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Materials: Surprising strength of silkworm silk



Commercial silkworm silk is presumed to be much weaker and less extensible than spider dragline silk, which has been hailed as a 'super-fibre'. But we show here that the mechanical properties of silkworm silks can approach those of spider dragline silk when reeled under controlled conditions. We suggest that silkworms might be able to produce threads that compare well with spider silk by changing their spinning habits, rather than by having their silk genes altered.
ommercial silkworm silk is presumed
to be much weaker and less extensible
than spider dragline silk, which has
been hailed as a ‘super-fibre
. But we
show here that the mechanical properties of
silkworm silks can approach those of spider
dragline silk when reeled under controlled
conditions. We suggest that silkworms might
be able to produce threads that compare
well with spider silk by changing their
spinning habits, rather than by having their
silk genes altered.
Typical commercial silkworm silk from
Bombyx mori cocoons has a tensile strength
of about 0.5 gigapascals (GPa), a breaking
elongation of 15%, and a breaking energy
(toughness) of 6210
J kg
(ref. 5). Nephila
spider dragline silk, on the other hand,
typically has a strength of 1.3 GPa, a break-
ing elongation of 40%, and a toughness of
J kg
(ref. 4).
These mechanical measures vary con-
siderably in both types of silk. For spider
silk, this variability is due to spinning
conditions, which are affected by the spi-
der’s body temperature and the speed of
. Silkworm silk is traditionally
obtained from a natural cocoon that is
spun by the moving silkworm, which accel-
erates and decelerates its head in arcs that
are attached at points that correspond to
each change of direction
We find that artificial reeling of silk from
immobilized silkworms under steady and
controlled conditions produces fibres that
are superior to naturally spun ones. The
silkworm, like the spider
, produces stronger,
more brittle fibres at faster spinning speeds,
whereas slower speeds lead to weaker, more
extensible fibres (Fig. 1).
Force-drawn silkworm fibres compared
favourably with Nephila spider dragline silk.
For example, slow spinning at 4 mm s
a breaking elongation for Bombyx silk that
is comparable to that of spider silk (37%
and 35%, respectively). Faster spinning (13
mm s
) gave a breaking energy approaching
that of Nephila silk (12210
Bombyx; versus 16210
J kg
for Nephila),
although the breaking strength remained
lower (0.7 GPa compared with 1.3 GPa). At
even greater spinning speeds, silk toughness
decreased, mainly because of loss of extensi-
bility (Fig. 1).
Other differences between silkworm and
spider silk may be linked to specific differ-
ences in the composition of the principal
silk molecules
, as well as to their arrange-
ment during and after spinning
. Depend-
ing on reeling speed, Bombyx silk is either
strong or stretchable (Fig. 1), whereas spider
dragline silk typically combines the two at
the speeds that are characteristic of web
(10–20 mm s
Although spider silk fibres are studied in
their native state
, silkworm silk must first be
to remove most of the sericin gum
coating the fibroin filaments from the ‘bave’
thread (Fig. 1, inset). Washing extraction is
necessary for the thread to be unravelled
from the cocoon (Fig. 2), and is generally
assumed to weaken the thread
. Our experi-
ments (each run four times) on silkworm silk
filaments reeled under controlled conditions
(12–24 filaments per treatment) showed that
spinning conditions affect the silks material
properties more than washing.
For example, washing experimental fibres
reeled at 4 mm s
changed neither the force
required to break a thread (washed, 3652
mN; native, 3854 mN, P40.34, t-test) nor
its breaking elongation (washed, 3753%;
native, 3954%, P40.53). However,
degumming’ significantly increased visco-
elastic recovery (washed, 4053%; native,
2952%, P*0.001), as shown by the
loading–unloading cycles of single fibres
stretched to 50% of typical breaking elonga-
tion values. Thus, more than anything else,
washing increases the elasticity of fibroin
filaments, presumably because of the com-
bined action of sericin removal and water
Our findings indicate that the mechanical
properties of Bombyx silk, like those of spider
, depend crucially on spinning condi-
tions — silkworm silks can be made
stronger, stiffer and more extensible simply
by adjusting the harvesting parameters. If we
could reel silk straight from the silkworm, as
from spiders, or if larvae could be bred to
spin their cocoons faster and more evenly
(with pretreatment washing appropriately
modified), then the silkworm would produce
fibres that might well give natural and artifi-
cially spun spider silks — genetically modi-
fied or not — a good run for their money.
