Stretching of supercontracted fibers: a link between spinning and the variability of spider silk
Departamento de Ciencia de Materiales, Universidad Politécnica de Madrid, ETS de Ingenieros de Caminos, Ciudad Universitaria, 28040 Madrid, Spain
* Author for correspondence (e-mail: melices{at}mater.upm.es)
Accepted 14 October 2004
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Summary |
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Key words: spider, silk, Argiope trifasciata, spinning, tensile properties, supercontraction.
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Introduction |
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The spinning of spider silk faces two major technological challenges: (i)
synthesising an insoluble fiber from an aqueous solution of extremely large
proteins (Xu and Lewis, 1990)
and (ii) ensuring the capability to modify the tensile properties of the fiber
within a time scale of fractions of a second
(Garrido et al., 2002a
). Not
surprisingly, spider silk glands have evolved as highly specialized and
sophisticated systems, where some distinct anatomic and physiological features
can be identified. The spinning of silk starts with a highly concentrated
protein solution produced in a sac in the initial region of the silk glands
(Vollrath and Knight, 2001
).
The sac is anchored by the funnel to an S-shaped duct that tapers down to the
spigot. The initially isotropic protein solution acquires a liquid crystal
structure in the funnel (Kerkam et al.,
1991
; Jin and Kaplan,
2003
), so that it can flow more easily through the narrow duct.
The liquid crystal structure could also promote the pre-alignment of the
fibers prior to full fiber formation. The fiber is formed from the solution in
the third limb of the duct, as is seen from the separation of the lateral
surface of the fiber from the walls of the duct
(Vollrath and Knight, 2001
),
and emerges through the lips of the spigot. The identification of several
structures along the duct and in the spigot responsible for recovering water
initially present in the aqueous solution suggests that water content might be
a critical parameter during spinning.
Interestingly, MAS fibers show a peculiar behaviour if unrestrained and
submerged in water, known as supercontraction
(Work, 1977). This effect is
characterized by a shrinkage of the fiber, exceeding 50% of its initial
length, and by a dramatic change in its mechanical behaviour. Dry fibers,
either naturally spun (NS) or forcibly silked (FS), behave as glassy polymers
(Fig. 1). Initially the
stress-strain curve is stiff and shows a linear-elastic regime, up to a
yielding point. At this point the slope of the curve decreases to a minimum
and starts to increase gradually until breaking point. By contrast,
supercontracted fibers tested in water (SCW) show a tensile behaviour that
resembles that of an elastomer (Gosline et
al., 1984
), characterized by a very low initial elastic modulus
(Fig. 1) and large (in excess
of 100%) strain at breaking. When dried, supercontracted fibers show an
initial high stiffness elastic regime with strain at breaking that exceeds
100%. This state is labelled as maximum supercontraction (MS). The different
tensile properties of dry and supercontracted spider silk fibers have been
modelled through a double network of hydrogen bonds and protein chains
(Termonia, 2000
). The model
assumes that supercontracted fibers correspond to a state where the hydrogen
bond network is molten and the tensile properties are controlled by the
stretching of the protein chains, leading to the observed elastomeric
behaviour. Stretching promotes the re-alignment of the protein chains, which
can be frozen by re-establishing the hydrogen bond network. Despite its
success in simulating the tensile properties of spider silk
(Termonia, 2000
), it was not
clear whether this process did actually occur at any step during spinning or
during the in-service life of the spider silk fiber, contributing to the
controversy on the biological significance of the supercontraction effect
(Bell et al., 2002
).
|
The search for a biological role of supercontraction, possibly in
combination with stretching, led us to consider the processing of the fiber as
a situation where the conditions required by both effects meet. In this
context, mimicking the range of tensile properties displayed by naturally spun
silk fibers by stretching supercontracted fibers would support the hypothesis
that the combination of a fiber under supercontrated state and simultaneous
stretching play a significant role during the spinning of spider silk. In this
work we show that the whole range of tensile properties exhibited by spider
silk (Madsen et al., 1999;
Garrido et al., 2002b
) can be
predictably reproduced by simply controlling the deformation of
supercontracted fibers stretched in water. Since the fiber is subjected to
similar influences during spinning, these results point to a critical
biological function of the supercontracted state during the spinning process
of silk fibers.
