The effect of spinning forces on spider silk properties
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.pm.es)
Accepted 10 May 2005
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Summary |
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Key words: spider silk, forced silking, silking force, Argiope trifasciata, tensile test
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Introduction |
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Tensile tests in natural as well as in artificial silk fibres
have shown a wide variability, as recognised in some of the earliest
works (Work, 1976;
Griffiths and Salinatri,
1980
). The variability of natural silk has been interpreted in
biological terms as a contribution to the capacity of survival of the spider
(Madsen et al., 1999
), and
efforts were made to establish a correlation between silking conditions and
the tensile properties. When testing fibres naturally spun by spiders
it was found that, even after a careful analysis of the web elements to be
tested (Work, 1976
;
Denny, 1976
) and with the use
of monofilaments instead of the whole thread retrieved from the web
(Pérez-Rigueiro et al.,
2001
), the tensile properties of the silk collected from the web
show an intrinsic large variability, even among fibres from a single web.
Fibres obtained from the safety line show similar variability
(Garrido et al., 2002a
).
Fig. 1 illustrates the range of
variation of naturally spun (NS) fibres. Forced silking allows silk
to be collected in significant quantities by a process that shares significant
features with the spinning of artificial silk. Forced silking consists of
pulling the fibre from the spider's spinneret, usually by winding it on a
rotating mandrel (Work and Emerson,
1982
). Despite the control exerted on the silking parameters, it
has been reported that forcibly silked (FS) fibres also show a significant
variability in their tensile properties
(Work, 1976
;
Cunniff et al., 1994
;
Madsen et al., 1999
).
Reproducibility has been found to improve significantly if precautions are
taken such as rescaling the force into stress and discarding the samples
obtained at the beginning of the reeling process or when the reeling has
proceeded for too long (Guinea et al.,
2005
). FS fibres are usually much stiffer than natural ones, as is
shown in Fig. 1.
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The possible influence of the silking force on the properties of spider
silk has been considered only recently
(Ortlepp and Gosline, 2004).
It was found that spiders possess a friction brake that allows them to control
the tension applied to their silks when drawn and that the forces exerted by
the friction brake differ between natural and forced silking. On the basis of
these observations, we have developed a procedure that allows us to measure
the silking force exerted by the spider during forced silking, and we have
analysed the retrieved fibres by the methodology of characterization of the
tensile properties published elsewhere
(Guinea et al., 2005
).
We found that the tensile behaviour of spider silk fibres can, as suggested
by Ortlepp and Gosline (2004),
be traced back to the silking force during the reeling process, there being a
strong correlation between the mean value of the silking force and the
intrinsic tensile properties (i.e. stressstrain curves) of FS fibres.
Interestingly, forced silking processes that proceeded under low silking force
i.e. loads much lower than the conventional yield stress have
yielded fibres with tensile properties similar either to NS or to maximum
supercontracted fibres tested in air
(Pérez-Rigueiro et al.,
2003
). These findings, to the authors' knowledge, are the first
report of spider silk fibres obtained by forced silking that are similar to
the NS or even to the supercontracted fibres.
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Materials and methods |
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Monitored force silking process
The immobilized spider was placed upside down on the base of an Instron
4411 testing machine (Canton, MA, USA), and the initial length of silk was
attached to a load cell HBM Q-11 (Darmstadt, Germany; resolution ±5 mg)
fixed to the crosshead of the machine (Fig.
2). Forced silking was done by displacing the crosshead of the
silking machine at constant speed while simultaneously measuring the silking
force. In this research, all forced silking processes proceeded either at 10
mm s1 or at 1 mm s1. Approximately 1 m of
silk fibre was obtained from each silking process.
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Tensile testing
Samples with a gauge length of 20 mm were glued on cardboard frames
(Pérez-Rigueiro et al.,
1998). Tensile tests were performed in an Instron 4411 testing
machine at a constant crosshead speed and an average strain rate of 2
x104 s1. The load applied to the
sample was measured with a balance (AND 1200 G; A&D Instruments, Oxford,
UK; resolution ±10 mg) attached to the lower end of the sample. The
crosshead displacement was taken as a direct measurement of the sample
deformation, since the compliance of silk has been estimated as 1000 times
greater than that of the equipment. The tests were performed in air under
nominal conditions of 20°C and 40% relative humidity.
Geometry characterisation
Both ends adjacent to the length of the fibre to be tested were secured and
retrieved before tensile testing to determine the fibre's cross-sectional
area. Samples were sputtered with gold and examined in a JEOL 6300 scanning
electron microscope (Tokyo, Japan; observation conditions V=10 kV,
I=0.06 nA; Pérez-Rigueiro
et al., 1998). Brin diameters were measured from each micrograph,
and the mean value of the diameters corresponding to both ends of a given
fibre was used to compute the cross-sectional area of the fibre, assuming a
circular cross-section
(Pérez-Rigueiro et al.,
2001
).
