Biomimetic fibers made of recombinant spidroins with the same toughness as natural spider silk

Biosynthetic Structural Proteins with Super Plasticity, Extraordinary Mechanical Performance, Biodegradability, Biocompatibility and Information Storage Ability

Degradable bioplastics have attracted growing interest worldwide. However, it is challenging to develop bioplastics with a simple processing procedure, strong mechanical performance, good biocompatibility, and adjustable physicochemical properties. Herein, we introduced structural proteins as building blocks and developed a simple environmentally friendly approach to fabricate diverse protein-based plastics. A cost-effective and high-level production approach was developed through batch fermentation of Escherichia coli to produce the biomaterials. These bioplastics possess super plasticity, biocompatibility, biodegradability, and high resistance to organic solvents. Their structural and mechanical properties can be precisely controlled. Besides, high density information storage and hemostatic applications were realized in the bioplastic system. The customizable bioplastics have great potential for applications in numerous fields and are capable to scale up to the industrial level.

Site-Specific Functionalization of Recombinant Spider Silk Janus Fibers

Biotechnological production is a powerful tool to design materials with customized properties. The aim of this work was to apply designed spider silk proteins to produce Janus fibers with two different functional sides. First, functionalization was established through a cysteine-modified silk protein, ntagCys eADF4(κ16). After fiber spinning, gold nanoparticles (AuNPs) were coupled via thiol-ene click chemistry. Significantly reduced electrical resistivity indicated sufficient loading density of AuNPs on such fiber surfaces. Then, Janus fibers were electrospun in a side-by-side arrangement, with "non-functional" eADF4(C16) on the one and "functional" ntagCys eADF4(κ16) on the other side. Post-treatment was established to render silk fibers insoluble in water. Subsequent AuNP binding was highly selective on the ntagCys eADF4(κ16) side demonstrating the potential of such silk-based systems to realize complex bifunctional structures with spatial resolutions in the nano scale.

Connecting materials, performance and evolution: a case study of the glue of moth-catching spiders (Cyrtarachninae)

Morphological structures and extended phenotypes are made possible by materials that are encoded by the genome. Nearly all biomaterials are viscoelastic, which means that to understand performance, one must understand the strain rate-dependent properties of these materials in relevant ecological interactions, as the behavior of a material can vary dramatically and rapidly. Spider silks are an example of materials whose properties vary substantially intra- and inter-specifically. Here, we focus on aggregate silk, which functions as a biological adhesive. As a case study to understand how a material manifests from genome through organism to ecology, we highlight moth-specialist spiders, the Cyrtarachninae, and their glues as an ideal experimental system to investigate the relationship between genomics and ecologically variable performance of a biological material. There is a clear eco-evolutionary innovation that Cyrtarachne akirai and related species have evolved, a unique trait not found in other spiders, a glue which overcomes the scales of moths. By examining traditional orb-weavers, C. akirai and other subfamily members using biomechanical testing and genomic analysis, we argue that we can track the evolution of this novel bioadhesive and comment on the selection pressures influencing prey specialization. The importance of the ecological context of materials testing is exemplified by the poor performance of C. akirai glue on glass and the exceptional spreading ability and adhesive strength on moths. The genetic basis for these performance properties is experimentally tractable because spider silk genes are minimally pleiotropic and advances in genomic technologies now make possible the discovery of complete silk gene sequences.

Spider Silk-Inspired Artificial Fibers

Spider silk is a natural polymeric fiber with high tensile strength, toughness, and has distinct thermal, optical, and biocompatible properties. The mechanical properties of spider silk are ascribed to its hierarchical structure, including primary and secondary structures of the spidroins (spider silk proteins), the nanofibril, the "core-shell", and the "nano-fishnet" structures. In addition, spider silk also exhibits remarkable properties regarding humidity/water response, water collection, light transmission, thermal conductance, and shape-memory effect. This motivates researchers to prepare artificial functional fibers mimicking spider silk. In this review, the authors summarize the study of the structure and properties of natural spider silk, and the biomimetic preparation of artificial fibers from different types of molecules and polymers by taking some examples of artificial fibers exhibiting these interesting properties. In conclusion, biomimetic studies have yielded several noteworthy findings in artificial fibers with different functions, and this review aims to provide indications for biomimetic studies of functional fibers that approach and exceed the properties of natural spider silk.

