Progress and Trends in Artificial Silk Spinning: A Systematic Review

Unconventional Spidroin Assemblies in Aqueous Dope for Spinning into Tough Synthetic Fibers

Spider dragline silk is a remarkable fiber made by spiders from an aqueous solution of spidroins, and this feat is largely attributed to the tripartite domain architecture of the silk proteins leading to the hierarchical assembly at the nano- and microscales. Although individual amino- and carboxy-terminal domains have been proposed to relate to silk protein assembly, their tentative synergizing roles in recombinant spidroin storage and spinning into synthetic fibers remain elusive. Here, we show biosynthesis and self-assembly of a mimic spidroin composed of amino- and carboxy-terminal domains bracketing 16 consensus repeats of the core region from spider Trichonephila clavipes. The presence of both termini was found essential for self-assembly of the mimic spidroin termed N16C into fibril-like (rather than canonical micellar) nanostructures in concentrated aqueous dope and ordered alignment of these nanofibrils upon extrusion into an acidic coagulation bath. This ultimately led to continuous, macroscopic fibers with a tensile fracture toughness of 100.9 ± 13.2 MJ m^-3, which is comparable to that of their natural counterparts. We also found that the recombinant proteins lacking one or both termini were unable to similarly preassemble into fibrillar nanostructures in dopes and thus yielded inferior fiber properties. This work thereby highlights the synergizing role of terminal domains in the storage and processing of recombinant analogues into tough synthetic fibers.

Mesoscale structure development reveals when a silkworm silk is spun

Silk fibre mechanical properties are attributed to the development of a multi-scale hierarchical structure during spinning. By careful ex vivo processing of a B. mori silkworm silk solution we arrest the spinning process, freezing-in mesoscale structures corresponding to three distinctive structure development stages; gelation, fibrilization and the consolidation phase identified in this work, a process highlighted by the emergence and extinction of 'water pockets'. These transient water pockets are a manifestation of the interplay between protein dehydration, phase separation and nanofibril assembly, with their removal due to nanofibril coalescence during consolidation. We modeled and validated how post-draw improves mechanical properties and refines a silk's hierarchical structure as a result of consolidation. These insights enable a better understanding of the sequence of events that occur during spinning, ultimately leading us to propose a robust definition of when a silkworm silk is actually 'spun'.

Host Systems for the Production of Recombinant Spider Silk

Spider silk is renowned for its impressive mechanical properties. It is one of the strongest known biomaterials, possessing mechanical properties that outmatch both steel and Kevlar. However, the farming of spiders for their silk is unfeasible. Consequently, production of recombinant spider silk proteins (spidroins) in more amenable hosts is an exciting field of research. For large-scale production to be viable, a heterologous silk production system that is both highly efficient and cost effective is essential. Genes encoding recombinant spidroin have been expressed in bacterial, yeast, insect, and mammalian cells, in addition to many other platforms. This review discusses the recent advances in exploiting an increasingly diverse range of host platforms in the heterologous production of recombinant spidroins.

From Protein Building Blocks to Functional Materials

Proteins are the fundamental building blocks for high-performance materials in nature. Such materials fulfill structural roles, as in the case of silk and collagen, and can generate active structures including the cytoskeleton. Attention is increasingly turning to this versatile class of molecules for the synthesis of next-generation green functional materials for a range of applications. Protein nanofibrils are a fundamental supramolecular unit from which many macroscopic protein materials are formed. In this Review, we focus on the multiscale assembly of such protein nanofibrils formed from naturally occurring proteins into new supramolecular architectures and discuss how they can form the basis of material systems ranging from bulk gels, films, fibers, micro/nanogels, condensates, and active materials. We review current and emerging approaches to process and assemble these building blocks in a manner which is different to their natural evolutionarily selected role but allows the generation of tailored functionality, with a focus on microfluidic approaches. We finally discuss opportunities and challenges for this class of materials, including applications that can be involved in this material system which consists of fully natural, biocompatible, and biodegradable feedstocks yet has the potential to generate materials with performance and versatility rivalling that of the best synthetic polymers.

