A noninvasive and versatile approach remains challenging to exfoliate different types of biomasses into nanofibrils. In this study, we showed a versatile and nonaggressive intercalative deprotonation agent of organic base, which could efficiently deprotonate various biomasses for energy-saving exfoliation and functionalization, including cellulose, chitin, and silk. Both carboxylic nanofibrils and nanofibrils with pristine chemical structures could be produced in high yields through manual shaking or sonication. By further grafting photoresponsive groups via transesterification, intelligent NFs were generated featuring ultraviolet-responsive fluorescence and hydrophilicity. These responsive fluorescence and actuation behaviors promised their potential as green encryption and anticounterfeiting nanomaterials.

Decoding silkworm spinning programmed by pH and metal ions

Silk is one of the toughest fibrous materials known despite spun at ambient temperature and pressure with water as a solvent. It is a great challenge to reproduce high-performance artificial fibers comparable to natural silk by bionic for the incomplete understanding of silkworm spinning in vivo. Here, we found that amphipol and digitonin stabilized the structure of natural silk fibroin (NSF) by a large-scale screening in vitro, and then studied the close-to-native ultrastructure and hierarchical assembly of NSF in the silk gland lumen. Our study showed that NSF formed reversible flexible nanofibrils mainly composed of random coils with a sedimentation coefficient of 5.8 S and a diameter of about 4 nm, rather than a micellar or rod-like structure assembled by the aggregation of globular NSF molecules. Metal ions were required for NSF nanofibril formation. The successive pH decrease from posterior silk gland (PSG) to anterior silk gland (ASG) resulted in a gradual increase in NSF hydrophobicity, thus inducing the sol-gelation transition of NSF nanofibrils. NSF nanofibrils were randomly dispersed from PSG to ASG-1, and self-assembled into anisotropic herringbone patterns at ASG-2 near the spinneret ready for silkworm spinning. Our findings reveal the controlled self-assembly mechanism of the multi-scale hierarchical architecture of NSF from nanofibrils to herringbone patterns programmed by metal ions and pH gradient, which provides novel insights into the spinning mechanism of silk-secreting animals and bioinspired design of high-performance fibers.

Protein-based nanoparticles for therapeutic nucleic acid delivery

To realize the full potential of emerging nucleic acid therapies, there is a need for effective delivery agents to transport cargo to cells of interest. Protein materials exhibit several unique properties, including biodegradability, biocompatibility, ease of functionalization via recombinant and chemical modifications, among other features, which establish a promising basis for therapeutic nucleic acid delivery systems. In this review, we highlight progress made in the use of non-viral protein-based nanoparticles for nucleic acid delivery in vitro and in vivo, while elaborating on key physicochemical properties that have enabled the use of these materials for nanoparticle formulation and drug delivery. To conclude, we comment on the prospects and unresolved challenges associated with the translation of protein-based nucleic acid delivery systems for therapeutic applications.

A Simplified Method for the Preparation of Highly Conductive and Flexible Silk Nanofibrils/MXene Membrane

Silk nanofibers (SNF) have great applications in high-performance functional nanocomposites due to their excellent mechanical properties, biocompatibility, and degradability. However, the preparation of SNF by traditional methods often requires the use of some environmentally harmful or toxic reagents, limiting its application in green chemistry. In this paper, we successfully prepared SNF using natural silk as raw material and solvent stripping technology by adjusting the solvent concentration and solution ratio (the diameter of about 120 nm). Using the above SNFs as raw materials, SNF membranes were prepared by vacuum filtration technology. In addition, we prepared an SNF/MXene nanocomposite material with excellent humidity sensitivity by simply coating MXene nanosheets with silk fibers. The conductivity of the material can approach 1400.6 S m-1 with excellent mechanical strength (51.34 MPa). The SNF/MXene nanocomposite material with high mechanical properties, high conductivity, and green degradability can be potentially applied in the field of electromagnetic interference (EMI) shielding, providing a feasible approach for the development of functional nanocomposite materials.

Biomimetic Silk Architectures Outperform Animal Horns in Strength and Toughness

Structural biomimicry is an intelligent approach for developing lightweight, strong, and tough materials (LSTMs). Current fabrication technologies, such as 3D printing and two-photon lithography often face challenges in constructing complex interlaced structures, such as the sinusoidal crossed herringbone structure that contributes to the ultrahigh strength and fracture toughness of the dactyl club of peacock mantis shrimps. Herein, bioinspired LSTMs with laminated or herringbone structures is reported, by combining textile processing and silk fiber "welding" techniques. The resulting biomimetic silk LSTMs (BS-LSTMs) exhibit a remarkable combination of lightweight with a density of 0.6-0.9 g cm-3 , while also being 1.5 times stronger and 16 times more durable than animal horns. These findings demonstrate that BS-LSTMs are among the toughest natural materials made from silk proteins. Finite element simulations further reveal that the fortification and hardening of BS-LSTMs arise primarily from the hierarchical organization of silk fibers and mechanically transferable meso-interfaces. This study highlights the rational, cost-effective, controllable mesostructure, and transferable strategy of integrating textile processing and fiber "welding" techniques for the fabrication of BS-LSTMs with advantageous structural and mechanical properties. These findings have significant implications for a wide range of applications in biomedicine, mechanical engineering, intelligent textiles, aerospace industries, and beyond.