Highly Stiff and Stretchable DNA Liquid Crystalline Organogels with Super Plasticity, Ultrafast Self-Healing, and Magnetic Response Behaviors

DNA-based gels are attractive materials as they allow intuitive rational design, respond to external physicochemical stimuli, and show great potential for biomedical applications. However, their relatively poor mechanical properties currently limit their technological application considerably as the latter requires mechanical integrity and tunability. With this work, a DNA organogel is reported that gels through supramolecular interactions, which induce mesophase ordering, and that exhibits exceptional stretchability, deformability, plasticity, and biocompatibility. Moreover, the nature of the supramolecular bond enables complete self-healing within 3 s. Most importantly, the DNA-based liquid crystalline organogels exhibit impressive ultimate tensile strengths above 1 MPa, stiffness higher than 20 MPa, and toughness up to 18 MJ m^-3 , rendering these materials the strongest among reported DNA networks. In addition, the facile access is demonstrated to composite DNA materials by blending magnetic nanoparticles with the organogel matrix giving access to magnetic field induced actuation. It is believed that these findings contribute significantly to the advancement of DNA gels for their use in smart materials and biomedical applications.

A review on advances in the applications of spider silk in biomedical issues

Spider silk, as one of the hardest natural and biocompatible substances with extraordinary strength and flexibility, have become an ideal option in various areas of science and have made their path onto the biomedical industry. Despite its growing popularity, the difficulties in the extraction of silks from spiders and farming them have made it unaffordable and almost impossible for industrial scale. Biotechnology helped production of spider silks recombinantly in different hosts and obtaining diverse morphologies out of them based on different processing and assembly procedures. Herein, the characteristics of these morphologies and their advantages and disadvantages are summarized. A detailed view about applications of recombinant silks in skin regeneration and cartilage, tendon, bone, teeth, cardiovascular, and neural tissues engineering are brought out, where there is a need for strong scaffolds to support cell growth. Likewise, spider silk proteins have applications as conduit constructs, medical sutures, and 3D printer bioinks. Other characteristics of spider silks, such as low immunogenicity, hydrophobicity, homogeneity, and adjustability, have attracted much attention in drug and gene delivery. Finally, the challenges and obstacles ahead for industrializing the production of spider silk proteins in sufficient quantities in biomedicine, along with solutions to overcome these barriers, are discussed.

Interfacial Crystallization and Supramolecular Self-Assembly of Spider Silk Inspired Protein at the Water-Air Interface

Macromolecular assembly into complex morphologies and architectural shapes is an area of fundamental research and technological innovation. In this work, we investigate the self-assembly process of recombinantly produced protein inspired by spider silk (spidroin). To elucidate the first steps of the assembly process, we examined highly concentrated and viscous pendant droplets of this protein in air. We show how the protein self-assembles and crystallizes at the water-air interface into a relatively thick and highly elastic skin. Using time-resolved in situ synchrotron x-ray scattering measurements during the drying process, we showed that the skin evolved to contain a high β-sheet amount over time. We also found that β-sheet formation strongly depended on protein concentration and relative humidity. These had a strong influence not only on the amount, but also on the ordering of these structures during the β-sheet formation process. We also showed how the skin around pendant droplets can serve as a reservoir for attaining liquid-liquid phase separation and coacervation from the dilute protein solution. Essentially, this study shows a new assembly route which could be optimized for the synthesis of new materials from a dilute protein solution and determine the properties of the final products.

