The Rheology behind Stress-Induced Solidification in Native Silk Feedstocks

Metal ions guide the production of silkworm silk fibers

Silk fibers' unique mechanical properties have made them desirable materials, yet their formation mechanism remains poorly understood. While ions are known to support silk fiber production, their exact role has thus far eluded discovery. Here, we use cryo-electron microscopy coupled with elemental analysis to elucidate the changes in the composition and spatial localization of metal ions during silk evolution inside the silk gland. During the initial protein secretion and storage stages, ions are homogeneously dispersed in the silk gland. Once the fibers are spun, the ions delocalize from the fibroin core to the sericin-coating layer, a process accompanied by protein chain alignment and increased feedstock viscosity. This change makes the protein more shear-sensitive and initiates the liquid-to-solid transition. Selective metal ion doping modifies silk fibers' mechanical performance. These findings enhance our understanding of the silk fiber formation mechanism, laying the foundations for developing new concepts in biomaterial design.

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

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

Silk fibroin-based inks for in situ 3D printing using a double crosslinking process

The shortage of tissues and organs for transplantation is an urgent clinical concern. In situ 3D printing is an advanced 3D printing technique aimed at printing the new tissue or organ directly in the patient. The ink for this process is central to the outcomes, and must meet specific requirements such as rapid gelation, shape integrity, stability over time, and adhesion to surrounding healthy tissues. Among natural materials, silk fibroin exhibits fascinating properties that have made it widely studied in tissue engineering and regenerative medicine. However, further improvements in silk fibroin inks are needed to match the requirements for in situ 3D printing. In the present study, silk fibroin-based inks were developed for in situ applications by exploiting covalent crosslinking process consisting of a pre-photo-crosslinking prior to printing and in situ enzymatic crosslinking. Two different silk fibroin molecular weights were characterized and the synergistic effect of the covalent bonds with shear forces enhanced the shift in silk secondary structure toward β-sheets, thus, rapid stabilization. These hydrogels exhibited good mechanical properties, stability over time, and resistance to enzymatic degradation over 14 days, with no significant changes over time in their secondary structure and swelling behavior. Additionally, adhesion to tissues in vitro was demonstrated.

Real-Time Monitoring of the Structural Transition of Bombyx mori Liquid Silk under Pressure by Solid-State NMR

Silk fibroin is stored in the silk glands of Bombyx mori silkworms as a condensed aqueous solution called liquid silk. It is converted into silk fibers at the silkworm's spinnerets under mechanical forces including shear stress and pressure. However, the detailed mechanism of the structural transition of liquid silk to silk fibers under pressure is not well understood. Magic angle spinning (MAS) in solid-state nuclear magnetic resonance (NMR) can exert pressure on liquid samples in a quantitative manner. In this study, solid-state NMR was used to quantitatively analyze the impact of pressure on the structural transition of liquid silk. A combination of 13C DD-MAS and CP-MAS NMR measurements enabled the conformation and dynamics of the crystalline region of the silk fibroin (both before (Silk Ip) and after (Silk IIp) the structural transition) to be detected in real time with atomic resolution. Spectral analyses proposed that the pressure-induced structural transition from Silk Ip to Silk IIp proceeds by a two-step autocatalytic reaction mechanism. The first reaction step is a nucleation step in which Silk Ip transforms to single lamellar Silk IIp, and the second is a growth step in which the single lamellar Silk IIp acts as a catalyst that reacts with Silk Ip molecules to further form Silk IIp molecules, resulting in stacked lamellar Silk IIp. Furthermore, the rate constant in the second step shows a significant pressure dependence, with an increase in pressure accelerating the formation of large stacked lamellar Silk IIp.

Continuous Wet Spinning of Regenerated Silk Fibers from Spinning Dopes Containing 4% Fibroin Protein

The wet spinning of fibers from regenerated silk fibroin has long been a research goal. Due to the degradation of the molecular structure of the fibroin protein during the preparation of the regenerated silk fibroin solution, fibroin concentrations with at least 10% protein content are required to achieve sufficient viscosity for wet spinning. In this study, a spinning dope formulation of regenerated silk fibroin is presented that shows a rheological behavior similar to that of native silk fibroin isolated from the glands of B. mori silkworm larvae. In addition, we present a wet-spinning process that enables, for the first time, the continuous wet spinning of regenerated silk fibroin with only 4% fibroin protein content into an endless fiber. Furthermore, the tensile strength of these wet-spun regenerated silk fibroin fibers per percentage of fibroin is higher than that of all continuous spinning approaches applied to regenerated and native silk fibroin published so far.

