Amorphous Silk Nanofiber Solutions for Fabricating Silk-Based Functional Materials

Nano-fibrous biopolymers as building blocks for gel networks: Interactions, characterization, and applications

Biopolymers derived from natural resources are highly abundant, biodegradable, and biocompatible, making them promising candidates to replace non-renewable fossil fuels and mitigate environmental and health impacts. Nano-fibrous biopolymers possessing advantages of biopolymers entangle with each other through inter-/intra-molecular interactions, serving as ideal building blocks for gel construction. These biopolymer nanofibers often synergize with other nano-building blocks to enhance gels with desirable functions and eco-friendliness across various applications in biomedical, environmental, and energy sectors. The inter-/intra-molecular interactions directly affect the assembly of nano-building blocks, which determines the structure of gels, and the integrity of connected nano-building blocks, influencing the mechanical properties and the performance of gels in specific applications. This review focuses on four biopolymer nanofibers (cellulose, chitin, silk, collagen), commonly used in gel preparations, as representatives for polysaccharides and polypeptides. The covalent and non-covalent interactions between biopolymers and other materials have been categorized and discussed in relation to the resulting gel network structures and properties. Nanomechanical characterization techniques, such as surface forces apparatus (SFA) and atomic force microscopy (AFM), have been employed to precisely quantify the intermolecular interactions between biopolymers and other building blocks. The applications of these gels are classified and correlated to the functions of their building blocks. The inter-/intra-molecular interactions act as "sewing threads", connecting all nano-building blocks to establish suitable network structures and functions. This review aims to provide a comprehensive understanding of the interactions involved in gel preparation and the design principles needed to achieve targeted functional gels.

Bioactive Silk Cryogel Dressing with Multiple Physical Cues to Control Cell Migration and Wound Regeneration

Introducing multiple physical cues to control cell behaviors effectively is considered as a promising strategy in developing bioactive wound dressings. Silk nanofiber-based cryogels are developed to favor angiogenesis and tissue regeneration through tuning hydrated state, microporous structure, and mechanical property, but remained a challenge to endow with more physical cues. Here, β-sheet rich silk nanofibers are used to develop cryogels with nanopore structure. Through optimizing crosslinking time and exposing the reactive group inside the nanofibers, the crosslinking reaction is improved to induce stable cryogel formation. Besides the hydrated state and macroporous structure, the nanopore structure formed on the macroporous walls, providing hierarchical microstructures to improve cell migration. Both in vitro and in vivo results reveal quicker cell migration inside the cryogels, which then accelerates angiogenesis and wound healing. The mechanical properties can further regulate to match with skin regeneration. The wound healing study in vivo reveals lower inflammatory factor secretion in the wounds treated with softer cryogels with nanopores, which then resulted in the best angiogenesis and wound healing with less scar. Therefore, the porous cryogels with multiple physical cues can be fabricated with silk nanofibers to control cell behaviors and tissue regeneration, providing a promising approach for designing bioactive wound dressings.

Structural and mechanical comparison of Eri and Mulberry silk fibroin nanofibers films through advanced mechanical treatments for sustainable applications

Mulberry silk (Bombyx mori) and eri silk (Samia/Philosamia ricini) are widely used silks. Eri silk is a wild silk that contains an arginine-glycine-aspartic acid tripeptide sequence within its structure, making it a potential and sustainable biomaterial. However, its poor solubility using conventional methods has resulted in limited research compared with that of mulberry silk fibroin. This study investigated the differences between nanofibrillated fibroins from both silks to assess their biomedical potential, focusing on their structural, morphological, and mechanical properties, as well as their biodegradability. Both silks were degummed and processed into fibroin microfibers (FMF) and fibroin nanofibers (FNF) via high-pressure ultrasonication and grinding in an aqueous solution. Analyses were performed using fourier transform infrared spectroscopy (FT-IR), X-ray diffractometry (XRD), atomic force microscopy (AFM), scanning electron microscope (SEM), UV-visible spectrometry, and thermogravimetric analysis (TGA) techniques. The results showed that both silks were prepared by high-pressure ultrasonication and exhibited microfibers when treated with a grinder machine, which can produce fibroin nanofibers. In terms of the type of fibroin, when comparing the properties of both silks, it was found that they have similar chemical structures but differ in their physical properties. Moreover, eri fibroin films showed superior transparency, high thermal resistance, and high strength, but were more brittle than mulberry fibroin films, which was attributed to amino acid differences. Eri silk has unique features that are interesting for the development of new materials and can create new pathways for utilizing non-mulberry silk.

Preparation of carboxylated-silk nanofibers by the one-pot method of maleic acid hydrolysis

In this study, a maleic acid (MA) hydrolysis one-pot method is proposed to prepare carboxylated silk nanofibers (MA-SNFs). The MA concentration, hydrolysis temperature, and processing time were optimized. Combined with high-pressure homogenization, MA-SNFs with a carboxyl content of 0.617±0.019 mmol/g and length of 333±116 nm were obtained with a yield of 54.90±1.98 % at the optimal conditions: 50 wt% of MA concentration, 110 °C of hydrolysis temperature and 120 min of processing time. The morphology, chemical structure, and crystal structure of raw silk fibroin (SF), acid-hydrolyzed silk fibroin and nanofibers were studied. Furthermore, MA was reused several times with a recovery rate of >94 % and maintained almost the same treatment effect. Finally, due to the presence of carboxyl groups, MA-SNF hydrogels were successfully prepared by an acetic coagulation vapor bath which exhibited excellent self-supporting ability and mechanical properties as well as a more sensitive pH response compared with regenerated silk fibroin solution and other non-carboxylated silk nanofibers. The MA-SNF aerogels had the characteristics of light weight, high strength and porous with cross-linked nanostructures.