Microbially Synthesized Polymeric Amyloid Fiber Promotes β-Nanocrystal Formation and Displays Gigapascal Tensile Strength

The ability of amyloid proteins to form stable β-sheet nanofibrils has made them potential candidates for material innovation in nanotechnology. However, such a nanoscale feature has rarely translated into attractive macroscopic properties for mechanically demanding applications. Here, we present a strategy by fusing amyloid peptides with flexible linkers from spidroin; the resulting polymeric amyloid proteins can be biosynthesized using engineered microbes and wet-spun into macroscopic fibers. Using this strategy, fibers from three different amyloid groups were fabricated. Structural analyses unveil the presence of β-nanocrystals that resemble the cross-β structure of amyloid nanofibrils. These polymeric amyloid fibers have displayed strong and molecular-weight-dependent mechanical properties. Fibers made of a protein polymer containing 128 repeats of the FGAILSS sequence displayed an average ultimate tensile strength of 0.98 ± 0.08 GPa and an average toughness of 161 ± 26 MJ/m³, surpassing most recombinant protein fibers and even some natural spider silk fibers. The design strategy and the biosynthetic approach can be expanded to create numerous functional materials, and the macroscopic amyloid fibers will enable a wide range of mechanically demanding 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.

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.

Applications of bacteria and their derived biomaterials for repair and tissue regeneration

Microorganisms such as bacteria and their derived biopolymers can be used in biomaterials and tissue regeneration. Various methods have been applied to regenerate damaged tissues, but using probiotics and biomaterials derived from bacteria with improved economic-production efficiency and highly applicable properties can be a new solution in tissue regeneration. Bacteria can synthesize numerous types of biopolymers. These biopolymers possess many desirable properties such as biocompatibility and biodegradability, making them good candidates for tissue regeneration. Here, we reviewed different types of bacterial-derived biopolymers and highlight their applications for tissue regeneration.

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.

Bioactive hierarchical silk fibers created by bioinspired self-assembly

Artificial recapitulation of the hierarchy of natural protein fibers is crucial to providing strategies for developing advanced fibrous materials. However, it is challenging due to the complexity of the natural environment. Inspired by the liquid crystalline spinning of spiders, we report the development of natural silk-like hierarchical fibers, with bundles of nanofibrils aligned in their long-axis direction, by self-assembly of crystallized silk fibroin (SF) droplets. The formation of self-assembled SF fibers is a process of coalesced droplets sprouting to form a branched fibrous network, which is similar to the development of capillaries in our body. The as-assembled hierarchical SF fibers are highly bioactive and can significantly enhance the spreading and growth of human umbilical vein endothelial cells compared to the natural SF fibers. This work could help to understand the natural silk spinning process of spiders and provides a strategy for design and development of advanced fibrous biomaterials for various applications.

Xanthurenic Acid Is the Main Pigment of Trichonephila clavata Gold Dragline Silk

Spider silk is a natural fiber with remarkable strength, toughness, and elasticity that is attracting attention as a biomaterial of the future. Golden orb-weaving spiders (Trichonephila clavata) construct large, strong webs using golden threads. To characterize the pigment of golden T. clavata dragline silk, we used liquid chromatography and mass spectrometric analysis. We found that the major pigment in the golden dragline silk of T. clavata was xanthurenic acid. To investigate the possible function of the pigment, we tested the effect of xanthurenic acid on bacterial growth using gram-negative Escherichia coli and gram-positive Bacillus subtilis. We found that xanthurenic acid had a slight antibacterial effect. Furthermore, to investigate the UV tolerance of the T. clavata threads bleached of their golden color, we conducted tensile deformation tests and scanning electron microscope observations. However, in these experiments, no significant effect was observed. We therefore speculate that golden orb-weaving spiders use the pigment for other purposes, such as to attract their prey in the sunlight.

Recent Advances in 3D Printing with Protein-Based Inks

Three-dimensional (3D) printing is a transformative manufacturing strategy, allowing rapid prototyping, customization, and flexible manipulation of structure-property relationships. Proteins are particularly appealing to formulate inks for 3D printing as they serve as essential structural components of living systems, provide a support presence in and around cells and for tissue functions, and also provide the basis for many essential ex vivo secreted structures in nature. Protein-based inks are beneficial in vivo due to their mechanics, chemical and physical match to the specific tissue, and full degradability, while also to promoting implant-host integration and serving as an interface between technology and biology. Exploiting the biological, chemical, and physical features of protein-based inks can provide key opportunities to meet the needs of tissue engineering and regenerative medicine. Despite these benefits, protein-based inks impose nontrivial challenges to 3D printing such as concentration and rheological features and reconstitution of the structural hierarchy observed in nature that is a source of the robust mechanics and functions of these materials. This review introduces photo-crosslinking mechanisms and rheological principles that underpins a variety of 3D printing techniques. The review also highlights recent advances in the design, development, and biomedical utility of monolithic and composite inks from a range of proteins, including collagen, silk, fibrinogen, and others. One particular focus throughout the review is to introduce unique material characteristics of proteins, including amino acid sequences, molecular assembly, and secondary conformations, which are useful for designing printing inks and for controlling the printed structures. Future perspectives of 3D printing with protein-based inks are also provided to support the promising spectrum of biomedical research accessible to these materials.

