Hierarchical spidroin micellar nanoparticles as the fundamental precursors of spider silks

Disentangling the Web: An Interdisciplinary Review on the Potential and Feasibility of Spider Silk Bioproduction

The remarkable material properties of spider silk, such as its high toughness and tensile strength combined with its low density, make it a highly sought-after material with myriad applications. In addition, the biological nature of spider silk makes it a promising, potentially sustainable alternative to many toxic or petrochemical-derived materials. Therefore, interest in the heterologous production of spider silk proteins has greatly increased over the past few decades, making recombinant spider silk an important frontier in biomanufacturing. This has resulted in a diversity of potential host organisms, a large space for sequence design, and a variety of downstream processing techniques and product applications for spider silk production. Here, we highlight advances in each of these technical aspects as well as white spaces therein, still ripe for further investigation and discovery. Additionally, industry landscaping, patent analyses, and interviews with Key Opinion Leaders help define both the research and industry landscapes. In particular, we found that though textiles dominated the early products proposed by companies, the versatile nature of spider silk has opened up possibilities in other industries, such as high-performance materials in automotive applications or biomedical therapies. While continuing enthusiasm has imbued scientists and investors alike, many technical and business considerations still remain unsolved before spider silk can be democratized as a high-performance product. We provide insights and strategies for overcoming these initial hurdles, and we highlight the importance of collaboration between academia, industry, and policy makers. Linking technical considerations to business and market entry strategies highlights the importance of a holistic approach for the effective scale-up and commercial viability of spider silk bioproduction.

Nanoassembly of spider silk protein mediated by intrinsically disordered regions

Spider silk is the self-assembling product of silk proteins each containing multiple repeating units. Each repeating unit is entirely intrinsically disordered or contains a small disordered domain. The role of the disordered domain/unit in conferring silk protein storage and self-assembly is not fully understood yet. Here, we used biophysical and biochemical techniques to investigate the self-assembly of a miniature version of a minor ampullate spidroin (denoted as miniMiSp). miniMiSp consists of two identical intrinsically disordered domains, one folded repetitive domain, and two folded terminal domains. Our data indicated that miniMiSp self-assembles into oligomers and further into liquid droplets. The oligomerization is attributed to the aggregation-prone property of both the disordered domains and the folded repetitive domain. Our results support the model of micellar structure for silk proteins at high protein concentrations. The disordered domain is indispensable for liquid droplet formation via liquid-liquid phase separation, and tyrosine residues located in the disordered domain make dominant contributions to stability of the liquid droplets. As the same tyrosine residues are also critical to fibrillation, the liquid droplets are likely an intermediate state between the solution state and the fiber state. Additionally, the terminal domains contribute to the pH- and salt-dependent self-assembly properties.

Modulating Polyalanine Motifs of Synthetic Spidroin for Controllable Preassembly and Strong Fiber Formation

Spider dragline (major ampullate) silk is one of the toughest known fibers in nature and exhibits an excellent combination of high tensile strength and elasticity. Increasing evidence has indicated that preassembly plays a crucial role in facilitating the proper assembly of silk fibers by bridging the mesoscale gap between spidroin molecules and the final strong fibers. However, it remains challenging to control the preassembly of spidroins and investigate its influence on fiber structural and mechanical properties. In this study, we explored to bridge this gap by modulating the polyalanine (polyA) motifs in repetitive region of spidroins to tune their preassemblies in aqueous dope solutions. Three biomimetic silk proteins with varying numbers of alanine residues in polyA motif and comparable molecular weights were designed and biosynthesized, termed as N16C-5A, N15C-8A, and N13C-12A, respectively. It was found that all three proteins could form nanofibril assemblies in the concentrated aqueous dopes, but the size and structural stability of the fibrils were distinct from each other. The silk protein N15C-8A with 8 alanine residues in polyA motif allowed for the formation of stable nanofibril assemblies with a length of approximately 200 nm, which were not prone to disassemble or aggregate as that of N16C-5A and N13C-12A. More interestingly, the stable fibril assembly of N15C-8A enabled spinning of simultaneously strong (623.3 MPa) and tough (107.1 MJ m-3) synthetic fibers with fine molecular orientation and close interface packing of fibril bundles. This work highlights that modulation of polyA motifs is a feasible way to tune the morphology and stability of the spidroin preassemblies in dope solutions, thus controlling the structural and mechanical properties of the resulting fibers.