Enhanced specific surface area and mechanical property of silk nanofibers aerogel for potential hemostasis applications

Biopolymer aerogel is a new type of material with potential applications in the biomedical field. Silk fibroin is of particular interest as a raw material with good biocompatibility and degradable. However, the low mechanical strength and small specific surface area of silk fibroin aerogels limit its further development. Herein, a fast water absorption, highly specific surface area and mechanically strong of aerogels were prepared using low crystal silk fibroin nanofibers (SNF), sol-gel process, solvent exchange and supercritical carbon dioxide (CO2) drying method. The resulting Aero-Sc displayed highly specific surface area (251 m2/g), porosity (97.6 %) and water absorption capacity (1200 %). Furthermore, with rapid water absorption and stronger erythrocyte adhesion, the Aero-Sc showed highly effective hemostasis in vitro. In vivo, animal experiments on rat liver hemorrhage model confirmed that SNF aerogels have a less blood loss (312 ± 29 mg) and faster hemostatic time (92 ± 13 s) than commercially gelatin sponge (p < 0.05). The unique properties of silk fibroin nanofibers aerogel developed in this study has great potential to be a safe and effective hemostatic medical device.

Natural Deep Eutectic Solvent-Assisted Construction of Silk Nanofibrils/Boron Nitride Nanosheets Membranes with Enhanced Heat-Dissipating Efficiency

Natural polymer-derived nanofibrils have gained significant interest in diverse fields. However, production of bio-nanofibrils with the hierarchical structures such as fibrillar structures and crystalline features remains a great challenge. Herein, an all-natural strategy for simple, green, and scalable top-down exfoliation silk nanofibrils (SNFs) in novel renewable deep eutectic solvent (DES) composed by amino acids and D-sorbitol is innovatively developed. The DES-exfoliated SNFs with a controllable fibrillar structures and intact crystalline features, novelty preserving the hierarchical structure of natural silk fibers. Owing to the amphiphilic nature, the DES-exfoliated SNFs show excellent capacity of assisting the exfoliation of several 2D-layered materials, i.e., h-BN, MoS2, and WS2. More importantly, the SNFs-assisted dispersion of BNNSs with a concentration of 59.3% can be employed to construct SNFs/BNNSs nanocomposite membranes with excellent mechanical properties (tensile strength of 416.7 MPa, tensile modulus of 3.86 GPa and toughness of 1295.4 KJ·m-3) and thermal conductivity (in-plane thermal conductivity coefficient of 3.84 W·m-1·K-1), enabling it to possess superior cooling efficiency compared with the commercial silicone pad.

Silk Nanofiber Scaffolds with Multiple Angiogenic Cues to Accelerate Wound Regeneration

Niches with multiple physical and chemical cues can influence the fate of cells and tissues in vivo. Simulating the in vivo niche in the design of bioactive materials is a challenge, particularly to tune multiple cues simultaneously in the same system. Here, an assembly strategy was developed to regulate multiple cues in the same scaffold based on the use of two silk nanofiber components that respond differently during the fabrication processes. An aqueous solution containing the two components, amorphous silk nanofibers (ASNFs) and β-sheet-rich silk nanofibers (BSNFs), was sequentially treated with an electrical field and freeze-drying processes where the BSNFs oriented to the electrical field, while the ASNFs formed stable porous structures during the lyophilization process to impact the mechanical properties. Bioactive cargo, such as deferoxamine (DFO), was loaded on the BSNFs to enrich cell responses with the scaffolds. The in vitro results revealed that the loaded DFO and the anisotropic structures with improved mechanical properties resulted in better vascularization than those of the scaffolds without the anisotropic features. The multiple cues in the scaffolds provided angiogenic niches to accelerate wound healing.

Nanofibrous scaffolds for the healing of the fibrocartilaginous enthesis: advances and prospects

With the current developmental advancements in nanotechnology, nanofibrous scaffolds are being widely used. The healing of fibrocartilaginous enthesis is a slow and complex process, and while existing treatments have a certain effect on promoting their healing, these are associated with some limitations. The nanofibrous scaffold has the advantages of easy preparation, wide source of raw materials, easy adjustment, easy modification, can mimic the natural structure and morphology of the fibrocartilaginous enthesis, and has good biocompatibility, which can compensate for existing treatments and be combined with them to promote the repair of fibrocartilaginous enthesis. The nanofibrous scaffold can promote the healing of fibrocartilaginous enthesis by controlling the morphology and ensuring controlled drug release. Hence, the use of nanofibrous scaffold with stimulative response features in the musculoskeletal system has led us to imagine its potential application in fibrocartilaginous enthesis. Therefore, the healing of fibrocartilaginous enthesis based on a nanofibrous scaffold may be a novel therapeutic approach.

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.

