6 Cocoon Silk: From Mesoscopic Materials Design …
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structure was able to stabilize long silk fibers at a substantially reserved mechanical strength, which has greatly surpassed the breaking force predicted within the
entangled amyloid fibril model and the no friction strings model (Fig. 6.12f) [18].
Several types of crystallite patterns can be observed within the framework of
the nano-fishnet crystal network topology. For example, while β-crystallites can
be effectively oriented adjacent to each other along the fibrous axis, they can also
be oriented in a direction perpendicular to the fibrous axis. Recently, it has been
determined that the nanofibrils in natural SF fibers are dominated by parallel βsheets in which β-strands are parallel to the fibril axis. However, the cross-β-sheet
arrangement, in which β-strands are perpendicular to the fibril axis, can also exist
in some non-fibrous SF materials [46]. Gong et al. [46] reported that SF materials
can be selectively folded into β-sheets with either a cross-β-sheet or parallel-β-sheet
arrangement by incubating SF solutions quiescently or under shear force, respectively. Considering the existence of shear forces during the natural spinning process
of silk fibers, it is reasonable to assume that such shear forces play a dominant role
in determining the arrangement of β-strands. Specifically, we have made a series of
ex situ attempts to mimic the in situ natural spinning process in silkworm glands by
adjusting the pH, changing the metallic ion concentrations, or applying shear force.
The experimental results show that only the RSF nanofibrils created by shear forces
adopt the para-β-sheet arrangement (not yet published). Different β-sheet arrangements might contribute to the distinct mechanical properties between silk fibers and
hydrogels. This is because parallel-β-sheets are more regularly patterned along the
fibrous axis; consequently, they are stronger. In this regard, the application of shear
force during β-sheet formation might be a promising strategy for the preparation of
various non-fiber SF materials with enhanced mechanical properties.
Although β-crystallites are mostly responsible for the outstanding mechanical
performance of silk fibers, the intermolecular β-sheet is also equally important and
plays a unique role. Specifically, mechanical tests show that A. pernyi silk fibers
display a unique strain-stiffening characteristic [33, 43]. Recently, the structural
origin of this strain-stiffening effect has been comprehensively studied [43]. The
mechanism of strain stiffening is described as follows:
At relatively low stretching ratios (i.e., less than 5% or before the so-named yield
point S, shown in Fig. 6.13a), the content of the intramolecular β-sheets in B. mori
silk slightly decrease [33]. In comparison, the intramolecular β-sheet content in A.
pernyi silk fibers decreases significantly [33, 43]. In other words, the intramolecular β-sheets in A. pernyi silk fibers are unfolded prior to the splitting of the intermolecular β-crystallites [33, 43]. This is attributed to the fact that the intramolecular
β-sheets show a lower level of morphological perfection and stacking compactness
than the intermolecular β-crystallites. Thus, the unfolding of intramolecular β-sheets
results in the release of the entire length of the protein chains, which consequently
leads to the extension of draglines without causing the breakage of the intermolecular linkage of molecular networks (i.e., the β-crystallites). In terms of macroscopic
261
structure was able to stabilize long silk fibers at a substantially reserved mechanical strength, which has greatly surpassed the breaking force predicted within the
entangled amyloid fibril model and the no friction strings model (Fig. 6.12f) [18].
Several types of crystallite patterns can be observed within the framework of
the nano-fishnet crystal network topology. For example, while β-crystallites can
be effectively oriented adjacent to each other along the fibrous axis, they can also
be oriented in a direction perpendicular to the fibrous axis. Recently, it has been
determined that the nanofibrils in natural SF fibers are dominated by parallel βsheets in which β-strands are parallel to the fibril axis. However, the cross-β-sheet
arrangement, in which β-strands are perpendicular to the fibril axis, can also exist
in some non-fibrous SF materials [46]. Gong et al. [46] reported that SF materials
can be selectively folded into β-sheets with either a cross-β-sheet or parallel-β-sheet
arrangement by incubating SF solutions quiescently or under shear force, respectively. Considering the existence of shear forces during the natural spinning process
of silk fibers, it is reasonable to assume that such shear forces play a dominant role
in determining the arrangement of β-strands. Specifically, we have made a series of
ex situ attempts to mimic the in situ natural spinning process in silkworm glands by
adjusting the pH, changing the metallic ion concentrations, or applying shear force.
The experimental results show that only the RSF nanofibrils created by shear forces
adopt the para-β-sheet arrangement (not yet published). Different β-sheet arrangements might contribute to the distinct mechanical properties between silk fibers and
hydrogels. This is because parallel-β-sheets are more regularly patterned along the
fibrous axis; consequently, they are stronger. In this regard, the application of shear
force during β-sheet formation might be a promising strategy for the preparation of
various non-fiber SF materials with enhanced mechanical properties.
Although β-crystallites are mostly responsible for the outstanding mechanical
performance of silk fibers, the intermolecular β-sheet is also equally important and
plays a unique role. Specifically, mechanical tests show that A. pernyi silk fibers
display a unique strain-stiffening characteristic [33, 43]. Recently, the structural
origin of this strain-stiffening effect has been comprehensively studied [43]. The
mechanism of strain stiffening is described as follows:
At relatively low stretching ratios (i.e., less than 5% or before the so-named yield
point S, shown in Fig. 6.13a), the content of the intramolecular β-sheets in B. mori
silk slightly decrease [33]. In comparison, the intramolecular β-sheet content in A.
pernyi silk fibers decreases significantly [33, 43]. In other words, the intramolecular β-sheets in A. pernyi silk fibers are unfolded prior to the splitting of the intermolecular β-crystallites [33, 43]. This is attributed to the fact that the intramolecular
β-sheets show a lower level of morphological perfection and stacking compactness
than the intermolecular β-crystallites. Thus, the unfolding of intramolecular β-sheets
results in the release of the entire length of the protein chains, which consequently
leads to the extension of draglines without causing the breakage of the intermolecular linkage of molecular networks (i.e., the β-crystallites). In terms of macroscopic