Zhengzhong Shao*, Fritz Vollrath†
*Department of Macromolecular Science and Key
Laboratory of Polymer Engineering of Education
Ministry, Fudan University, Shanghai 200433,
Department of Zoology, Oxford University,
South Parks Road, Oxford OX1 PX3, UK
1. Vollrath, F. & Knight, D. P. Nature 410, 541–548 (2001).
2. Kaplan, D. L., Adams, W. W., Viney, C. & Farmer, B. L. Silk
Polymers: Materials Science and Biotechnology (ACS,
Washington, 1994).
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4. Vollrath, F., Madsen, B. & Shao, Z. Proc. R. Soc. Lond. B 268,
2339–2346 (2001).
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Encyclopaedia Vol. 11 (ed. Salamone, J. C.) 8307–8322 (CRC,
Boca Raton, Florida, 1996).
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8433–8439 (2000).
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10.Monti, P., Freddi, G., Bertoluzza, A., Kasai, N. & Tsukada, M.
J. Raman Spectrosc. 29, 297–304 (1998).
Competing financial interests: declared none.
brief communications
VOL 418
15 AUGUST 2002
| 741
Surprising strength of silkworm silk
Silk fibres produced by artificial reeling are superior to those that are spun naturally.
Figure 1 Comparison of silks drawn at different speeds from the
Bombyx mori
. Stress–strain curves of washed and
degummed single-filament silkworm silk (motor-reeled at 25 7C at
the indicated speeds),
spider dragline silk (20 mm s
25 7C) and standard, degummed commercial silk from a cocoon
spun by the animal in the natural ‘figure of eight’
at speeds
oscillating between 4 and 15 mm s
at 20 7C. The area under the
stress–strain curve represents the energy that a fibre can take up
before breaking, and thus indicates its toughness. Scale bar,
10 mm. Immobilized silkworms (
44) were forcibly silked
, each
providing 3–6 single filaments, which were tested in a stretching
rig (force resolution, 30 mN; time resolution, *5 ms for 1 mN;
strain rate, 50% per min)
; further details are available from the
authors. For silk ‘degumming’, a traditional aqueous solution
standard wash of 1% sodium hydrogen carbonate
was used,
which led to a 30–40% reduction in fibre diameter. Inset,
unwashed native silkworm silk (photo courtesy of Giuliano Freddi).
0 0.05 0.15 0.25 0.35
27 mm s
20 mm s
13 mm s
4 mm s
Stress (GPa)
Figure 2 A cocoon produced by the silkworm
Bombyx mori
Changing the spinning conditions can improve the silk’s quality.
© 2002
... 因苯硼酸(PBA)中的相关基团可以与 RSF 中的碱性 基团, 如氨基和胍基等, 发生路易斯酸-碱配对反应 [31] , 本研究尝试苯硼酸衍生物在 RSF 水溶液中原位聚合的 方式, 以聚苯硼酸诱导 RSF 微球的形成并改善其分散 性、稳定性和特异性等. 根据 RSF 的一级结构 [17] , RSF 的分子链被认为是由亲水段和疏水段交替构成的"两亲 性多嵌段共聚物" [32] , 因此, 在 RSF 水溶液中聚合 3-丙烯酰胺基苯硼酸(APBA), 得到的疏水聚苯硼酸衍生 物(PAPBA) 有可能与 纳米药物传输载体的大小决定了细胞摄取和体内 生物分布, 更小尺寸的颗粒表现出清除率下降和血液循 环时间延长 [35][36][37][38][39][40][41] . 牛血清白蛋白(BSA)同样具有如氨基 和胍基等的碱性基团, 而猪皮明胶(GEL)分子链上除了 有大量的羟基外, 还有许多羧基和氨基, 因此, 我们设 想其同样能够形成相应的 PAPBA 复合微球. ...
... Moreover due to the steadiness of these fibers as compared to spherical proteins due to presence hydrogen bonding and hydrophobic nature like the protein. Silk fibers are obtained from silkworms [15]. Wray et al. [16] used Silk fibroin for biomedical application in tissue regeneration. ...
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.