A brief review on the mechanisms and approaches of silk spinning-inspired biofabrication

Silk spinning, observed in spiders and insects, exhibits a remarkable biological source of inspiration for advanced polymer fabrications. Because of the systems design, silk spinning represents a holistic and circular approach to sustainable polymer fabrication, characterized by renewable resources, ambient and aqueous processing conditions, and fully recyclable "wastes." Also, silk spinning results in structures that are characterized by the combination of monolithic proteinaceous composition and mechanical strength, as well as demonstrate tunable degradation profiles and minimal immunogenicity, thus making it a viable alternative to most synthetic polymers for the development of advanced biomedical devices. However, the fundamental mechanisms of silk spinning remain incompletely understood, thus impeding the efforts to harness the advantageous properties of silk spinning. Here, we present a concise and timely review of several essential features of silk spinning, including the molecular designs of silk proteins and the solvent cues along the spinning apparatus. The solvent cues, including salt ions, pH, and water content, are suggested to direct the hierarchical assembly of silk proteins and thus play a central role in silk spinning. We also discuss several hypotheses on the roles of solvent cues to provide a relatively comprehensive analysis and to identify the current knowledge gap. We then review the state-of-the-art bioinspired fabrications with silk proteins, including fiber spinning and additive approaches/three-dimensional (3D) printing. An emphasis throughout the article is placed on the universal characteristics of silk spinning developed through millions of years of individual evolution pathways in spiders and silkworms. This review serves as a stepping stone for future research endeavors, facilitating the in vitro recapitulation of silk spinning and advancing the field of bioinspired polymer fabrication.

Advances in Preparation and Properties of Regenerated Silk Fibroin

Over the years, silk fibroin (SF) has gained significant attention in various fields, such as biomedicine, tissue engineering, food processing, photochemistry, and biosensing, owing to its remarkable biocompatibility, machinability, and chemical modifiability. The process of obtaining regenerated silk fibroin (RSF) involves degumming, dissolving, dialysis, and centrifugation. RSF can be further fabricated into films, sponges, microspheres, gels, nanofibers, and other forms. It is now understood that the dissolution method selected greatly impacts the molecular weight distribution and structure of RSF, consequently influencing its subsequent processing and application. This study comprehensively explores and summarizes different dissolution methods of SF while examining their effects on the structure and performance of RSF. The findings presented herein aim to provide valuable insights and references for researchers and practitioners interested in utilizing RSF in diverse fields.

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.

Regulable preparation of silk fibroin composite cryogel by dual-directional crosslink for achieving self-cleaning, superelasticity and multifunctional water purification

Recently, the cryogel as a special type of hydrogel was widely used in the field of medicine due to its porous structure and good biocompatibilit. However, great challenges existed for its irregular pore size and incompressible property, limiting its application in other fields. In this study, a novel silk fibroin-based cryogel (named SF@PVA/CS) with regulable pore size, excellent elasticity and durability was constructed using a green dual-directional crosslink strategy. The SF@PVA/CS was prepared by using silk fibroin (SF) as bone scaffold, and chitosan (CS) and polyvinyl alcohol (PVA) as polymer hydrogel which was introduced into the inner bone scaffold of SF. Such a brand-new cryogel possessed three-dimensional dual network structure, which can overcome the shortcoming of unregulatable pore size and incompressibility of traditional cryogel. Additionally, the developed SF@PVA/CS membrane was used for water purification for the first time, which exhibited superior selective permeation, excellent anti-fouling and brilliant self-cleaning property, and it can achieve the purification of both oil/water emulsion and methylene blue solution. This study expanded the application of SF-based cryogel, providing a novel routine for designing new-type composite cryogel and widening the application of dual-directional crosslink strategy developed in this study for facilitating the purification of wastewater.

Tough Porous Silk Nanofiber-Derived Cryogels with Osteogenic and Angiogenic Capacity for Bone Repair

Tough porous cryogels with angiogenesis and osteogenesis features remain a design challenge for utility in bone regeneration. Here, building off of the recent efforts to generate tough silk nanofiber-derived cryogels with osteogenic activity, deferoxamine (DFO) is loaded in silk nanofiber-derived cryogels to introduce angiogenic capacity. Both the mechanical cues (stiffness) and the sustained release of DFO from the gels are controlled by tuning the concentration of silk nanofibers in the system, achieving a modulus above 400 kPa and slow release of the DFO over 60 days. The modulus of the cryogels and the released DFO induce osteogenic and angiogenic activity, which facilitates bone regeneration in vivo in femur defects in rat, resulting in faster regeneration of vascularized bone tissue. The tunable physical and chemical cues derived from these nanofibrous-microporous structures support the potential for silk cryogels in bone tissue regeneration.

Advanced strategies for constructing interfacial tissues of bone and tendon/ligament

Enthesis, the interfacial tissue between a tendon/ligament and bone, exhibits a complex histological transition from soft to hard tissue, which significantly complicates its repair and regeneration after injury. Because traditional surgical treatments for enthesis injury are not satisfactory, tissue engineering has emerged as a strategy for improving treatment success. Rapid advances in enthesis tissue engineering have led to the development of several strategies for promoting enthesis tissue regeneration, including biological scaffolds, cells, growth factors, and biophysical modulation. In this review, we discuss recent advances in enthesis tissue engineering, particularly the use of biological scaffolds, as well as perspectives on the future directions in enthesis tissue engineering.

High Concentration Crystalline Silk Fibroin Solution for Silk-Based Materials

As a functional biomaterial, silk fibroin has been widely used in drug release, cell encapsulation and tissue regeneration. To meet the requirements of these applications, the properties of silk fibroin-based materials should be finely tunable. Many useful properties of biomaterials emerge from the collective interactions among ordered and disordered domains. Thus, increasing subtle control of silk hierarchical structures is required. As a characteristic of ordered silk fibroin, crystalline silk fibroin (CSF) is an important part of silk fibroin-based biomaterials, but the preparation of CSF solution, especially high concentration CSF solution, remains a challenge. Here, a solution composed of β-sheet-rich silk fibroin is reported. These CSF were obtained by the sonication of silk fibroin hydrogel, destroying the hydrogel network, and turning silk fibroin hydrogels into CSF solution. These β-sheet-rich CSF solutions were stable enough for several days or even weeks. In addition, they were typically ordered crystalline domains, which could be mixed with disordered domains and fabricated into porous scaffolds, films, hydrogels and other silk fibroin-based scaffolds with different properties.

