Chris Holland

Silk and Synthetic Polymers: Reconciling 100 Degrees of Separation

IO N The control of crystallization in polymers and synthetic fiber formation through melt processing has been one of the great technological successes of the 20th century.[1] And yet even this achievement is dwarfed by the silk spinners.[2,3] Silk filaments produced by spiders and silk moths demonstrate combinations of strength and toughness that still outperformes their synthetic counterparts, which has been attributed to their complex hierarchical nanostructure.[4,5] However, this is not the only advantage. Crucially for cross-disciplinary inspiration, we can assume that the fiber formation in nature is energy optimized, as natural processes usually are. Here, we compare, for the first time, the energetic cost of silk and synthetic polymer fiber formation and demonstrate that if we can learn how to spin like the spider, we should be able to cut the energy costs for polymer fiber processing by over 90%. Recent fundamental advances in our understanding of synthetic polymers have shown that the onset of shear induced fibrillation requires the polymer chains to undergo i) stretching of long-chain molecules, ii) formation of persistent point nuclei, and iii) flow alignment of these nuclei into rows with iv) a subsequent growth to create a contiguous crystalline fiber (Scheme 1).[6–8] Importantly, the fibrilogenesis of natural silkpolymers also requires a stress induced phase transition in order to transform stored liquid gel to final solid fiber during the few seconds it moves through the spinning duct.[9] Thus, analogous to the industrial melt spinning of a synthetic poly mer, in the natural spinning of a silk the molecules (proteins) align (refold), nucleate (denature), and crystallize (aggregate).[10] The mechanism behind this similarity, we believe, is due to the native protein’s unique interaction with its bound water.[10] Thus much like an individual polymer chain in a melt, silk proteins and their associated water molecules may be considered as a single processable entity, a nanocomposite state of biological matter we define as an “aquamelt” (Scheme 1). Support for this term stems from a comparison of native and reconstituted silk (i.e., spun silk that has been dissolved by strong chaotropic chemicals); we have shown that while native silk behaves like a

There are many more lessons still to be learned from spider silks

At the heart of ‘copying from Nature’ lies the implied assumptions that, firstly, researchers understand what they propose to copy and that, secondly, industrialists have found ways to successfully integrate the many requirements necessary for a reliable bio-mimetic copying process. Here we explore the traits of native silks and examine the assertions associated with man-made copies focusing on the question: why do the various assertions and claims matter, and why is a critical evaluation appropriate for a special themed issue on biomimetic soft matter?

Forced Reeling of Bombyx mori Silk: Separating Behavior and Processing Conditions

Controlled reeling is a powerful tool to investigate the details of silk processing. However, consistent forced reeling of silkworms is hindered by the significant degree of behaviorally induced variation caused by the animal. This paper proposes silkworm paralysis as a novel method to control the animal and thus in vivo spinning conditions. Using these methods, we achieve low and consistent reeling forces during the collection of over 500 m of individual silk fiber while monitoring filament variability, morphology, and properties. Novel techniques to measure the irregular silk cross-sectional areas lead to the more accurate calculation of the true engineering values and mechanical property variation of individual silk fibers. Combining controlled reeling and accurate thread measurement techniques allows us to present the relative contributions of processing and behavior in the performance envelope of Bombyx mori silk.

Direct visualization of shear dependent silk fibrillogenesis

Silks desirable properties originate from the development of a multiscale hierarchical structure produced during spinning, but little is known regarding the origin of the micro and nano fibrils that are consistent and prominent features of spun fibres. This raises the important question: can shearing alone generate these filaments, or is further input and control by the physiology/morphology of the silk duct required? Combining confocal microscopy with rheology provides unique insights into the direct connection between applied shear and the fibrillation of silk proteins in as close to in vivo conditions as currently possible. Our measurements demonstrate that unlike typical synthetic polymers, native silk proteins are able to spontaneously self-assemble, fibrillate and develop hierarchical structures upon controlled shearing, and that the shear induced fibrilogenesis is accompanied by dramatic changes in the rheological response. These observations suggest that natural spinning may be far less complex than previously assumed.

Tensile and shear mechanical properties of rotator cuff repair patches.

