6 Cocoon Silk: From Mesoscopic Materials Design …
269
Fig. 6.16 A Hierachical Structures of spider and cocoon silk fibers; Reproduced with permission
[32]. Copyright 2014, the Royal Society of Chemistry. B Schema of hierarchical network structures
of SF fibers and non-fiber SF materials. (i) Level 1: amino acid sequence; (ii) Level 2: α-helix &
β-sheet. They are stabilized by intermolecular H-bonds, and β-sheets are crystallized. (iii) Level
3: β-crystallites. The formation is attributed to inter-sheet interactions, and several neighboring
β-sheets (from different molecules) can crystallize into intermolecular β-crystallites (iv) Level 4:
crystal network. It is composed of numerous β-crystallites, which are connected to each other by
amorphous chains. A crystal network is indeed an individual nanofibril. During the process of
crystal network formation, shear forces can help to orient directions of crystallites. (v) Level 5:
nanofibril network. Based on nanofibril network topology and inter-nanofibril interaction strength,
silk fibers and non-fiber silk materials are of different fibril arrangements: among silk fibers, silk
nanofibrils are bundled along the fibrous axis, while for non-fiber silk materials, silk nanofibrils
are interconnected in a nearly random manner. Reproduced with permission [37]. Copyright 2019,
Wiley-VCH
nanofibrils, which prevents adjacent nanofibrils from slipping, and (2) the nanofibril
bundle architecture as well as the non-slipperiness between nanofibrils causes extra
stress to be shared equally among the unbroken nanofibrils.
As shown in Fig. 6.14, nanofibrils in silk fibers and SF hydrogels have similar
diameters. In addition, most structural parameters, including level 1 to level 4 structures, except the orientation of β-crystallites, are also nearly the same [18]. However,
different types of interactions between nanofibrils are observed in these two level
5 SF materials. Specifically, weak nanofibril–nanofibril interactions are observed
in non-fiber SF materials, while strong inter-nanofibril interactions are observed in
silk fibers. They both exhibit different, distinct nanofibril network architectures. For
instance, the nanofibrils in silk fibers are well aligned along the fibrous axis, whereas
the nanofibrils in regenerated SF hydrogels are much less ordered or randomly
distributed. Hence, natural silk fibers are much stronger than SF films (measured
at approximately 300–400 MPa and 40 MPa for silk fiber and SF films, respectively.
6.3.6 Summary of Hierarchical Structure of SF Materials
Based on the structural analyses of SF materials, the extraordinary properties of B.
mori silkworm silk materials are attributed to the five levels of hierarchical network
structures illustrated in Fig. 6.16. Figure 6.16a shows the key hierarchical structures
of cocoon silk fibers. In detail and following a bottom-to-top manner, the hierarchical network structure can be summarized as follows: (1) amino acid sequence,
(2) secondary structure, (3) β-crystallites, (4) crystal networks or nanofibrils, and (5)
nanofibril networks.
Within the framework of the hierarchical structure, the crystalline binding interaction is very important in the stabilization of SF materials. The crystallite networks or
nanofibrils and the networks of nanofibrils (Fig.6.16b) are two of the most essential
structural elements, which, to a large extent, determine the macroscopic performance
of SF materials.
269
Fig. 6.16 A Hierachical Structures of spider and cocoon silk fibers; Reproduced with permission
[32]. Copyright 2014, the Royal Society of Chemistry. B Schema of hierarchical network structures
of SF fibers and non-fiber SF materials. (i) Level 1: amino acid sequence; (ii) Level 2: α-helix &
β-sheet. They are stabilized by intermolecular H-bonds, and β-sheets are crystallized. (iii) Level
3: β-crystallites. The formation is attributed to inter-sheet interactions, and several neighboring
β-sheets (from different molecules) can crystallize into intermolecular β-crystallites (iv) Level 4:
crystal network. It is composed of numerous β-crystallites, which are connected to each other by
amorphous chains. A crystal network is indeed an individual nanofibril. During the process of
crystal network formation, shear forces can help to orient directions of crystallites. (v) Level 5:
nanofibril network. Based on nanofibril network topology and inter-nanofibril interaction strength,
silk fibers and non-fiber silk materials are of different fibril arrangements: among silk fibers, silk
nanofibrils are bundled along the fibrous axis, while for non-fiber silk materials, silk nanofibrils
are interconnected in a nearly random manner. Reproduced with permission [37]. Copyright 2019,
Wiley-VCH
nanofibrils, which prevents adjacent nanofibrils from slipping, and (2) the nanofibril
bundle architecture as well as the non-slipperiness between nanofibrils causes extra
stress to be shared equally among the unbroken nanofibrils.
As shown in Fig. 6.14, nanofibrils in silk fibers and SF hydrogels have similar
diameters. In addition, most structural parameters, including level 1 to level 4 structures, except the orientation of β-crystallites, are also nearly the same [18]. However,
different types of interactions between nanofibrils are observed in these two level
5 SF materials. Specifically, weak nanofibril–nanofibril interactions are observed
in non-fiber SF materials, while strong inter-nanofibril interactions are observed in
silk fibers. They both exhibit different, distinct nanofibril network architectures. For
instance, the nanofibrils in silk fibers are well aligned along the fibrous axis, whereas
the nanofibrils in regenerated SF hydrogels are much less ordered or randomly
distributed. Hence, natural silk fibers are much stronger than SF films (measured
at approximately 300–400 MPa and 40 MPa for silk fiber and SF films, respectively.
6.3.6 Summary of Hierarchical Structure of SF Materials
Based on the structural analyses of SF materials, the extraordinary properties of B.
mori silkworm silk materials are attributed to the five levels of hierarchical network
structures illustrated in Fig. 6.16. Figure 6.16a shows the key hierarchical structures
of cocoon silk fibers. In detail and following a bottom-to-top manner, the hierarchical network structure can be summarized as follows: (1) amino acid sequence,
(2) secondary structure, (3) β-crystallites, (4) crystal networks or nanofibrils, and (5)
nanofibril networks.
Within the framework of the hierarchical structure, the crystalline binding interaction is very important in the stabilization of SF materials. The crystallite networks or
nanofibrils and the networks of nanofibrils (Fig.6.16b) are two of the most essential
structural elements, which, to a large extent, determine the macroscopic performance
of SF materials.