Functionalization of Colored/Fluorescent Silkworm Silk Fibrous Materials - Soft Fibrillar Materials: Fabrication and Applications - page 209

Table 7.1 A comparison of the structure of spider N. pilipes dragline silk and silkworm Bom-
byx mori cocoon silk [34].

Sample Overall content Crystallinity Content of Crystallite Inter-crystallite

name of β -sheets (%) intra-molecular size (nm) distance (nm)

(%) β -sheets (%)

bb cc

a Meridional Equatorial

direction direction

Spider
dragline
(10mms − 1 )

51 (100%) 22 (43%) 29 (57%) 2.1 2.7 6.5 17.8 13.5

Silkworm
cocoon silk

49 (100%) 40 (82%) 9 (18%) 2.3 4.1 10.3 4.8 7.2

(amorphous) segments. Among the fibrils, the protein molecules are joined by
β -crystallites to form a sort of molecular network (cf. Figure 7.1a) [40, 41]. The

primary structure of silkworm silk fibroin consists of repetitive sequences that can
be divided into small blocks. The amino acid sequence in the crystalline region of
Bombyx mori silk fibroin is considered to be (GAGAGS) n ,andthesequenceinthe
amorphous region contains Tyr-rich domains (cf. Figure 7.1b) [34].
Wu et al . correlated both the structure of spider and silkworm silks with their
mechanical properties theoretically and experimentally [35]. From the theoretical
model (Figure 7.2a), it follows that silkworm silk is composed of intermolecular
β -crystallites, while spider dragline silk is composed of both intermolecular- β -

crystallites and intramolecular -sheets in their fibrils. The molecular dynamics

(MD) simulations based on this model predicted that the stress-strain profiles
of the two types of silks were in good agreement with the experimental data for
appropriate values (Figure 7.2b). As expected, the breakage of

β

β

-crystallites in
silkworm silk fibrils weakens the linkage between protein molecules. By varying
the reeling speeds to simulate the dynamic stretching process (Figure 7.2c), it was
found that the elasticity of silk increased with reeling speed. This implies that the
protein macromolecules in the amorphous state are better aligned at higher reeling
rates, resulting in more efficient resistance to external stress. This model has been
verified by various experiments, and turns out to be able to explain well the strain
hardening observed in spider dragline silk fibers [34].
Notice that silk fibroin dissolved from Bombyxmori silk fibers can be reformulated
into silk fibroin films by casting solutions of silk proteins onto a substrate and
allowing the evaporation of the solvent. As-cast films made from aqueous solutions
of silk fibroin are mechanically weak and typically unstructured or -helix-rich [5,

42]. In tissue engineering studies, in order to modulate the mechanical properties
and the rate/extent of degradation, the crystalline state (

α

β -sheet content) and

morphology need to be controlled [8].