Graphene oxide bulk material reinforced by heterophase platelets with multiscale interface crosslinking

Graphene oxide (GO) and reduced GO possess robust mechanical, electrical and chemical properties. Their nanocomposites have been extensively explored for applications in diverse fields. However, due to the high flexibility and weak interlayer interactions of GO nanosheets, the flexural mechanical properties of GO-based composites, especially in bulk materials, are largely constrained, which hinders their performance in practical applications. Here, inspired by the amorphous/crystalline feature of the heterophase within nacreous platelets, we present a centimetre-sized, GO-based bulk material consisting of building blocks of GO and amorphous/crystalline leaf-like MnO₂ hexagon nanosheets adhered together with polymer-based crosslinkers. These building blocks are stacked and hot-pressed with further crosslinking between the layers to form a GO/MnO₂-based layered (GML) bulk material. The resultant GML bulk material exhibits a flexural strength of 231.2 MPa. Moreover, the material exhibits sufficient fracture toughness and strong impact resistance while being light in weight. Experimental and numerical analyses indicate that the ordered heterophase structure and synergetic crosslinking interactions across multiscale interfaces lead to the superior mechanical properties of the material. These results are expected to provide insights into the design of structural materials and potential applications of high-performance GO-based bulk materials in aerospace, biomedicine and electronics.

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.

Engineered Tough Silk Hydrogels through Assembling β-Sheet Rich Nanofibers Based on a Solvent Replacement Strategy

β-Sheet rich silk nanofiber hydrogels are suitable scaffolds in tissue regeneration and carriers for various drugs. However, unsatisfactory mechanical performance limits its applications. Here, insight into the silk nanofibers stimulates the remodeling of previous solvent systems to actively regulate the assembly of silk nanofibers. Formic acid, a solvent of regenerated silk fibroin, is used to shield the charge repulsion of silk nanofibers to facilitate the nanofiber assembly under concentrated solutions. Formic acid was replaced with water to solidify the assembly, which induced the formation of a tough hydrogel. The hydrogels generated with this process possessed a modulus of 5.88 ± 0.82 MPa, ultimate stress of 1.55 ± 0.06 MPa, and toughness of 0.85 ± 0.03 MJ m^-3, superior to those of previous silk hydrogels prepared through complex cross-linking processes. Benefiting from the dense gel network and high β-sheet content, these silk nanofiber hydrogels had good stability and antiswelling ability. The modulus could be modulated via changing the silk nanofiber concentration to provide differentiation signals to stem cells. Improved mechanical and bioactive properties with these hydrogels suggest utility in biomedical and engineering fields. More importantly, our present study reveals that the in-depth understanding of silk nanofibers could infuse power into traditional fabrication systems to achieve more high performance biomaterials, which is seldom considered in silk material studies.

Intraarticularly injectable silk hydrogel microspheres with enhanced mechanical and structural stability to attenuate osteoarthritis

A silk fibroin (silk) hydrogel was prepared by using diglycidyl ether (BDDE), a chemical crosslinker commonly used to generate Food and Drug Administration (FDA)-approved hyaluronic acid (HA) medical products. The silk/BDDE hydrogels exhibited high elasticity (compressive modulus of 166 ± 15.0 kPa), anti-fatigue properties, and stable structure and mechanical strength in aqueous solution. Chemical crosslinking was conducted in a high concentration (9.3 M) of lithium bromide (LiBr) solution, a salt that is commonly used to dissolve degummed silk fibers during silk solubilization. The unfolded and extended structure of silk molecules with these reaction conditions, as well as the unique ionic environment provided by LiBr facilitated a high degree of crosslinking in the hydrogel. Similar hydrogels were not obtained when the silk was dissolved in other silk fiber-dissolving reagents (e.g., Ajisawa's, formic acid (FA)/LiBr, FA/CaCl2 solutions), likely because partially folded silk structures and the ionic conditions with these reagents were less favorable for the crosslinking reaction. Based on these findings, silk/BDDE hydrogel spheres were prepared using an oil/water (o/w) emulsification method and biocompatibility and biodegradation were evaluated in vivo, along with other silk gel control systems (e.g., enzyme-catalyzed di-tyrosine and pulverized silk/BDDE gel particles with irregular shapes). Histological and immunohistochemical analyses demonstrated that the silk/BDDE hydrogel spheres were biocompatible and served as a bio-lubricant to treat osteoarthritis (OA). The intra-articular injection of the gel spheres reduced pain as measured with OA rats, reduced cartilage damage and resisted the digestive environment in the articular cavity for extended time frames (>4 weeks), suggesting utility for pain relief and sustained drug release for future OA treatments.

Macroporous Silk Nanofiber Cryogels with Tunable Properties

Cryogels are widely used in tissue regeneration due to their porous structures and friendly hydrogel performance. Silk-based cryogels were developed but failed to exhibit desirable tunable properties to adapt various biomedical applications. Here, amorphous short silk nanofibers (SSFs) were introduced to fabricate silk cryogels with versatile cues. Compared to previous silk cryogels, the SSF cryogels prepared under same conditions showed significantly enhanced mechanical properties. The microporous cryogels were achieved under lower silk concentrations, confirming better tunability. Versatile cryogels with the modulus in the range of 0.5-283.7 kPa were developed through adjusting silk concentration and crosslinking conditions, superior to previous silk cryogel systems. Besides better cytocompatibility, the SSF cryogels were endowed with effective mechanical cues to control osteogenetic differentiation behaviors of BMSCs. The mechanical properties could be further regulated finely through the introduction of β-sheet-rich silk nanofibers (SNFs), which suggested possible optimization of mechanical niches. Bioactive cargo-laden SNFs were introduced to the SSF cryogel systems, bringing biochemical signals without the compromise of mechanical properties. Versatile SNF-based cryogels with different physical and biological cues were developed here to facilitate the applications in various tissue engineering.