This research was done with silk from the orb-web weaving spider, Argiope trifasciata Forskäl, specifically with fibres spun by the major ampullate gland, which are used by the spider for the dragline and for web frames and radii.
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Results and discussion |
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If the silking force is no longer constant during the reeling process, but the average value is still high, the results remain similar to the above, although some variability appears. These findings are shown in Fig. 4. Fig. 4A shows a plot of the silking force, which is seen to change substantially in each of the six samples. Here, the reeling speed was 1 mm s1. Tensile tests are shown in Fig. 4B; after the initial region below the yielding point dispersion is apparent. This improves somewhat when tests are drawn as stressstrain curves, as seen in Fig. 4C, but some variability remains. Here, the diameters of the samples differ, in contrast to those of samples obtained at constant silking force, even though the fibre is still of type FS (Fig. 1).
Forced silking at low loads
The usual reeling processes normally provide silking forces around the
yielding limit, y, as shown in Figs
3A and
4A. However, lower forces were
measured sporadically; silking forces well below the yielding limit (between
0.3
y and 0.1
y) were measured in 10% of
tests performed at a reeling speed of 1 mm s1, as discussed
below.
Fig. 5A shows the force
measured along the fibre of a forced silking process at low load. Four samples
were tensile tested and the results are plotted in
Fig. 5B. The silking forces are
much lower than the yield limit (3 mN). When plotted as
stressstrain curves (Fig.
5C), the results come within the domain of fibres naturally spun
(NS) by the spider. This is a completely different finding from previously
(see Figs 3C,
4C), where fibres were of type
FS, were stiffer, had low values of maximum strain and were well outside the
realm of NS fibres.
Results obtained with silking forces well below the conventional yielding
limit (<0.1 mN) are shown in Fig.
6A. Three samples were tensile tested; the
forcedisplacement curves are shown in
Fig. 6B, and the corresponding
stressstrain curves in Fig.
6C. Again, another interesting result was found; at such low
silking forces, the curves are much more compliant than the NS fibres, they
are below the NS region and are similar to those of the maximum
supercontracted fibres (MS) tested in air
(Pérez-Rigueiro et al.,
2003). As for the data shown in
Fig. 3 for almost constant
silking forces, the forcedisplacement curves here were reproducible, a
behaviour that improves when stressstrain curves are considered.
Modulation of the tensile behaviour of spider silk through the silking forces
Table 1 outlines the results
presented above and compares the mean silking force (Fs)
in each process with the spider's weight (W) and the values of
yielding force (Fy) and breaking force
(Fu). The absolute values of the silking force span a
range of two orders of magnitude (Fs=0.066.5 mN).
Since the absolute values are likely to depend on the spider's size, the
results have been re-scaled by the spider's weight to show that a similar
range of values is found (Fs/W=0.010.96).
Classification of the silking process according to the silking force has
proceeded by comparing the silking force and the tensile parameters of the
fibre. The comparison of the silking force and the breaking force is an
intuitive way of describing the stresses to which the fibres are subjected
during processing. However, breaking force is affected by large variations
(Garrido et al., 2002b) related
to the low Weibull modulus of spider silk [Weibull modulus is a statistical
property of fibres (Chou, 1992
)
that measures the spread of the tensile strength of different samples of a
given material; a low Weibull modulus indicates large spread in the tensile
strength]; consequently, the value of the yield force has been favoured as the
classification parameter. The intervals spanned by both ratios,
Fs/Fy and
Fs/Fu, are comparable and narrower
than the interval spanned by the Fs/W
parameter.
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The results obtained with A. trifasciata are consistent with
previously published results on garden spider Araneus diadematus
(Ortlepp and Gosline, 2004).
It has been found that spiders during vertical descents can exert forces on
the silk fibres even up to 2.2 x the spider's weight. This value is
comparable with the silking force measured in the high silking force
processes, which lead to fibres with FS properties. At the other extreme of
the silking force range, it has been estimated that spiders in free fall spin
silk at forces of approximately 0.1 x the spider's weight. These results
are consistent with the data presented in
Fig. 5, since fibres spun at
0.09 x body weight show the characteristics of NS fibres. Combining the
results on free-falling spiders (Ortlepp
and Gosline, 2004
) with those on undisturbed climbing spiders
(Garrido et al., 2002) can account for an intriguing property of the latter.
It has been found that fibres spun during undisturbed climbing represent the
lower limit of the stressstrain curves retrieved either from the web or
from the safety line. The results obtained from free-falling spiders suggest
that these fibres could be spun at the lowest silking force available to the
spider.
