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 degrees C and 55 degrees 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.

Naturally spun silks generate fibres with unique properties, including strength, elasticity and biocompatibility. Here we describe a microfluidics-based strategy to spin liquid native silk, obtained directly from the silk gland of Bombyx mori silkworms, into micron-scale capsules with controllable geometry and variable levels of intermolecular β-sheet content in their protein shells. We demonstrate that such micrococoons can store internally the otherwise highly unstable liquid native silk for several months and without apparent effect on its functionality. We further demonstrate that these native silk micrococoons enable the effective encapsulation, storage and release of other aggregation-prone proteins, such as functional antibodies. These results show that native silk micrococoons are capable of preserving the full activity of sensitive cargo proteins that can aggregate and lose function under conditions of bulk storage, and thus represent an attractive class of materials for the storage and release of active biomolecules.

Silks are a family of semi-crystalline structural materials, spun naturally by insects, spiders and even crustaceans. Compared to the characteristic β-sheet crystalline structure in silks, the non-crystalline structure and its composition deserves more attention as it is equally critical to the filaments' high toughness and strength. Here we further unravel the structure-property relationship in silks using Dynamic Mechanical Thermal Analysis (DMTA). This technique allows us to examine the most important structural relaxation event of the disordered structure the disordered structure, the glass transition (GT), in native silk fibres of the lepidopteran Bombyx mori and Antheraea pernyi and the spider Nephila edulis. The measured glass transition temperature Tg, loss tangent tan δ and dynamic storage modulus are quantitatively modelled based on Group Interaction Modelling (GIM). The "variability" issue in native silks can be conveniently explained by the different degrees of structural disorder as revealed by DMTA. The new insights will facilitate a more comprehensive understanding of the structure-property relations for a wide range of biopolymers.

The mineralization templating of Bombyx mori silkworm silk fibroin film on calcium carbonate is found to depend on the β-structure of the surface.

Here we present a set of measurements using Differential Scanning Fluorimetry (DSF) as an inexpensive, high throughput screening method to investigate the folding of silk protein molecules as they abandon their first native melt conformation, dehydrate and denature into their final solid filament conformation. Our first data and analyses comparing silks from spiders, mulberry and wild silkworms as well as reconstituted ‘silk’ fibroin show that DSF can provide valuable insights into details of silk denaturation processes that might be active during spinning. We conclude that this technique and technology offers a powerful and novel tool to analyse silk protein transitions in detail by allowing many changes to the silk solutions to be tested rapidly with microliter scale sample sizes. Such transition mechanisms will lead to important generic insights into the folding patterns not only of silks but also of other fibrous protein (bio)polymers.

The housing tube material of the marine worm Chaetopterus sp. exhibits thermal stability up to 250°C, similar to other biological materials such as mulberry silkworm cocoons. Interestingly, however, dynamic mechanical thermal analysis conducted in both air and water elucidated the lack of a glass transition in the organic tube wall material. In fact, the viscoelastic properties of the anhydrous and undried tube were remarkably stable (i.e. constant and reversible) between -75°C and 200°C in air, and 5°C and 75°C in water, respectively. Moreover, it was found that hydration and associated-water plasticization were key to the rubber-like flexible properties of the tube; dehydration transformed the material behaviour to glass-like. The tube is made of bionanocomposite fibrils in highly oriented arrangement, which we argue favours the biomaterial to be highly crystalline or cross-linked, with extensive hydrogen and/or covalent bonds. Mechanical property characterization in the longitudinal and transverse directions ascertained that the tubes were not quasi-isotropic structures. In general, the higher stiffness and strength in the transverse direction implied that there were more nanofibrils orientated at ± 45° and ± 65° than at 0° to the tube axis. The order of the mechanical properties of the soft-tough tubes was similar to synthetic rubber-like elastomers and even some viscid silks. The complex structure-property relations observed indicated that the worm has evolved to produce a tubular housing structure which can (i) function stably over a broad range of temperatures, (ii) endure mechanical stresses from specific planes/axes, and (iii) facilitate rapid growth or repair.

The forced reeling of silkworms offers the potential to produce a spectrum of silk filaments, spun from natural silk dope and subjected to carefully controlled applied processing conditions. Here we demonstrate that the envelope of stress–strain properties for forced reeled silks can encompass both naturally spun cocoon silk and unnaturally processed artificial silk filaments. We use dynamic mechanical thermal analysis (DMTA) to quantify the structural properties of these silks. Using this well-established mechanical spectroscopic technique, we show high variation in the mechanical properties and the associated degree of disordered hydrogen-bonded structures in forced reeled silks. Furthermore, we show that this disorder can be manipulated by a range of processing conditions and even ameliorated under certain parameters, such as annealing under heat and mechanical load. We conclude that the powerful combination of forced reeling silk and DMTA has tied together native/natural and synthetic/unnatural extrusion spinning. The presented techniques therefore have the ability to define the potential of Bombyx-derived proteins for use in fibre-based applications and serve as a roadmap to improve fibre quality via post-processing.

