Molecular Orientation Enhancement of Silk by the Hot-Stretching-Induced Transition from α-Helix-HFIP Complex to β-Sheet

Unravelling the antioxidant behaviour of self-assembly β-Sheet in silk fibroin

Local oxidative stress in diseases or injury severely hinders cell homeostasis and organ regeneration. Antioxidant therapy is an effective strategy for oxidative stress treatment. Biomaterials with good biocompatibility and reactive oxygen species (ROS) scavenging ability are good choices for antioxidant therapeutics. However, there are few natural biomaterials that are identified with both biocompatibility and strong antioxidant activity. Here, we show, for the first time, that silk fibroin (SF) is a strong antioxidant, which can eliminate ROS in both cells and zebrafish. We further demonstrate that the β-sheet structures turn into a random coiled structure when SF is treated with hydrogen peroxide. The content of β-sheet structures can be increased by heating, thus enhancing the antioxidation properties of SF. Therefore, SF can serve as a good antioxidant biomaterial for therapeutics, and its β-sheet structure-based antioxidation mechanism provides a novel theoretical basis, which could be a new cue for more antioxidant biomaterial discovery and identification.

Tissue/Organ Adaptable Bioelectronic Silk-Based Implants

Implantable bioelectronic devices, designed for both monitoring and modulating living organisms, require functional and biological adaptability. Pure silk is innovatively employed, which is known for its excellent biocompatibility, to engineer water-triggered, geometrically reconfigurable membranes, on which functions can be integrated by Micro Electro Mechanical System (MEMS) techniques and specially functionalized silk. These devices can undergo programmed shape deformations within 10 min once triggered by water, and thus establishing stable bioelectronic interfaces with natively fitted geometries. As a testament to the applicability of this approach, a twining peripheral nerve electrode is designed, fabricated, and rigorously tested, demonstrating its efficacy in nerve modulation while ensuring biocompatibility for successful implantation.

Molecular organization of fibroin heavy chain and mechanism of fibre formation in Bombyx mori

Fibroins' transition from liquid to solid is fundamental to spinning and underpins the impressive native properties of silk. Herein, we establish a fibroin heavy chain fold for the Silk-I polymorph, which could be relevant for other similar proteins, and explains mechanistically the liquid-to-solid transition of this silk, driven by pH reduction and flow stress. Combining spectroscopy and modelling we propose that the liquid Silk-I fibroin heavy chain (FibH) from the silkworm, Bombyx mori, adopts a newly reported β-solenoid structure. Similarly, using rheology we propose that FibH N-terminal domain (NTD) templates reversible higher-order oligomerization driven by pH reduction. Our integrated approach bridges the gap in understanding FibH structure and provides insight into the spatial and temporal hierarchical self-assembly across length scales. Our findings elucidate the complex rheological behaviour of Silk-I, solutions and gels, and the observed liquid crystalline textures within the silk gland. We also find that the NTD undergoes hydrolysis during standard regeneration, explaining key differences between native and regenerated silk feedstocks. In general, in this study we emphasize the unique characteristics of native and native-like silks, offering a fresh perspective on our fundamental understanding of silk-fibre production and applications.

Raman, WAXS, and Solid-State NMR Characterizations of Regenerated Silk Fibroin Using Lanthanide Ions as Chaotropic Agents

Bombyx mori silk fibroin (SF) has been reported as a convenient natural material for regenerative medicine, optoelectronics, and many other technological applications. SF owes its unique features to the hierarchical organization of the fibers. Many efforts have been made to set up protocols for dissolution since many applications of SF are based on regenerated solutions and fibers, but chaotropic conditions required to disassemble the packing of the polymer afford solutions with poor crystalline behavior. Our previous research has disclosed a dissolution and regeneration process of highly crystalline fibers involving lanthanide ions as chaotropic agents, demonstrating that each lanthanide has its own unique interaction with SF. Herein, we report elucidation of the structure of Ln-SF fibers by the combined use of Raman spectroscopy, wide-angle X-ray scattering (WAXS), and solid-state NMR techniques. Raman spectra confirmed the coordination of metal ions to SF, WAXS results highlighted the crystalline content of fibers, and solid-state NMR enabled the assessment of different ratios of secondary structures in the protein.