Natural spider silk nanofibrils produced by assembling molecules or disassembling fibers

Spider silk is biocompatible, biodegradable, and rivals some of the best synthetic materials in terms of strength and toughness. Despite extensive research, comprehensive experimental evidence of the formation and morphology of its internal structure is still limited and controversially discussed. Here, we report the complete mechanical decomposition of natural silk fibers from the golden silk orb-weaver Trichonephila clavipes into ≈10 nm-diameter nanofibrils, the material's apparent fundamental building blocks. Furthermore, we produced nanofibrils of virtually identical morphology by triggering an intrinsic self-assembly mechanism of the silk proteins. Independent physico-chemical fibrillation triggers were revealed, enabling fiber assembly from stored precursors "at-will". This knowledge furthers the understanding of this exceptional material's fundamentals, and ultimately, leads toward the realization of silk-based high-performance materials. STATEMENT OF SIGNIFICANCE: Spider silk is one of the strongest and toughest biomaterials, rivaling the best man-made materials. The origins of these traits are still under debate but are mostly attributed to the material's intriguing hierarchical structure. Here we fully disassembled spider silk into 10 nm-diameter nanofibrils for the first time and showed that nanofibrils of the same appearance can be produced via molecular self-assembly of spider silk proteins under certain conditions. This shows that nanofibrils are the key structural elements in silk and leads toward the production of high-performance future materials inspired by spider silk.

Simulation of Cortical and Cancellous Bone to Accelerate Tissue Regeneration

Different tissues have complex anisotropic structures to support biological functions. Mimicking these complex structures in vitro remains a challenge in biomaterials designs in support of tissue regeneration. Here, inspired by different types of silk nanofibers, a composite materials strategy was pursued towards this challenge. A combination of fabrication methods was utilized to achieve separate control of amorphous and beta-sheet rich silk nanofibers in the same solution. Aqueous solutions containing these two structural types of silk nanofibers were then simultaneously treated with an electric field and with ethylene glycol diglycidyl ether (EGDE). Under these conditions, the beta-sheet rich silk nanofibers in the mixture responded to the electric field while the amorphous nanofibers were active in the crosslinking process with the EGDE. As a result, cryogels with anisotropic structures were prepared, including mimics for cortical- and cancellous-like bone biomaterials as a complex osteoinductive niche. In vitro studies revealed that mechanical cues of the cryogels induced osteodifferentiation of stem cells while the anisotropy inside the cryogels influenced immune reactions of macrophages. These bioactive cryogels also stimulated improved bone regeneration in vivo through modulation of inflammation, angiogenesis and osteogenesis responses, suggesting an effective strategy to develop bioactive matrices with complex anisotropic structures beneficial to tissue regeneration.

The Fractal Network Structure of Silk Fibroin Molecules and Its Effect on Spinning of Silkworm Silk

Animal silk is usually considered to exist as a solid fiber with a highly ordered structure, formed by the hierarchical assembly starting from a single silk fibroin (SF) chain. However, this study showed that silk protein molecules existed in the form of a fractal network structure in aqueous solution, rather than as a single chain. This type of network was relatively rigid with low fractal dimension. Finite element analysis revealed that this network structure significantly helped in the stable storage of SF prior to the spinning process and in the rapid formation of a β-sheeted nanocrystalline and nematic texture during spinning. Further, the strong but brittle mechanical properties of Bombyx mori silk could also be well-explained through the fractal network model of silk fibroin. The strength was mainly derived from the dual network structure, consisting of nodes and β-sheet cross-links, whereas the brittleness could be attributed to the rigidity of the SF chains between these nodes and cross-links. In summary, this study presents insights from network topology for understanding the spinning process of natural silk and the structure-property relationship in silk materials.

Controllable Production of Natural Silk Nanofibrils for Reinforcing Silk-Based Orthopedic Screws

As a natural high-performance material with a unique hierarchical structure, silk is endowed with superior mechanical properties. However, the current approaches towards producing regenerated silk fibroin (SF) for the preparation of biomedical devices fail to fully exploit the mechanical potential of native silk materials. In this study, using a top-down approach, we exfoliated natural silk fibers into silk nanofibrils (SNFs), through the disintegration of interfibrillar binding forces. The as-prepared SNFs were employed to reinforce the regenerated SF solution to fabricate orthopedic screws with outstanding mechanical properties (compression modulus > 1.1 GPa in a hydrated state). Remarkably, these screws exhibited tunable biodegradation and high cytocompatibility. After 28 days of degradation in protease XIV solution, the weight loss of the screw was ~20% of the original weight. The screws offered a favorable microenvironment to human bone marrow mesenchymal stem cell growth and spread as determined by live/dead staining, F-action staining, and Alamar blue staining. The synergy between native structural components (SNFs) and regenerated SF solutions to form bionanocomposites provides a promising design strategy for the fabrication of biomedical devices with improved performance.

Natural silk nanofibers as building blocks for biomimetic aerogel scaffolds

Protein nanofibers offer great promise for tissue engineering scaffolds owing to biomimetic architecture and exceptional biocompatibility. Natural silk nanofibrils (SNFs) are promising but unexplored protein nanofibers for biomedical applications. In this study, the SNF-assembled aerogel scaffolds with ECM-mimicking architecture and ultra-high porosity are developed based on a polysaccharides-assisted strategy. The SNFs exfoliated from silkworm silks can be utilized as building blocks to construct 3D nanofibrous scaffolds with tunable densities and desirable shapes on a large scale. We demonstrate that the natural polysaccharides can regulate SNF assembly through multiple binding modes, endowing the scaffolds with structural stability in water and tunable mechanical properties. As a proof of concept, the biocompatibility and biofunctionality of the chitosan-assembled SNF aerogels were investigated. The nanofibrous aerogels have excellent biocompatibility, and their biomimetic structure, ultra-high porosity, and large specific surface area endow the scaffolds with enhanced cell viability to mesenchymal stem cells. The nanofibrous aerogels were further functionalized by SNF-mediated biomineralization, demonstrating their potential as a bone-mimicking scaffold. Our results show the potential of natural nanostructured silks in the field of biomaterials and provide a feasible strategy to construct protein nanofiber scaffolds.