Kinetic Analysis Reveals the Role of Secondary Nucleation in Regenerated Silk Fibroin Self-Assembly

Silk proteins obtained from the Bombyx mori silkworm have been extensively studied due to their remarkable mechanical properties. One of the major structural components of this complex material is silk fibroin, which can be isolated and processed further in vitro to form artificial functional materials. Due to the excellent biocompatibility and rich self-assembly behavior, there has been sustained interest in such materials formed through the assembly of regenerated silk fibroin feedstocks. The molecular mechanisms by which the soluble regenerated fibroin molecules self-assemble into protein nanofibrils remain, however, largely unknown. Here, we use the framework of chemical kinetics to connect macroscopic measurements of regenerated silk fibroin self-assembly to the underlying microscopic mechanisms. Our results reveal that the aggregation of regenerated silk fibroin is dominated by a nonclassical secondary nucleation processes, where the formation of new fibrils is catalyzed by the existing aggregates in an autocatalytic manner. Such secondary nucleation pathways were originally discovered in the context of polymerization of disease-associated proteins, but the present results demonstrate that this pathway can also occur in functional assembly. Furthermore, our results show that shear flow induces the formation of nuclei, which subsequently accelerate the process of aggregation through an autocatalytic amplification driven by the secondary nucleation pathway. Taken together, these results allow us to identify the parameters governing the kinetics of regenerated silk fibroin self-assembly and expand our current understanding of the spinning of bioinspired protein-based fibers, which have a wide range of applications in materials science.

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.

Use of Bombyx mori silk fibroin in tissue engineering: From cocoons to medical devices, challenges, and future perspectives

Silk fibroin has become a prominent material in tissue engineering (TE) over the last 20 years with almost 10,000 published works spanning in all the TE applications, from skeleton to neuronal regeneration. Fibroin is an extremely versatile biopolymer that, due to its ease of processing, has enabled the development of an entire plethora of materials whose properties and architectures can be tailored to suit target applications. Although the research and development of fibroin TE materials and devices is mature, apart from sutures, only a few medical products made of fibroin are used in the clinical routines. <40 clinical trials of Bombyx mori silk-related products have been reported by the FDA and few of them resulted in a commercialized device. In this review, after explaining the structure and properties of silk fibroin, we provide an overview of both fibroin constructs existing in the literature and fibroin devices used in clinic. Through the comparison of these two categories, we identified the burning issues faced by fibroin products during their translation to the market. Two main aspects will be considered. The first is the standardization of production processes, which leads both to the standardization of the characteristics of the issued device and the correct assessment of its failure. The second is the FDA regulations, which allow new devices to be marketed through the 510(k) clearance by demonstrating their equivalence to a commercialized medical product. The history of some fibroin medical devices will be taken as a case study. Finally, we will outline a roadmap outlining what actions we believe are needed to bring fibroin products to the market.

Seeking Solvation: Exploring the Role of Protein Hydration in Silk Gelation

The mechanism by which arthropods (e.g., spiders and many insects) can produce silk fibres from an aqueous protein (fibroin) solution has remained elusive, despite much scientific investigation. In this work, we used several techniques to explore the role of a hydration shell bound to the fibroin in native silk feedstock (NSF) from Bombyx mori silkworms. Small angle X-ray and dynamic light scattering (SAXS and DLS) revealed a coil size (radius of gyration or hydrodynamic radius) around 12 nm, providing considerable scope for hydration. Aggregation in dilute aqueous solution was observed above 65 °C, matching the gelation temperature of more concentrated solutions and suggesting that the strength of interaction with the solvent (i.e., water) was the dominant factor. Infrared (IR) spectroscopy indicated decreasing hydration as the temperature was raised, with similar changes in hydration following gelation by freezing or heating. It was found that the solubility of fibroin in water or aqueous salt solutions could be described well by a relatively simple thermodynamic model for the stability of the protein hydration shell, which suggests that the affected water is enthalpically favoured but entropically penalised, due to its reduced (vibrational or translational) dynamics. Moreover, while the majority of this investigation used fibroin from B. mori, comparisons with published work on silk proteins from other silkworms and spiders, globular proteins and peptide model systems suggest that our findings may be of much wider significance.