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Materials and methods |
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The experimental procedure for stretching supercontracted fibers is
sketched in Fig. 2. Initially,
an FS fiber glued by its ends to the aluminium foil frame is immersed in water
at 20°C for 10 min. The fiber is allowed to contract unrestrained up to
the supercontracted length, LSC
(Fig. 2A). The fiber is
stretched up to the selected length, LA, and the ends are
clamped in this position (Fig.
2B). Water is removed after 10 min and the fiber allowed to dry
overnight at 20°C and 35%RH (Fig.
2C).Stresses build up in the fiber during drying, probably due to
the restoration of the hydrogen network that tends to shrink the fiber
(Guinea et al., 2003;
Savage et al., 2004
). The
fibers are allowed to relax by unloading down to the final length
LC (Fig.
2D).
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Results and discussion |
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Spider silk fibers tested in air behave as glassy polymers. This tensile
behaviour can be explained as the result of a network of hydrogen bonds
between chains (Termonia,
2000), which accounts for the high initial tensile modulus
(E
10 GPa) and breaks after yielding. Subsequent stretching leads
to the extension of a network of protein chains and a rotation of
microcrystallites, and induces a net molecular alignment parallel to the fiber
axis (Gosline et al., 1999
;
Grubb and Gending, 1999
). The
stress-strain curve of supercontracted FS fibers tested in water (SCW) shows
an elastomeric behaviour, which can be explained as the result of water
molecules acting as a plasticizer (Gosline
et al., 1984
), by disrupting the network of hydrogen bonds. This
explanation is supported by X-ray diffraction
(Work and Morosoff, 1982
;
Grubb and Gending, 1999
) and
Raman spectroscopy data (Shao et al.,
1999a
,b
).
The chain conformation of proteins would be kept highly flexible in
supercontracted fibers, but it could be frozen after drying by promoting the
formation of protein-protein hydrogen bonds, thus re-establishing the hydrogen
bond network and leading to behaviour observed in MS fibers.
The effect of stretching on supercontracted fibers and subsequent drying is
illustrated in Fig. 3 (test
conditions: 20°C, 35%RH, strain rate 0.0002 s-1). Engineering
strain is calculated from the length of the unloaded fiber after drying,
LC, and the engineering stress is calculated under the
assumption that volume remains constant throughout the process. The stretching
process is characterized by the alignment parameter defined as
=LC/LSC-1. Very good
repeatability (unusual when dealing with spider silk fibers) was achieved
using this process, as is shown by the similar stress-strain curves presented
by the three fibers tested for each alignment parameter.
|
A value of zero for the alignment parameter corresponds to dried
supercontracted fibers, i.e. supercontracted fibers not subjected to
stretching. The tensile behaviour of dried supercontracted fibers (labelled as
MS) have been described previously
(Pérez-Rigueiro et al.,
2003), and it has been found that MS fibers represent a lower
limit of the tensile properties that can be reached by spider silk fibers
tested in air, regardless of the previous loading history of the fiber.
Stretching the fiber leads to an increase in LC and,
consequently, of the alignment parameter. The collection of stress-strain
curves displayed by NS fibers (see inset in
Fig. 3) is regained for
alignment parameters ranging from
0.4 to
0.8, and it is
illustrated by the curves corresponding to
0.45. Alignment
parameters in the range from
0.9 to
1.1 yield fibers
with stress-strain curves similar to those of forcibly silked fibers (FS;
shown to allow direct comparison). These values of the alignment parameter
indicate that the FS fiber length is double that of supercontracted fibers, as
expected from the approximate 50% reduction of the FS fiber length when
subjected to supercontraction. Consistent with the hypothesis that stretching
leads to a better alignment of the protein chains, it is apparent from
Fig. 3 that overall stiffness
increases monotonically with the alignment parameter, although the behaviour
of the elastic modulus and the proportional limit is slightly more complex
(see Table 1). Tensile strength
follows a comparable trend, despite the low Weibull modulus of spider silk
(Pérez-Rigueiro et al.,
2001
) that implies a large scattering on the values of this
parameter.
|
Our results suggest that a similar stretching and drying process can
account for the variability observed in the tensile properties of MAS fibers,
and places supercontraction, or more accurately the supercontracted state of
spider fibers, as a central feature of the spinning process. Although the
details of the spinning process remain largely unknown, the results of the
present work are consistent with the finding that spiders use a friction brake
to control the force exerted on the fiber
(Ortlepp and Gosline, 2004).
