6 Cocoon Silk: From Mesoscopic Materials Design …
263
Fig. 6.12 Schema of crystal network of silk materials. a AFM morphology of regenerated nanofibrils in SF solutions. b Typical force-extension curves and schematic structure of amyloid fibrils.
Reproduced with permission [35]. Copyright 2006, Elsevier. c Results reported by Oroudjev et al.;
heights of peaks are random. Reproduced with permission [36]. Copyright 2002, National Academy
of Science. d Latest report of force spectra and corresponding mechanism (e) Molecular fishnet
structure of a silk nanofibril. e Simulated mechanical strength of fishnet, no friction strings, and
entangled amyloid fibril structures of silk fibrils. Breaking forces are plotted as functions of length
of fibrils, i.e., number of rows along fibril axes. Insets show a drawing of the different types of
networks. f Microscopic mechanism of silk yielding behavior and effect of ordering function of
crystallite network. (i) illustrates the interspersed β-crystallites in nanofibrils for which the orientations deviated from those of the fiber axis, (ii) illustrates splitting of β-crystallites in an arbitrary
direction, and (iii) is a sketch presenting the number of H-bonds involved in splitting dynamics,
indicated using red dashed lines. Better alignment along fiber axis would cause more H-bonds
to be recruited to undergo the splitting force and consequently give rise to stronger crystalline
binding interactions within β-crystallites. d–f Reproduced with permission [18]. Copyright 2016,
Wiley-VCH
performance, during the above process (strain less than 10%), the modulus of the
silk drops to nearly zero, as fiber extension is mainly caused by the breaking of weak
intramolecular hydrogen bonds within the intramolecular β-sheets. Afterwards, with
the progressive unfolding of fibers, the intermolecular β-crystallites begin to support
the load. The silk fibers become stiffer owing to the contribution of the enthalpic
component [33]. This is how strain-stiffening occurs. It was also found that the intermolecular β-crystallites in A. pernyi silks are reoriented and become aligned along
the fibrous axis (Fig. 6.13b) [43], which further contributes to the stiffening of the
entire crystal network (Fig. 6.13c). The stretching of fibers beyond inflection point
H causes a failure in the function of β-crystallites. Demolishing nodes of the crystal
network in fibers results in the weakening of the fiber (called strain weakening). This
breakage process has been verified by XRD measurements, which showed that once
the silk is stretched beyond point H, the β-crystallinity drops immediately [33]. B.
mori fibers with significantly lower intramolecular β-sheet content in non-crystalline
regions (only 9%) have lower flexibility and exhibit only strain-weakening behavior
after the yield point. Thus, it is possible that if there is a method for increasing the
number of intramolecular β-sheets in SF materials, the mechanical properties should
be reinforced accordingly. However, it should be noted that the content of intramolecular β-sheets is greatly controlled by intrinsic primary structures. As discussed above,
the core repetitive motifs forming β-sheet crystallites in B. mori and A. pernyi silks
are (GAGAGS) n and poly-alanine motifs, respectively. In addition, it was confirmed
that the (GAGAGS) n motifs in B. mori silk fibers are significantly longer. Longer
lengths of repetitive (GAGAGS) n sequences can increase the likelihood of their
association with each other and consequently lead to the formation of intermolecular
β-crystallites instead of intramolecular β-crystallites. On the other hand, a statistical
comparison of the primary fibroin sequences of B. mori and A. pernyi further revealed
that the B. mori silk fibroin contains approximately 65% of the repetitive GAGAGS
motif, whereas A. pernyi contains only approximately 18% of the repetitive poly(A)
263
Fig. 6.12 Schema of crystal network of silk materials. a AFM morphology of regenerated nanofibrils in SF solutions. b Typical force-extension curves and schematic structure of amyloid fibrils.
Reproduced with permission [35]. Copyright 2006, Elsevier. c Results reported by Oroudjev et al.;
heights of peaks are random. Reproduced with permission [36]. Copyright 2002, National Academy
of Science. d Latest report of force spectra and corresponding mechanism (e) Molecular fishnet
structure of a silk nanofibril. e Simulated mechanical strength of fishnet, no friction strings, and
entangled amyloid fibril structures of silk fibrils. Breaking forces are plotted as functions of length
of fibrils, i.e., number of rows along fibril axes. Insets show a drawing of the different types of
networks. f Microscopic mechanism of silk yielding behavior and effect of ordering function of
crystallite network. (i) illustrates the interspersed β-crystallites in nanofibrils for which the orientations deviated from those of the fiber axis, (ii) illustrates splitting of β-crystallites in an arbitrary
direction, and (iii) is a sketch presenting the number of H-bonds involved in splitting dynamics,
indicated using red dashed lines. Better alignment along fiber axis would cause more H-bonds
to be recruited to undergo the splitting force and consequently give rise to stronger crystalline
binding interactions within β-crystallites. d–f Reproduced with permission [18]. Copyright 2016,
Wiley-VCH
performance, during the above process (strain less than 10%), the modulus of the
silk drops to nearly zero, as fiber extension is mainly caused by the breaking of weak
intramolecular hydrogen bonds within the intramolecular β-sheets. Afterwards, with
the progressive unfolding of fibers, the intermolecular β-crystallites begin to support
the load. The silk fibers become stiffer owing to the contribution of the enthalpic
component [33]. This is how strain-stiffening occurs. It was also found that the intermolecular β-crystallites in A. pernyi silks are reoriented and become aligned along
the fibrous axis (Fig. 6.13b) [43], which further contributes to the stiffening of the
entire crystal network (Fig. 6.13c). The stretching of fibers beyond inflection point
H causes a failure in the function of β-crystallites. Demolishing nodes of the crystal
network in fibers results in the weakening of the fiber (called strain weakening). This
breakage process has been verified by XRD measurements, which showed that once
the silk is stretched beyond point H, the β-crystallinity drops immediately [33]. B.
mori fibers with significantly lower intramolecular β-sheet content in non-crystalline
regions (only 9%) have lower flexibility and exhibit only strain-weakening behavior
after the yield point. Thus, it is possible that if there is a method for increasing the
number of intramolecular β-sheets in SF materials, the mechanical properties should
be reinforced accordingly. However, it should be noted that the content of intramolecular β-sheets is greatly controlled by intrinsic primary structures. As discussed above,
the core repetitive motifs forming β-sheet crystallites in B. mori and A. pernyi silks
are (GAGAGS) n and poly-alanine motifs, respectively. In addition, it was confirmed
that the (GAGAGS) n motifs in B. mori silk fibers are significantly longer. Longer
lengths of repetitive (GAGAGS) n sequences can increase the likelihood of their
association with each other and consequently lead to the formation of intermolecular
β-crystallites instead of intramolecular β-crystallites. On the other hand, a statistical
comparison of the primary fibroin sequences of B. mori and A. pernyi further revealed
that the B. mori silk fibroin contains approximately 65% of the repetitive GAGAGS
motif, whereas A. pernyi contains only approximately 18% of the repetitive poly(A)