Custom-designed, mass silk production in genetically engineered silkworms

Genetically engineered silkworms have been widely used to obtain silk with modified characteristics especially by introducing spider silk genes. However, these attempts are still challenging due to limitations in transformation strategies and difficulties in integration of the large DNA fragments. Here, we describe three different transformation strategies in genetically engineered silkworms, including transcription-activator-like effector nuclease (TALEN)-mediated fibroin light chain (FibL) fusion (BmFibL-F), TALEN-mediated FibH replacement (BmFibH-R), and transposon-mediated genetic transformation with the silk gland-specific fibroin heavy chain (FibH) promoter (BmFibH-T). As the result, the yields of exogenous silk proteins, a 160 kDa major ampullate spidroin 2 (MaSp2) from the orb-weaving spider Nephila clavipes and a 226 kDa fibroin heavy chain protein (EvFibH) from the bagworm Eumeta variegate, reach 51.02 and 64.13% in BmFibH-R transformed cocoon shells, respectively. Moreover, the presence of MaSp2 or EvFibH significantly enhances the toughness of genetically engineered silk fibers by ∼86% in BmFibH-T and ∼80% in BmFibH-R silkworms, respectively. Structural analysis reveals a substantial ∼40% increase in fiber crystallinity, primarily attributed to the presence of unique polyalanines in the repetitive sequences of MaSp2 or EvFibH. In addition, RNA-seq analysis reveals that BmFibH-R system only causes minor impact on the expression of endogenous genes. Our study thus provides insights into developing custom-designed silk production using the genetically engineered silkworm as the bioreactor.

Decoding silkworm spinning programmed by pH and metal ions

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

Replicating shear-mediated self-assembly of spider silk through microfluidics

The development of artificial spider silk with properties similar to native silk has been a challenging task in materials science. In this study, we use a microfluidic device to create continuous fibers based on recombinant MaSp2 spidroin. The strategy incorporates ion-induced liquid-liquid phase separation, pH-driven fibrillation, and shear-dependent induction of β-sheet formation. We find that a threshold shear stress of approximately 72 Pa is required for fiber formation, and that β-sheet formation is dependent on the presence of polyalanine blocks in the repetitive sequence. The MaSp2 fiber formed has a β-sheet content (29.2%) comparable to that of native dragline with a shear stress requirement of 111 Pa. Interestingly, the polyalanine blocks have limited influence on the occurrence of liquid-liquid phase separation and hierarchical structure. These results offer insights into the shear-induced crystallization and sequence-structure relationship of spider silk and have significant implications for the rational design of artificially spun fibers.

The role of flow in the self-assembly of dragline spider silk proteins

Hydrodynamic flow in the spider duct induces conformational changes in dragline spider silk proteins (spidroins) and drives their assembly, but the underlying physical mechanisms are still elusive. Here we address this challenging multiscale problem with a complementary strategy of atomistic and coarse-grained molecular dynamics simulations with uniform flow. The conformational changes at the molecular level were analyzed for single-tethered spider silk peptides. Uniform flow leads to coiled-to-stretch transitions and pushes alanine residues into β sheet and poly-proline II conformations. Coarse-grained simulations of the assembly process of multiple semi-flexible block copolymers using multi-particle collision dynamics reveal that the spidroins aggregate faster but into low-order assemblies when they are less extended. At medium-to-large peptide extensions (50%-80%), assembly slows down and becomes reversible with frequent association and dissociation events, whereas spidroin alignment increases and alanine repeats form ordered regions. Our work highlights the role of flow in guiding silk self-assembly into tough fibers by enhancing alignment and kinetic reversibility, a mechanism likely relevant also for other proteins whose function depends on hydrodynamic flow.