Toughening Wet-Spun Silk Fibers by Silk Nanofiber Templating

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

Study on the Effect of Stretching on the Strength of Natural Silk Based on Different Feeding Methods

Silk is an important biological protein fiber, which has been widely developed and used in textile and biomedical fields due to its excellent mechanical properties and good biocompatibility. Strength is an important indicator that determines the value and use of silk. Although investigations have been made on the mechanical properties of silkworm silks and their dependence relationship with the microstructures, the variation of silk strength formed in the process of silkworm spinning has not been reported. By feeding the same strain of silkworms with mulberry leaves, mulberry leaves + artificial feed, and artificial feed, silks with three filament sizes were obtained, respectively. The tensile test results showed that the strength and filament size of silk are inversely proportional. The structure and fibrosis process of different-strength silks were analyzed. The results showed that, compared with ordinary silk, the β-sheet and crystallinity content of high-strength silk is higher, indicating that its fibrosis process is more sufficient. We proposed that the stretched degree of silk protein determines its structure and properties. During the spinning process of individual silkworms, the secretion of silk protein is not stable, which will cause changes in the stretched degree. The measurement results of the intraindividual stretched degree and strength verified that the degree of stretch determines the strength of the silk. This study not only provides a deeper understanding of the properties of silk protein but also is of interest for the design and development of advanced biomimetic silk materials.

Structure of Silk I (Bombyx mori Silk Fibroin before Spinning) -Type II β-Turn, Not α-Helix

Recently, considerable attention has been paid to Bombyx mori silk fibroin by a range of scientists from polymer chemists to biomaterial researchers because it has excellent physical properties, such as strength, toughness, and biocompatibility. These appealing physical properties originate from the silk fibroin structure, and therefore, structural determinations of silk fibroin before (silk I) and after (silk II) spinning are a key to make wider applications of silk. There are discrepancies about the silk I structural model, i.e., one is type II β-turn structure determined using many solid-state and solution NMR spectroscopies together with selectively stable isotope-labeled model peptides, but another is α-helix or partially α-helix structure speculated using IR and Raman methods. In this review, firstly, the process that led to type II β-turn structure by the authors was introduced in detail. Then the problems in speculating silk I structure by IR and Raman methods were pointed out together with the problem in the assignment of the amide I band in the spectra. It has been emphasized that the conformational analyses of proteins and peptides from IR and Raman studies are not straightforward and should be very careful when the proteins contain β-turn structure using many experimental data by Vass et al. In conclusion, the author emphasized here that silk I structure should be type II β-turn, not α-helix.

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.

Complex fluids in animal survival strategies

Animals have evolved distinctive survival strategies in response to constant selective pressure. In this review, we highlight how animals exploit flow phenomena by manipulating their habitat (exogenous) or by secreting (endogenous) complex fluids. Ubiquitous endogenous complex fluids such as mucus demonstrate rheological versatility and are therefore involved in many animal behavioral traits ranging from sexual reproduction to protection against predators. Exogenous complex fluids such as sand can be used either for movement or for predation. In all cases, time-dependent rheological properties of complex fluids are decisive for the fate of the biological behavior and vice versa. To exploit these rheological properties, it is essential that the animal is able to sense the rheology of their surrounding complex fluids in a timely fashion. As timing is key in nature, such rheological materials often have clearly defined action windows matching the time frame of their direct biological behavior. As many rheological properties of these biological materials remain poorly studied, we demonstrate with this review that rheology and material science might provide an interesting quantitative approach to study these biological materials in particular in context towards ethology and bio-mimicking material design.

Stretching of Bombyx mori Silk Protein in Flow

The flow-induced self-assembly of entangled Bombyx mori silk proteins is hypothesised to be aided by the ‘registration’ of aligned protein chains using intermolecularly interacting ‘sticky’ patches. This suggests that upon chain alignment, a hierarchical network forms that collectively stretches and induces nucleation in a precisely controlled way. Through the lens of polymer physics, we argue that if all chains would stretch to a similar extent, a clear correlation length of the stickers in the direction of the flow emerges, which may indeed favour such a registration effect. Through simulations in both extensional flow and shear, we show that there is, on the other hand, a very broad distribution of protein–chain stretch, which suggests the registration of proteins is not directly coupled to the applied strain, but may be a slow statistical process. This qualitative prediction seems to be consistent with the large strains (i.e., at long time scales) required to induce gelation in our rheological measurements under constant shear. We discuss our perspective of how the flow-induced self-assembly of silk may be addressed by new experiments and model development.