Methods for Silk Property Analyses across Structural Hierarchies and Scales

Silk from silkworms and spiders is an exceptionally important natural material, inspiring a range of new products and applications due to its high strength, elasticity, and toughness at low density, as well as its unique conductive and optical properties. Transgenic and recombinant technologies offer great promise for the scaled-up production of new silkworm- and spider-silk-inspired fibres. However, despite considerable effort, producing an artificial silk that recaptures the physico-chemical properties of naturally spun silk has thus far proven elusive. The mechanical, biochemical, and other properties of pre-and post-development fibres accordingly should be determined across scales and structural hierarchies whenever feasible. We have herein reviewed and made recommendations on some of those practices for measuring the bulk fibre properties; skin-core structures; and the primary, secondary, and tertiary structures of silk proteins and the properties of dopes and their proteins. We thereupon examine emerging methodologies and make assessments on how they might be utilized to realize the goal of developing high quality bio-inspired fibres.

Novel Dry Spinning Process of Natural Macromolecules for Sustainable Fiber Material -1- Proof of the Concept Using Silk Fibroin

Researchers around the world are developing technologies to minimize carbon dioxide emissions or carbon neutrality in various fields. In this study, the dry spinning of regenerated silk fibroin (RSF) was achieved as a proof of concept for a process using ionic liquids as dissolution aids and plasticizers in developing natural polymeric materials. A dry spinning equipment system combining a stainless-steel syringe and a brushless motor was built to generate fiber compacts from a dope of silk fibroin obtained by degumming silkworm silk cocoons and ionic liquid 1-hexyl-3-methyl-imidazolium chloride ([HMIM][Cl]) according to a general method. The maximum stress and maximum elongation of the RSF fibers were 159.9 MPa and 31.5%, respectively. RSF fibers containing ionic liquids have a homogeneous internal structure according to morphological investigations. Elemental analysis of fiber cross sections revealed the homogeneous distribution of nonvolatile ionic liquid [HMIM][Cl] in RSF fibers. Furthermore, the removal of ionic liquids from RSF fibers through impregnation washing with organic solvents was verified to enhance industrial applications. Tensile testing showed that the fiber strength could be maintained even after removing the ionic liquid. Thermogravimetric analysis results show that the organic solvent 1,1,1,3,3,3-hexafluoro-2-propanol is chemically coordinated to silk fibroin and, as a natural polymer, can withstand heat up to 250 °C.

Toughening Wet-Spun Silk Fibers by Silk Nanofiber Templating

Regenerated silk fibers typically fall short of silkworm cocoon fibers in mechanical properties due to reduced fiber crystal structure and alignment. One approach to address this has been to employ inorganic materials as reinforcing agents. The present study avoids the need for synthetic additives, demonstrating the first use of exfoliated silk nanofibers to control silk solution crystallization, resulting in all-silk pseudocomposite fibers with remarkable mechanical properties. Incorporating only 0.06 wt. % silk nanofibers led to a ∼44% increase in tensile strength (over 600 MPa) and ∼33% increase in toughness (over 200 kJ/kg) compared with fibers without silk nanofibers. These remarkable properties can be attributed to nanofiber crystal seeding in conjunction with fiber draw. The crystallinity nearly doubled from ∼17% for fiber spun from pure silk solution to ∼30% for the silk nanofiber reinforced sample. The latter fiber also shows a high degree of crystal orientation with a Herman's orientation factor of 0.93, a value which approaches that of natural degummed B. mori silk cocoon fiber (0.96). This study provides a strong foundation to guide the development of simple, eco-friendly methods to spin regenerated silk with excellent properties and a hierarchical structure that mimics natural silk. This article is protected by copyright. All rights reserved.

An Insight into Tunable Innate Piezoelectricity of Silk for Green Bioelectronics

Commercially available bioelectronics account for significant percentage of e-waste, especially battery waste, that demand immediate intervention due to rising environmental concerns. Consumers are becoming increasingly aware and cautious of their contribution to carbon footprint on a regular basis. It has become imperative to adopt sustainability in every aspect of production of bioelectronics taking into consideration the growing market for wearable healthcare monitoring system. Green electronics is a relatively new concept gaining tremendous attention within the scientific and industrial community with the ultimate goal of employing organic, biodegradable, and self-sustainable system to replace the conventional inorganic battery-powered electronics. Silk is a green material that has been extensively explored for its use in functional electronics due to its tunable biodegradability and flexibility. Nevertheless, an intriguing property of Silk is its innate piezoelectricity. This review highlights the importance of crystal orientation and structure of Silk Fibroin to display piezoelectric response and documents possible strategies for its enhancement. It also provides insight into the possibility of using piezoelectric Silk as a piezoelectric sensor, actuator, and energy harvester to form self-powered hybrid systems for autonomous bioelectronics.