Electrospun Silk Fibroin Scaffolds for Tissue Regeneration: Chemical, Structural, and Toxicological Implications of the Formic Acid-Silk Fibroin Interaction

The dissolution of Bombyx mori silk fibroin (SF) films in formic acid (FA) for the preparation of electrospinning dopes is widely exploited to produce electrospun SF scaffolds. The SILKBridge^® nerve conduit is an example of medical device having in its wall structure an electrospun component produced from an FA spinning dope. Though highly volatile, residual FA remains trapped into the bulk of the SF nanofibers. The purpose of this work is to investigate the type and strength of the interaction between FA and SF in electrospun mats, to quantify its amount and to evaluate its possible toxicological impact on human health. The presence of residual FA in SF mats was detected by FTIR and Raman spectroscopy (new carbonyl peak at about 1,725 cm⁻¹) and by solid state NMR, which revealed a new carbonyl signal at about 164.3 ppm, attributed to FA by isotopic ¹³C substitution. Changes occurred also in the spectral ranges of hydroxylated amino acids (Ser and Thr), demonstrating that FA interacted with SF by forming formyl esters. The total amount of FA was determined by HS-GC/MS analysis and accounted for 247 ± 20 μmol/g. The greatest part was present as formyl ester, a small part (about 3%) as free FA. Approximately 17% of the 1,500 μmol/g of hydroxy amino acids (Ser and Thr) theoretically available were involved in the formation of formyl esters. Treatment with alkali (Na₂CO₃) succeeded to remove the greatest part of FA, but not all. Alkali-treated electrospun SF mats underwent morphological, physical, and mechanical changes. The average diameter of the fibers increased from about 440 nm to about 480 nm, the mat shrunk, became stiffer (the modulus increased from about 5.5 MPa to about 7 MPa), and lost elasticity (the strain decreased from about 1 mm/mm to about 0.8 mm/mm). Biocompatibility studies with human adult dermal fibroblasts did not show significant difference in cell proliferation (313 ± 18 and 309 ± 23 cells/mm² for untreated and alkali-treated SF mat, respectively) and metabolic activity. An in-depth evaluation of the possible toxicological impact of residual FA was made using the SILKBridge^® nerve conduit as case study, following the provisions of the ISO 10993-1 standard. The Potential Patient Daily Intake, calculated from the total amount of FA determined by HS-GC/MS, was 2.4 mg/day and the Tolerable Exposure level was set to 35.4 mg/day. This allowed to obtain a value of the Margin of Safety of 15, indicating that the amount of FA left on SF mats after electrospinning does not raise concerns for human health.

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.

Structural and Morphological Properties of Wool Keratin and Cellulose Biocomposites Fabricated Using Ionic Liquids

In this study, the structural, thermal, and morphological properties of biocomposite films composed of wool keratin mixed with cellulose and regenerated with ionic liquids and various coagulation agents were characterized and explored. These blended films exhibit different physical and thermal properties based on the polymer ratio and coagulation agent type in the fabrication process. Thus, understanding their structure and molecular interaction will enable an understanding of how the crystallinity of cellulose can be modified in order to understand the formation of protein secondary structures. The thermal, morphological, and physiochemical properties of the biocomposites were investigated by Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), and X-ray scattering. Analysis of the results suggests that both the wool keratin and the cellulose structures can be manipulated during dissolution and regeneration. Specifically, the β-sheet content in wool keratin increases with the increase of the ethanol solution concentration during the coagulation process; likewise, the cellulose crystallinity increases with the increase of the hydrogen peroxide concentration via coagulation. These findings suggest that the different molecular interactions in a biocomposite can be tuned systematically. This can lead to developments in biomaterial research including advances in natural based electrolyte batteries, as well as implantable bionics for medical research.

Fragile-Tough Mechanical Reversion of Silk Materials via Tuning Supramolecular Assembly

Regenerated silk nanofibers are interesting as protein-based material building blocks due to their unique structure and biological origin. Here, a new strategy based on control of supramolecular assembly was developed to regulate interactions among silk nanofibers by changing the solvent, achieving tough mechanical features for silk films. Formic acid was used to replace water related to charge repulsion of silk nanofibers in solution, inducing interactions among the nanofibers. The films formed under these conditions had an elastic modulus of 3.4 ± 0.3 GPa, an ultimate tensile strength of 76.9 ± 1.6 MPa, and an elongation at break of 3.5 ± 0.1%, while the materials formed from aqueous solutions remained fragile. The mechanical performance of the formic acid-derived nanofiber films was further improved through post-stretching or via the addition of graphene. In addition, the silk nanofiber films could be functionalized with various bioactive ingredients such as curcumin. These new silk nanofiber films with a unique combination of mechanical properties and functions provide new biomaterials achieved using traditional solvents and processes through insight and control of their assembly mechanisms in solution.