Filtration-processed biomass nanofiber electrodes for flexible bioelectronics

An increasing demand for bioelectronics that interface with living systems has driven the development of materials to resolve mismatches between electronic devices and biological tissues. So far, a variety of different polymers have been used as substrates for bioelectronics. Especially, biopolymers have been investigated as next-generation materials for bioelectronics because they possess interesting characteristics such as high biocompatibility, biodegradability, and sustainability. However, their range of applications has been restricted due to the limited compatibility of classical fabrication methods with such biopolymers. Here, we introduce a fabrication process for thin and large-area films of chitosan nanofibers (CSNFs) integrated with conductive materials. To this end, we pattern carbon nanotubes (CNTs), silver nanowires, and poly (3,4-ethylenedioxythiophene):poly (styrenesulfonate) (PEDOT:PSS) by a facile filtration process that uses polyimide masks fabricated via laser ablation. This method yields feedlines of conductive material on nanofiber paper and demonstrates compatibility with conjugated and high-aspect-ratio materials. Furthermore, we fabricate a CNT neural interface electrode by taking advantage of this fabrication process and demonstrate peripheral nerve stimulation to the rapid extensor nerve of a live locust. The presented method might pave the way for future bioelectronic devices based on biopolymer nanofibers.

Outstanding Bio-Tribological Performance Induced by the Synergistic Effect of 2D Diamond Nanosheet Coating and Silk Fibroin

Due to their excellent biocompatibility, outstanding mechanical properties, high strength-to-weight ratio, and good corrosion resistance, titanium (Ti) alloys are extensively used as implant materials in artificial joints. However, Ti alloys suffer from poor wear resistance, resulting in a considerably short lifetime. In this study, we demonstrate that the chemical self-assembly of novel two-dimensional (2D) diamond nanosheet coatings on Ti alloys combined with natural silk fibroin used as a novel lubricating fluid synergistically results in excellent friction and wear performance. Linear-reciprocating sliding tests verify that the coefficient of friction and the wear rate of the diamond nanosheet coating under silk fibroin lubrication are reduced by 54 and 98%, respectively, compared to those of the uncoated Ti alloy under water lubrication. The lubricating mechanism of the newly designed system was revealed by a detailed analysis of the involved microstructural and chemical changes. The outstanding tribological behavior was attributed to the establishment of artificial joint lubrication induced by the cross binding between the diamond nanosheets and silk fibroin. Additionally, excellent biocompatibility of the lubricating system was verified by cell viability, which altogether paves the way for the application of diamond coatings in artificial Ti joint implants.

Preparation of Highly Crystalline Silk Nanofibrils and Their Use in the Improvement of the Mechanical Properties of Silk Films

Due to their commendable biocompatibility, regenerated silk fibroin (RSF) films have attracted considerable research interest. However, the poor mechanical properties of RSF films have limited their use in various biomedical applications. In this study, a novel, highly crystalline silk fibril was successfully extracted from silk by combining degumming with ultrasonication. Ultrasonication accelerated the development of silk nanofibrils measuring 130–200 nm on the surface of the over-degummed silk fibers, which was confirmed via scanning electron microscopy. Additionally, the crystallinity index of silk fibril was found to be significantly higher (~68%) than that of conventionally degummed silk (~54%), as confirmed by the Fourier-transform infrared (FTIR) spectroscopy results. Furthermore, the breaking strength and elongation of the RSF film were increased 1.6 fold and 3.4 fold, respectively, following the addition of 15% silk nanofibrils. Thus, the mechanical properties of the RSF film were remarkably improved by the addition of the silk nanofibrils, implying that it can be used as an excellent reinforcing material for RSF films.

Tiny NaOH Assisted Facile Preparation of Silk Nanofibers and Their Nanotube-Compositing Strong, Flexible, and Conductive Films

Natural silk nanofibers (SNFs) can not only be used as good building blocks for two- or three-dimensional biomaterials but also provide a clue for understanding the theory of structure-function relationships. Nevertheless, it is still difficult to directly extract SNFs from natural silk fibers due to their high crystallinity and recalcitrant complex structures. In the present study, a dilute alkali-assisted separation of high-yield SNFs is proposed. The degummed silk was first treated with a tiny amount of alkali at a mild temperature, followed by high-pressure homogenization. Under the optimized conditions (2% sodium hydroxide, 0 °C, 48 h), SNFs with diameters of 8-42 nm and lengths of 0.9 ± 0.3 μm were prepared with yields higher than 75%, which retained the natural structures at the nanoscale and some inherent properties of silk fibers. Interestingly, SNFs can be used as a stabilizing matrix to assist carbon nanotubes (CNTs) to disperse, aiming to form a uniform and stable CNT/SNF dispersion. Thereafter, a strong and flexible conductive composite film was fabricated with good mechanical properties. The composite film showed good piezoelectric properties and electric thermal response, which has promising application prospects for SNFs, such as in optical devices, nanoelectronics, and biosensors.