In this context, applying a controlled stress to the fiber in the final stages
of silk production, when the silk exits the spigot but is likely to remain
hydrated for a short period of time, would lead to the observed variability in
the tensile properties of spider silk. It has also been shown that the
stresses exerted on the fiber by the friction brake can account for the
stresses involved in the supercontraction process
(Guinea et al., 2003
;
Savage et al., 2004
). It is
interesting to notice that the low forces involved in stretching
supercontracted fibers up to large deformations (strain values up to 1.0)
permit the modification of the tensile properties with a minimum expense of
energy.
From a technological perspective, the stretching and drying process enables production of spider silk fibers with a tailored and pre-established stress-strain profile in a reliable and repetitive way. The procedure illustrated in this work casts light on the natural spinning process and should be helpful for the design of spinning processes currently being developed by the biomimetics industry. It also offers the possibility of reproducing the full range of tensile properties exhibited by MAS fibers, so that the variability found in natural silk fibers should not represent a significant drawback in future research work.
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Summary and conclusions |
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The combination of an aqueous environment and stretching is an essential feature of the spinning process of silk, and in consequence it is suggested that a similar process accounts for the broad range of tensile properties displayed by spider silk fibers. The ubiquitous presence of supercontraction and the simplicity of the process allows it to be implemented in the operation of spinning of artificial silk fibers, improving the reproducibility of the fiber tensile properties.
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List of abbreviations |
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Acknowledgments |
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References |
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Bell, F. I., McEwen, I. J. and Viney, C. (2002). Supercontraction stress in wet spider dragline. Nature 416,37 .[CrossRef][Medline]
Craig, C. L. (1997). Evolution of arthropod silks. Annu. Rev. Entomol. 42,231 -267.[CrossRef]
Elices, M., Pérez-Rigueiro, J., Plaza, G. and Guinea, G. V. (2004). Recovery in spider silk fibers. J. Appl. Polym. Sci. 92,3537 -3541.[CrossRef]
Garrido, M. A., Elices, M., Viney, C. and Pérez-Rigueiro, J. (2002a). Active control of spider silk strength: comparison of drag line spun on vertical and horizontal surfaces. Polymer 43,1537 -1540.[CrossRef]
Garrido, M. A., Elices, M., Viney, C. and Pérez-Rigueiro, J. (2002b). The variability and interdependence of spider drag line tensile properties. Polymer 43,4495 -4502.[CrossRef]
Gosline, J. M., Denny, M. and DeMont, M. E. (1984). Spider silk as rubber. Nature 309,551 -552.
Gosline, J. M., Guerette, P. A., Ortlepp, C. S. and Savage, K.
N. (1999). The mechanical design of spider silks: from
fibroin sequence to mechanical function. J. Exp. Biol.
202,3295
-3303.
Griffiths, J. R. and Salinatri, V. R. (1980). The strength of spider silk. J. Mater. Sci. 15,491 -496.[CrossRef]
Grubb, D. T. and Gending, J. (1999). Molecular chain orientation in supercontracted and re-extended spider silk. Int. J. Biol. Macromol. 24,203 -210.[CrossRef][Medline]
Guinea, G. V., Elices, M., Pérez-Rigueiro, J. and Plaza, G. R. (2003). Selftightening of spider silk fibers induced by moisture. Polymer 44,5785 -5788.[CrossRef]
Jelinski, J. W., Blye, A., Liivak, O., Michal, C., LaVerde, G., Seidel, A., Shah, N. and Yang, Z. (1999). Orientation, structure, wet-spinning and molecular basis for supercontraction of spider dragline silk. Int. J. Biol. Macromol. 24,197 -201.[CrossRef][Medline]
Jin, H.-J. and Kaplan, D. L. (2003). Mechanisms of silk processing in insects and spiders. Nature 424,1057 -1061.[CrossRef][Medline]
Kaplan, D. L., Lombardi, S. J., Muller, W. S. and Fossey, S. A. (1991). Silks. In Biomaterials. Novel Materials from Biological Sources (ed. D. Byrom), pp.1 -53. New York: Stockton Press.
Kerkam, K., Viney, C., Kaplan, D. and Lombardi, S. (1991). Liquid crystallinity of natural silk secretions. Nature 424,596 -598.