Pressure-driven spreadable deferoxamine-laden hydrogels for vascularized skin flaps

The development of hydrogels that support vascularization to improve the survival of skin flaps, yet establishing homogeneous angiogenic niches without compromising the ease of use in surgical settings remains a challenge. Here, pressure-driven spreadable hydrogels were developed utilizing beta-sheet rich silk nanofiber materials. These silk nanofiber-based hydrogels exhibited excellent spreading under mild pressure to form a thin coating to cover all the regions of the skin flaps. Deferoxamine (DFO) was loaded onto the silk nanofibers to support vascularization and these DFO-laden hydrogels were implanted under skin flaps in rats to fill the interface between the wound bed and the flap using the applied pressure. The thickness of the spread hydrogels was below 200 μm, minimizing the physical barrier effects from the hydrogels. The distribution of the hydrogels provided homogeneous angiogenic stimulation, accelerating rapid blood vessel network formation and significantly improving the survival of the skin flaps. The hydrogels also modulated the immune reactions, further facilitating the regeneration of the skin flaps. Considering the homogeneous distribution at the wound sites, improved vascularization, reduced barrier effects and low inflammation, these hydrogels appear to be promising candidates for use in tissue repair where a high blood supply is in demand. The pressure-driven spreading properties should simplify the use of the hydrogels in surgical settings to facilitate clinical translation.

Injectable Desferrioxamine-Laden Silk Nanofiber Hydrogels for Accelerating Diabetic Wound Healing

Dysangiogenesis and chronic inflammation are two critical reasons for diabetic foot ulcers. Desferrioxamine (DFO) was used clinically in the treatment of diabetic foot ulcers by repeated injections because of its capacity to induce vascularization. Biocompatible carriers that release DFO slowly and facilitate healing simultaneously are preferable options to accelerate the healing of diabetic wounds. Here, DFO-laden silk nanofiber hydrogels that provided a sustained release of DFO for more than 40 days were used to treat diabetic wounds. The DFO-laden hydrogels stimulated the healing of diabetic wounds. In vitro cell studies revealed that the DFO-laden hydrogels modulated the migration and gene expression of endothelial cells, and they also tuned the inflammation behavior of macrophages. These results were confirmed in an in vivo diabetic wound model. The DFO-laden hydrogels alleviated dysangiogenesis and chronic inflammation in the diabetic wounds, resulting in a more rapid wound healing and increased collagen deposition. Both in vitro and in vivo studies suggested potential clinical applications of these DFO-laden hydrogels in the treatment of diabetic ulcers.

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.

Gradient Biomineralized Silk Fibroin Nanofibrous Scaffold with Osteochondral Inductivity for Integration of Tendon to Bone

Enthesis injury repair remains a huge challenge because of the unique biomolecular composition, microstructure, and mechanics in the interfacial region. Surgical reconstruction often creates new bone-scaffold interfaces with mismatched properties, resulting in poor osseointegration. To mimic the natural interface tissue structures and properties, we fabricated a nanofibrous scaffold with gradient mineral coating based on 10 × simulated body fluid (SBF) and silk fibroin (SF). We then characterized the physicochemical properties of the scaffold and evaluated its biological functions both in vitro and in vivo. The results showed that different areas of SF nanofibrous scaffold had varying levels of mineralization with disparate mechanical properties and had different effects on bone marrow mesenchymal stem cell growth and differentiation. Furthermore, the gradient scaffolds exhibited an enhancement of integration in the tendon-to-bone interface with a higher ultimate load and more fibrocartilage-like tissue formation. These findings demonstrate that the silk-based nanofibrous scaffold with gradient mineral coating can regulate the formation of interfacial tissue and has the potential to be applied in interface tissue engineering.

Microskin-Inspired Injectable MSC-Laden Hydrogels for Scarless Wound Healing with Hair Follicles

Scarless skin regeneration with functional tissue remains a challenge for full-thickness wounds. Here, mesenchymal stem cell (MSC)-laden hydrogels are developed for scarless wound healing with hair follicles. Microgels composed of aligned silk nanofibers are used to load MSCs to modulate the paracrine. MSC-laden microgels are dispersed into injectable silk nanofiber hydrogels, forming composites biomaterials containing the cells. The injectable hydrogels protect and stabilize the MSCs in the wounds. The synergistic action of silk-based composite hydrogels and MSCs stimulated angiogenesis and M1-M2 phenotype switching of macrophages, provides a suitable niche for functional recovery of wounds. Compared to skin defects treated with MSC-free hydrogels, the defects treated with the MSC-laden composite hydrogels heal faster and form scarless tissues with hair follicles. Wound healing can be further improved by adjusting the ratio of silk nanofibers and particles and the loaded MSCs, suggesting tunability of the system. To the best of current knowledge, this is the first time scarless skin regeneration with hair follicles based on silk material systems is reported. The improved wound healing capacity of the systems suggests future in vivo studies to compare to other biomaterial systems related to clinical goals in skin regeneration in the absence of scarring.

Electric field-driven building blocks for introducing multiple gradients to hydrogels

Gradient biomaterials are considered as preferable matrices for tissue engineering due to better simulation of native tissues. The introduction of gradient cues usually needs special equipment and complex process but is only effective to limited biomaterials. Incorporation of multiple gradients in the hydrogels remains challenges. Here, beta-sheet rich silk nanofibers (BSNF) were used as building blocks to introduce multiple gradients into different hydrogel systems through the joint action of crosslinking and electric field. The blocks migrated to the anode along the electric field and gradually stagnated due to the solution-hydrogel transition of the systems, finally achieving gradient distribution of the blocks in the formed hydrogels. The gradient distribution of the blocks could be tuned easily through changing different factors such as solution viscosity, which resulted in highly tunable gradient of mechanical cues. The blocks were also aligned under the electric field, endowing orientation gradient simultaneously. Different cargos could be loaded on the blocks and form gradient cues through the same crosslinking-electric field strategy. The building blocks could be introduced to various hydrogels such as Gelatin and NIPAM, indicating the universality. Complex niches with multiple gradient cues could be achieved through the strategy. Silk-based hydrogels with suitable mechanical gradients were fabricated to control the osteogenesis and chondrogenesis. Chondrogenic-osteogenic gradient transition was obtained, which stimulated the ectopic osteochondral tissue regeneration in vivo. The versatility and highly controllability of the strategy as well as multifunction of the building blocks reveal the applicability in complex tissue engineering and various interfacial tissues.