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.

Factors Influencing Properties of Spider Silk Coatings and Their Interactions within a Biological Environment

Biomaterials are an indispensable part of biomedical research. However, although many materials display suitable application-specific properties, they provide only poor biocompatibility when implanted into a human/animal body leading to inflammation and rejection reactions. Coatings made of spider silk proteins are promising alternatives for various applications since they are biocompatible, non-toxic and anti-inflammatory. Nevertheless, the biological response toward a spider silk coating cannot be generalized. The properties of spider silk coatings are influenced by many factors, including silk source, solvent, the substrate to be coated, pre- and post-treatments and the processing technique. All these factors consequently affect the biological response of the environment and the putative application of the appropriate silk coating. Here, we summarize recently identified factors to be considered before spider silk processing as well as physicochemical characterization methods. Furthermore, we highlight important results of biological evaluations to emphasize the importance of adjustability and adaption to a specific application. Finally, we provide an experimental matrix of parameters to be considered for a specific application and a guided biological response as exemplarily tested with two different fibroblast cell lines.

Adenylate Kinase Fused to Spidroin as a Catalyst for Decreasing Leakage out of 3D-Bioprinted Hydrogels and for ATP Regeneration

Numerous metabolic reactions and pathways use adenosine 5'-triphosphate (ATP) as an energy source and as a phosphorous or pyrophosphorous donor. Based on three-dimensional (3D)-printing, enzyme immobilization can be used to improve ATP regeneration and operability and reduce cost. However, due to the relatively large mesh size of 3D-bioprinted hydrogels soaked in a reaction solution, the lower-molecular-weight enzymes cannot avoid leaking out of the hydrogels readily. Here, a chimeric adenylate-kinase-spidroin (ADK-RC) is created, with ADK serving as the N-terminal domain. The chimera is capable of self-assembling to form micellar nanoparticles at a higher molecular scale. Although fused to spidroin (RC), ADK-RC remains relatively consistent and exhibits high activity, thermostability, pH stability, and organic solvent tolerance. Considering different surface-to-volume ratios, three shapes of enzyme hydrogels are designed, 3D bioprinted, and measured. In addition, a continuous enzymatic reaction demonstrates that ADK-RC hydrogels have higher specific activity and substrate affinity but a lower reaction rate and catalytic power compared to free enzymes in solution. With ATP regeneration, the ADK and ADK-RC hydrogels significantly increase the production of d-glucose-6-phosphate and obtain an efficient usage frequency. In conclusion, enzymes fused to spidroin might be an efficient strategy for maintaining activity and reducing leakage in 3D-bioprinted hydrogels under mild conditions.

Tyrosine's Unique Role in the Hierarchical Assembly of Recombinant Spider Silk Proteins: From Spinning Dope to Fibers

Producing recombinant spider silk fibers that exhibit mechanical properties approaching native spider silk is highly dependent on the constitution of the spinning dope. Previously published work has shown that recombinant spider silk fibers spun from dopes with phosphate-induced pre-assembly (biomimetic dopes) display a toughness approaching native spider silks far exceeding the mechanical properties of fibers spun from dopes without pre-assembly (classical dopes). Dynamic light scattering experiments comparing the two dopes reveal that biomimetic dope displays a systematic increase in assembly size over time, while light microscopy indicates liquid-liquid-phase separation (LLPS) as evidenced by the formation of micron-scale liquid droplets. Solution nuclear magnetic resonance (NMR) shows that the structural state in classical and biomimetic dopes displays a general random coil conformation in both cases; however, some subtle but distinct differences are observed, including a more ordered state for the biomimetic dope and small chemical shift perturbations indicating differences in hydrogen bonding of the protein in the different dopes with notable changes occurring for Tyr residues. Solid-state NMR demonstrates that the final wet-spun fibers from the two dopes display no structural differences of the poly(Ala) stretches, but biomimetic fibers display a significant difference in Tyr ring packing in non-β-sheet, disordered helical domains that can be traced back to differences in dope preparations. It is concluded that phosphate pre-orders the recombinant silk protein in biomimetic dopes resulting in LLPS and fibers that exhibit vastly improved toughness that could be due to aromatic ring packing differences in non-β-sheet domains that contain Tyr.