Silk Fibroin: An Ancient Material for Repairing the Injured Nervous System

Silk refers to a family of natural fibers spun by several species of invertebrates such as spiders and silkworms. In particular, silkworm silk, the silk spun by Bombyx mori larvae, has been primarily used in the textile industry and in clinical settings as a main component of sutures for tissue repairing and wound ligation. The biocompatibility, remarkable mechanical performance, controllable degradation, and the possibility of producing silk-based materials in several formats, have laid the basic principles that have triggered and extended the use of this material in regenerative medicine. The field of neural soft tissue engineering is not an exception, as it has taken advantage of the properties of silk to promote neuronal growth and nerve guidance. In addition, silk has notable intrinsic properties and the by-products derived from its degradation show anti-inflammatory and antioxidant properties. Finally, this material can be employed for the controlled release of factors and drugs, as well as for the encapsulation and implantation of exogenous stem and progenitor cells with therapeutic capacity. In this article, we review the state of the art on manufacturing methodologies and properties of fiber-based and non-fiber-based formats, as well as the application of silk-based biomaterials to neuroprotect and regenerate the damaged nervous system. We review previous studies that strategically have used silk to enhance therapeutics dealing with highly prevalent central and peripheral disorders such as stroke, Alzheimer's disease, Parkinson's disease, and peripheral trauma. Finally, we discuss previous research focused on the modification of this biomaterial, through biofunctionalization techniques and/or the creation of novel composite formulations, that aim to transform silk, beyond its natural performance, into more efficient silk-based-polymers towards the clinical arena of neuroprotection and regeneration in nervous system diseases.

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.

Dichroic Fourier Transform Infrared Spectroscopy Characterization of the β-Sheet Orientation in Spider Silk Films on Silicon Substrates

Orientation analysis of the β-sheet structure within films of the established recombinant spider silk protein eADF4(C16) was performed using a concept based on dichroic transmission- and attenuated total reflection-Fourier transform infrared spectroscopy, lineshape analysis, assignment of amide I components to specific vibration modes, and transition dipole moment directions of β-sheet structures. Based on the experimental dichroic ratio R, the order parameter S of β-sheet structures was calculated with respect to uniaxial orientation. Films of eADF4(C16) were deposited on untexturized (Si) and unidirectionally scratched silicon substrates (Si-sc) and post-treated with MeOH vapor. Freshly cast thin and thick eADF4(C16) films out of hexafluoroisopropanol featured β-sheet contents of ≈6%, which increased to >30% after MeOH post-treatment in dependence of time. Pseudo-first order folding kinetics were obtained, suggesting a transition from an unfolded to a folded state. In MeOH post-treated thin films with diameters in the nanometer range, a significant orientation of β-sheets was obtained regardless of the texturization of the silicon substrate (Si, Si-sc). This was rationalized by dichroic ratios of the amide I component at 1696 cm^-1 assigned to the (0, π) mode of antiparallel β-sheet structures, whose transition dipole moment M is located in parallel to both β-sheet plane and chain direction. The calculated high molecular order parameter S ≈ 0.40 suggested vertically (out-of-plane) oriented antiparallel β-sheet stacks with tilt angles of γ ≈ 39° to the surface normal. Microscale (thick) films, in contrast, revealed low order parameters S ≈ 0. Scanning force microscopy on thin eADF4 films at silicon substrates showed dewetted polymer film structures rather at the micro-scale. These findings give new insights in the role of the β-sheet crystallite orientation for the mechanical properties of spider silk materials.