Bioinspired silk fibroin materials: From silk building blocks extraction and reconstruction to advanced biomedical applications

Silk fibroin has become a promising biomaterial owing to its remarkable mechanical property, biocompatibility, biodegradability, and sufficient supply. However, it is difficult to directly construct materials with other formats except for yarn, fabric and nonwoven based on natural silk. A promising bioinspired strategy is firstly extracting desired building blocks of silk, then reconstructing them into functional regenerated silk fibroin (RSF) materials with controllable formats and structures. This strategy could give it excellent processability and modifiability, thus well meet the diversified needs in biomedical applications. Recently, to engineer RSF materials with properties similar to or beyond the hierarchical structured natural silk, novel extraction and reconstruction strategies have been developed. In this review, we seek to describe varied building blocks of silk at different levels used in biomedical field and their effective extraction and reconstruction strategies. This review also present recent discoveries and research progresses on how these functional RSF biomaterials used in advanced biomedical applications, especially in the fields of cell-material interactions, soft tissue regeneration, and flexible bioelectronic devices. Finally, potential study and application for future opportunities, and current challenges for these bioinspired strategies and corresponding usage were also comprehensively discussed. In this way, it aims to provide valuable references for the design and modification of novel silk biomaterials, and further promote the high-quality-utilization of silk or other biopolymers.

Multi-Point Nanoindentation Method to Determine Mechanical Anisotropy in Nanofibrillar Thin Films

Biomaterials with outstanding mechanical properties, including spider silk, wood, and cartilage, often feature an oriented nanofibrillar structure. The orientation of nanofibrils gives rise to a significant mechanical anisotropy, which is extremely challenging to characterize, especially for microscopically small or inhomogeneous samples. Here, a technique utilizing atomic force microscope indentation at multiple points combined with finite element analysis to sample the mechanical anisotropy of a thin film in a microscopically small area is reported. The system studied here is the tape-like silk of the Chilean recluse spider, which entirely consists of strictly oriented nanofibrils giving rise to a large mechanical anisotropy. The most detailed directional nanoscale structure-property characterization of spider silk to date is presented, revealing the tensile and transverse elastic moduli as 9 and 1 GPa, respectively, and the binding strength between silk nanofibrils as 159 ± 13 MPa. Furthermore, based on this binding strength, the nanofibrils' surface energy is derived as 37 mJ m^-2 , and concludes that van der Waals forces play a decisive role in interfibrillar binding. Due to its versatility, this technique has many potential applications, including early disease diagnostics, as underlying pathological conditions can alter the local mechanical properties of tissues.

Natural Polymer in Soft Electronics: Opportunities, Challenges, and Future Prospects

Pollution caused by nondegradable plastics has been a serious threat to environmental sustainability. Natural polymers, which can degrade in nature, provide opportunities to replace petroleum-based polymers, meanwhile driving technological advances and sustainable practices. In the research field of soft electronics, regenerated natural polymers are promising building blocks for passive dielectric substrates, active dielectric layers, and matrices in soft conductors. Here, the natural-polymer polymorphs and their compatibilization with a variety of inorganic/organic conductors through interfacial bonding/intermixing and surface functionalization for applications in various device modalities are delineated. Challenges that impede the broad utilization of natural polymers in soft electronics, including limited durability, compromises between conductivity and deformability, and limited exploration in controllable degradation, etc. are explicitly inspected, while the potential solutions along with future prospects are also proposed. Finally, integrative considerations on material properties, device functionalities, and environmental impact are addressed to warrant natural polymers as credible alternatives to synthetic ones, and provide viable options for sustainable soft electronics.

Silk-based bioinspired structural and functional materials

Natural biological materials provide a rich source of inspiration for building high-performance materials with extensive applications. By mimicking their chemical compositions and hierarchical architectures, the past decades have witnessed the rapid development of bioinspired materials. As a very promising biosourced raw material, silk is drawing increasing attention due to excellent mechanical properties, favorable versatility, and good biocompatibility. In this review, we provide an overview of the recent progress in silk-based bioinspired structural and functional materials. We first give a brief introduction of silk, covering its sources, features, extraction, and forms. We then summarize the preparation and application of silk-based materials mimicking four typical biological materials including bone, nacre, skin, and polar bear hair. Finally, we discuss the current challenges and future prospects of this field.

From Mesoscopic Functionalization of Silk Fibroin to Smart Fiber Devices for Textile Electronics and Photonics

Bombyx mori silk fibers exhibit significant potential for applications in smart textiles, such as fiber sensors, fiber actuators, optical fibers, and energy harvester. Silk fibroin (SF) from B. mori silkworm fibers can be reconstructed/functionalized at the mesoscopic scale during refolding from the solution state into fibers. This facilitates the mesoscopic functionalization by engaging functional seeds in the refolding of unfolded SF molecules. In particular, SF solutions can be self-assembled into regenerated fiber devices by artificial spinning technologies, such as wet spinning, dry spinning, microfluidic spinning, electrospinning, and direct writing. Meso-functionalization manipulates the SF property from the mesoscopic scale, transforming the original silk fibers into smart fiber devices with smart functionalities, such as sensors, actuators, optical fibers, luminous fibers, and energy harvesters. In this review, the progress of mesoscopic structural construction from SF materials to fiber electronics/photonics is comprehensively summarized, along with the spinning technologies and fiber structure characterization methods. The applications, prospects, and challenges of smart silk fibers in textile devices for wearable personalized healthcare, self-propelled exoskeletons, optical and luminous fibers, and sustainable energy harvesters are also discussed.

Structure of Animal Silks

As an abundant fibrous protein, animal silks have received a variety of interests in both traditional and high-tech industries, such as textiles, decoration, and biomedicine, due to their unique advantages in mechanical performance, sustainability, biocompatibility, and biodegradability. While developing applications of animal silks, the structure of animal silks has also received more and more attention in these decades. Briefly, most animal silks can be considered as semicrystalline fibers, which are composed of β-sheet nanocrystals and amorphous regions. However, different animal silks have similarities and also have obvious differences at different structural levels. In this chapter, we will introduce the structures of the three most representative animal silks, that is, spider dragline silk, tussah silk, and mulberry silk. The similarities and differences in their structures will be highlighted, so as to provide fundamental guidance for the research and use of these animal silks.