Interpenetrated biosurfactant-silk fibroin networks - a SANS study

Silk fibroin (SF) based hydrogels have been exploited for years for their inherent biocompatibility and favorable mechanical properties which makes them interesting for biotechnology applications. In this study we investigate silk based composite hydrogels where pH-sensitive, anionic biosurfactant assemblies (sophorolipids SL-C18 : 1 and SL-C18 : 0), are employed to improve the present properties of SF. Results suggest that the presence of SL surfactant assemblies leads to faster gelling of SF by accelerating the refolding from random coil to β-sheet as shown by infrared and UV-visible spectroscopy. Small angle neutron scattering (SANS) including contrast matching studies show that SF and SL assemblies coexist in a fibrillary network that is, in the case of SL-C18 : 0, interpenetrating. The resulting overall network structure in composite gels is slightly more affected by SL-C18 : 1 than by SL-C18 : 0, whereas the structure of both SF and surfactant assemblies remains unchanged. No disassembly of SL surfactant structures is observed, which gives a new perspective on SF-surfactant interactions. The hydrophobic effect within SF is favored in the presence of SL, leading to faster refolding of SF into β-sheet conformation. The presented composite gels, being an interpenetrating network of which one compound (SL-C18 : 0) can be tweaked by pH, open an interesting option towards improved workability and stimuli responsive mechanical properties of SF based hydrogels with possible applications in controlled cell culture and tissue engineering or drug delivery. The presented SANS analysis approach has the potential to be expanded to other protein-surfactant systems and composite hydrogels.

The influence of metal ions on native silk rheology

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

In-Situ Forming pH and Thermosensitive Injectable Hydrogels to Stimulate Angiogenesis: Potential Candidates for Fast Bone Regeneration Applications

Biomaterials that promote angiogenesis are required for repair and regeneration of bone. In-situ formed injectable hydrogels functionalised with bioactive agents, facilitating angiogenesis have high demand for bone regeneration. In this study, pH and thermosensitive hydrogels based on chitosan (CS) and hydroxyapatite (HA) composite materials loaded with heparin (Hep) were investigated for their pro-angiogenic potential. Hydrogel formulations with varying Hep concentrations were prepared by sol-gel technique for these homogeneous solutions were neutralised with sodium bicarbonate (NaHCO3) at 4 °C. Solutions (CS/HA/Hep) constituted hydrogels setting at 37 °C which was initiated from surface in 5-10 minutes. Hydrogels were characterised by performing injectability, gelation, rheology, morphology, chemical and biological analyses. Hydrogel solutions facilitated manual dropwise injection from 21 Gauge which is highly used for orthopaedic and dental administrations, and the maximum injection force measured through 19 G needle (17.191 ± 2.296N) was convenient for manual injections. Angiogenesis tests were performed by an ex-ovo chick chorioallantoic membrane (CAM) assay by applying injectable solutions on CAM, which produced in situ hydrogels. Hydrogels induced microvascularity in CAM assay this was confirmed by histology analyses. Hydrogels with lower concentration of Hep showed more efficiency in pro-angiogenic response. Thereof, novel injectable hydrogels inducing angiogenesis (CS/HA/Hep) are potential candidates for bone regeneration and drug delivery applications.

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.

The Biomedical Use of Silk: Past, Present, Future

Humans have long appreciated silk for its lustrous appeal and remarkable physical properties, yet as the mysteries of silk are unraveled, it becomes clear that this outstanding biopolymer is more than a high-tech fiber. This progress report provides a critical but detailed insight into the biomedical use of silk. This journey begins with a historical perspective of silk and its uses, including the long-standing desire to reverse engineer silk. Selected silk structure-function relationships are then examined to appreciate past and current silk challenges. From this, biocompatibility and biodegradation are reviewed with a specific focus of silk performance in humans. The current clinical uses of silk (e.g., sutures, surgical meshes, and fabrics) are discussed, as well as clinical trials (e.g., wound healing, tissue engineering) and emerging biomedical applications of silk across selected formats, such as silk solution, films, scaffolds, electrospun materials, hydrogels, and particles. The journey finishes with a look at the roadmap of next-generation recombinant silks, especially the development pipeline of this new industry for clinical use.