Bio-functionalized titanium surfaces with modified silk fibroin carrying titanium binding motif to enhance the ossific differentiation of MC3T3-E1

Silk fibroin (SF) from Bombyx mori has superior properties as both a textile and a biomaterial, and has been used to functionalize the surfaces of various medical inorganic materials including titanium (Ti). In this study, we endowed SF with reversible binding ability to Ti by embedding a titanium binding motif (minTBP-1 and RKLPDA). Artificial SF proteins were first created by conjugating gene cassettes for SF motif (AGSGAG) and minTBP-1 motif with different ratios, which have been shown to bind reversibly to Ti surfaces in quartz crystal microbalance analyses. Based on these results, the functionalized SF (TiBP-SF) containing the designed peptide [TS[(AGSGAG)₃ AS]₂ RKLPDAS]₈ was prepared from the cocoon of transgenic B. mori, which accelerates the ossific differentiation of MC3T3-E1 cells when coated on titanium substrates. Thus, TiBP-SF presents an alternative for endowing the surfaces of titanium materials with osseointegration functionality, which would allow the exploration of potential applications in the medical field.

Emerging Silk Material Trends: Repurposing, Phase Separation and Solution-Based Designs

Silk continues to amaze. This review unravels the most recent progress in silk science, spanning from fundamental insights to medical silks. Key advances in silk flow are examined, with specific reference to the role of metal ions in switching silk from a storage to a spinning state. Orthogonal thermoplastic silk molding is described, as is the transfer of silk flow principles for the triggering of flow-induced crystallization in other non-silk polymers. Other exciting new developments include silk-inspired liquid-liquid phase separation for non-canonical fiber formation and the creation of "silk organelles" in live cells. This review closes by examining the role of silk fabrics in fashioning facemasks in response to the SARS-CoV-2 pandemic.

The solubility of N-acetyl amino acid amides in organic acid and alcohol solutions: Mechanistic insight into structural protein solubilization

Structural proteins such as spider silk and silkworm silk are generally poorly soluble in aqueous and organic solutions, making them difficult to manipulate in manufacturing processes. Although some organic acids and alcohols, such as formic acid and hexafluoroisopropanol (HFIP), effectively solubilize poorly soluble proteins, little is known about their protein solubilization mechanism. In this study, the solubility of N-acetyl amino acid amide compounds in organic solvents-formic acid, acetic acid, HFIP and isopropanol-was measured to clarify the protein solubilization mechanism at the amino acid residue level. On the basis of thermodynamic analyses of the solubility in terms of the transfer free energy (from water to organic solvents), every organic solvent was found to be effective in thermodynamically stabilizing hydrophobic amino acid side chains in the liquid phase. Formic acid and HFIP were comparably effective in the stabilization of the polypeptide backbone, whereas acetic acid and isopropanol were ineffective. Therefore, the significant solubilizing effect of formic acid and HFIP on the structural proteins was attributed to their favorable interactions with hydrophobic amino acid side chains and with the polypeptide backbone of the proteins. The present findings are useful for the optimization of protein manipulation and amino acid sequence design.

Doing What Spiders Cannot-A Road Map to Supreme Artificial Silk Fibers

Fabricating artificial spider silk fibers in bulk scale has been a major goal in materials science for centuries. Two main routes have emerged for making such fibers. One method uses biomimetics in which the spider silk proteins (spidroins) are produced under nativelike conditions and then spun into fibers in a process that captures the natural, complex molecular mechanisms. However, these fibers do not yet match the mechanical properties of native silk fibers, potentially due to the small size of the designed spidroin used. The second route builds on biotechnological progress that enables production of large spidroins that can be spun into fibers by using organic solvents. With this approach, fibers that equal the native material in terms of mechanical properties can be manufactured, but the yields are too low for economically sustainable production. Hence, the need for new ideas is urgent. Herein, we introduce a structural-biology-based approach for engineering artificial spidroins that circumvents the laws with which spidroins, being secretory proteins, have to comply in order to avoid membrane insertion and provide a road map to the production of biomimetic silk fibers with improved mechanical properties.