The balance of crystalline and amorphous regions in the fibroin structure underpins the tensile strength of bagworm silk

Protein-based materials are considered versatile biomaterials, and their biodegradability is an advantage for sustainable development. Bagworm produces strong silk for use in unique situations throughout its life stages. Rigorous molecular analyses of Eumeta variegata suggested that the particular mechanical properties of its silk are due to the coexistence of poly-A and GA motifs. However, little molecular information on closely related species is available, and it is not understood how these properties were acquired evolutionarily or whether the motif combination is a conserved trait in other bagworms. Here, we performed a transcriptome analysis of two other bagworm species (Canephora pungelerii and Bambalina sp.) belonging to the family Psychidae to elucidate the relationship between the fibroin gene and silk properties. The obtained transcriptome assemblies and tensile tests indicated that the motif combination and silk properties were conserved among the bagworms. Furthermore, our analysis showed that C. pungelerii produces extraordinarily strong silk (breaking strength of 1.4 GPa) and indicated that the cause may be the C. pungelerii -specific balance of crystalline/amorphous regions in the H-fibroin repetitive domain. This particular H-fibroin architecture may have been evolutionarily acquired to produce strong thread to maintain bag stability during the relatively long development period of Canephora species relative to other bagworms.

Recombinant Silk Proteins with Additional Polyalanine Have Excellent Mechanical Properties

This paper explores the structures of exogenous protein molecules that can effectively improve the mechanical properties of silkworm silk. Several transgenic vectors fused with the silkworm fibroin light chain and type 3 repeats in different multiples of the ampullate dragline silk protein 1 (MaSp1) from black widow spider with different lengths of the polyalanine motifs were constructed for this study. Transgenic silkworms were successfully obtained by piggyBac-mediated microinjection. Molecular detection showed that foreign proteins were successfully secreted and contained within the cocoon shells. According to the prediction of PONDR^® VSL2 and PONDR^® VL-XT, the type 3 repeats and the polyalanine motif of the MaSp1 protein were amorphous. The results of FTIR analysis showed that the content of β-sheets in the silk of transgenic silkworms engineered with transgenic vectors with additional polyalanine was significantly higher than that of wild-type silkworm silk. Additionally, silk with a higher β-sheet content had better fracture strength and Young’s modulus. The mechanical properties of silk with longer chains of exogenous proteins were improved. In general, our results provide theoretical guidance and technical support for the large-scale production of excellent bionic silk.

Durably Ductile, Transparent Polystyrene Based on Extensional Stress-Induced Rejuvenation Stabilized by Styrene-Butadiene Block Copolymer Nanofibrils

The glassy polymer of polystyrene (PS) enjoys a good reputation as a promising optical material; however, the inherent brittleness hinders its further applications. Conventional toughening methods are realized based on the premise of a sacrifice in transparency and stiffness. In this work, we found an unprecedented strategy to address these obstacles by combining extensional stress-induced ductility and suppressing physical aging. PS-based film with a high stiffness, long-term ductility, and excellent transparency is achieved by introducing a styrene-butadiene block copolymer into the PS matrix and subsequently annealing stretched. A nanofibrillar structure of the polybutadiene (PB) phase is formulated surrounded by a PS matrix, and thus, the elongation at break enhances from 3.1% up to 86.8%, accompanying the yield strength enhanced from 25.5 to 62.2 MPa. More significantly, compared with neat PS, these films survive from physical aging and persistent ductility over time. The morphology deformation induced by stress makes an obvious contribution to the improvement of transparency. Investigating the dynamics of chain segments indicates that the incorporation of the copolymer can restrict rearrangement and local relaxation to the PS chain. This work could pave a potential route toward high-performance PS and might be transferable to other glassy polymers with a fragile character.

