Dragline spider silks have
relatively high mass-based mechanical properties (tensile strength, elongation
to break and rupture energy) and are environmentally responsive
(supercontraction). In order to produce new synthetic fibers with these
properties, many research groups have focused on identifying the chemical composition
of these fibers and the structure of the fiber core. Since each fiber also has
an outer skin, our study will provide a detailed understanding of the silk
surface morphology, the response of the surface morphology to environmental
conditions and processing variables, and also determine if the silk surface has
a definitive patterning of charged amino acids. Specifically, by using force
microscopy and functionalized nanoparticles, the present study examines 1) how
the silk surface (topography, average roughness) is altered due to prior
mechanical loading (viz. reeling speed), 2) alterations in morphology due to
conditions (supercontraction, storage time), and 3) the negatively and
positively charged regions along with the surface using both force and nanoparticle mapping. Roughness data
taken on dragline silk collected from Nephila
clavipes spiders revealed that the surface comprised both smooth (5 nm RMS)
and rough (65 nm RMS) regions. Supercontracted silk (from immersion in0.01 MPBS during AFM testing) showed higher surface roughness values compared to
spider silk tested in the air, indicating that the surface might be reorganized during
supercontraction. No correlation was found between surface roughness and
neither collection speed nor aging time for the as-spun or supercontracted
fiber, demonstrating the surface stability of the dragline silk over time in
terms of roughness. Both the force microscopy and the nanoparticle methods
suggested that the density of negatively charged amino acids (glutamic acid,
aspartic acid) was higher than that of the positively charged amino acids
(lysine, asparagine, and histidine).
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