Lazaris, A., Arcidiacono, S., Huang, Y., Zhou, J.-F., Duguay,
F., Chretien, N., Welsh, E. A., Soares, J. W. and Karatzas, C. N.
(2002). Spider silk fibers spun from soluble recombinant silk
produced in mammalian cells. Science
295,472
-476.
Madsen, B., Shao, Z. Z. and Vollrath, F. (1999). Variability in the mechanical properties of spider silks on three levels: interspecific, intraspecific and intraindividual. Int. J. Biol. Macromol. 24,301 -306.[CrossRef][Medline]
Marsh, R. B., Corey, L. and Pauling, L. (1955). Structure of silk. Biochem. Biophys. Acta 16, 1-34.[CrossRef]
Ortlepp, C. S. and Gosline, J. M. (2004). Consequences of forced silking. Biomacromol. 5, 727-731.[CrossRef][Medline]
Pérez-Rigueiro, J., Viney, C., Llorca, J. and Elices, M. (1998). Silkworm silk as an engineering material. J. Appl. Polym. Sci. 70,2439 -2447.[CrossRef]
Pérez-Rigueiro, J., Elices, M., Llorca, J. and Viney, C. (2001). Tensile properties of Argiope trifasciata drag line silk obtained from the spider's web. J. Appl. Polym. Sci. 82,2245 -2251.[CrossRef]
Pérez-Rigueiro, J., Elices, M. and Guinea, G. V. (2003). Supercontraction tailors the tensile properties of spider silk. Polymer 44,3733 -3736.[CrossRef]
Plaza, G. (2004). Thermo-hydro-mechanical behaviour of spider silk fibers (in Spanish). PhD thesis, Universidad Politécnica de Madrid, Madrid.
Savage, K. N., Guerette, P. A. and Gosline, J. M. (2004). Supercontraction stress in spider webs. Biomacromol. 5,675 -679.[CrossRef][Medline]
Shao, Z., Vollrath, F., Sirichaisit, J. and Young, R. J. (1999a). Analysis of spider silk in native and supercontracted states using Raman spectroscopy. Polymer 40,2493 -2500.[CrossRef]
Shao, Z., Young, R. J. and Vollrath, F. (1999b). The effect of solvents on the contraction and mechanical properties of spider silk. Polymer 40,1799 -1806.[CrossRef]
Simmons, A. H., Michal, C. A. and Jelinski, L. W. (1996). Molecular orientation and two-component nature of the crystalline fraction of spider dragline silk. Science 271, 84-87.[Abstract]
Termonia, Y. (2000). Molecular modelling of the stress/strain behavior of spider dragline. In Structural Biological Materials (ed. M. Elices), pp. 335-349. Amsterdam: Pergamon Press.
van Beek, J. D., Kümmerlen, J., Vollrath, F. and Meier, B. H. (1999). Supercontracted spider dragline silk: a solid-state NMR study of the local structure. Int. J. Biol. Macromol. 24,173 -178.[CrossRef][Medline]
Viney, C. (2000). Silk Fibres: Origins, Nature and Consequences of Structure. In Structural Biological Materials (ed. M. Elices), pp. 293-333. Amsterdam: Pergamon Press.
Vollrath, F. and Knight, D. P. (2001). Liquid crystalline spinning of spider silk. Nature 410,541 -548.[CrossRef][Medline]
Work, R. W. (1976). The force-elongation behavior of web fibers and silks forcibly obtained from orb-web-spinning spiders. Textile Res. J. 46,485 -492.
Work, R. W. (1977). Dimensions, birefringences, and force-elongation behavior of major and minor ampullate silk fibers from orb-web-spinning spiders. The effects of wetting on these properties. Textile Res. J. 47,650 -662.
Work, R. W. and Emerson, P. D. (1982). An apparatus and technique for the forcible silking of spiders. J. Arachnol. 10,1 -10.
Work, R. W. and Morosoff, N. (1982). A physico-chemical study of the supercontraction of spider major ampullate silk fibers. Textile Res. J. 52,349 -356.
Work, R. W. and Young, C. T. (1987). The amino acid compositions of major and minor ampullate silks of certain orb-web-building spiders (Aranae, Araneidae). J. Arachnol. 15,65 -80.
Xu, M. and Lewis, R. V. (1990). Structure of a protein superfiber: spider dragline silk. Proc. Natl. Acad. Sci. 87,7120 -7124.[Abstract]
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