Wild silkworm silk fibers have garnered attention owing to their softness, natural color, lightweight, and excellent mechanical properties. Because most wild silkworm cocoons obtained are pierced or dirty after the eclosion process, it is difficult to reel the long filament from the pierced cocoons to use as textile materials. Therefore, damaged wild silkworm cocoons are typically removed during the industrial process. Artificial silk spinning has been developed to transform domesticated silkworm silk solutions into regenerated silk fibers. However, regenerated fibers derived from wild silkworm silk have not been reported. Here, we produced regenerated silk fibers using a dry-wet spinning method using a dope solution derived from wild silkworm silk cocoon wastes. These regenerated silk fibers have thick and uniform diameters, unlike native silk fibers, contributing to their usefulness for sterilization and handling in medical applications. Moreover, they exhibited the same level of mechanical strength as their native counterparts. The molecular orientation and crystallinity of the regenerated silk fibers were adjustable by the drawing process, enabling the realization of their various tensile properties. This study promotes the utilization of unused protein resources to produce mechanically stable and tough silk-based fibers.
Silkworms spin different silks at different growth stages for specific purposes. The silk spun before the end of each instar is stronger than that at the beginning of each instar and cocoon silk. However, the compositional changes in silk proteins during this process are unknown. Consequently, we performed histomorphological and proteomic analyses of the silk gland to characterize changes from the instar end to the next instar beginning. The silk glands were collected on day 3 of third- and fourth-instar larvae (III-3 and IV-3) and the beginning of fourth-instar larvae (IV-0). Proteomic analysis identified 2961 proteins from all silk glands. Silk proteins P25 and Ser5 were significantly more abundant in III-3 and IV-3 than in IV-0, and many cuticular proteins and protease inhibitors increased significantly in IV-0 compared with III-3 and IV-3. This shift may cause mechanical property differences between the instar end and beginning silk. Using section staining, qPCR, and western blotting, we found for the first time that silk proteins were degraded first and then resynthesized during the molting stage. Furthermore, we revealed that fibroinase mediated the changes of silk proteins during molting. Our results provide insights into the molecular mechanisms of silk proteins dynamic regulation during molting.
Tissue engineering provides a promising approach for regenerative medicine. The ideal engineered tissue should have the desired structure and functional properties suitable for uniform cell distribution and stable shape fidelity in the full period of in vitro culture and in vivo implantation. However, due to insufficient cell infiltration and inadequate mechanical properties, engineered tissue made from porous scaffolds may have an inconsistent cellular composition and a poor shape retainability, which seriously hinders their further clinical application. In this study, silk fibroin was integrated with silk short fibers with a physical and chemical double-crosslinking network to fabricate fiber-reinforced silk fibroin super elastic absorbent sponges (Fr-SF-SEAs). The Fr-SF-SEAs exhibited the desirable synergistic properties of a honeycomb structure, hygroscopicity and elasticity, which allowed them to undergo an unconventional cyclic compression inoculation method to significantly promote cell diffusion and achieve a uniform cell distribution at a high-density. Furthermore, the regenerated cartilage of the Fr-SF-SEAs scaffold withstood a dynamic pressure environment after subcutaneous implantation and maintained its precise original structure, ultimately achieving human-scale ear-shaped cartilage regeneration. Importantly, the SF-SEAs preparation showed valuable universality in combining chemicals with other bioactive materials or drugs with reactive groups to construct microenvironment bionic scaffolds. The established novel cell inoculation method is highly versatile and can be readily applied to various cells. Based on the design concept of dual-network Fr-SF-SEAs scaffolds, homogenous and mature cartilage was successfully regenerated with precise and complicated shapes, which hopefully provides a platform strategy for tissue engineering for various cartilage defect repairs.
Biodegradable radiative cooling materials can achieve both zero‐energy cooling and environmental friendliness. Natural silk cocoon exhibits both high solar reflectivity and mid‐infrared emissivity owing to its unique structure and composition, while it possesses excellent biodegradability. Herein, we restructure the natural silk by electrospinning to obtain scalable and biodegradable silk fibroin (SF) fiber membranes with enhanced optical properties. Specially, nano‐sized fibers exhibit better optical properties than micrometer‐sized ones. The fabricated SF nanofiber membrane can achieve ultra‐high average solar reflectance and infrared emittance of 96% and ~97%, respectively. Such outstanding optical properties enables the fiber membrane to yield an average sub‐ambient cooling temperature of ~6°C in the outdoor environment even under a peak solar intensity of ~800 W m−2. Overall, the resulted SF fiber membranes promise a wide range of applications such as building energy efficiency, cold‐chain transportation, in‐vehicle temperature control, outdoor precision instrument protection and outdoor personal thermal management.