Differential Scanning Calorimetry of Native Silk Feedstock

Native silk proteins, extracted directly from the silk gland prior to spinning, offer access to a naturally hydrated protein that has undergone little to no processing. Combined with differential scanning calorimetry (DSC), it is possible to probe the thermal stability and hydration status of silk and thus investigate its denaturation and solidification, echoing that of the natural spinning process. It is found that native silk is stable between -10 °C and 55 °C, and both the high-temperature enthalpy of denaturation (measured via modulated temperature DSC) and a newly reported low-temperature ice-melting transition may serve as useful quality indicators in the future for artificial silks. Finally, compared to albumin, silk's denaturation enthalpy is much lower than expected, which is interpreted within a recently proposed entropic desolvation framework which can serve to unveil the low-energy aquamelt processing pathway.

Extensional flow behaviour and spinnability of native silk

Silk fibres are assembled via flow. While changes in the physiological environment of the gland as well as the shear rheology of silk are largely understood, the effect of extensional flow fields on native silk proteins is almost completely unknown. Here we demonstrate that filament stretching on a conventional tensile tester is a suitable technique to assess silk's extensional flow properties and its ability to form fibres under extensional conditions characteristic of natural spinning. We report that native Bombyx mori silk responds differently to extensional flow fields when compared to synthetic linear polymers, as evidenced by a higher Trouton ratio which we attribute to silk's increased interchain interactions. Finally, we show that native silk proteins can only be spun into stable fibres at low extension rates as a result of dehydration, suggesting that extensional fields alone are unable to induce natural fibre formation.

Changes in Silk Feedstock Rheology during Cocoon Construction: The Role of Calcium and Potassium Ions

Variation in silk feedstocks is a barrier both to our understanding of natural spinning and biomimetic endeavors. To address this, compositional changes are investigated in feedstock specimens from the domesticated silkworm (Bombyx mori). It is found that the feedstock viscosity decreased systematically by over two orders of magnitude during cocoon construction. Potential factors such as protein concentration, molecular weight, pH, or the presence of trehalose are excluded, whereas a clear correlation appear between viscosity and the relative concentrations of Ca²⁺ and K⁺ ions. It is expected that Ca²⁺ ions would favor "salt bridges" between acidic (Asp and Glu) amino acids, leading to an increased viscosity, whereas K⁺ ions would compete for these sites, thereby reducing viscosity. Thus, these findings suggest a simple, systematic yet sophisticated control of feedstock viscosity in the silkworm, which in turn can be applied to future industrial silk production.

Biophotonics of Native Silk Fibrils

Native silk fibroin (NSF) is a unique biomaterial with extraordinary mechanical and biochemical properties. These key characteristics are directly associated with the physical transformation of unstructured, soluble NSF into highly organized nano- and microscale fibrils rich in β-sheet content. Here, it is shown that this NSF fibrillation process is accompanied by the development of intrinsic fluorescence in the visible range, upon near-UV excitation, a phenomenon that has not been investigated in detail to date. Here, the optical and fluorescence characteristics of NSF fibrils are probed and a route for potential applications in the field of self-assembled optically active biomaterials and systems is explored. In particular, it is demonstrated that NSF can be structured into autofluorescent microcapsules with a controllable level of β-sheet content and fluorescence properties. Furthermore, a facile and efficient fabrication route that permits arbitrary patterns of NSF microcapsules to be deposited on substrates under ambient conditions is shown. The resulting fluorescent NSF patterns display a high level of photostability. These results demonstrate the potential of using native silk as a new class of biocompatible photonic material.

The rheological properties of native sericin

Unlike spider silk, spinning silkworm silk has the added intricacy of being both fibre and micron-thick glue-like coating. Whilst the natural flow properties of the fibre feedstock fibroin are now becoming more established, our understanding of the coating sericin is extremely limited and thus presents both a gap in our knowledge and a hindrance to successful exploitation of these materials. In this study we characterise sericin feedstock from the silkworm Bombyx mori in its native state and by employing both biochemical, rheological and spectroscopic tools, define a natural gold standard. Our results demonstrate that native sericin behaves as a viscoelastic shear thinning fluid, but that it does so at a considerably lower viscosity than its partner fibroin, and that its upper critical shear rate (onset of gelation) lies above that of fibroin. Together these findings provide the first evidence that in addition to acting as a binder in the construction of the cocoon, sericin is capable of lubricating the flow of fibroin within the silk gland, which has implications for future processing, modelling and biomimetic use of these materials.

STATEMENT OF SIGNIFICANCE

This study addresses one of the major gaps in our knowledge regarding natural silk spinning by providing rigorous rheological characterisation of the other major protein involved - sericin. This allows progress in silk flow modelling, biomimetic system design, and in assessing the quality of bioinspired and waste sericin materials by providing a better understanding of the native, undegraded system.