Functionalization of biopolymer fibers with magnetic nanoparticles

Hybrid fibers consisting of biopolymers and inorganic nanoparticles are receiving increasing attention due to their unique properties. Commonly, the nanoparticles are chosen for their intrinsic properties such as magnetic, thermal, or electrical conductivity. The biopolymer component of the hybrid fiber is chosen for its mechanical properties and ability to act as a scaffold or matrix for the nanoparticles. While there are many fiber-forming synthetic polymers, there has been a recent interest in replacing these systems with biopolymers due to their sustainability, biocompatibility, nontoxicity, and biodegradability. Fibers made from biopolymers have one additional benefit over synthetic polymers as they make good scaffolds for embedding nanoparticles without the need of any additional bonding agents. In particular, naturally occurring biopolymers such as proteins exhibit a myriad of interactions with nanoparticles, including ionic, H-bonding, covalent, Van der Waals, and electrostatic interactions. The diverse range of interactions between magnetic nanoparticles and biopolymers makes resulting hybrid fibers of particular interest as magnetic-responsive materials. Magnetically responsive hybrid biopolymer fibers have many features, including enhanced thermal stabilities, strong mechanical toughness, and perhaps most interestingly multifunctionality, allowing for a wide range of applications. These applications range from biosensing, filtration, UV shielding, antimicrobial, and medical applications, to name a few. Here, we review established hybrid fibers consisting of biopolymers and nanoparticles with a primary focus on biopolymers doped with magnetic nanoparticles and their various putative applications.

Characterization of the second type of aciniform spidroin (AcSp2) provides new insight into design for spidroin-based biomaterials

Spiders spin a range of silks from different glands for distinct functions, and each silk type exhibits distinct material properties. Silk extruded by the aciniform gland is used for prey wrapping and egg case construction and displays high toughness and extensibility. So far, only the aciniform spidroin 1 (AcSp1) gene which was firstly identified as a silk gene in aciniform gland has been obtained. Here we present the gene sequence for the second type of full-length aciniform silk protein, AcSp2. Analysis of the AcSp2 primary sequence reveals relatively conserved terminal regions and a distinct repetitive sequence relative to AcSp1. A fraction of the gene can be expressed in recombinant systems. Secondary structure analysis of the recombinant AcSp2 protein in solution reveals that the protein adopts mainly an α-helical conformation. Artificial spinning of recombinant AcSp2 demonstrates that the spidroins can be spun into fine fibers which display up to 142% extensibility. The silk fibers are dominated by β-sheet and β-turn secondary structures. Moreover, the mechanical data collected from these synthetic fibers revealed that the mechanical properties are partly correlated with the molecular weights. Overall, our studies enrich our knowledge of spidroin gene family members and provide a new insight into creation of high-performance silk fibers for next generation biomaterials. STATEMENT OF SIGNIFICANCE: In this study, we presented the second type of aciniform silk protein (AcSp2) gene sequence of orb-weaving spider Araneus ventricosus, expanding the spider silk gene family members. The primary structure revealed the central repetitive sequence of the new spidroin gene is distinctly different from other AcSp1 genes. Characterization of the recombinant minispidroin fibers of AcSp2 revealed the mechanical properties are partly correlate with the molecular weights, and the spidroins can be spun into fine fibers which display up to 142% extensibility. Overall, our studies enrich our knowledge of spidroin gene family members and provide a new insight into creation of high-performance silk fibers for next generation biomaterials.

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.

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.

Surface Modification of Spider Silk Particles to Direct Biomolecular Corona Formation