Native-like Flow Properties of an Artificial Spider Silk Dope

Recombinant spider silk has emerged as a biomaterial that can circumvent problems associated with synthetic and naturally derived polymers, while still fulfilling the potential of the native material. The artificial spider silk protein NT2RepCT can be produced and spun into fibers without the use of harsh chemicals and here we evaluate key properties of NT2RepCT dope at native-like concentrations. We show that NT2RepCT recapitulates not only the overall secondary structure content of a native silk dope but also emulates its viscoelastic rheological properties. We propose that these properties are key to biomimetic spinning and that optimization of rheological properties could facilitate successful spinning of artificial dopes into fibers.

Optimization of the pre-tension and separation distance for measurement of the dynamic elastic modulus and macromolecular orientation of a polypropylene monofilament via the sonic velocity method

Using a fiber orientation degree measurement instrument (i.e., a dynamic modulus tester), 28 groups of averaged sonic pulse travel times in a polypropylene monofilament were measured and recorded under five pre-tensions across eight separation distances. The zero-time (or delay time) T₀, sonic velocity C, sonic modulus E, Hermans orientation factor F, and orientation angle θ were calculated via two- and multi-point methods. The good agreement observed between the scatter plots of calculated data and the regression lines shows that the multi-point method provides reliable, accurate determination of the sonic modulus (or the dynamic elastic modulus) and the orientation parameters. Surprisingly, the zero-time for sonic pulse propagation depends significantly on the separation distance in practice, although it does not in theory. For easy and rapid measurement or relative comparisons using the two-point method, the optimal range of pre-tension is 0.1 gf/den-0.2 gf/den, and the optimal separation distances are 200 mm and 400 mm. The two-point method is appropriate for industrial applications, while because of its greater accuracy, the multi-point method is preferred for scientific research.

Engineering of sustainable biomaterial composites from cellulose and silk fibroin: Fundamentals and applications

This review addresses composites prepared from cellulose (Cel) and silk fibroin (SF) to generate multifunctional, biocompatible, biodegradable materials such as fibers, films and scaffolds for tissue engineering. First, we discuss briefly the molecular structures of Cel and SF. Their structural features explain why certain solvents, e.g., ionic liquids, inorganic electrolyte solutions dissolve both biopolymers. We discuss the mechanisms of Cel dissolution because in many cases they also apply to (much less studied) SF dissolution. Subsequently, we discuss the fabrication and characterization of Cel/SF composite biomaterials. We show how the composition of these materials beneficially affects their mechanical properties, compared to those of the precursor biopolymers. We also show that Cel/SF materials are excellent and versatile candidates for biomedical applications because of the inherent biocompatibility of their components.

The influence of metal ions on native silk rheology

Whilst flow is the basis for silk fibre formation, subtle changes in a silk feedstocks' chemical environment may serve to increase both energetic efficiency and control hierarchical structure development during spinning. Despite the role of pH being largely understood, the influence of metal ions is not, only being inferred by correlative work and observations. Through a combination of rheology and microscopy, we provide a causative study of how the most abundant metal ions in the silk feedstock, Ca²⁺ and K⁺, affect its flow properties and structure. Our results show that Ca²⁺ ions increase viscosity and prevent molecular alignment and aggregation, providing ideal storage conditions for unspun silk. In contrast, the addition of K⁺ ions promotes molecular alignment and aggregation and therefore seems to transfer the silk feedstock into a spinning state which confirms recent 'sticky reptation' modelling hypotheses. Additionally, we characterised the influence of the ubiquitous kosmotropic agent Li⁺, used to prepare regenerated silk solutions, and find that it promotes molecular alignment and prevents aggregation which may permit a range of interesting artificial silk processing techniques to be developed. In summary, our results provide a clearer picture of how metal ions co-ordinate, control and thus contribute towards silk protein self-assembly which in turn can inspire structuring approaches in other biopolymer systems.