Forcibly spun dragline silk fibers from web-building spider Trichonephila clavata ensure robustness irrespective of spinning speed and humidity

Web-building spiders secrete dragline silk fibers to sustain their body and use them as frameworks during web construction. They spin dragline silk fibers at various spinning speed and humidity conditions depending on their natural habitat. Here, we investigated the effect of spinning speed and humidity on the structural and mechanical properties of dragline silk fibers from web-building spider Trichonephila clavata obtained by the forcibly spinning method. We found that the crystal and morphological structures did not rely on the spinning speed and humidity. Furthermore, the mechanical strength and extensibility of the dragline silk fibers were maintained, demonstrating that dragline silk fibers ensure robustness irrespective of the spinning speed and humidity. The results obtained in the present study are helpful not only to understand the biological basis of the silk fiber formation of spiders but also contribute to consider the spinning conditions for the process of creating synthetic silk fibers.

Structure Water-Solubility Relationship in α-Helix-Rich Films Cast from Aqueous and 1,1,1,3,3,3-Hexafluoro-2-Propanol Solutions of S. c. ricini Silk Fibroin

Silk fibroin (SF) produced by the domesticated wild silkworm, Samia cynthia ricini (S. c. ricini) is attracting increasing interest owing to its unique mechanical properties, biocompatibility, and abundance in nature. However, its utilization is limited, largely due to lack of appropriate processing strategies. Various strategies have been assessed to regenerate cocoon SF, as well as the use of aqueous liquid fibroin (LFaq) prepared by dissolution of silk dope obtained from the silk glands of mature silkworm larvae in water. However, films cast from these fibroin solutions in water or organic solvents are often water-soluble and require post-treatment to render them water-stable. Here, we present a strategy for fabrication of water-stable films from S. c. ricini silk gland fibroin (SGF) without post-treatment. Aqueous ethanol induced gelation of fibroin in the posterior silk glands (PSG), enabling its separation from the rest of the silk gland. When dissolved in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), the SGF-gel gave a solution from which a transparent, flexible, and water-insoluble film (SGFHFIP) was cast. Detailed structural characterization of the SGFHFIP as-cast film was carried out and compared to a conventional, water-soluble film cast from LFaq. FTIR and 13C solid-state NMR analyses revealed both cast films to be α-helix-rich. However, gelation of SGF induced by the 40%-EtOH-treatment resulted in an imperfect β-sheet structure. As a result, the SGF-gel was soluble in HFIP, but some β-sheet structural memory remains, and the SGFHFIP as-cast film obtained has some β-sheet content which renders it water-resistant. These results reveal a structure water-solubility relationship in S. c. ricini SF films that may offer useful insights towards tunable fabrication of novel biomaterials. A plausible model of the mechanism that leads to the difference in water resistance of the two kinds of α-helix-rich films is proposed.

Aggregation State of Residual α-Helices and Their Influence on Physical Properties of S. c. ricini Native Fiber

Formation of the α-helical conformation in the poly-l-alanine (PA) sequence regions, subsequent structural transition to β-sheet during natural spinning, and presence of residual α-helices in Samia cynthia ricini (S. c. ricini) native silk fiber have been experimentally proven. However, the aggregation state of the residual α-helices, and their influence on the mechanical deformation behavior in native fiber remain unclear. Here we show that the α-helices form an ordered aggregation state with a hexagonal packing in the aqueous solution, some of which remain during natural spinning. X-ray scattering and differential scanning calorimetry (DSC) analyses revealed occurrence of a structural transition of the residual α-helices to the β-sheet structure, accompanied by disappearance of the plateau region in the force-strain curve, due to heat-treatment at ~220 °C. On the basis of X-ray scattering before and after tensile stretching of S. c. ricini native silk, a direct connection between the plateau region and the α-helix to β-sheet structural transition was confirmed. Our findings demonstrate the importance of the PA sequence regions in fiber structure formation and their influence on the tensile deformation behavior of S. c. ricini silk, features believed to be essentially similar in other saturniid silks. We strongly believe the residual ordered α-helices to be strategically and systematically designed by S. c. ricini silkworms to impart flexibility in native silk fiber. We anticipate that these knowledge forms a basis for fruitful strategies in the design and development of amino acid sequences for artificial silks with desired mechanical properties.