In recent years, spider silk-based materials have attracted attention because of their biocompatibility, processability, and biodegradability. For their potential use in biomaterial applications, i.e., as drug delivery systems and implant coatings for tissue regeneration, it is vital to understand the interactions between the silk biomaterial surface and the biological environment. Like most polymeric carrier systems, spider silk material surfaces can adsorb proteins when in contact with blood, resulting in the formation of a biomolecular corona. Here, we assessed the effect of surface net charge of materials made of recombinant spider silk on the biomolecular corona composition. In-depth proteomic analysis of the biomolecular corona revealed that positively charged spider silk materials surfaces interacted predominantly with fibrinogen-based proteins. This fibrinogen enrichment correlated with blood clotting observed for both positively charged spider silk films and particles. In contrast, negative surface charges prevented blood clotting. Genetic engineering allows the fine-tuning of surface properties of the spider silk particles providing a whole set of recombinant spider silk proteins with different charges or peptide tags to be used for, for example, drug delivery or cell docking, and several of these were analyzed concerning the composition of their biomolecular corona. Taken together this study demonstrates how the surface net charge of recombinant spider silk surfaces affects the composition of the biomolecular corona, which in turn affects macroscopic effects such as fibrin formation and blood clotting.

Tough synthetic spider-silk fibers obtained by titanium dioxide incorporation and formaldehyde cross-linking in a simple wet-spinning process

Due to its unique mechanical properties, spider silk shows great promise as a strong super-thin fiber in many fields. Although progress has been made in the field of synthesizing spider-silk fiber from recombinant spidroin (spider silk protein) in the last few decades, methods to obtain synthetic spider-silk fibers as tough as natural silk from small-sized recombinant protein with a simple spinning process have eluded scientists. In this paper, a recombinant spidroin (MW: 93.4 kDa) was used to spin tough synthetic spider-silk fibers with a simple wet-spinning process. Titanium oxide incorporation and formaldehyde cross-linking were used to improve the mechanical properties of synthetic spider-silk fibers. Fibers treated with incorporation or/and cross-linking varied in microstructure, strength and extensibility while all exhibited enhanced strength and toughness. In particular, one fiber possessed a toughness of 249 ± 22 MJ/m³. This paper presents a new method to successfully spin tough spider-silk fibers in a simple way.

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.

Modulating the Mechanical Performance of Macroscale Fibers through Shear-Induced Alignment and Assembly of Protein Nanofibrils

Protein-based fibers are used by nature as high-performance materials in a wide range of applications, including providing structural support, creating thermal insulation, and generating underwater adhesives. Such fibers are commonly generated through a hierarchical self-assembly process, where the molecular building blocks are geometrically confined and aligned along the fiber axis to provide a high level of structural robustness. Here, this approach is mimicked by using a microfluidic spinning method to enable precise control over multiscale order during the assembly process of nanoscale protein nanofibrils into micro- and macroscale fibers. By varying the flow rates on chip, the degree of nanofibril alignment can be tuned, leading to an orientation index comparable to that of native silk. It is found that the Young's modulus of the resulting fibers increases with an increasing level of nanoscale alignment of the building blocks, suggesting that the mechanical properties of macroscopic fibers can be controlled through varying the level of ordering of the nanoscale building blocks. Capitalizing on strategies evolved by nature, the fabrication method allows for the controlled formation of macroscopic fibers and offers the potential to be applied for the generation of further novel bioinspired materials.

Engineering the Architecture of Elastin-Like Polypeptides: From Unimers to Hierarchical Self-Assembly

Well-defined tunable nanostructures formed through the hierarchical self-assembly of peptide building blocks have drawn significant attention due to their potential applications in biomedical science. Artificial protein polymers derived from elastin-like polypeptides (ELPs), which are based on the repeating sequence of tropoelastin (the water-soluble precursor to elastin), provide a promising platform for creating nanostructures due to their biocompatibility, ease of synthesis, and customizable architecture. By designing the sequence and composition of ELPs at the gene level, their physicochemical properties can be controlled to a degree that is unmatched by synthetic polymers. A variety of ELP-based nanostructures are designed, inspired by the self-assembly of elastin and other proteins in biological systems. The choice of building blocks determines not only the physical properties of the nanostructures, but also their self-assembly into architectures ranging from spherical micelles to elongated nanofibers. This review focuses on the molecular determinants of ELP and ELP-hybrid self-assembly and formation of spherical, rod-like, worm-like, fibrillar, and vesicle architectures. A brief discussion of the potential biomedical applications of these supramolecular assemblies is also included.