Cyclic Expansion/Compression of the Air-Liquid Interface as a Simple Method to Produce Silk Fibers

Self-assembly of recombinant spider silk protein at air-liquid interfaces is used as a starting point to produce homogeneous fiber bundles. The film that is formed on a silk protein solution in a vertically placed syringe is subjected to repeated controlled extension and compression by an oscillating vertical motion. Thereby, a precise breakup of the film can be achieved, followed by transport and roll-up against the syringe wall prior to extraction. Advantages of the method are that it 1) is simple to use; 2) requires a small volume of protein solution (1 mL) at relatively low concentration (1 mg mL^-1 ); 3) can be performed under sterile conditions; 4) does not require any use of coagulants; and 5) is compatible with the addition of viable cells during the process, which thereby are integrated uniformly throughout the fiber.

Structural Diversity of Native Major Ampullate, Minor Ampullate, Cylindriform, and Flagelliform Silk Proteins in Solution

The foundations of silk spinning, the structure, storage, and activation of silk proteins, remain highly debated. By combining solution small-angle neutron and X-ray scattering (SANS and SAXS) alongside circular dichroism (CD), we reveal a shape anisotropy of the four principal native spider silk feedstocks from Nephila edulis. We show that these proteins behave in solution like elongated semiflexible polymers with locally rigid sections. We demonstrated that minor ampullate and cylindriform proteins adopt a monomeric conformation, while major ampullate and flagelliform proteins have a preference for dimerization. From an evolutionary perspective, we propose that such dimerization arose to help the processing of disordered silk proteins. Collectively, our results provide insights into the molecular-scale processing of silk, uncovering a degree of evolutionary convergence in protein structures and chemistry that supports the macroscale micellar/pseudo liquid crystalline spinning mechanisms proposed by the community.

From Silk Spinning to 3D Printing: Polymer Manufacturing using Directed Hierarchical Molecular Assembly

Silk spinning offers an evolution-based manufacturing strategy for industrial polymer manufacturing, yet remains largely inaccessible as the manufacturing mechanisms in biological and synthetic systems, especially at the molecular level, are fundamentally different. The appealing characteristics of silk spinning include the sustainable sourcing of the protein material, the all-aqueous processing into fibers, and the unique material properties of silks in various formats. Substantial progress has been made to mimic silk spinning in artificial manufacturing processes, despite the gap between natural and artificial systems. This report emphasizes the universal spinning conditions utilized by both spiders and silkworms to generate silk fibers in nature, as a scientific and technical framework for directing molecular assembly into high-performance structures. The preparation of regenerated silk feedstocks and mimicking native spinning conditions in artificial manufacturing are discussed, as is progress and challenges in fiber spinning and 3D printing of silk-composites. Silk spinning is a biomimetic model for advanced and sustainable artificial polymer manufacturing, offering benefits in biomedical applications for tissue scaffolds and implantable devices.

Tensegrity Modelling and the High Toughness of Spider Dragline Silk

This work establishes a tensegrity model of spider dragline silk. Tensegrity systems are ubiquitous in nature, being able to capture the mechanics of biological shapes through simple and effective modes of deformation via extension and contraction. Guided by quantitative microstructural characterization via air plasma etching and low voltage scanning electron microscopy, we report that this model is able to capture experimentally observed phenomena such as the Poisson effect, tensile stress-strain response, and fibre toughness. This is achieved by accounting for spider silks' hierarchical organization into microfibrils with radially variable properties. Each fibril is described as a chain of polypeptide tensegrity units formed by crystalline granules operating under compression, which are connected to each other by amorphous links acting under tension. Our results demonstrate, for the first time, that a radial variability in the ductility of tensegrity chains is responsible for high fibre toughness, a defining and desirable feature of spider silk. Based on this model, a discussion about the use of graded tensegrity structures for the optimal design of next-generation biomimetic fibres is presented.

