Materials: Surprising strength of silkworm silk

Abstract

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.

Full text

C
ommercial silkworm silk is presumed
to be much weaker and less extensible
than spider dragline silk, which has
been hailed as a ‘super-fibre’
1–4
. 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
4
J kg
11
(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
16210
4
J kg
11
(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
drawing
4
. 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
6
.
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
4
, 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
11
gave
a breaking elongation for Bombyx silk that
is comparable to that of spider silk (37%
and 35%, respectively). Faster spinning (13
mm s
11
) gave a breaking energy approaching
that of Nephila silk (12210
4
Jkg
11
for
Bombyx; versus 16210
4
J kg
11
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
2
, as well as to their arrange-
ment during and after spinning
1
. 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
construction
4
(10–20 mm s
11
).
Although spider silk fibres are studied in
their native state
4
, silkworm silk must first be
washed
7
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
8
. 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 silk’s material
properties more than washing.
For example, washing experimental fibres
reeled at 4 mm s
11
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
plasticization.
Our findings indicate that the mechanical
properties of Bombyx silk, like those of spider
silk
4
, 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,
China
e-mail: zzshao@fudan.edu.cn
†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).
3. Lazaris, A. et al. Science 295, 472–476 (2002).
4. Vollrath, F., Madsen, B. & Shao, Z. Proc. R. Soc. Lond. B 268,
2339–2346 (2001).
5. Wilding, M. A. & Hearle, J. W. S. in Polymeric Materials
Encyclopaedia Vol. 11 (ed. Salamone, J. C.) 8307–8322 (CRC,
Boca Raton, Florida, 1996).
6. Miura, M., Pan, Z. J., Aoyama, S., Morikawa, H. &
Mochizuki, S. J. Seric. Sci. Jpn 67, 51–56 (1998).
7. Perez-Rigueiro, J., Elices, M. J., Llorca, J. & Viney, C.
J. Appl. Polymer Sci. 82, 1928–1935 (2001).
8. Perez-Rigueiro, J., Viney, C., Llorca, J. & Elices, M. Polymer 41,
8433–8439 (2000).
9. Wiedbrauck, J. Z. Tierpsychol. 12, 176–202 (1955).
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
NATURE
|
VOL 418
|
15 AUGUST 2002
|
www.nature.com/nature 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
silkworm
Bombyx mori
. Stress–strain curves of washed and
degummed single-filament silkworm silk (motor-reeled at 25 7C at
the indicated speeds),
Nephila
spider dragline silk (20 mm s
11
at
25 7C) and standard, degummed commercial silk from a cocoon
spun by the animal in the natural ‘figure of eight’
9
at speeds
oscillating between 4 and 15 mm s
11
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 (
n
44) were forcibly silked
4
, 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)
4
; further details are available from the
authors. For silk ‘degumming’, a traditional aqueous solution
standard wash of 1% sodium hydrogen carbonate
10
was used,
which led to a 30–40% reduction in fibre diameter. Inset,
unwashed native silkworm silk (photo courtesy of Giuliano Freddi).
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
0 0.05 0.15 0.25 0.35
27 mm s
–1
Spider
20 mm s
–1
13 mm s
–1
Cocoon
4 mm s
–1
Strain
Stress (GPa)
Figure 2 A cocoon produced by the silkworm
Bombyx mori
.
Changing the spinning conditions can improve the silk’s quality.
© 2002
Nature
Publishing
Group