The Rheology behind Stress-Induced Solidification in Native Silk Feedstocks (original) (raw)
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Rheological Behaviour of Native Silk Feedstocks
Polymer, 2015
Whilst much is known about the properties of silks, the means by which native silk feedstocks are spun still represent a gap in our knowledge. Rheology of the native silk feedstocks is germane to an understanding of the natural spinning process. Yet, an overview of the literature reveals subtle limitations and inconsistencies between studies, which has been largely attributed to sample-tosample variation when testing these exquisitely flow-sensitive materials. This ambiguity has prevented reliable, consistent inferences from standard polymer rheology and constitutes an obstacle to further development. To address this challenge, we present the largest study to date into the rheological properties of native silk feedstocks from Bombyx mori larvae. A combination of shear and oscillatory measurements were used to examine in detail the relationships between concentration, low shear viscosity, relaxation times, complex modulus and estimates of the molecular weights between entanglements. The results from this highly detailed survey will provide a sound basis for further experimental or theoretical work and lay the foundations for future bio-inspired processing of proteins.
Thermo-rheological behaviour of native silk feedstocks
European Polymer Journal, 2017
The rheology of native silk protein feedstock specimens was characterised by shear and oscillatory measurements, over the temperature range from 2 to 55°C, producing no evidence of thermally-driven phase change behaviour. All specimens exhibited flow characteristics typical of a concentrated polymer solution, with visco-elastic behaviour dominated by two main relaxation modes exhibiting time constants around 0.44 and 0.055 s at 25°C. The specimens showed well-behaved temperature dependence following the Arrhenius equation, consistent with the kinetics being governed by an activation energy of flow, which ranged from 30.9 to 55.4 kJ mol À1 based on oscillatory data. Consequently, for the first time, it was possible to compile master-curves for native silk feedstock specimens following the principles of time-temperature superposition, using oscillatory data demonstrating visco-elastic behaviour typical of a polymer solution across a wide temperature range. Our work has highlighted the processing range of natural silks and furthered our stance on the molecular mechanisms governing the flow behaviour of these interesting and important materials.
Native Silk Feedstock as a Model Biopolymer: A Rheological Perspective
Biomacromolecules, 2016
Variability in silk's rheology is often regarded as an impediment to understanding or successfully copying the natural spinning process. We have previously reported such variability in unspun native silk extracted straight from the gland of the domesticated silkworm Bombyx mori and discounted classical explanations such as differences in molecular weight and concentration. We now report that variability in oscillatory measurements can be reduced onto a simple master-curve through normalizing with respect to the crossover. This remarkable result suggests that differences between silk feedstocks are rheologically simple and not as complex as originally thought. By comparison, solutions of poly(ethylene-oxide) and hydroxypropyl-methyl-cellulose showed similar normalization behavior; however, the resulting curves were broader than for silk, suggesting greater polydispersity in the (semi)synthetic materials. Thus, we conclude Nature may in fact produce polymer feedstocks that are mor...
Regenerated Bombyx silk solutions studied with rheometry and FTIR
Polymer, 2001
Several different solvent systems are commonly used to dissolve Bombyx mori silk ®broin to prepare regenerated silk membranes and ®bers though differences in the behavior of these solvents have not been fully investigated. Here we compare the effects of four of these on the rheology of silk ®broin solutions and on protein secondary structure as revealed by FTIR spectroscopy of cast membranes. The results demonstrated that Ca(NO 3 ) 2 ±MeOH±H 2 O and LiBr±EtOH±H 2 O had the strongest solvation on the silk ®broin chains, which showed an almost constant viscosity (Newtonian behavior) over most of the shear rate range (0.1±500 s 21 ). In contrast, the 9.5 M aqueous LiBr appeared to have the weakest solvation with similar effects on the silk ®broin molecules to pure water as indicated by rheological behavior. It was also found that the silk ®broin membranes prepared using all four solvent system showed mainly random coil conformation with a small proportion of b-sheet by FTIR spectroscopy. We discuss the implications of these ®ndings for the preparation of regenerated silk for different applications. q
Concentration State Dependence of the Rheological and Structural Properties of Reconstituted Silk
Biomacromolecules, 2009
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.
Fibroin is the main polypeptidic component of the silk fibre generated by the larvae of the domesticated silk moth (Bombyx mori), and it has been extensively studied as a biomaterial with applications in tissue engineering and regenerative medicine. Due to their inherent practical advantages, the hydrogels constitute the preferred format for the B. mori silk fibroin (BMSF) biomaterials, which can conveniently be obtained by crosslinking processes. While the physically or chemically crosslinked BMSF hydrogels have been frequently described, the self-crosslinking of BMSF solutions induced by the catalytic effect of the enzyme horseradish peroxidase (HRP) has been barely reported. Following a previous preliminary study, where we demonstrated the advantages of using this enzyme for crosslinking BMSF, in the present work we investigated factors (amount of enzyme, initial fibroin concentration) that may affect the gelation. The measurement of dynamic moduli resulting from application of an oscillatory shear stress in a rheometer was our method to estimate the gelation time and to investigate the influence of certain factors on the process of crosslinking. It was found that both a higher initial concentration of the BMSF solution and a higher amount of the catalyst HRP induced a significant reduction of the gelation time.