Fiber-Based Biopolymer Processing as a Route toward Sustainability

Some of the most abundant biomass on earth is sequestered in fibrous biopolymers like cellulose, chitin, and silk. These types of natural materials offer unique and striking mechanical and functional features that have driven strong interest in their utility for a range of applications, while also matching environmental sustainability needs. However, these material systems are challenging to process in cost-competitive ways to compete with synthetic plastics due to the limited options for thermal processing. This results in the dominance of solution-based processing for fibrous biopolymers, which presents challenges for scaling, cost, and consistency in outcomes. However, new opportunities to utilize thermal processing with these types of biopolymers, as well as fibrillation approaches, can drive renewed opportunities to bridge this gap between synthetic plastic processing and fibrous biopolymers, while also holding sustainability goals as critical to long-term successful outcomes.

Deconstruction and Reassembly of Renewable Polymers and Biocolloids into Next Generation Structured Materials

This review considers the most recent developments in supramolecular and supraparticle structures obtained from natural, renewable biopolymers as well as their disassembly and reassembly into engineered materials. We introduce the main interactions that control bottom-up synthesis and top-down design at different length scales, highlighting the promise of natural biopolymers and associated building blocks. The latter have become main actors in the recent surge of the scientific and patent literature related to the subject. Such developments make prominent use of multicomponent and hierarchical polymeric assemblies and structures that contain polysaccharides (cellulose, chitin, and others), polyphenols (lignins, tannins), and proteins (soy, whey, silk, and other proteins). We offer a comprehensive discussion about the interactions that exist in their native architectures (including multicomponent and composite forms), the chemical modification of polysaccharides and their deconstruction into high axial aspect nanofibers and nanorods. We reflect on the availability and suitability of the latter types of building blocks to enable superstructures and colloidal associations. As far as processing, we describe the most relevant transitions, from the solution to the gel state and the routes that can be used to arrive to consolidated materials with prescribed properties. We highlight the implementation of supramolecular and superstructures in different technological fields that exploit the synergies exhibited by renewable polymers and biocolloids integrated in structured materials.

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.

Biospired Janus Silk E-Textiles with Wet-Thermal Comfort for Highly Efficient Biofluid Monitoring

Functionalized textiles capable of biofluid administration are favorable for enhancing the wet-thermal comfort of the wearer and healthcare performance. Herein, inspired by the Janus wettability of lotus leaf, we propose a skin-comfortable Janus electronic textile (e-textile) based on natural silk materials for managing and analysis of biofluid. Silk materials are chosen and modified as both a textile substrate and a sensing electrode due to its natural biocompatibility. The unidirectional biofluid behavior of such Janus silk substrate facilitates a comfortable skin microenvironment, including weakening the undesired wet adhesion (∼0 mN cm^-2) and avoiding excessive heat or cold on the epidermis. We noninvasively analyze multiple targets of human sweat with less required liquid volume (∼5 μL) and a faster (2-3 min) response time based on the silk-based yarn electrode woven into the hydrophilic side of Janus silk. This work bridges the gap between physiological comfort and sensing technology using biomass-derived elements, presenting a new type of smart textiles for wet-thermal management and health monitoring.

Isolation of Nanofibrils from Animal Silks

The presence of well-organized nanofibrils in animal silks is considered to provide them excellent mechanical and biochemical properties. To direct utilize these unique natural nanomaterials, a variety of physical and/or chemical processes have been developed for directly isolating silk nanofibrils from animal silks. The yield and processability of these techniques as well as the morphologies of resultant silk nanofibrils have apparent differences but also have their own merits. In this chapter, I presented the protocols for isolation silk nanofibrils, including a physical approach of sonication, a chemical approach of salt-formic acid dissolution, as well as three combination approaches, hexafluoroisopropanol liquid exfoliation, urea-guanidine hydrochloride dissolution, and sodium hypochlorite partial dissolution.

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 β-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.

Protein nanofibrils for next generation sustainable water purification

Water scarcity is rapidly spreading across the planet, threatening the population across the five continents and calling for global sustainable solutions. Water reclamation is the most ecological approach for supplying clean drinking water. However, current water purification technologies are seldom sustainable, due to high-energy consumption and negative environmental footprint. Here, we review the cutting-edge technologies based on protein nanofibrils as water purification agents and we highlight the benefits of this green, efficient and affordable solution to alleviate the global water crisis. We discuss the different protein nanofibrils agents available and analyze them in terms of performance, range of applicability and sustainability. We underline the unique opportunity of designing protein nanofibrils for efficient water purification starting from food waste, as well as cattle, agricultural or dairy industry byproducts, allowing simultaneous environmental, economic and social benefits and we present a case analysis, including a detailed life cycle assessment, to establish their sustainable footprint against other common natural-based adsorbents, anticipating a bright future for this water purification approach.

Robust and double-layer micro-patterned bioadhesive based on silk nanofibril/GelMA-alginate for stroma tissue engineering

We develop a robust micro-patterned double-layer film that can adhere firmly to the tissue and provide a sustained release of ascorbic acid (AA) for corneal regeneration. This double-layer film consists of a AA reservoir sodium alginate (SA) adhesive and an anisotropic layer made of micro-patterned silk nanofibrils (SNF) incorporated gelatin methacrylate (GelMA) (S/G). The S/G layer facilitates the adhesion and orientation of corneal stroma cells, depending on the pattern sizes (50 μm (P1) and 100 (P2) μm). Results reveal that more than 90% and 80% of the cells are located at angles close to the vertical axis (0-20°) in the sample with the smaller and larger pattern size, respectively. The mechanical robustness and 90% light transmission of this hybrid film originate from the micro-patterned S/G layer. However, the micro-pattern size does not show a significant role in the mechanical properties of hybrid films (tensile strength of S/G-SA, S/G-SA(P₁), and S/G-SA(P₂) is 3.4 ± 0.1 MPa, 3.6 ± 0.6 MPa and 3.3 ± 0.2 MPa, respectively). In addition, the strong adhesion to the tissue of this double-layer film is related to the alginate layer. AA can release in a controlled manner, which can significantly promote corneal stroma cells' attachment, alignment, and proliferation compared to the control (AA-free micro-patterned film). Our results reveal that this innovative multifunctional S/G-SA + AA film can be a proper candidate for use in stroma tissue engineering of the human cornea.