Tough Anisotropic Silk Nanofiber Hydrogels with Osteoinductive Capacity

Multiple physical cues such as hierarchical microstructures, topography, and stiffness influence cell fate during tissue regeneration. Yet, introducing multiple physical cues to the same biomaterial remains a challenge. Here, a synergistic cross-linking strategy was developed to fabricate protein hydrogels with multiple physical cues based on combinations of two types of silk nanofibers. β-sheet-rich silk nanofibers (BSNFs) were blended with amorphous silk nanofibers (ASNFs) to form composite nanofiber systems. The composites were transformed into tough hydrogels through horseradish peroxidase (HRP) cross-linking in an electric field, where ASNFs were cross-linked with HRP, while BSNFs were aligned by the electrical field. Anisotropic morphologies and higher stiffness of 120 kPa were achieved. These anisotropic hydrogels induced osteogenic differentiation and the aligned aggregation of stem cells in vitro while also exhibiting osteoinductive capacity in vivo. Improved tissue outcomes with the hydrogels suggest promising applications in bone tissue engineering, as the processing strategy described here provides options to form hydrogels with multiple physical cues.

Natural Fibrous Protein for Advanced Tissue Engineering Applications: Focusing on Silk Fibroin and Keratin

As one of the important branches of natural biopolymer, natural fibrous protein has a lot of advantages including good mechanical properties, excellent biocompatibility, controllable biodegradability, renewability, abundant sources, and so on. Moreover, natural fibrous protein is also a protein that could only be used for structure supporting without any bioactivities, which attracts a lot of attentions in the field of tissue engineering scaffold. This chapter is taking silk fibroin and keratin as model materials of natural fibrous protein, focusing on their protein structure, chemical compositions, processing and extraction methods, chemical modification methods, and their applications in tissue engineering through advanced manufacturing.

Silk Fibroin-Based Biomaterials for Biomedical Applications: A Review

Since it was first discovered, thousands of years ago, silkworm silk has been known to be an abundant biopolymer with a vast range of attractive properties. The utilization of silk fibroin (SF), the main protein of silkworm silk, has not been limited to the textile industry but has been further extended to various high-tech application areas, including biomaterials for drug delivery systems and tissue engineering. The outstanding mechanical properties of SF, including its facile processability, superior biocompatibility, controllable biodegradation, and versatile functionalization have allowed its use for innovative applications. In this review, we describe the structure, composition, general properties, and structure-properties relationship of SF. In addition, the methods used for the fabrication and modification of various materials are briefly addressed. Lastly, recent applications of SF-based materials for small molecule drug delivery, biological drug delivery, gene therapy, wound healing, and bone regeneration are reviewed and our perspectives on future development of these favorable materials are also shared.

Water-insoluble amorphous silk fibroin scaffolds from aqueous solutions

Regenerated silk fibroin (RSF) is emerging as promising biomaterial for regeneration, drug delivery and optical devices, with continued demand for mild, all-aqueous processes to control microstructure and the performance. Here, temperature control of assembly kinetics was introduced to prepare the water-insoluble scaffolds from neutral aqueous solutions of RSF protein. Higher temperatures were used to accelerate the assembly rate of the silk fibroin protein chains in aqueous solution and during the lyophilization process, resulting in water-insoluble scaffold formation. The scaffolds were mainly composed of amorphous states of the silk fibroin chains, endowing softer mechanical properties. These scaffolds also showed nanofibrous structures, improved cell proliferation in vitro and enhanced neovascularization and tissue regeneration in vivo than previously reported silk fibroin scaffolds. These results suggest utility of silk scaffolds in soft tissue regeneration.

Design, Fabrication, and Function of Silk-Based Nanomaterials

Animal silks are built from pure protein components and their mechanical performance, such as strength and toughness, often exceed most engineered materials. The secret to this success is their unique nanoarchitectures that are formed through the hierarchical self-assembly of silk proteins. This natural material fabrication process in sharp contrast to the production of artificial silk materials, which usually are directly constructed as bulk structures from silk fibroin (SF) molecular. In recent years, with the aim of understanding and building better silk materials, a variety of fabrication strategies have been designed to control nanostructures of silks or to create functional materials from silk nanoscale building blocks. These emerging fabrication strategies offer an opportunity to tailor the structure of SF at the nanoscale and provide a promising route to produce structurally and functionally optimized silk nanomaterials. Here, we review the critical roles of silk nanoarchitectures on property and function of natural silk fibers, outline the strategies of utilization of these silk nanobuilding blocks, and we provide a critical summary of state of the art in the field to create silk nanoarchitectures and to generate silk-based nanocomponents. Further, such insights suggest templates to consider for other materials systems.

Single Molecular Layer of Silk Nanoribbon as Potential Basic Building Block of Silk Materials

In this study, nascent silk nanoribbons (SNRs) with an average thickness of 0.4 nm were extracted from natural silkworm silk by partially dissolving degummed silk (DS) in sodium hydroxide (NaOH)/urea solution at -12 °C. In this gentle treatment, the solvent could not destroy the nanofibrillar structure completely, but the chosen conditions would influence the dimensions of resulting SNRs. Molecular dynamics simulations of silk models indicated that the potential of mean force required to break hydrogen bonds between silk fibroin chains was 40% larger than that of van der Waals interactions between β-sheet layers, allowing the exfoliating treatment. It was found that the resulting SNRs contained a single β-sheet layer and amorphous silk fibroin molecules, which could be considered as the basic building block of DS consisting of hierarchical structures. The demonstrated technique for extracting ultrathin SNRs having the height of a single β-sheet layer may provide a useful pathway for creating stronger and tougher silk-based materials and/or adding functionality and durability in materials for various applications. The hierarchical structure model based on SNRs may afford more insight into the structure and property relationship of fabricating silk-based materials.