In this context, Fig. 6,
which shows fibres with tensile features characteristic of MS fibres, would
represent an anomalous behaviour of the spider. The extremely low forces
involved in this silking process could be related to changes in the anatomy or
physiology of the spinning process as a consequence of the advanced age of the
spinning spider (the specimen died a few days after the silking process).
However, Fig. 6 indicates that,
even under the minimum silking force, the tensile properties of the fibres are
within the previously established range of stressstrain curves
described for A. trifasciata spiders
(Pérez-Rigueiro et al.,
2003).
The identification of the influence of protein sequence and processing on
the final properties of spider silk fibres has prompted an intense debate,
driven by the need to determine the essential features of spider silk in order
to synthesize artificial silk fibres. The observation that a reduced number of
motifs appears conserved in the protein sequence of spiders belonging to
widely different groups (Gatesy et al.,
2001) suggests that the protein sequence plays an essential role
in the properties of spider silk. Our results suggest that, although the basic
properties of the silk fibre can be determined by the protein sequence, its
tensile behaviour can be modulated through the silking force in a range that
spans maximum supercontracted fibres and forcibly silked fibres and includes
the naturally spun fibres produced during web building and as a safety
line.
Conclusions
(1) The forced silking technique described here allows us to measure the
silking force in each silk segment along the fibre length and, additionally,
retrieve the sample to determine its tensile properties. A correlation between
silking force and tensile properties was verified. The experimental procedure
is complicated due to the small forces involved in spinning (forces as low as
0.1 mN have been measured) and by the control exerted by the spider on the
spinning process, which excludes the possibility of performing a silking
process at a constant spinning force.
(2) The silking force greatly influences the type of silk fibre. When the force is high, i.e. around the conventional yield limit, the fibres are stiffer than those naturally spun by spiders. As the load decreases, fibre stiffness decreases, and forcibly silked fibres come to resemble the naturally spun ones. At very low loads, the fibre is even more compliant, and its behaviour is similar to that of the supercontracted fibre, tested in air. In addition, our results open a new and promising scenario for forced silking and its application in the spinning of artificial silk, since this method can be used to obtain all kinds of fibre and not only stiff FS ones.
(3) When tensile tests on samples taken from the same fibre are compared, variability was found to decrease if stressstrain curves are considered instead of forcedisplacement curves. In this respect, the diameter of fibres reeled at constant force was more homogeneous than that of fibres reeled at variable force.
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Acknowledgments |
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References |
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Chou, T.-W. (1992). Microstructural Design Of Fiber Composites. Cambridge: Cambridge University Press.
Cunniff, P. M., Fossey, S. A., Auerbach, M. A. and Song, J. W. (1994). Mechanical properties of major ampulate gland silk fibers extracted from Nephila clavipes spiders. In Silk Polymers. Materials Science and Biotechnology (ed. D. Kaplan, W. W. Adams, B. Farmer and C. Viney), pp. 234-251. Washington, DC: American Chemical Society.
Denny, M. W. (1976). The physical properties of spider's silk and their role in the design of orb-webs. J. Exp. Biol. 65,483 -506.
Elices, M. (ed.) (2000). Structural Biological Materials. Amsterdam: Pergamon Press.
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]
Gatesy, J., Hayashi, C., Motriuk, D., Woods, J. and Lewis,
R. (2001). Extreme diversity, conservation, and convergence
of spider silk fibroin sequences. Science
291,2603
-2605.
Guess, K. B. and Viney, C. (1998). Thermal analysis of major ampullate (drag line) spider silk: the effect of spinning rate on tensile modulus. Thermochimica Acta 315, 61-66.[CrossRef]
Guinea, G. V., Elices, M., Real, J. I., Gutiérrez, S. and Pérez-Rigueiro, J. (2005). Reproducibility of the tensile properties of spider (Argiope trifasciata) silk obtained by forced silking. J. Exp. Zool. A 303, 37-44.
Griffiths, J. R. and Salinatri, V. R. (1980). The strength of spider silk. J. Mater. Sci. 15,491 -496.[CrossRef]
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.
Kaplan, D., Adams, W. W., Farmer, B. and Viney, C. (eds.) (1994). Silk Polymers. Materials Science and Biotechnology. Washington, DC: American Chemical Society.
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. and Vollrath, F. (2000). Mechanics and morphology of silk drawn from anesthetized spiders. Naturwissenschaften 87,148 -153.[CrossRef][Medline]
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]
Ortlepp, C. S. and Gosline, J. M. (2004). Consequences of forced silking. Biomacromolecules 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]
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. and Emerson, P. D. (1982). An apparatus and technique for the forcible silking of spiders. J. Arachnol. 10,1 -10.