Spider Silk: Structure, Engineering, and Applications - Soft Fibrillar Materials: Fabrication and Applications - page 184

A-zone B-zone Duct

Funnel

Epithelial
cells

Draw down

Spigot

Hexagonal
columnar
packing

Flow elongation
and β -sheet formation

Rod like
amphiphilic
block copolymer

Figure 6.3 Illustration of an assembly model and the extracellular transport of spider silk
major ampullate (MAA) protein in orb web spiders. (Reprinted with permission from Ref.
[7]. Copyright (2006) Elsevier.)

takes place at the draw-down taper, where the solid fiber is eventually formed.
The lumen of the gland has been represented as much wider in proportion to its
length, and only a small number of bilayer discs (top left) and the epithelium have
been shown on only one side of the duct and gland. The dotted lines represent the
molecular director field. This lies at right angles to the slow axis of polarization as
a result of the assembly of the compactly wound, rod-shaped molecules of spidroin
into bilayered discs of the nematic discotic phase. These are present as an escaped
nematic texture in the gland and first half of the duct [7]. In summary, the silk fiber
is formed near the end of the duct during a dramatic phase transition resulting
from rapid flow elongation in a draw-down taper within the spinning duct [9, 25].
The linear velocity of the protein through the duct increases exponentially before
the draw-down taper, suggesting that wall shear may play a role in assisting the
transition from protein solution to solid silk [7]. A controlled flow elongation and
water extraction provide a progressive increase in birefringence, which is correlated
to an increase in β -sheet structure in the duct [25-27].

6.2.2
Strain Hardening of Spider Dragline Silk

Compared to silkworm silk, spider dragline fibers exhibit a characteristic strain-
hardening feature in the non-linear region of the stress-strain curve. As shown
in Figure 6.4, both spider and silkworm silk fibers exhibit a linear stress-strain
relationship up to a yield point. The linear portion of the curve is the elastic
region and the slope is defined as Young's modulus [28]. After the yield point,
the spider dragline initially displays a characteristic J -shaped behavior of strain
hardening (the slope of the stress-strain curves increases with strain) until point