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Silk: fibers, films, and compositesdtypes, processing, structure, and mechanics 153

5.2.2.1 Composition

The diversity of spiders produces a wide variety of silks. Indeed, depending on the
species, the creature can possess from two to nine glands producing different polymers
and fibers. The glands can act like a spinneret leading to a fiber with good orientation
and mechanical characteristics or used as an extruder producing silk more used as a
glue than a fiber but still composed of macromolecular polymers. These silks can
have different uses: protection, feeding, eggs, etc Spiders can produce their polymer,
named spidroin, throughout their life, and in contrast to silkworms, spiders have inner-
vated spinnerets permitting a closer control of the spinning (Craig, 1997).
The set of spider glands (seven for Araneid) is much more complex. The major and
minor ampullate glands produce the dragline and the frame threads of the web,
whereas the minor ones provide fibers that can be added to the web. The aciniform
glands make the silk for wrapping prey, the cylindrical glands make the wrapping
silk for the egg sac and the pyriform glands make the silk for attachments and for
joining threads (Vollrath and Knight, 2001). The flagelliform glands provide the
core of the threads used in the web to capture prey, whereas the coating for the thread
is made by the aggregate glands.
In the production of spider silk, the amino acid composition can change depending
on the gland used. Thus, for example, the minor ampullate gland of Araneus diadematus
produces a silk rich in Gly and Ala (80%), which results in medium values of extensi-
bility and low values of strength and toughness. The major ampullate gland produces a
silk (the dragline and the web frame) with almost the same quantity of Ala but much less
of Gly with a global ratio of these amino acids of 0.5. This results in higher strength but
lower extensibility. Finally, aciniform gland silk has much less Ala and Gly (25%) and
shows low strength but high extensibility and toughness. Stiffness is very similar for all
of these silks (Hayashi et al., 2004). We will discuss dragline silk produced by the major
ampullate gland because the silk produced by this gland has the better mechanical prop-
erties and can be compared to the silkmoth fiber.
The polymers found in spider silks are called spidroins. There are two kinds of
spidroin moleculesdSpidroin I and IIdboth fibrous, in contrast with those produced
by the B. mori. The apparent molecular weight has being determined (by dissolution
and electrophoresis) as 275e320 kDa. It is reported that intermolecular disulfide
bonds permit these two chains to be linked together as in keratin (Paquin and
Colomban, 2007) to form an oligomer with a molecular weight of w725 kDa. First
of all, spidroins are characterized by repetitions of alanines (A) n . Then, another motif
is present in all species: GPGG(Xaa) or simply GG(Xaa), where Xaa is an unspecific
amino acid (Bittencourt et al., 2007). However, the sequence can be much more com-
plex for species like A. diadematus, the only commonality being the richness of
glycine and alanine. It has been shown that silks produced by different glands, major
and minor ampullate gland, aciniform, flagelliform, or tubuliform glands are different.
The functions and the mechanical properties also change, as has been shown for
different kinds of fibers produced by Argiope trifasciata (Hayashi et al., 2004).
To give a view of the possible conformations, sketches were made by Colomban
et al. (2008a) using the Chem3D software (Fig. 5.4(a)) with the sequence reported