Bäcklund F, Barth A, Johansson J, M. Pugno N, Rising A. Properties of Biomimetic Artificial Spider Silk Fibers Tuned by PostSpin Bath Incubation

Efficient production of artificial spider silk fibers with properties that match its natural counterpart has still not been achieved. Recently, a biomimetic process for spinning recombinant spider silk proteins (spidroins) was presented, in which important molecular mechanisms involved in native spider silk spinning were recapitulated. However, drawbacks of these fibers included inferior mechanical properties and problems with low resistance to aqueous environments. In this work, we show that ≥5 h incubation of the fibers, in a collection bath of 500 mM NaAc and 200 mM NaCl, at pH 5 results in fibers that do not dissolve in water or phosphate buffered saline, which implies that the fibers can be used for applications that involve wet/humid conditions. Furthermore, incubation in the collection bath improved the strain at break and was associated with increased β-sheet content, but did not affect the fiber morphology. In summary, we present a simple way to improve artificial spider silk fiber strain at break and resistance to aqueous solvents.

The effect of terminal globular domains on the response of recombinant mini-spidroins to fiber spinning triggers

Spider silk spidroins consist of long repetitive protein strands, flanked by globular terminal domains. The globular domains are often omitted in recombinant spidroins, but are thought to be essential for the spiders' natural spinning process. Mimicking this spinning process could be an essential step towards producing strong synthetic spider silk. Here we describe the production of a range of mini-spidroins with both terminal domains, and characterize their response to a number of biomimetic spinning triggers. Our results suggest that mini-spidroins which are able to form protein micelles due to the addition of both terminal domains exhibit shear-thinning, a property which native spidroins also show. Furthermore, our data also suggest that a pH drop alone is insufficient to trigger assembly in a wet-spinning process, and must be combined with salting-out for effective fiber formation. With these insights, we applied these assembly triggers for relatively biomimetic wet spinning. This work adds to the foundation of literature for developing improved biomimetic spinning techniques, which ought to result in synthetic silk that more closely approximates the unique properties of native spider silk.

Transgenic and Diet-Enhanced Silk Production for Reinforced Biomaterials: A Metamaterial Perspective

Silk fibers, which are protein-based biopolymers produced by spiders and silkworms, are fascinating biomaterials that have been extensively studied for numerous biomedical applications. Silk fibers often have remarkable physical and biological properties that typical synthetic materials do not exhibit. These attributes have prompted a wide variety of silk research, including genetic engineering, biotechnological synthesis, and bioinspired fiber spinning, to produce silk proteins on a large scale and to further enhance their properties. In this review, we describe the basic properties of spider silk and silkworm silk and the important production methods for silk proteins. We discuss recent advances in reinforced silk using silkworm transgenesis and functional additive diets with a focus on biomedical applications. We also explain that reinforced silk has an analogy with metamaterials such that user-designed atypical responses can be engineered beyond what naturally occurring materials offer. These insights into reinforced silk can guide better engineering of superior synthetic biomaterials and lead to discoveries of unexplored biological and medical applications of silk.

Microfluidic Silk Fibers with Aligned Hierarchical Microstructures

The hierarchical structure of the ECM provides specific niches for tissues to regulate cell behavior, yet the challenge remains to design biomaterial systems for tissue regeneration to recreate such features in vitro. Here, we achieved this goal through the use of aligned hierarchical structures of native silk fibers, generated through the integration of "bottom-up" and "top-down" strategies to generate regenerated silk fibers with aligned nano- to micro-hierarchical structures. To achieve these designs, we assembled and dispersed silk nanofibers (SNF) in formic acid and spun them into fibers using bioinspired microfluidic chips with a geometry mimicking the native silk gland. The fibers generated using this device exhibited aligned hierarchical structure with fiber mechanical properties superior to fibers derived from more traditional spinning approaches with regenerated silk solutions. Besides the improved mechanical properties, Raman spectroscopic results indicated similarly aligned structures to native fibers and active control of cell proliferation, migration, and aggregate orientation. The results indicate the feasibility of developing bioactive silk fiber materials with hierarchical structures to facilitate utility in a range of cell and tissue regeneration scenarios.