Pseudosolvent Intercalator of Chitin: Self-Exfoliating into Sub-1 nm Thick Nanofibrils for Multifunctional Chitinous Materials

Traditionally, energy-intensive and time-consuming postmechanical disintegration processes are inevitable in extracting biopolymer nanofibrils from natural materials and thereby hinder their practical applications. Herein, a new, convenient, scalable, and energy-efficient method for exfoliating nanofibrils (ChNFs) from various chitin sources via pseudosolvent-assisted intercalation process is proposed. These self-exfoliated ChNFs possess controllable thickness from 2.2 to 0.8 nm, average diameter of 4-5 nm, high aspect ratio up to 10³ and customized surface chemistries. Particularly, compared with elementary nanofibrils, ChNFs with few molecular layers thick exhibit greater potential to construct high-performance structural materials, e.g., ductile nanopapers with large elongation up to 70.1% and toughness as high as 30.2 MJ m^-3 , as well as soft hydrogels with typical nonlinear elasticity mimicking that of human-skin. The proposed self-exfoliation concept with unique advantages in the combination of high yield, energy efficiency, scalable productivity, less equipment requirements, and mild conditions opens up a door to extract biopolymer nanofibrils on an industrial scale. Moreover, the present modular ChNFs exfoliation will facilitate researchers to study the effect of thickness on the properties of nanofibrils and provide more insight into the structure-function relationship of biopolymer-based materials.

Thermochromic Silks for Temperature Management and Dynamic Textile Displays

HIGHLIGHTS

Wearable and smart textiles are constructed by integrating embroidery technology and 5G cloud communication, showing promising applications in temperature management and real-time dynamic textile displays. Thermochromism is introduced into the natural silk to produce high-performance thermochromic silks (TCSs) through a low cost, sustainable, efficient, and scalable strategy. The interfacial bonding of the continuously produced TCSs is in situ analyzed and improved through pre-solvent treatment and is confirmed using synchrotron Fourier transform infrared microspectroscopy.

ABSTRACT

Silks have various advantages compared with synthetic polymer fibers, such as sustainability, mechanical properties, luster, as well as air and humidity permeability. However, the functionalization of silks has not yet been fully developed. Functionalization techniques that retain or even improve the sustainability of silk production are required. To this end, a low-cost, effective, and scalable strategy to produce TCSs by integrating yarn-spinning and continuous dip coating technique is developed herein. TCSs with extremely long length (> 10 km), high mechanical performance (strength of 443.1 MPa, toughness of 56.0 MJ m⁻³, comparable with natural cocoon silk), and good interfacial bonding were developed. TCSs can be automatically woven into arbitrary fabrics, which feature super-hydrophobicity as well as rapid and programmable thermochromic responses with good cyclic performance: the response speed reached to one second and remained stable after hundreds of tests. Finally, applications of TCS fabrics in temperature management and dynamic textile displays are demonstrated, confirming their application potential in smart textiles, wearable devices, flexible displays, and human–machine interfaces. Moreover, combination of the fabrication and the demonstrated applications is expected to bridge the gap between lab research and industry and accelerate the commercialization of TCSs. [Image: see text]

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s40820-021-00591-w.

Top-down extraction of surface carboxylated-silk nanocrystals and application in hydrogel preparation

Bionanomaterial based hydrogels originated from natural biopolymer have drawn much attention for advanced applications. However, nanosilk-based hydrogels derived from top-down approaches remain in their infancy. First, nanosilks based on existing methods fail to prepare hydrogels; second, both nanosilk extraction and surface modification remain a challenge due to high crystallinity and sophisticated hierarchical structures. To produce nanosilk-based hydrogels, pretreatment and oxidation are necessary. In this work, pretreatments were conducted first to loosen the sophisticated structures of natural silk fibers, NaClO oxidation was utilized in succession to introduce carboxyl groups onto silk fibroin. Combined with moderate mechanical disintegration, silk nanocrystals with additional carboxyl groups were prepared facilely. Finally, silk nanocrystal-based hydrogels were prepared successfully through gas phase coagulation. An optimization of pretreatment approaches and oxidation conditions was carried out. The morphologies, chemical and crystalline structures of original, pretreated and oxidized silk fibroin as well as nanofibrillated silk were investigated. In addition, the silk nanocrystal-based hydrogel exhibited outstanding mechanical properties compared to those of dissolved and regenerated silk fibroin-based hydrogels. Moreover, silk nanocrystal-based aerogels present highly porous, interconnected, and crisscrossed network nanostructures, which are ideal candidates for tissue regeneration and provide new prospects as porous scaffolds for bioengineering applications.