Biological Materials Processing: Time-Tested Tricks for Sustainable Fiber Fabrication

There is an urgent need to improve the sustainability of the materials we produce and use. Here, we explore what humans can learn from nature about how to sustainably fabricate polymeric fibers with excellent material properties by reviewing the physical and chemical aspects of materials processing distilled from diverse model systems, including spider silk, mussel byssus, velvet worm slime, hagfish slime, and mistletoe viscin. We identify common and divergent strategies, highlighting the potential for bioinspired design and technology transfer. Despite the diversity of the biopolymeric fibers surveyed, we identify several common strategies across multiple systems, including: (1) use of stimuli-responsive biomolecular building blocks, (2) use of concentrated fluid precursor phases (e.g., coacervates and liquid crystals) stored under controlled chemical conditions, and (3) use of chemical (pH, salt concentration, redox chemistry) and physical (mechanical shear, extensional flow) stimuli to trigger the transition from fluid precursor to solid material. Importantly, because these materials largely form and function outside of the body of the organisms, these principles can more easily be transferred for bioinspired design in synthetic systems. We end the review by discussing ongoing efforts and challenges to mimic biological model systems, with a particular focus on artificial spider silks and mussel-inspired materials.

Structural insight on the liquid silk from the middle silk gland of non-mulberry silkworm Antheraea assamensis

This study highlights the preliminary characterization of liquid silk from the middle silk gland (MSG) along with the in-silico analysis of the sericin protein of a less explored non mulberry silkworm Antheraea assamensis which is endemic to the North Eastern region of India. Various biophysical methods have been applied to elucidate the conformational patterns of the liquid silk present inside the MSG without removing the sericin layer. This will help us to know the actual features of the in vivo transitional status of the silk in the MSG which travel towards the anterior silk gland (ASG) prior to spinning. The SDS PAGE analysis represented the existence of the both fibroin and sericin bands in the sample. The structural pattern of the MSG liquid silk as revealed by various methods denoted the occurrence of β-sheet component along with some random coil and β-turn components which in turn suggests the transitional state of the liquid silk attributed to the existence of both the crystalline and amorphous contents. The thermo gravimetric study and the aggregation behavior analysis results proposed the occurrence of intermolecular hydrogen bonding between the sericin and fibroin in the MSG. This study will sensitize the better understanding of the behavior of the liquid silk in the MSG of non-mulberry silkworm A. assamensis and will open avenues for various application-based studies of this silk.Communicated by Ramaswamy H. Sarma.

Micro and nano-scale compartments guide the structural transition of silk protein monomers into silk fibers

Silk is a unique, remarkably strong biomaterial made of simple protein building blocks. To date, no synthetic method has come close to reproducing the properties of natural silk, due to the complexity and insufficient understanding of the mechanism of the silk fiber formation. Here, we use a combination of bulk analytical techniques and nanoscale analytical methods, including nano-infrared spectroscopy coupled with atomic force microscopy, to probe the structural characteristics directly, transitions, and evolution of the associated mechanical properties of silk protein species corresponding to the supramolecular phase states inside the silkworm's silk gland. We found that the key step in silk-fiber production is the formation of nanoscale compartments that guide the structural transition of proteins from their native fold into crystalline β-sheets. Remarkably, this process is reversible. Such reversibility enables the remodeling of the final mechanical characteristics of silk materials. These results open a new route for tailoring silk processing for a wide range of new material formats by controlling the structural transitions and self-assembly of the silk protein's supramolecular phases.