Structure-Function Relationship of Artificial Spider Silk Fibers Produced by Straining Flow Spinning

The production of large quantities of artificial spider silk fibers that match the mechanical properties of the native material has turned out to be challenging. Recent advancements in the field make biomimetic spinning approaches an attractive way forward since they allow the spider silk proteins to assemble into the secondary, tertiary, and quaternary structures that are characteristic of the native silk fiber. Straining flow spinning (SFS) is a newly developed and versatile method that allows production under a wide range of processing conditions. Here, we use a recombinant spider silk protein that shows unprecedented water solubility and that is capable of native-like assembly, and we spin it into fibers by the SFS technique. We show that fibers may be spun using different hydrodynamical and chemical conditions and conclude that these spinning conditions affect fiber mechanics. In particular, it was found that the addition of acetonitrile and polyethylene glycol to the collection bath results in fibers with increased β-sheet content and improved mechanical properties.

Artificial ligament made from silk protein/Laponite hybrid fibers

With developments in tissue engineering, artificial ligaments are expected to be future materials for anterior cruciate ligament (ACL) reconstruction. However, poor healing of the intraosseous part after ACL reconstruction significantly hinders their applications in this field. In this study, a bioactive clay Laponite (LAP) was introduced into the regenerated silk fibroin (RSF) spinning dope to produce functional RSF/LAP hybrid fibers by wet-spinning. These RSF/LAP hybrid fibers were then woven into artificial ligament for ACL reconstruction. The structure and mechanical properties of RSF/LAP hybrid fibers were extensively studied by different means. Results confirmed the presence of LAP in RSF fibers and revealed that the addition of LAP slightly deteriorated the comprehensive mechanical properties of RSF fibers. However, they were still much tougher (with higher breaking energy) than those of degummed natural silkworm silk that was earlier used for making artificial ligament. The artificial ligament woven from RSF/LAP hybrid fibers showed better cytocompatibility and osteogenic differentiation with mouse pre-osteoblasts in vitro than those made from degummed natural silkworm silks and pure RSF fibers. Furthermore, in vivo study in a rat ACL reconstruction model demonstrated that the presence of LAP in the artificial ligament could significantly enhance the graft osseointegration process and also improve the corresponding biomechanical properties of the artificial ligament. Based upon these results, the RSF/LAP hybrid fibers, which can be mass produced by wet-spinning process, are believed to have a great potential for use as artificial ligament materials for ACL reconstruction. STATEMENT OF SIGNIFICANCE: In this study, we successfully introduced Laponite (LAP), a kind of clay that has the function of osteogenic induction, into regenerated silk fibroin (RSF) fibers, which was prepared by a mature wet-spinning method developed in our lab. We believe that through artificial spinning, additional functional components can be added into RSF fibers, which one can hardly achieve with natural silks. We showed that the artificial ligament woven from RSF/LAP hybrid fibers had better cytocompatibility and osteogenic differentiation for mouse pre-osteoblasts in vitro, and significantly enhanced the graft osseointegration process and improved the corresponding biomechanical properties in a rat ACL reconstruction model in vivo, compared to those artificial ligaments made from degummed natural silkworm silks and pure RSF fibers.

Towards engineering and production of artificial spider silk using tools of synthetic biology

Spider silk is one of the strongest biomaterials available in nature. Its mechanical properties make it a good candidate for applications in various fields ranging from protective armour to bandages for wound dressing to coatings for medical implants. Spider silk is formed by an intricate arrangement of spidroins, which are extremely large proteins containing long stretches of repeating segments rich in alanine and glycine. A large amount of research has been directed towards harnessing the spectacular potential of spider silks and using them for different applications. The interdisciplinary approach of synthetic biology is an ideal tool to study these spider silk proteins and work towards the engineering and production of synthetic spider silk. This review aims to highlight the recent progress that has been made in the study of spider silk proteins using different branches of synthetic biology. Here, the authors discuss the different computational approaches, directed evolution techniques and various expression platforms that have been tested for the successful production of spider silk. Future challenges facing the field and possible solutions offered by synthetic biology are also discussed.