Liquid-Exfoliated Mesostructured Collagen from the Bovine Achilles Tendon as Building Blocks of Collagen Membranes

Mesoscaled assemblies are organized in native collagen tissues to achieve remarkable and diverse performance and functions. In this work, a facile, low-cost, and controllable liquid exfoliation method was applied to directly extract these collagen mesostructures from bovine Achilles tendons using a sodium hydroxide (NaOH)/urea aqueous system with freeze-thaw cycles and sonication. A series of collagen fibrils with diameters of 26-230 nm were harvested using this process, and in situ observations under polarizing microscopy (POM) and using molecular dynamics simulations revealed the influence of the NaOH/urea system on the tendon collagen. FTIR and XRD results confirmed that these collagen fibrils preserved typical structural characteristics of type I collagen. These isolated collagen fibrils were then utilized as building blocks to fabricate free-standing collagen membranes, which exhibited good stability in solvents and outstanding mechanical properties and transparency, with potential for utility in optical and electronic sensors. Moreover, in vitro and vivo evaluations demonstrated that these new resulting collagen membranes had good cytocompatibility, biocompatibility, and degradability for potential applications in biomedicine. This work provides a new approach for collagen processing by liquid exfoliation with utility for the formation of robust collagen materials that consist of native collagen mesostructures as building blocks.

Ethanol-induced coacervation in aqueous gelatin solution for constructing nanospheres and networks: Morphology, dynamics and thermal sensitivity

Ethanol was used to induce coacervation in aqueous solutions of gelatin. Coacervation resulted from phase separation driven by ethanol as a poor solvent for gelatin, impacting aggregation of gelatin chains. Static coacervation was performed to investigate coacervate morphology, and gelatin concentration and ethanol temperature influenced the morphologies of the gelatin coacervates. High-concentration gelatin solutions (>4.8 wt%) treated with lower temperature ethanol (<25 °C) formed network morphologies, while low-concentration gelatin solution (<4.8 wt%) treated with ethanol near room temperature formed nanosphere assemblies. Dispersive nanospheres were obtained after treatment with higher temperature ethanol (~45 °C). Stirring the mixture of gelatin solution and ethanol resulted in dispersed nanospheres where the size was adjusted by changing the volume ratio of aqueous gelatin solution and ethanol (V_Gel:V_EtOH) and the gelatin concentration. Turbidity and absorbance measurements were carried out to further investigate coacervation dynamics. The cocervation system reached dynamic equilibrium according to the V_Gel:V_EtOH, suggesting phase separation and molecular arrangements were key. DLS results showed that reversible changes in coacervate radius could be attained by periodic heating and cooling cycles (25-60 °C). This work provides useful information for constructing gelatin-based materials using a facile coacervation method.

Generation of Nano-pores in Silk Fibroin Films Using Silk Nanoparticles for Full-Thickness Wound Healing

Silk fibroin films are used in tissue engineering due to their biocompatibility, optical clarity, and slow biodegradability. However, the relatively smooth surface and low permeability of these systems may limit some applications; thus, here, a method was developed to generate nano-pores in methanol or ethanol-treated silk fibroin films. The first step was to induce the formation of nanoparticles (50-300 nm diam.) in silk fibroin solutions by autoclaving. After drying in air, the films formed were treated to induce silk β-sheet structures, which condense the bulk silk phase and nanoparticles and phase separation and enlarge the space of bulk silk phase and nanoparticles. These films were then extracted with water to allow the condensed nanoparticles to escape, leaving homogeneous nano-pores (50-300 nm) in the silk fibroin matrix. The introduction of nano-pores resulted in enhanced permeability and minimized loss of the mechanical properties of the nano-porous silk fibroin films (NSFs) when compared to the un-autoclaving-treated silk fibroin films. NSFs promoted cell (human fibroblasts) proliferation and oxygen/nutrition perfusion and significantly enhanced the complete skin-thickness wound healing in a rat model, suggesting the potential use in tissue regeneration or as wound dressing biomaterials for clinical applications.

Biopolymer Nanofibers for Nanogenerator Development

The development of nanogenerators (NGs) with optimal performances and functionalities requires more novel materials. Over the past decade, biopolymer nanofibers (BPNFs) have become critical sustainable building blocks in energy-related fields because they have distinctive nanostructures and properties and can be obtained from abundant and renewable resources. This review summarizes recent advances in the use of BPNFs for NG development. We will begin by introducing various strategies for fabricating BPNFs with diverse structures and performances. Then, we will systematically present the utilization of polysaccharide and protein nanofibers for NGs. We will mainly focus on the use of BPNFs to generate bulk materials with tailored structures and properties for assembling of triboelectric and piezoelectric NGs. The use of BPNFs to construct NGs for the generation of electricity from moisture and osmosis is also discussed. Finally, we illustrate our personal perspectives on several issues that require special attention with regard to future developments in this active field.

Quaternized Silk Nanofibrils for Electricity Generation from Moisture and Ion Rectification

Protein nanostructures in living organisms have attracted intense interests in biology and material science owing to their intriguing abilities to harness ion transportation for matter/signal transduction and bioelectricity generation. Silk nanofibrils, serving as the fundamental building blocks for silk, not only have the advantages of natural abundance, low cost, biocompatibility, sustainability, and degradability but also play a key role in mechanical toughness and biological functions of silk fibers. Herein, cationic silk nanofibrils (SilkNFs), with an ultrathin thickness of ∼4 nm and a high aspect ratio up to 500, were successfully exfoliated from natural cocoon fibers via quaternization followed by mechanical homogenization. Being positively charged in a wide pH range of 2-12, these cationic SilkNFs could combine with different types of negatively charged biological nanofibrils to produce asymmetric ionic membranes and aerogels that have the ability to tune ion translocation. The asymmetric ionic aerogels could create an electric potential as high as 120 mV in humid ambient air, whereas asymmetric ionic membranes could be used in ionic rectification with a rectification ratio of 5.2. Therefore, this green exfoliation of cationic SilkNFs may provide a biological platform of nanomaterials for applications as diverse as ion electronics, renewable energy, and sustainable nanotechnology.