Spidroin N-terminal domain forms amyloid-like fibril based hydrogels and provides a protein immobilization platform

Recombinant spider silk proteins (spidroins) have multiple potential applications in development of novel biomaterials, but their multimodal and aggregation-prone nature have complicated production and straightforward applications. Here, we report that recombinant miniature spidroins, and importantly also the N-terminal domain (NT) on its own, rapidly form self-supporting and transparent hydrogels at 37 °C. The gelation is caused by NT α-helix to β-sheet conversion and formation of amyloid-like fibrils, and fusion proteins composed of NT and green fluorescent protein or purine nucleoside phosphorylase form hydrogels with intact functions of the fusion moieties. Our findings demonstrate that recombinant NT and fusion proteins give high expression yields and bestow attractive properties to hydrogels, e.g., transparency, cross-linker free gelation and straightforward immobilization of active proteins at high density.

Recombinant spider silks are of interest but the multimodal and aggregation-prone nature of them is a limitation. Here, the authors report on a miniature spidroin based on the N-terminal domain which forms a hydrogel at 37 °C which allows for ease of production and fusion protein modification to generate functional biomaterials.

Complexity of Spider Dragline Silk

The tiny spider makes dragline silk fibers with unbeatable toughness, all under the most innocuous conditions. Scientists have persistently tried to emulate its natural silk spinning process using recombinant proteins with a view toward creating a new wave of smart materials, yet most efforts have fallen short of attaining the native fiber’s excellent mechanical properties. One reason for these shortcomings may be that artificial spider silk systems tend to be overly simplified and may not sufficiently take into account the true complexity of the underlying protein sequences and of the multidimensional aspects of the natural self-assembly process that give rise to the hierarchically structured fibers. Here, we discuss recent findings regarding the material constituents of spider dragline silk, including novel spidroin subtypes, nonspidroin proteins, and possible involvement of post-translational modifications, which together suggest a complexity that transcends the two-component MaSp1/MaSp2 system. We subsequently consider insights into the spidroin domain functions, structures, and overall mechanisms for the rapid transition from disordered soluble protein into a highly organized fiber, including the possibility of viewing spider silk self-assembly through a framework relevant to biomolecular condensates. Finally, we consider the concept of “biomimetics” as it applies to artificial spider silk production with a focus on key practical aspects of design and evaluation that may hopefully inform efforts to more closely reproduce the remarkable structure and function of the native silk fiber using artificial methods.

A Novel Approach for the Production of Aggregation-Prone Proteins Using the Spidroin-Derived NT* Tag

Spiders have evolved proteins that can be kept in a highly concentrated soluble form in the silk gland yet rapidly assemble into stable silk fibers under certain environmental conditions. The transition between soluble and fibrillar states is partly regulated by the pH-sensitive N-terminal (NT) domain which has emerged as nature's own solubility-enhancing domain. NT has an inherent capacity to keep the silk proteins' partly hydrophobic and very aggregation-prone regions from premature fibrillation in spite of storage at enormous concentrations. The genetically engineered double-mutant NT* shows increased solubility and stability and has arisen as a powerful tool for the production of aggregation-prone as well as other recombinant proteins. Here we describe a robust and highly efficient protocol for improved soluble expression of peptides and proteins by fusion to the NT* tag.