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.

Robust Biological Fibers Based on Widely Available Proteins: Facile Fabrication and Suturing Application

Lightweight and mechanically strong protein fibers are promising for many technical applications. Despite the widespread investigation of biological fibers based on spider silk and silkworm proteins, it remains a challenge to develop low-cost proteins and convenient spinning technology for the fabrication of robust biological fibers. Since there are plenty of widely available proteins in nature, it is meaningful to investigate the preparation of fibers by the proteins and explore their biomedical applications. Here, a facile microfluidic strategy is developed for the scalable construction of biological fibers via a series of easily accessible spherical and linear proteins including chicken egg, quail egg, goose egg, bovine serum albumin, milk, and collagen. It is found that the crosslinking effect in microfluidic chips and double-drawn treatment after spinning are crucial for the formation of fibers. Thus, high tensile strength and toughness are realized in the fibers, which are comparable or even higher than that of many recombinant spider silks or regenerated silkworm fibers. Moreover, the suturing applications in rat and minipig models are realized by employing the mechanically strong fibers. Therefore, this work opens a new direction for the production of biological fibers from natural sources.

Plasticised Regenerated Silk/Gold Nanorods Hybrids as Sealant and Bio-Piezoelectric Materials

Manual and mechanical suturing are currently the gold standard for bowel anastomosis. If tissue approximation fails, anastomotic leaks occur. Anastomotic leaks may have catastrophic consequences. The development of a fully absorbable, biocompatible sealant material based on a bio-ink silk fibroin can reduce the chance of anastomotic leaks. We have produced a Ca-modified plasticised regenerated silk (RS) with gold nanorods sealant. This sealant was applied to anastomosed porcine intestine. Water absorption from wet tissue substrate applied compressive strains on hybrid RS films. This compression results in a sealant effect on anastomosis. The increased toughness of the hybrid plasticised RS resulted in the designing of a bio-film with superior elongation at break (i.e., ≈200%) and bursting pressure. We have also reported structure-dependent piezoelectricity of the RS film that shows a piezoelectric effect out of the plane. We hope that in the future, bowel anastomosis can be simplified by providing a multifunctional bio-film that makes feasible the mechanical tissue joint without the need for specific tools and could be used in piezoelectric sealant heads.

Transgenic PDGF-BB/sericin hydrogel supports for cell proliferation and osteogenic differentiation

The present study demonstrates fabrication of PDGF-BB functionalized sericin hydrogel to explore biomaterials-related utility in bone tissue engineering.

3D Printing of Silk Protein Structures by Aqueous Solvent-Directed Molecular Assembly

Hierarchical molecular assembly is a fundamental strategy for manufacturing protein structures in nature. However, to translate this natural strategy into advanced digital manufacturing like three-dimensional (3D) printing remains a technical challenge. This work presents a 3D printing technique with silk fibroin to address this challenge, by rationally designing an aqueous salt bath capable of directing the hierarchical assembly of the protein molecules. This technique, conducted under aqueous and ambient conditions, results in 3D proteinaceous architectures characterized by intrinsic biocompatibility/biodegradability and robust mechanical features. The versatility of this method is shown in a diversity of 3D shapes and a range of functional components integrated into the 3D prints. The manufacturing capability is exemplified by the single-step construction of perfusable microfluidic chips which eliminates the use of supporting or sacrificial materials. The 3D shaping capability of the protein material can benefit a multitude of biomedical devices, from drug delivery to surgical implants to tissue scaffolds. This work also provides insights into the recapitulation of solvent-directed hierarchical molecular assembly for artificial manufacturing.