While it is common knowledge in natural fibre composites manufacture that plant fibre reinforcements are considerably less compactable than synthetic fibre reinforcements, the through-thickness compaction behaviour of animal-fibre silk reinforcements has not been characterised thus far. We find that not only are silk reinforcements significantly more compressible than plant fibre reinforcements, but their compactibility exceeds that of even glass fibre textiles. For instance, the fibre volume fraction (at a compaction pressure of 2.0bar) of woven biaxial fabrics of silk, plant fibres and E-glass are 54–57%, 30–40% and 49–54%, respectively. Therefore, silks provide an opportunity to manufacture high fibre content natural fibre composites; this is a bottleneck of plant fibre textiles. Analysing the structure of silk textiles through scanning electron microscopy, we show that favourable fibre/yarn/fabric geometry, high degree of fibre alignment and dispersion, and suitable technical fibre properties enable optimal packing and arrangement of silk textiles.

With the growing interest in material technologies incorporating bio-based constituents, substitutes to both conventional polymer foams and their synthetic reinforcements are being sought. In this article, we characterise the physical, mechanical, thermal and environmental resistance properties of novel syntactic foams that are based on silkworm cocoons as natural, hollow particulate fillers for a rigid, bio-based polyurethane matrix. Syntactic foams were cast with the natural cocoons replacing 60-90% by weight (40-70% by volume) of the polymer matrix. Although reinforcing the foam with domesticated and wild cocoons increased its density from 45 kg/m(3) to similar to 60 kg/m(3) and similar to 120 kg/m(3) respectively, a marked increase in both absolute and specific compressive properties was also observed. Comparable thermal and dimensional stability of the proteinaceous cocoon reinforced polymer foams with the un-reinforced foam attest to comparable environmental resistance. (C) 2014 Elsevier Ltd. All rights reserved.

With the growing interest in bio-based composites as alternatives to traditional glass fibre reinforced composites (GFRPs), there has been a persistent rise in the commercial use of plant fibre composites (PFRPs). In contrast, nature’s ‘wonder-fibre’ silk has had no commercial applications, and only limited scientific investigations, as a composite reinforcement. To produce silk fibre composites (SFRPs) with useful properties, three key recommendations from our critical literature review were followed: (i) a high-failure strain, low-processing temperature thermoset matrix was used to (a) maximise the reinforcing effect of low-stiffness, ductile silk, and (b) facilitate impregnation and avoid fibre degradation, (ii) high fibre volume fractions were employed to ensure that fibres carried a larger fraction of the load, and (iii) given the lack of studies investigating fracture energy dissipation mechanisms in SFRPs, interface modification was avoided due to its complex, sometimes detrimental, effects on toughness. In directly addressing the question, ‘is there a case for silks as polymer reinforcements?’, we evaluated various mechanical properties of nonwoven and plain woven SFRPs against similar flax and glass composites. In all cases, woven composites performed better than nonwoven composites. While SFRPs were weak in terms of stiffness, their flexural and tensile strength was comparable to PFRPs, but much below that of GFRPs. Notably, the low density of SFRPs, like PFRPs, made them comparable to GFRPs in terms of specific flexural properties. Woven SFRPs exhibited much higher fracture strain capacities than both flax and glass composites, making SFRPs suitable for applications where high compliance is required. The Achilles’ heels of PFRPs have been their reportedly (i) inadequate interfacial properties, (ii) inferior impact properties, (iii) poor strength performance, and (iv) high moisture sensitivity. We found that SFRPs outperformed their flax counterparts in areas (i)–(iii), and were more comparable to, but not better than, GFRPs. While concerns such as cost and ‘sustainability’ of silk are acknowledged, potential applications for SFRPs are discussed.

A continuous polydiacetylene fibre based on a peptide amphiphile is developed to exhibit ultrafast, reversible thermochromism, and a general and effective model is discovered to quantitatively predict the critical temperature of the chromatic transition.