Ceramic nanofibrous nanostructure-based sponges have attracted significant attention due to ultrahigh porosity, low thermal conductivity, large specific area, and chemical stability. From the regulation of the fiber itself to the construction method of 3D networks, efforts are being made to improve the mechanical properties of ceramic sponges for practical applications. So far resilient compressibility has been realized in ceramic nanofibrous-based sponges via structural design, but they still show brittle fracture under a more complex stress state. Herein, we introduced a highly aligned and interwoven Si3N4 nanofiber sponge, which exhibits superflexibility, large break elongation (>80%), large-strain reversible stretch (20%), and good resistance to tensile fatigue. The ceramic sponge also displays reversible compressibility up to 60% strain, puncture resistance, high air filtration efficiency (>99.8%), and low pressure drop (38% of cotton fiber), making the ceramic sponge a high-performance wearable respirator to protect us from harm due to PM2.5 pollution and possible microorganisms.
Full-text available
Spider dragline silk is a unique biomaterial and represents nature's strongest known fibre. As it is almost as strong as many commercial synthetic fibres, it is suitable for use in many industrial and medical applications. The prerequisite for such a widespread use is the cost-effective production in sufficient quantities for commercial fibre manufacturing. Agricultural biotechnology and the production of recombinant dragline silk proteins in transgenic plants offer the potential for low-cost, large-scale production. The purpose of this work was to examine the feasibility of producing the two protein components of dragline silk (MaSp1 and MaSp2) from Nephila clavipes in transgenic tobacco. Two different promoters, the enhanced CaMV 35S promoter (Kay et al., 1987) and a new tobacco cryptic constitutive promoter, tCUP (Foster et al., 1999) were used, in conjunction with a plant secretory signal (PR1b), a translational enhancer (alfalfa mosaic virus, AMV) and an endoplasmic reticulum (ER) retention signal (KDEL), to express the MaSp1 and MaSp2 genes in the leaves of transgenic plants. Both genes expressed successfully and recombinant protein accumulated in transgenic plants grown in both greenhouse and field trials.
Full-text available
Spider silk has outstanding mechanical properties despite being spun at close to ambient temperatures and pressures using water as the solvent. The spider achieves this feat of benign fibre processing by judiciously controlling the folding and crystallization of the main protein constituents, and by adding auxiliary compounds, to create a composite material of defined hierarchical structure. Because the 'spinning dope' (the material from which silk is spun) is liquid crystalline, spiders can draw it during extrusion into a hardened fibre using minimal forces. This process involves an unusual internal drawdown within the spider's spinneret that is not seen in industrial fibre processing, followed by a conventional external drawdown after the dope has left the spinneret. Successful copying of the spider's internal processing and precise control over protein folding, combined with knowledge of the gene sequences of its spinning dopes, could permit industrial production of silk-based fibres with unique properties under benign conditions.
Full-text available
We studied the mechanical properties of dragline threads of the edible golden silk spider Nephila edulis that are produced under spinning speeds ranging from 0.1 to 400 mm s(-1) and temperatures ranging from 5 to 40 degrees C. These conditions affected the silk in all of the mechanical traits we tested (strain at breaking, breaking energy, initial Young's modulus and point of yielding). We argue that both trade-offs (between mechanical properties) and constraints (in the manufacturing process) have a large role in defining spider silk fibres.
After a silkworm's body shape was represented by two variables, the characteristics of their distributions were investigated for different kinds of species. The results showed that they were significantly different from normal distributions in more than half the cases. Shape difference between silkworms was also investigated by using two shape variables at different spinning stages. As a result, it was found that there were significant differences both between individuals and spinning stages. We then analyzed serial changes of two shape values. A time-series model was used to express the changes of the front part of a silkworm's body shape. The results showed that serial shape changes of a silkworm's body could be represented by an appropriate bivariate autoregressive model.