Pressure-induced silk spinning mechanism in webspinners (Insecta: Embioptera)

The articulated appendages of arthropods are highly adaptable and potentially multifunctional, used for walking, swimming, feeding, prey capture, or other functions. Webspinners (Order Embioptera) are a paragon in this context. In contrast to other arthropods producing silk, they utilize their front feet for silk production. However, employing the same leg for alternative functions rather than for pure locomotion potentially imposes constraints and compromises. We here present morphological and experimental evidence for a "passive" pressure-induced silk spinning mechanism induced by external mechanical stimuli. Furthermore, we demonstrate that, as a consequence of the conflicting functions for their front feet, webspinners have evolved a unique style of walking that reduces the potentially problematic contact between silk ejectors and the substrate. Here we answer for the first time a long-term question within this enigmatic group of insects-how webspinners can use their front feet to spin their nanoscale silk. This knowledge may open the door for experimental studies on an artificial spinning process and for future utilization in applied fields of robotics or chemistry.

Scalable Spider-Silk-Like Supertough Fibers using a Pseudoprotein Polymer

Spider silks are tougher than almost all other materials in the world and thus are considered ideal materials by scientists and the industry. Although there have been tremendous attempts to prepare fibers from genetically engineered spider-silk proteins, it is still a very large challenge to artificially produce materials with a very high fracture energy, not to mention the high scaling-up requirements because of the extremely low productivity and high cost levels. Here, a facile spider-silk-mimicking strategy is first reported for preparing scalable supertough fibers using the chemical synthesis route. Supertoughness (≈387 MJ m^-3 ), more than twice the reported value of common spider dragline silk and comparable to the value of the toughest spider silk, the aciniform silk of Argiope trifasciata, is achieved by introducing β-sheet crystals and α-helical peptides simultaneously in a pseudoprotein polymer. The process opens up a very promising avenue for obtaining excellent spider fibers.

Artificial spinning of natural silk threads

Silk producing arthropods spin solid fibres from an aqueous protein feedstock apparently relying on the complex structure of the silk protein and its controlled aggregation by shear forces, alongside biochemical changes. This flow-induced phase-transition of the stored native silk molecules is irreversible, environmentally sound and remarkably energy efficient. The process seemingly relies on a self-assembling, fibrillation process. Here we test this hypothesis by biomimetically spinning a native-based silk feedstock, extracted by custom processes, into silk fibres that equal their natural models' mechanical properties. Importantly, these filaments, which featured cross-section morphologies ranged from large crescent-like to small ribbon-like shapes, also had the slender cross-sectional areas of native fibres and their hierarchical nanofibrillar structures. The modulation of the post-draw conditions directly affected mechanical properties, correlated with the extent of fibre crystallinity, i.e. degree of molecular order. We believe our study contributes significantly to the understanding and development of artificial silks by demonstrating successful biomimetic spinning relies on appropriately designed feedstock properties. In addition, our study provides inspiration for low-energy routes to novel synthetic polymers.

Synthetic biology for fibres, adhesives and active camouflage materials in protection and aerospace

Synthetic biology has huge potential to produce the next generation of advanced materials by accessing previously unreachable (bio)chemical space. In this prospective review, we take a snapshot of current activity in this rapidly developing area, focussing on prominent examples for high-performance applications such as those required for protective materials and the aerospace sector. The continued growth of this emerging field will be facilitated by the convergence of expertise from a range of diverse disciplines, including molecular biology, polymer chemistry, materials science and process engineering. This review highlights the most significant recent advances and address the cross-disciplinary challenges currently being faced.

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.

Spinning and Applications of Bioinspired Fiber Systems

Natural fiber systems provide inspirations for artificial fiber spinning and applications. Through a long process of trial and error, great progress has been made in recent years. The natural fiber itself, especially silks, and the formation mechanism are better understood, and some of the essential factors are implemented in artificial spinning methods, benefiting from advanced manufacturing technologies. In addition, fiber-based materials produced via bioinspired spinning methods find an increasingly wide range of biomedical, optoelectronic, and environmental engineering applications. This paper reviews recent developments in the spinning and application of bioinspired fiber systems, introduces natural fiber and spinning processes and artificial spinning methods, and discusses applications of artificial fiber materials. Views on remaining challenges and the perspective on future trends are also proposed.