BACKGROUND Augmentation of rotator cuff tears aims to strengthen the repair and reduce rerupture, yet studies still report high failure rates. This study determines key mechanical properties of rotator cuff repair patches, including establishing values for toughness and measuring the shear properties of repair patches and human rotator cuff tendons. We hypothesized that different repair grafts would (1) have varying material parameters, and (2) not all have mechanical properties similar to human rotator cuff tendons. MATERIALS AND METHODS Eight specimens each from the Restore, GraftJacket, Zimmer Collagen Repair, and SportsMesh repair patches were tested to failure in tension and for suture pullout. We assessed ultimate tensile strength, tensile (Youngs) modulus, and failure strain. This study also established toughness values and shear data. Storage modulus was calculated using dynamic shear analysis for the patches and 18 samples of normal rotator cuff tendon. RESULTS We report significant variability in important mechanical properties of repair patches, with the mechanical parameters of the patches diverting variously-and often significantly-from values for human rotator cuff tendon. CONCLUSIONS The repair grafts tested all displayed significant variation in their mechanical properties and had at least some reduced parameters compared with human rotator cuff tendons. This study offers experimentally derived information of value to surgeons when selecting rotator cuff repair grafts. A better understanding of the mechanical suitability of repair grafts for supporting human rotator cuffs is needed if repair patches are to provide a solution for the clinical problem of failure of rotator cuff repairs.

Concentration State Dependence of the Rheological and Structural Properties of Reconstituted Silk

The ability to control the processing of artificial silk is key to the successful application of this important and high performance biopolymer. Understanding where our current reconstitution process can be improved will not only aid us in the creation of better materials, but will also provide insight into the natural material along the way. This study aims to understand what proportion of reconstituted silk contributes to its rheological properties and what conformational state the silk proteins are in. It shows, for the first time, that a change in rheological properties can be related to a change in silk structures present in solution and reveals a low concentration gel state for silk that may have important implications for future successful artificial processing of silk.

Identification and classification of silks using infrared spectroscopy

ABSTRACT Lepidopteran silks number in the thousands and display a vast diversity of structures, properties and industrial potential. To map this remarkable biochemical diversity, we present an identification and screening method based on the infrared spectra of native silk feedstock and cocoons. Multivariate analysis of over 1214 infrared spectra obtained from 35 species allowed us to group silks into distinct hierarchies and a classification that agrees well with current phylogenetic data and taxonomies. This approach also provides information on the relative content of sericin, calcium oxalate, phenolic compounds, poly-alanine and poly(alanine-glycine) β-sheets. It emerged that the domesticated mulberry silkmoth Bombyx mori represents an outlier compared with other silkmoth taxa in terms of spectral properties. Interestingly, Epiphora bauhiniae was found to contain the highest amount of β-sheets reported to date for any wild silkmoth. We conclude that our approach provides a new route to determine cocoon chemical composition and in turn a novel, biological as well as material, classification of silks. Summary: FTIR analysis performed on unspun silks and cocoons from 35 different species provides information regarding the biochemical diversity and evolution of this group of materials.

A poisonous surprise under the coat of the African crested rat

Plant toxins are sequestered by many animals and the toxicity is frequently advertised by aposematic displays to deter potential predators. Such ‘unpalatability by appropriation’ is common in many invertebrate groups and also found in a few vertebrate groups. However, potentially lethal toxicity by acquisition has so far never been reported for a placental mammal. Here, we describe complex morphological structures and behaviours whereby the African crested rat, Lophiomys imhausi, acquires, dispenses and advertises deterrent toxin. Roots and bark of Acokanthera schimperi (Apocynaceae) trees are gnawed, masticated and slavered onto highly specialized hairs that wick up the compound, to be delivered whenever the animal is bitten or mouthed by a predator. The poison is a cardenolide, closely resembling ouabain, one of the active components in a traditional African arrow poison long celebrated for its power to kill elephants.

Comparing the rheology of mulberry and “wild” silkworm spinning dopes †

Lepidoperan silks provide a superb opportunity for comparative studies of spinning and fiber characteristics. Comparing the four species, Bombyx mori (China), Actias selene (India), Antheraea yamamai (Japan), Gonometa postica (Africa), allows us to examine differences on the family, species, and race levels. Measured rheological properties were consistent with phylogenetic relationships and in the context of resource allocation and gland morphology. We propose that the thorough domestication of the mulberry silkworm B. mori for high silk yield has resulted in a compensatory optimization for spinning efficiency. This is in stark contrast to the wild silkworms, where Saturnids appear to minimize their energetic input toward silk output and G. postica seems to balance both. We conclude that comparative studies provide valuable baseline information for future biomimetic applications and modeling, as well as illuminating biologically important details of silk processing.

Linking naturally and unnaturally spun silks through the forced reeling of Bombyx mori.

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.