Tensile tests have been performed on silkworm silk fibres submerged in liquid environments (water, acetone, ethanol and isopropanol). Liquid media were initially chosen in order to weaken non-covalent interactions specifically. However, only immersion in water leads to a decrease in the mechanical properties of silk, indicating the weakening of hydrogen bonds. Immersion in acetone, ethanol and isopropanol leads to an increase in the stiffness of the fibre. In addition, all three organic solvents produce similar force–displacement curves, which can be explained by the desiccating effect that these solvents exert on silk.These results indicate that water disrupts hydrogen bonds initially present in the amorphous phase, while the other solvents eliminate water and contribute to the formation of new hydrogen bonds in the amorphous phase of silk. This interpretation was developed through the shear lag model of the elastic modulus (E) of silk, and a good agreement has been found between the model and the experimental values of E.
This study was focused on the conformational characterization of Bombyx mori silk fibroin in film, fiber and powder form by means of Fourier transform Raman spectroscopy. Native and regenerated silk fibroin films prepared by casting dilute silk fibroin solutions (<1%, w/v) display characteristic conformationally sensitive bands at 1660 cm-1 (amide I), in the range 1276–1244 cm-1 (a complex amide III region with multiple detectable maxima) and at 1107 and 938 cm-1. This spectral pattern can be related to a prevalently random coil conformation, with traces of α-helix. Liquid silk, prepared by casting the silk gland content (fibroin concentration 20–25%, w/v), shows almost the same wavenumbers in the amide I and III ranges, while differences appear below 1000 cm-1, where three bands at 952, 930 and 867 cm-1 increase in intensity. The spectral differences between films and liquid silk are discussed with a view to identifying possible markers for silk I structure, a crystalline modification of silk fibroin. The treatment of both native and regenerated films with 50% (v/v) methanol solution induces the conformational transition to a β-sheet structure, as demonstrated by the shift of amide I to 1665 cm-1 and the appearance of new maxima at 1262 and 1236 cm-1 (amide III) and at 1084 cm-1. When liquid silk is cast at above 50°C, the prevailing conformation taken by silk fibroin is β-sheet, whatever the rate of drying. By comparing the Raman spectra of silk fibroin fiber and powder, both having a β-sheet structure, a difference in the tyrosine doublet bands and in the amide I band can be observed. The value of the I853/I830 (Rtyr) intensity ratio increases in the powder while amide I shifts to lower wavenumbers, suggesting that the hydrogen bonds involving the tyrosil residues are weaker in the powder than in the fiber. © 1998 John Wiley & Sons, Ltd.
Bave was acquired by the forced silking of three Bombyx mori silkworms, and its tensile properties were characterized. The material collected from any given silkworm yielded reproducible force-displacement plots, which were qualitatively similar to the plots obtained from silk collected from the other silkworms. This uniformity contrasts with the highly variable properties exhibited by silk which had been reeled from degummed cocoons. Scanning electron microscopy images were used to obtain information about the sample cross-sectional area, so that force-displacement plots could be rescaled as stress–strain curves. Surprisingly, the scatter in the tensile properties increases after such rescaling. This finding can be explained in terms of the sericin coating of the bave (which contributes to the cross-sectional area but not significantly to the load-bearing capacity) having a variable thickness. When the sericin coating was eliminated by a degumming treatment, it was found that the fibers showed more consistent cross-sectional areas. Therefore, stress–strain curves of forced B. mori silk are reproducible, provided that force-displacement data are rescaled by the correct cross-sectional area. Finally, the Weibull parameters of the forced silk were determined. The Weibull modulus, m, has a value of 13.0 ± 0.3, which is more than double the value obtained previously from silk reeled from a cocoon, demonstrating that the process of degumming cocoons has a detrimental effect on the distribution of defects in the silk microstructure. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 1928–1935, 2001
  • A Lazaris
Lazaris, A. et al. Science 295, 472–476 (2002).
  • M A Wilding
  • Hearle
Wilding, M. A. & Hearle, J. W. S. in Polymeric Materials Encyclopaedia Vol. 11 (ed. Salamone, J. C.) 8307–8322 (CRC, Boca Raton, Florida, 1996).
  • J Wiedbrauck
Wiedbrauck, J. Z. Tierpsychol. 12, 176–202 (1955).