Spider Silk for Tissue Engineering Applications

Due to its properties, such as biodegradability, low density, excellent biocompatibility and unique mechanics, spider silk has been used as a natural biomaterial for a myriad of applications. First clinical applications of spider silk as suture material go back to the 18th century. Nowadays, since natural production using spiders is limited due to problems with farming spiders, recombinant production of spider silk proteins seems to be the best way to produce material in sufficient quantities. The availability of recombinantly produced spider silk proteins, as well as their good processability has opened the path towards modern biomedical applications. Here, we highlight the research on spider silk-based materials in the field of tissue engineering and summarize various two-dimensional (2D) and three-dimensional (3D) scaffolds made of spider silk. Finally, different applications of spider silk-based materials are reviewed in the field of tissue engineering in vitro and in vivo.

Fabrication and Mechanical Properties of Engineered Protein-Based Adhesives and Fibers

Protein-based structural biomaterials are of great interest for various applications because the sequence flexibility within the proteins may result in their improved mechanical and structural integrity and tunability. As the two representative examples, protein-based adhesives and fibers have attracted tremendous attention. The typical protein adhesives, which are secreted by mussels, sandcastle worms, barnacles, and caddisfly larvae, exhibit robust underwater adhesion performance. In order to mimic the adhesion performance of these marine organisms, two main biological adhesives are presented, including genetically engineered protein-based adhesives and biomimetic chemically synthetized adhesives. Moreover, various protein-based fibers inspired by spider and silkworm proteins, collagen, elastin, and resilin are studied extensively. The achievements in synthesis and fabrication of structural biomaterials by DNA recombinant technology and chemical regeneration certainly will accelerate the explorations and applications of protein-based adhesives and fibers in wound healing, tissue regeneration, drug delivery, biosensors, and other high-tech applications. However, the mechanical properties of the biological structural materials still do not match those of natural systems. More efforts need to be devoted to the study of the interplay of the protein structure, cohesion and adhesion effects, fiber processing, and mechanical performance.

Determination of the Complete Elasticity of Nephila pilipes Spider Silk

Spider silks are remarkable materials designed by nature to have extraordinary elasticity. Their elasticity, however, remains poorly understood, as typical stress–strain experiments only allow access to the axial Young’s modulus. In this work, micro-Brillouin light spectroscopy (micro-BLS), a noncontact, nondestructive technique, is utilized to probe the direction-dependent phonon propagation in the Nephila pilipes spider silk and hence solve its full elasticity. To the best of our knowledge, this is the first demonstration on the determination of the anisotropic Young’s moduli, shear moduli, and Poisson’s ratios of a single spider fiber. The axial and lateral Young’s moduli are found to be 20.9 ± 0.8 and 9.2 ± 0.3 GPa, respectively, and the anisotropy of the Young’s moduli further increases upon stretching. In contrast, the shear moduli and Poisson’s ratios exhibit very weak anisotropy and are robust to stretching.

Fiber Formation from Liquid Crystalline Collagen Vesicles Isolated from Mussels

Marine mussels (Mytilus edulis) fabricate byssal threads, high-performance biopolymeric fibers, which exhibit exceptional toughness and self-healing capacity. These properties are associated with collagenous proteins in the fibrous thread core known as preCols that self-organize into a hierarchical semicrystalline structure. Threads assemble individually in a bottom-up process lasting just minutes via secretion of membrane bound vesicles filled with preCols. However, very little is understood about the details and dynamics of this assembly process. Here, we explore the hypothesis that preCols are stored within the vesicles in a liquid crystalline phase, which contributes to fiber assembly by preordering molecules. To achieve this, a protocol was developed for extracting and isolating intact preCol secretory vesicles in high yield and purity. Vesicles were characterized and were manipulated in vitro, clearly indicating the dynamic liquid crystalline nature of the proteins within. Moreover, mechanical shearing of vesicles led to formation of highly birefringent preCol fibers. These findings have relevance for efforts toward sustainable production of advanced polymeric materials, and possibly for engineering biomedical scaffolds based on collagenous proteins.