Fibrous Scaffolds From Elastin-Based Materials

Current cutting-edge strategies in biomaterials science are focused on mimicking the design of natural systems which, over millions of years, have evolved to exhibit extraordinary properties. Based on this premise, one of the most challenging tasks is to imitate the natural extracellular matrix (ECM), due to its ubiquitous character and its crucial role in tissue integrity. The anisotropic fibrillar architecture of the ECM has been reported to have a significant influence on cell behaviour and function. A new paradigm that pivots around the idea of incorporating biomechanical and biomolecular cues into the design of biomaterials and systems for biomedical applications has emerged in recent years. Indeed, current trends in materials science address the development of innovative biomaterials that include the dynamics, biochemistry and structural features of the native ECM. In this context, one of the most actively studied biomaterials for tissue engineering and regenerative medicine applications are nanofiber-based scaffolds. Herein we provide a broad overview of the current status, challenges, manufacturing methods and applications of nanofibers based on elastin-based materials. Starting from an introduction to elastin as an inspiring fibrous protein, as well as to the natural and synthetic elastin-based biomaterials employed to meet the challenge of developing ECM-mimicking nanofibrous-based scaffolds, this review will follow with a description of the leading strategies currently employed in nanofibrous systems production, which in the case of elastin-based materials are mainly focused on supramolecular self-assembly mechanisms and the use of advanced manufacturing technologies. Thus, we will explore the tendency of elastin-based materials to form intrinsic fibers, and the self-assembly mechanisms involved. We will describe the function and self-assembly mechanisms of silk-like motifs, antimicrobial peptides and leucine zippers when incorporated into the backbone of the elastin-based biomaterial. Advanced polymer-processing technologies, such as electrospinning and additive manufacturing, as well as their specific features, will be presented and reviewed for the specific case of elastin-based nanofiber manufacture. Finally, we will present our perspectives and outlook on the current challenges facing the development of nanofibrous ECM-mimicking scaffolds based on elastin and elastin-like biomaterials, as well as future trends in nanofabrication and applications.

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.

Investigating the Atomic and Mesoscale Interactions that Facilitate Spider Silk Protein Pre-Assembly

Black widow spider dragline silk is one of nature's high-performance biological polymers, exceeding the strength and toughness of most man-made materials including high tensile steel and Kevlar. Major ampullate (Ma), or dragline silk, is primarily comprised of two spidroin proteins (Sp) stored within the Ma gland. In the native gland environment, the MaSp1 and MaSp2 proteins self-associate to form hierarchical 200-300 nm superstructures despite being intrinsically disordered proteins (IDPs). Here, dynamic light scattering (DLS), three-dimensional (3D) triple resonance solution NMR, and diffusion NMR is utilized to probe the MaSp size, molecular structure, and dynamics of these protein pre-assemblies diluted in 4 M urea and identify specific regions of the proteins important for silk protein pre-assembly. 3D NMR indicates that the Gly-Ala-Ala and Ala-Ala-Gly motifs flanking the poly(Ala) runs, which comprise the β-sheet forming domains in fibers, are perturbed by urea, suggesting that these regions may be important for silk protein pre-assembly stabilization.

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'.

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.

Interplay of Different Major Ampullate Spidroins during Assembly and Implications for Fiber Mechanics

Major ampullate (MA) spider silk has fascinating mechanical properties combining strength and elasticity. All known natural MA silks contain at least two or more different spidroins; however, it is unknown why and if there is any interplay in the spinning dope. Here, two different spidroins from Araneus diadematus are co-produced in Escherichia coli to study the possible dimerization and effects thereof on the mechanical properties of fibers. During the production of the two spidroins, a mixture of homo- and heterodimers is formed triggered by the carboxyl-terminal domains. Interestingly, homodimeric species of the individual spidroins self-assemble differently in comparison to heterodimers, and stoichiometric mixtures of homo- and heterodimers yield spidroin networks upon assembly with huge impact on fiber mechanics upon spinning. The obtained results provide the basis for man-made tuning of spinning dopes to yield high-performance fibers.

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.

Spider silk self-assembly via modular liquid-liquid phase separation and nanofibrillation

Spider silk fiber rapidly assembles from spidroin protein in soluble state via an incompletely understood mechanism. Here, we present an integrated model for silk formation that incorporates the effects of multiple chemical and physical gradients on the different spidroin functional domains. Central to the process is liquid-liquid phase separation (LLPS) that occurs in response to multivalent anions such as phosphate, mediated by the carboxyl-terminal and repetitive domains. Acidification coupled with LLPS triggers the swift self-assembly of nanofibril networks, facilitated by dimerization of the amino-terminal domain, and leads to a liquid-to-solid phase transition. Mechanical stress applied to the fibril structures yields macroscopic fibers with hierarchical organization and enriched for β-sheet conformations. Studies using native silk gland material corroborate our findings on spidroin phase separation. Our results suggest an intriguing parallel between silk assembly and other LLPS-mediated mechanisms, such as found in intracellular membraneless organelles and protein aggregation disorders.