Variability is a common feature of natural silk fibres, caused by a range of natural processing conditions. Better understanding of variability will not only be favourable for explaining the enviable mechanical properties of animal silks but will provide valuable information for the design of advanced artificial and biomimetic silk-like materials. In this work, we have investigated the origin of variability in forcibly reeled Antheraea pernyi silks from different individuals using dynamic mechanical thermal analysis (DMTA) combined with the effect of polar solvent penetration. Quasi-static tensile curves in different media have been tested to show the considerable variability of tensile properties between samples from different silkworms. The DMTA profiles (as a function of temperature or humidity) through the glass transition region of different silks as well as dynamic mechanical properties after high temperature and water annealing are analysed in detail to identify the origin of silk variability in terms of molecular structures and interactions, which indicate that different hydrogen bonded structures exist in the amorphous regions and they are notably different for silks from different individuals. Solubility parameter effects of solvents are quantitatively correlated with the different glass transitions values. Furthermore, the overall ordered fraction is shown to be a key parameter to quantify the variability in the different silk fibres, which is consistent with DMTA and FTIR observations.

The morphology and polymorphs of CaCO3that crystallized at the air/water interface depend on the assembly of silk fibroin therein.

SUMMARY Silk cocoons, constructed by silkmoths (Lepidoptera), are protective structural composites. Some cocoons appear to have evolved towards structural and material optimisation in order to sustain impact strikes from predators and hinder parasite ingress. This study investigates the protective properties of silk cocoons with different morphologies by evaluating their impact resistance and damage tolerance. Finite element analysis was used to analyse empirical observations of the quasi-static impact response of the silk cocoons, and to evaluate the separate benefits of the structures and materials through the deformation and damage mechanism. We use design principles from composite engineering in order to understand the structure–property–function relationship of silkworm cocoons. Understanding the highly evolved survival strategies of the organisms building natural cocoons will hopefully lead to inspiration that in turn could lead to improved composite design.

Comparative silk research has begun to provide us with valuable insights into a class of biopolymers that have evolved an enormous range of material performance based on highly adapted structure-property interactions. As outlined in this article, such insights cover the analysis of biological and bioinspired spinning technologies, the discovery of a novel type of melt spinning at ambient temperatures, and an advanced predictive modeling framework that uses ab initio calculations. Importantly, the huge diversity and extensive range of material properties found in natural silks is providing a fertile field for discoveries that could change the polymer paradigm and our approach to using proteins as structural materials. For example, highly relevant to both sustainability and engineering properties is the role of water in silk processing and function, as this article will explore in some detail.

This study reveals that an “old” mechanism for shape memory in oriented polymers is in fact just one separate contribution for “supercontraction” in Nephila spider major ampulate silks. When Nephila spider silks are in contact with liquid water, they “super”-contract up to 28% of the original stretched length. However, we discovered that under glass transition conditions these silks only relax with a maximum shrinkage of 13%, and this phenomenon is defined as Tg-contraction. Structural components permanent order (PO), permanent disorder (PD), meta order (MO) and meta disorder (MD) were proposed from the primary amino-acid sequence of the silk protein to explain morphological changes in the two contraction phenomena: MD contributes 13% of the full supercontraction and contributes to Tg-contraction; whereas MO (the proline-containing motifs) contributes the rest for the full super-contraction and does not contribute to Tg-contraction. The morphology in Nephila spider silk structure suggests two separate mechanisms to generate the shape memory effect in synthetic polymers.

The interaction between silk and polar solvents of different molecular size can be an important tool for understanding the structural features of natural silk; in particular, the disordered regions associated with the key property of mechanical toughness. In this work, we investigate the transitions induced in the tensile performance and structure of as-reeled Antheraea pernyi silks from different silkworms by a range of solvents that can only soften the protein chains in the amorphous regions. The results indicate that polar solvents with different molecular sizes affect the silk to different degrees, and silks with slightly different structures display significantly different tensile performance in the same solvent. The solvent molecular size is quantitatively correlated with the accessible volume in the amorphous regions before and after the yield point, which suggests that the volume accessible to the solvent molecules decreases as the solvent radius increases. Moreover, silks with more ordered structure (less free volume) in the amorphous regions are less sensitive to solvents than those with more disordered structures. However, silks with higher free volume have higher toughness due to the greater strain to failure.

Quantum dynamics simulations are used to test and validate the hypothesis that enhanced electronic activity and energetic efficiency in hydrated proteins are due to water-mediated proton hopping between peptide and peptenol states. Water is thereby an integral part of the electronic structure of a 'live' protein prior to denaturation.

This work challenges the popular view that silk has exceptional intrinsic mechanical properties of strength and toughness. All polymer fibres are shown to follow one fracture mechanics relationship between failure strength and the ratio of elastic modulus to fibre diameter. Predictive structure–property relations are derived for strength and fracture toughness, and small diameter plays a large part in silk properties. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.