Hierarchical Structure of Silk Materials Versus Mechanical Performance and Mesoscopic Engineering Principles

A comprehensive review on the five levels of hierarchical structures of silk materials and the correlation with macroscopic properties/performance of the silk materials, that is, the toughness, strain-stiffening, etc., is presented. It follows that the crystalline binding force turns out to be very important in the stabilization of silk materials, while the β-crystallite networks or nanofibrils and the interactions among helical nanofibrils are two of the most essential structural elements, which to a large extent determine the macroscopic performance of various forms of silk materials. In this context, the characteristic structural factors such as the orientation, size, and density of β-crystallites are very crucial. It is revealed that the formation of these structural elements is mainly controlled by the intermolecular nucleation of β-crystallites. Consequently, the rational design and reconstruction of silk materials can be implemented by controlling the molecular nucleation via applying sheering force and seeding (i.e., with carbon nanotubes). In general, the knowledge of the correlation between hierarchical structures and performance provides an understanding of the structural reasons behind the fascinating behaviors of silk materials.

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.

Biomimetic composites with enhanced toughening using silk-inspired triblock proteins and aligned nanocellulose reinforcements

Silk and cellulose are biopolymers that show strong potential as future sustainable materials. They also have complementary properties, suitable for combination in composite materials where cellulose would form the reinforcing component and silk the tough matrix. A major challenge concerns balancing structure and functional properties in the assembly process. We used recombinant proteins with triblock architecture, combining structurally modified spider silk with terminal cellulose affinity modules. Flow alignment of cellulose nanofibrils and triblock protein allowed continuous fiber production. Protein assembly involved phase separation into concentrated coacervates, with subsequent conformational switching from disordered structures into β sheets. This process gave the matrix a tough adhesiveness, forming a new composite material with high strength and stiffness combined with increased toughness. We show that versatile design possibilities in protein engineering enable new fully biological materials and emphasize the key role of controlled assembly at multiple length scales for realization.

Understanding Secondary Structures of Silk Materials via Micro- and Nano-Infrared Spectroscopies

The secondary structures (also termed conformations) of silk fibroin (SF) in animal silk fibers and regenerated SF materials are critical in determining mechanical performance and function of the materials. In order to understand the structure-mechanics-function relationships of silk materials, a variety of advanced infrared spectroscopic techniques, such as micro-infrared spectroscopies (micro-IR spectroscopies for short), synchrotron micro-IR spectroscopy, and nano-infrared spectroscopies (nano-IR spectroscopies for short), have been used to determine the conformations of SF in silk materials. These IR spectroscopic methods provide a useful toolkit to understand conformations and conformational transitions of SF in various silk materials with spatial resolution from the nano-scale to the micro-scale. In this Review, we first summarize progress in understanding the structure and structure-mechanics relationships of silk materials. We then discuss the state-of-the-art micro- and nano-IR spectroscopic techniques used for silk materials characterization. We also provide a systematic discussion of the strategies to collect high-quality spectra and the methods to analyze these spectra. Finally, we demonstrate the challenges and directions for future exploration of silk-based materials with IR spectroscopies.

Shear-Induced β-Crystallite Unfolding in Condensed Phase Nanodroplets Promotes Fiber Formation in a Biological Adhesive

Natural materials provide an increasingly important role model for the development and processing of next-generation polymers. The velvet worm Euperipatoides rowelli hunts using a projectile, mechanoresponsive adhesive slime that rapidly and reversibly transitions into stiff glassy polymer fibers following shearing and drying. However, the molecular mechanism underlying this mechanoresponsive behavior is still unclear. Previous work showed the slime to be an emulsion of nanoscale charge-stabilized condensed droplets comprised primarily of large phosphorylated proteins, which under mechanical shear coalesce and self-organize into nano- and microfibrils that can be drawn into macroscopic fibers. Here, we utilize wide-angle X-ray diffraction and vibrational spectroscopy coupled with in situ shear deformation to explore the contribution of protein conformation and mechanical forces to the fiber formation process. Although previously believed to be unstructured, our findings indicate that the main phosphorylated protein component possesses a significant β-crystalline structure in the storage phase and that shear-induced partial unfolding of the protein is a key first step in the rapid self-organization of nanodroplets into fibers. The insights gained here have relevance for sustainable production of advanced polymeric materials.

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.