Abstract
The excellent mechanical properties of spider dragline silk are closely linked to its multiscale hierarchical structuring which develops as it is spun. If this is to be understood and mimicked, multiscale models must emerge which effectively bridge the length scales. This study aims to contribute to this goal by exposing structures within Nephila dragline silk using low-temperature plasma etching and advanced Low Voltage Scanning Electron Microscopy (LV-SEM). It is shown that Secondary Electron Hyperspectral Imaging (SEHI) is sensitive to compositional differences on both the micro and nano scale. On larger scales it can distinguish the lipids outermost layer from the protein core, while at smaller scales SEHI is effective in better resolving nanostructures present in the matrix. Key results suggest that the silks spun at lower reeling speeds tend to have a greater proportion of smaller nanostructures in closer proximity to one-another in the fiber, which we associate with the fiber's higher toughness but lower stiffness. The bimodal size distribution of ordered domains, their radial distribution, nanoscale spacings, and crucially their interactions may be key in bridging the length scale gaps which remain in current spider silk structure-property models. Ultimately this will allow successful biomimetic implementation of new models.
Highlights
Spider silk is of great interest to a range of scientific communities due to its high-performance and unique mechanical properties (Vollrath and Porter, 2009; Walker et al, 2015; Koeppel and Holland, 2017; Holland et al, 2018b)
This nanostructure has been extensively explored by bulk and space-averaging techniques such as calorimetry (Cebe et al, 2013; Vollrath et al, 2014; Holland et al, 2018a), spectroscopy (Dicko et al, 2007; Boulet-Audet et al, 2015) small angle scattering X-ray and neutron diffraction (Termonia, 1994; Riekel et al, 2000; Greving et al, 2010; Wagner et al, 2017) and solid state nuclear magnetic resonance (NMR) (Willis et al, 1972; Hijirida et al, 1996; Kümmerlen et al, 1996; Yang et al, 2000; Holland et al, 2008; McGill et al, 2018), which together have provided the fuel for a range of modeling approaches (Giesa et al, 2011; Cranford, 2013; Ebrahimi et al, 2015; Rim et al, 2017)
The underlying skin is of similar thickness to the coating and has been found to compose mostly of minor ampullate spidroin protein (MiSp) which is the main component of minor ampullate fibers (Sponner et al, 2007)
Summary
Spider silk is of great interest to a range of scientific communities due to its high-performance and unique mechanical properties (Vollrath and Porter, 2009; Walker et al, 2015; Koeppel and Holland, 2017; Holland et al, 2018b). In a typical dragline fiber of 5 μm diameter, the coating is approximately 100 nm in thickness and can be further differentiated into a waxy lipids and a glycoprotein layer, which together are attributed with the control of moisture content, antimicrobial properties, and pheromonal communication (Augsten et al, 2000; Sponner et al, 2005, 2007) It is chemically diverse, the contribution of the coating to the fiber’s overall tensile behavior has been proposed to be very small (Yazawa et al, 2018). The underlying skin is of similar thickness to the coating and has been found to compose mostly of minor ampullate spidroin protein (MiSp) which is the main component of minor ampullate fibers (Sponner et al, 2007)
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