A study of the extraordinarily strong and tough silk produced by bagworms

Global ecological damage has heightened the demand for silk as 'a structural material made from sustainable resources'. Scientists have earnestly searched for stronger and tougher silks. Bagworm silk might be a promising candidate considering its superior capacity to dangle a heavy weight, summed up by the weights of the larva and its house. However, detailed mechanical and structural studies on bagworm silks have been lacking. Herein, we show the superior potential of the silk produced by Japan's largest bagworm, Eumeta variegata. This bagworm silk is extraordinarily strong and tough, and its tensile deformation behaviour is quite elastic. The outstanding mechanical property is the result of a highly ordered hierarchical structure, which remains unchanged until fracture. Our findings demonstrate how the hierarchical structure of silk proteins plays an important role in the mechanical property of silk fibres.

Annealing and Stretching Induced High Energy Storage Properties in All-Organic Composite Dielectric Films

High discharged energy density and charge⁻discharge efficiency, in combination with high electric breakdown strength, maximum electric displacement and low residual displacement, are very difficult to simultaneously achieve in single-component polymer dielectrics. Plenty of researches have reported polymer based composite dielectrics filled with inorganic fillers, through complex surface modification of inorganic fillers to improve interface compatibility. In this work, a novel strategy of introducing environmentally-friendly biological polyester into fluoropolymer matrix has been presented to prepare all-organic polymer composites with desirable high energy storage properties by solution cast process (followed by annealing or stretching post-treatment), in order to simplify the preparation steps and lower the cost. Fluoropolymer with substantial ferroelectric domains (contributing to high dielectric response) as matrix and poly (3-hydroxybutyrate-co-3-hydroxyvalerate) with excellent linear polarization property (resulting in high breakdown strength) as filler were employed. By high-temperature annealing, the size of ferroelectric domains could be improved and interfacial air defects could be removed, leading to elevated high energy storage density and efficiency in composite. By mono-directional stretching, the ferroelectric domains and polyester could be regularly oriented along stretching direction, resulting in desired high energy storage performances as well. Besides, linear dielectric components could contribute to high efficiency from their strong rigidity restrain effect on ferroelectric component. This work might open up the way for a facile fabrication of promising all-organic composite dielectric films with high energy storage properties.

Transformation of Coiled α-Helices into Cross-β-Sheets Superstructure

The fibrous silk produced by bees, wasps, ants, or hornets is known to form a four-strand α-helical coiled coil superstructure. We have succeeded in showing the formation of this coiled coil structure not only in natural fibers, but also in artificial films made of regenerated silk of the hornet Vespa simillima xanthoptera using wide- and small-angle X-ray scatterings and polarized Fourier transform infrared spectroscopy. On the basis of time-resolved simultaneous synchrotron X-ray scattering observations for in situ monitoring of the structural changes in regenerated silk material during tensile deformation, we have shown that the application of tensile force under appropriate conditions induces a transition from the coiled α-helices to a cross-β-sheet superstructure. The four-stranded tertiary superstructure remains unchanged during this process. It has also been shown that the amorphous protein chains in the regenerated silk material are transformed into conventional β-sheet arrangements with varying orientation.

Orientational Mapping Augmented Sub-Wavelength Hyper-Spectral Imaging of Silk

Molecular alignment underpins optical, mechanical, and thermal properties of materials, however, its direct measurement from volumes with micrometer dimensions is not accessible, especially, for structurally complex bio-materials. How the molecular alignment is linked to extraordinary properties of silk and its amorphous-crystalline composition has to be accessed by a direct measurement from a single silk fiber. Here, we show orientation mapping of the internal silk fiber structure via polarisation-dependent IR absorbance at high spatial resolution of 4.2 μm and 1.9 μm in a hyper-spectral IR imaging by attenuated total reflection using synchrotron radiation in the spectral fingerprint region around 6 μm wavelength. Free-standing longitudinal micro-slices of silk fibers, thinner than the fiber cross section, were prepared by microtome for the four polarization method to directly measure the orientational sensitivity of absorbance in the molecular fingerprint spectral window of the amide bands of β-sheet polypeptides of silk. Microtomed lateral slices of silk fibers, which may avoid possible artefacts that affect spectroscopic measurements with fibers of an elliptical cross sections were used in the study. Amorphisation of silk by ultra-short laser single-pulse exposure is demonstrated.