Anisotropic Biomimetic Silk Scaffolds for Improved Cell Migration and Healing of Skin Wounds

Improved and more rapid healing of full-thickness skin wounds remains a major clinical need. Silk fibroin (SF) is a natural protein biomaterial that has been used in skin repair. However, there has been little effort aimed at improving skin healing through tuning the hierarchical microstructure of SF-based matrices and introducing multiple physical cues. Recently, enhanced vascularization was achieved with SF scaffolds with nanofibrous structures and tunable secondary conformation of the matrices. We hypothesized that anisotropic features in nanofibrous SF scaffolds would promote cell migration, neovascularization, and tissue regeneration in wounds. To address this hypothesis, SF nanofibers were aligned in an electric field to form anisotropic porous scaffolds after lyophilization. In vitro and in vivo studies indicated good cytocompatibility, and improved cell migration and vascularization than nanofibrous scaffolds without these anisotropic features. These improvements resulted in more rapid wound closure, tissue ingrowth, and the formation of new epidermis, as well as higher collagen deposition with a structure similar to the surrounding native tissue. The new epidermal layers and neovascularization were achieved by day 7, with wound healing complete by day 28. It was concluded that anisotropic SF scaffolds alone, without a need for growth factors and cells, promoted significant cell migration, vascularization, and skin regeneration and may have the potential to effectively treat dermal wounds.

Silk-Graphene Hybrid Hydrogels with Multiple Cues to Induce Nerve Cell Behavior

Cell behavior is dependent in part on chemical and physical cues from the extracellular matrix. Although the influence of various cues on cell behavior has been studied, challenges remain to incorporate multiple cues to matrix systems to optimize and control cell outcomes. Here, aligned silk fibroin (SF)-graphene hydrogels with preferable stiffness were developed through arranging SF nanofibers and SF-modified graphene sheets under an electric field. Different signals, such as bioactive graphene, nanofibrous structure, aligned topography, and mechanical stiffness, were tailored into the hydrogel system, providing niches for nerve cell responses. The desired adhesion, proliferation, differentiation, extensio,n and growth factor secretion of multiple nerve-related cells was achieved on these hydrogels, suggesting strong synergistic action through the combination of different cues. Based on the fabrication strategy, our present study provides a useful materials engineering platform for revealing cooperative influences of different signals on nerve cell behavior, to help in the understanding of cell-biomaterial interactions, with potential toward studies related to nerve regeneration.

Biopolymer nanofibrils: structure, modeling, preparation, and applications

Biopolymer nanofibrils exhibit exceptional mechanical properties with a unique combination of strength and toughness, while also presenting biological functions that interact with the surrounding environment. These features of biopolymer nanofibrils profit from their hierarchical structures that spun angstrom to hundreds of nanometer scales. To maintain these unique structural features and to directly utilize these natural supramolecular assemblies, a variety of new methods have been developed to produce biopolymer nanofibrils. In particular, cellulose nanofibrils (CNFs), chitin nanofibrils (ChNFs), silk nanofibrils (SNFs) and collagen nanofibrils (CoNFs), as the four most abundant biopolymer nanofibrils on earth, have been the focus of research in recent years due to their renewable features, wide availability, low-cost, biocompatibility, and biodegradability. A series of top-down and bottom-up strategies have been accessed to exfoliate and regenerate these nanofibrils for versatile advanced applications. In this review, we first summarize the structures of biopolymer nanofibrils in nature and outline their related computational models with the aim of disclosing fundamental structure-property relationships in biological materials. Then, we discuss the underlying methods used for the preparation of CNFs, ChNFs, SNF and CoNFs, and discuss emerging applications for these biopolymer nanofibrils.

A Hierarchical Model To Understand the Processing of Polysaccharides/Protein-Based Films in Ionic Liquids

In recent years, biomaterials from abundant and renewable sources have shown potential in medicine and materials science alike. In this study, we combine theoretical modeling, molecular dynamics simulations, and several experimental techniques to understand the regeneration of cellulose/silk-, chitin/silk-, and chitosan/silk-based biocomposites after dissolution in ionic liquid and regeneration in water. We propose a novel theoretical model that correlates the composite's microscopic structure to its bulk properties. We rely on modeling non-cross-linked biopolymers that present layer-like structures such as β-sheets and we successfully predict structural, thermal, and mechanical properties of a mixture of these biomolecules. Our model and experiments show that the solubility of the pure substance in the chosen solvent can be used to modulate the amount of crystallinity of the biopolymer blend, as measured by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). Thermogravimetric analysis (TGA) shows that the decomposition temperature of the blended biocomposites compared to their pure counterparts is reduced in accordance with our theoretical predictions. The morphology of the material is further characterized through scanning electron microscopy (SEM) and shows differently exposed surface area depending on the blend. Finally, differential scanning calorimetry (DSC) is performed to characterize the residual water content in the material, essential for explaining the regeneration process in water. As a final test of the model, we compare our model's prediction of the Young's modulus with existing data in the literature. The model correctly reproduces experimental trends observed in the Young's modulus due to varying the concentration of silk in the biopolymer blend.