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.

Recombinant Spidroins as the Basis for New Materials

Spider web proteins are unique materials created by nature that, considering the combination of their properties, do not have analogues among natural or human-created materials. Obtaining significant amounts of these proteins from natural sources is not feasible. Biotechnological manufacturing in heterological systems is complicated by the very high molecular weight of spidroins and their specific amino acid composition. Obtaining recombinant analogues of spidroins in heterological systems, mainly in bacteria and yeast, has become a compromise solution. Because they can self-assemble, these proteins can form various materials, such as fibers, films, 3D-foams, hydrogels, tubes, and microcapsules. The effectiveness of spidroin hydrogels in deep wound healing, as 3D scaffolds for bone tissue regeneration and as oriented fibers for axon growth and nerve tissue regeneration, was demonstrated in animal models. The possibility to use spidroin micro- and nanoparticles for drug delivery was demonstrated, including the use of modified spidroins for virus-free DNA delivery into animal cell nuclei. In the past few years, significant interest has arisen concerning the use of these materials as biocompatible and biodegradable soft optics to construct photonic crystal super lenses and fiber optics and as soft electronics to use in triboelectric nanogenerators. This review summarizes the latest achievements in the field of spidroin production, the creation of materials based on them, the study of these materials as a scaffold for the growth, proliferation, and differentiation of various types of cells, and the prospects for using these materials for medical applications (e.g., tissue engineering, drug delivery, coating medical devices), soft optics, and electronics. Accumulated data suggest the use of recombinant spidroins in medical practice in the near future.

Hydrothermal Effect on Mechanical Properties of Nephila pilipes Spidroin

The superlative mechanical properties of spider silk and its conspicuous variations have instigated significant interest over the past few years. However, current attempts to synthetically spin spider silk fibers often yield an inferior physical performance, owing to the improper molecular interactions of silk proteins. Considering this, herein, a post-treatment process to reorganize molecular structures and improve the physical strength of spider silk is reported. The major ampullate dragline silk from Nephila pilipes with a high β-sheet content and an adequate tensile strength was utilized as the study material, while that from Cyrtophora moluccensis was regarded as a reference. Our results indicated that the hydrothermal post-treatment (50-70 °C) of natural spider silk could effectively induce the alternation of secondary structures (random coil to β-sheet) and increase the overall tensile strength of the silk. Such advantageous post-treatment strategy when applied to regenerated spider silk also leads to an increment in the strength by ~2.5-3.0 folds, recapitulating ~90% of the strength of native spider silk. Overall, this study provides a facile and effective post-spinning means for enhancing the molecular structures and mechanical properties of as-spun silk threads, both natural and regenerated.

High intracellular stability of the spidroin N-terminal domain in spite of abundant amyloidogenic segments revealed by in-cell hydrogen/deuterium exchange mass spectrometry

Proteins require an optimal balance of conformational flexibility and stability in their native environment to ensure their biological functions. A striking example is spidroins, spider silk proteins, which are stored at extremely high concentrations in soluble form, yet undergo amyloid-like aggregation during spinning. Here, we elucidate the stability of the highly soluble N-terminal domain (NT) of major ampullate spidroin 1 in the Escherichia coli cytosol as well as in inclusion bodies containing fibrillar aggregates. Surprisingly, we find that NT, despite being largely composed of amyloidogenic sequences, showed no signs of concentration-dependent aggregation. Using a novel intracellular hydrogen/deuterium exchange mass spectrometry (HDX-MS) approach, we reveal that NT adopts a tight fold in the E. coli cytosol and in this manner conceals its aggregation-prone regions by maintaining a tight fold under crowded conditions. Fusion of NT to the unstructured amyloid-forming Aβ₄₀ peptide, on the other hand, results in the formation of fibrillar aggregates. However, HDX-MS indicates that the NT domain is only partially incorporated into these aggregates in vivo. We conclude that NT is able to control its aggregation to remain functional under the extreme conditions in the spider silk gland.