A Transparent, Skin-Inspired Composite Film with Outstanding Tear Resistance Based on Flat Silk Cocoon

Flexible and transparent substrates play a fundamental role as a mechanical support in advanced electronic devices. However, commonly used polymer films, such as polydimethylsiloxane, show low tear resistance because of their crack sensitivity. Herein, inspired by the excellent mechanical robustness of the skin and its fibrous structure, an epoxy-resin-based composite with a flat silk cocoon as a reinforcing fiber network is fabricated. With only 1 wt% of silk fiber, the tensile strength and modulus of the as-prepared composite film are considerably increased by 300% and 612% compared to those of pure resin, while still maintaining flexibility and transparency. More importantly, the composite shows remarkable tear resistance: without fracture after ≈30 000 tensile cycles. The potential application of such transparent composite films as mechanically robust substrates for flexible electronics is also demonstrated. In addition, this study represents a bioinspired strategy to construct high-performance functional composite materials.

Light, Strong, and Ductile Architectures Achieved by Silk Fiber "Welding" Processing

Light, strong, and ductile materials (LSDMs) are desired in many emerging fields, such as biomedicine, aerospace industries, and structural engineering materials. However, producing such materials remains a significant challenge because their structures cannot confer the desired mechanical properties. In this study, we developed a silk fiber "welding" strategy to construct bioinspired LSDMs. The key to the welding process is to etch the surface of silk fiber through a partial dissolution process. The dissolved silk proteins further serve as welding materials or glues to bond the silk fibers together. Remarkably, these silk-LSDMs are not only lightweight (with the densities of around 0.28 g cm^-3) but also strong and tough. Their compression strength reaches up to 13.8 ± 3.4 MPa, which is higher than those of most natural and engineered porous materials. These favorable structural and mechanical characteristics, together with outstanding biocompatibility of silk proteins, render these silk-LSDMs applicable in regenerated engineered tissues and water treatment materials.

Interplay between Light and Functionalized Silk Fibroin and Applications

Silkworm silk has been considered to be a luxurious textile for more than five thousand years. Native silk fibroin (SF) films have excellent (ca. 90%) optical transparency and exhibit fluorescence under UV light. The silk dyeing process is very important and difficult, and methods such as pigmentary coloration and structural coloration have been tested for coloring silk fabrics. To functionalize silk that exhibits fluorescence, the in vivo and in vitro assembly of functional compounds with SF and the resulting amplification of fluorescence emission are examined. Finally, we discuss the applications of SF materials in basic optical elements, light energy conversion devices, photochemical reactions, sensing, and imaging. This review is expected to provide insight into the interaction between light and silk and to inspire researchers to develop silk materials with a consideration of history, material properties, and future prospects.

Biopolymeric photonic structures: design, fabrication, and emerging applications

Biological photonic structures can precisely control light propagation, scattering, and emission via hierarchical structures and diverse chemistry, enabling biophotonic applications for transparency, camouflaging, protection, mimicking and signaling. Corresponding natural polymers are promising building blocks for constructing synthetic multifunctional photonic structures owing to their renewability, biocompatibility, mechanical robustness, ambient processing conditions, and diverse surface chemistry. In this review, we provide a summary of the light phenomena in biophotonic structures found in nature, the selection of corresponding biopolymers for synthetic photonic structures, the fabrication strategies for flexible photonics, and corresponding emerging photonic-related applications. We introduce various photonic structures, including multi-layered, opal, and chiral structures, as well as photonic networks in contrast to traditionally considered light absorption and structural photonics. Next, we summarize the bottom-up and top-down fabrication approaches and physical properties of organized biopolymers and highlight the advantages of biopolymers as building blocks for realizing unique bioenabled photonic structures. Furthermore, we consider the integration of synthetic optically active nanocomponents into organized hierarchical biopolymer frameworks for added optical functionalities, such as enhanced iridescence and chiral photoluminescence. Finally, we present an outlook on current trends in biophotonic materials design and fabrication, including current issues, critical needs, as well as promising emerging photonic applications.

Engineering Silk Materials: From Natural Spinning to Artificial Processing

Silks spun by the arthropods are "ancient' materials historically utilized for fabricating high-quality textiles. Silks are natural protein-based biomaterials with unique physical and biological properties, including particularly outstanding mechanical properties and biocompatibility. Current goals to produce artificially engineered silks to enable additional applications in biomedical engineering, consumer products, and device fields, have prompted considerable effort towards new silk processing methods using bio-inspired spinning and advanced biopolymer processing. These advances have redefined silk as a promising biomaterial past traditional textile applications and into tissue engineering, drug delivery, and biodegradable medical devices. In this review, we highlight recent progress in understanding natural silk spinning systems, as well as advanced technologies used for processing and engineering silk into a broad range of new functional materials.

Superomniphobic Silk Fibroin/Ag Nanowires Membrane for Flexible and Transparent Electronic Sensor

Superwetting surfaces that repel various liquids have been exciting for biomimetic research and have displayed versatile potential applications. Generally, superhydrophobic coatings can allow for droplet rolling off and antifouling, whereas it is a challenge to achieve superomniphobic surfaces with transparency, flexibility, and conductivity. Here, we adopt an effective and simple method to fabricate a superomniphobic, transparent, and flexible smart silk fibroin (SF) membrane by spray-coating long AgNWs dispersed in polydimethylsiloxane (PDMS), followed by treatment with vacuum drying. The resulting SF/AgNWs membranes are super-repellent to different liquids with low surface tension and water, and demonstrate high contact angles (CAs) more than 150° and low rolling-off angles (RAs) even less than 10°. Moreover, the obtained membranes display superior sensitivity under stretching and bending, as well as intact stability of high transparency, which can be considered as promising flexible sensing electronics to detect human motions under wet conditions.