Silk nanofibril self-assembly versus electrospinning

Natural silk fibers represent one of the most advanced blueprints for (bio)polymer scientists, displaying highly optimized mechanical properties due to their hierarchical structures. Biotechnological production of silk proteins and implementation of advanced processing methods enabled harnessing the potential of these biopolymer not just based on the mechanical properties. In addition to fibers, diverse morphologies can be produced, such as nonwoven meshes, films, hydrogels, foams, capsules and particles. Among them, nanoscale fibrils and fibers are particularly interesting concerning medical and technical applications due to their biocompatibility, environmental and mechanical robustness as well as high surface-to-volume ratio. Therefore, we introduce here self-assembly of silk proteins into hierarchically organized structures such as supramolecular nanofibrils and fabricated materials based thereon. As an alternative to self-assembly, we also present electrospinning a technique to produce nanofibers and nanofibrous mats. Accordingly, we introduce a broad range of silk-based dopes, used in self-assembly and electrospinning: natural silk proteins originating from natural spinning glands, natural silk protein solutions reconstituted from fibers, engineered recombinant silk proteins designed from natural blueprints, genetic fusions of recombinant silk proteins with other structural or functional peptides and moieties, as well as hybrids of recombinant silk proteins chemically conjugated with nonproteinaceous biotic or abiotic molecules. We highlight the advantages but also point out drawbacks of each particular production route. The scope includes studies of the natural self-assembly mechanism during natural silk spinning, production of silk fibrils as new nanostructured non-native scaffolds allowing dynamic morphological switches, as well as studying potential applications. This article is categorized under:  Biology-Inspired Nanomaterials > Peptide-Based Structures  Nanotechnology Approaches to Biology > Nanoscale Systems in Biology  Biology-Inspired Nanomaterials > Protein and Virus-Based Structures.

Clarity of objectives and working principles enhances the success of biomimetic programs

Biomimetics, the transfer of functional principles from living systems into product designs, is increasingly being utilized by engineers. Nevertheless, recurring problems must be overcome if it is to avoid becoming a short-lived fad. Here we assess the efficiency and suitability of methods typically employed by examining three flagship examples of biomimetic design approaches from different disciplines: (1) the creation of gecko-inspired adhesives; (2) the synthesis of spider silk, and (3) the derivation of computer algorithms from natural self-organizing systems. We find that identification of the elemental working principles is the most crucial step in the biomimetic design process. It bears the highest risk of failure (e.g. losing the target function) due to false assumptions about the working principle. Common problems that hamper successful implementation are: (i) a discrepancy between biological functions and the desired properties of the product, (ii) uncertainty about objectives and applications, (iii) inherent limits in methodologies, and (iv) false assumptions about the biology of the models. Projects that aim for multi-functional products are particularly challenging to accomplish. We suggest a simplification, modularisation and specification of objectives, and a critical assessment of the suitability of the model. Comparative analyses, experimental manipulation, and numerical simulations followed by tests of artificial models have led to the successful extraction of working principles. A searchable database of biological systems would optimize the choice of a model system in top-down approaches that start at an engineering problem. Only when biomimetic projects become more predictable will there be wider acceptance of biomimetics as an innovative problem-solving tool among engineers and industry.

Analysis of the pressure requirements for silk spinning reveals a pultrusion dominated process

Silks are remarkable materials with desirable mechanical properties, yet the fine details of natural production remain elusive and subsequently inaccessible to biomimetic strategies. Improved knowledge of the natural processes could therefore unlock development of a host of bio inspired fibre spinning systems. Here, we use the Chinese silkworm Bombyx mori to review the pressure requirements for natural spinning and discuss the limits of a biological extrusion domain. This provides a target for finite element analysis of the flow of silk proteins, with the aim of bringing the simulated and natural domains into closer alignment. Supported by two parallel routes of experimental validation, our results indicate that natural spinning is achieved, not by extruding the feedstock, but by the pulling of nascent silk fibres. This helps unravel the oft-debated question of whether silk is pushed or pulled from the animal, and provides impetus to the development of pultrusion-based biomimetic spinning devices.The natural production of silks remains elusive and subsequently inaccessible to biomimetic strategies. Here the authors show that silks cannot be spun by pushing alone, and that natural spinning is dominated by pultrusion, which provides design guidelines for future biomimetic spinning systems.