Chemical Synthesis of Silk-Mimetic Polymers

Silk is a naturally occurring high-performance material that can surpass man-made polymers in toughness and strength. The remarkable mechanical properties of silk result from the primary sequence of silk fibroin, which bears semblance to a linear segmented copolymer with alternating rigid ("crystalline") and flexible ("amorphous") blocks. Silk-mimetic polymers are therefore of great emerging interest, as they can potentially exhibit the advantageous features of natural silk while possessing synthetic flexibility as well as non-natural compositions. This review describes the relationships between primary sequence and material properties in natural silk fibroin and furthermore discusses chemical approaches towards the synthesis of silk-mimetic polymers. In particular, step-growth polymerization, controlled radical polymerization, and copolymerization with naturally derived silk fibroin are presented as strategies for synthesizing silk-mimetic polymers with varying molecular weights and degrees of sequence control. Strategies for improving macromolecular solubility during polymerization are also highlighted. Lastly, the relationships between synthetic approach, supramolecular structure, and bulk material properties are explored in this review, with the aim of providing an informative perspective on the challenges facing chemical synthesis of silk-mimetic polymers with desirable properties.

The transcriptome of Darwin's bark spider silk glands predicts proteins contributing to dragline silk toughness

Darwin's bark spider (Caerostris darwini) produces giant orb webs from dragline silk that can be twice as tough as other silks, making it the toughest biological material. This extreme toughness comes from increased extensibility relative to other draglines. We show C. darwini dragline-producing major ampullate (MA) glands highly express a novel silk gene transcript (MaSp4) encoding a protein that diverges markedly from closely related proteins and contains abundant proline, known to confer silk extensibility, in a unique GPGPQ amino acid motif. This suggests C. darwini evolved distinct proteins that may have increased its dragline's toughness, enabling giant webs. Caerostris darwini's MA spinning ducts also appear unusually long, potentially facilitating alignment of silk proteins into extremely tough fibers. Thus, a suite of novel traits from the level of genes to spinning physiology to silk biomechanics are associated with the unique ecology of Darwin's bark spider, presenting innovative designs for engineering biomaterials.

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.

Recombinant Silk Fiber Properties Correlate to Prefibrillar Self-Assembly

Spider silks are desirable materials with mechanical properties superior to most synthetic materials coupled with biodegradability and biocompatibility. In order to replicate natural silk properties using recombinant spider silk proteins (spidroins) and wet-spinning methods, the focus to date has typically been on modifying protein sequence, protein size, and spinning conditions. Here, an alternative approach is demonstrated. Namely, using the same ≈57 kDa recombinant aciniform silk protein with a consistent wet-spinning protocol, fiber mechanical properties are shown to significantly differ as a function of the solvent used to dissolve the protein at high concentration (the "spinning dope" solution). A fluorinated acid/alcohol/water dope leads to drastic improvement in fibrillar extensibility and, correspondingly, toughness compared to fibers produced using a previously developed fluorinated alcohol/water dope. To understand the underlying cause for these mechanical differences, morphology and structure of the two classes of silk fiber are compared, with features tracing back to dope-state protein structuring and preassembly. Specifically, distinct classes of spidroin nanoparticles appear to form in each dope prior to fiber spinning and these preassembled states are, in turn, linked to fiber morphology, structure, and mechanical properties. Tailoring of dope-state spidroin nanoparticle assembly, thus, appears a promising strategy to modulate fibrillar silk properties.