Advanced Materials, 2009
The dragline silk of certain spiders has excellent tensile properties [1] that are maintained over a remarkable temperature range. Although the commercial Mulberry Silkworm (Bombyx mori) silk is considerably weaker and less tough than the best spider dragline silks, fibers with comprehensive mechanical properties approaching that of spiders can be obtained by force-reeling directly from silkworms. Remarkably, both spider and silkworm silks are spun naturally from aqueous protein solutions at very low hydraulic pressures and at ambient temperature, not requiring a noxious coagulation bath. These considerations have lead to the search for methods to extrude strong and tough fibers from regenerated silk protein solutions. The processes developed so far depend on extruding silk fibroin dissolved in formic acid, N-methyl morpholine N-oxide (NMMO), 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), [7] trifluoro-acetic acid (TFA), [5a] hexafluoroacetone (HFA) or 1-ethyl-3-methylimidazolium chloride, usually into a methanol bath. However, most of these silk solvents either severely degrade the silk fibroin or are too expensive or toxic for use in industrial processes. Moreover, in all cases described, fibers spun from silk fibroin generated fibers weaker than their natural counterparts, with the exception of Ha et al.'s use of fibroin dissolved in TFA. [5a] However, the cold methanol coagulation bath used in these examples produced fibers that were much larger than the natural ones, lacked smooth surfaces, and were so stiff in their as-extruded form that their tensile properties could only be brought above those of natural fibers by manual neck drawing. This, and the cost of TFA, render Ha et al.'s process [5a] unsuitable for industrial scale up.
Interfacial Rheology of Natural Silk Fibroin at Air/Water and Oil/Water Interfaces
Langmuir, 2011
Silk produced by the domesticated silkworm, Bombyx mori, has attracted recent attention because of its excellent mechanical properties, outstanding biocompatibility, low biodegradability, and minimal inflammatory reaction. These properties have led to the exploration of its use in biotechnological and biomedical applications, such as controlled drug release and tissue engineering scaffolds. 1,2 Silk, as it is emitted from the glands of the silkworm, consists of a fibroin core (75 wt %) surrounded by gelatin-like sericin proteins (25 wt %), with the former constituent being a fibrous, structural protein with high crystallinity. As a result, fibers formed from this material display highly oriented crystalline alignment along the fiber axis. 3À5 The sericin protein, with a large fraction of water-soluble serine and other hydrophilic amino acids, serves as a binder and aids silk fiber spinning by the animal. The sericin can be easily removed from cocoons to eliminate its adverse effects in medical applications. The core protein of silk, silk fibroin (SF), is composed of highly repetitive amino acid sequences with alternating hydrophobic and hydrophilic blocks along the molecular chains and can be regarded as nature's counterpart of a synthetic, multiblock polyelectrolyte. This multiblock structure allows the protein to self-assemble into micelles and form gels in concentrated solution. 6,7 The relatively hydrophobic silk fibroin molecules consist of heavy and light chains of approximately 360 and 25 kDa in M w , respectively, connected by disulfide linkages. 8 The polypeptide chain is a known polymorph and can achieve at least three secondary conformations, two of which are stable and one of which is metastable. The two most commonly known conformations are called silk I and silk II, which exist as dimorphs of the fibroin from Bombyx mori. Silk I is a structure residing in 51 solution in the glands of the silk worm before the spinning 52 process. 9,10 Silk II gives rise to the crystalline structure of fibroin 53 in native silk fiber, with antiparallel β sheets crystallized in 54 hydrophobic regions and more or less random conformations 55 existing in hydrophilic regions. 11 It is the unique coexistence of 56 both crystallized hydrophobic pleated β sheets and amorphous 57 hydrophilic regions in the fibroin structure that gives silk extra-58 ordinary mechanical properties with both high tensile strength 59 and exceptional toughness. Silk III, reported by Valluzzi and 60 Gido, 12À14 is a crystalline structure formed at the interface of air 61 and water. The formation of different secondary structures in 62 fibroin depends on the charge density, pH, and salinity of its 63 aqueous solvent and its processing conditions. 64 Silk fibroin possesses regions of different hydrophobicity 65 when it folds into the appropriate secondary or higher-order 66 structures. The coexistence of distinct hydrophobic and hydro-67 philic regions endows the silk fibroin molecules with amphiphilic 68 character and surface activity, allowing silk fibroin to reside at 69 fluid interfaces and form stable viscoelastic films at the surface of 70 an aqueous medium and either air or oil. If this occurs, then the 71 silk fibroin can be used as an emulsifier to form stable emulsions 72 by creating viscoelastic shells on the surfaces of dispersed drops. 73 Similarly, this mechanism can be used to stabilize foams effec-74 tively. These shells resist droplet or bubble deformation and 75 prevent droplet and bubble coalescence and macroscopic phase 76 separation. The purpose of this article is to report on the ability of
Investigations on rheological properties and gelation of tasar regenerated silk fibroin solution
Journal of Applied Polymer Science, 2014
Tasar silk is a variety of non-mulberry silk indigenous to the Indian subcontinent. We present the measured frequencydependent viscoelastic moduli of Tasar regenerated silk fibroin (RSF) solution using optical tweezers at two concentrations (0.16% and 0.25% w/v) and extend these measurements to the low frequency regime using a video microscopy technique. We extend the investigation on the rheological behavior of Tasar RSF for four more RSF concentrations, viz., 0.50%, 1.00%, 2.50% and 5.00% using video microscopy. In all the RSF samples, both storage and loss moduli are found to increase with frequency. At lower frequencies the loss modulus is more than the storage modulus and exhibit similar behavior until a crossover frequency beyond which the storage modulus exceeds the loss modulus at all frequencies. The relaxation time which is inversely related to the crossover frequency is found to rise sharply at 5% w/v, indicating the onset of gelation in the sample. These results are examined in relation to the viscoelastic parameters of mulberry silk, wherein the larger crossover frequencies at the same higher concentrations indicate relaxation times that are an order of magnitude smaller than those measured for Tasar RSF.