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Materials Science in Additive Manufacturing Fibrous silk in biomedicine

advantages such as luster and fineness over other textile species capable of secreting up to seven distinct types of
fibers. 29,30 silk, each characterized by unique sequences that remain
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FS accounts for about 70 – 80% of the total mass of largely unexplored. Among these, Nephila silk stands
silk fiber, while sericin makes up about 20 – 30%; minor out as a well-studied model. This review focuses on the
constituents include wax, pigments, and inorganic matter. secondary structure of Nephila silk, whose amino acid
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FS is primarily composed of heavy (H) and light (L) chains, sequence features repetitive core segments alternating
along with the P25 glycoprotein. These components between hydrophilic and hydrophobic regions, flanked by
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adhere to a molecular ratio of H: L: P = 6:6:1. The H conserved N- and C-terminal domains. The hydrophobic
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and L chains are interconnected through disulfide bonds cores, rich in (A)n motifs (n = 4 – 6), promote crystal
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located at their C-termini, forming a stable H–L complex, formation, while the hydrophilic regions, abundant in
while the P25 glycoprotein associates non-covalently to glycine and tyrosine residues, favor random coil or helical
further stabilize this assembly. The primary sequence of conformations that transform into β-sheet nanocrystals
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the H-chain features repetitive GAGAGS segments, which during spinning. These nanocrystals act as cross-linkers,
constitute more than 80% of the FS composition. 35 binding amorphous segments into a robust network that
impart Nephila silk with exceptional mechanical and
Bombyx mori silk, renowned for its intricate structure, biological properties. 45,46
comprises heavy chain (Fib-H), light chain (Fib-L), and
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the P25 glycoprotein. Fib-H and Fib-L are covalently Spun FS exhibits a distinctive triangular cross-sectional
linked by disulfide bonds, forming a unique branched morphology and semi-crystalline characteristics within its
polymer, 34,37 while P25 physically interacts with this system, condensed structure. The sequences of Fib-H and Fib-L
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enhancing overall structural integrity. Fib-H, the core differ, and this sequence divergence is a key factor in
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structural component, consists of a 5,263-residue amino regulating the crystallinity and secondary structure of SF.
acid sequence (391.6 kDa), whereas Fib-L is shorter (262 The β-sheet configuration within the crystalline domains
residues, 27.7 kDa). The P25 glycoprotein (30 kDa) also serves as a pivotal determinant of silk’s mechanical
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plays a crucial stabilizing role. The structure of Fib-H is performance, primarily contributing to its exceptional
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characterized by a glycine-rich repetitive core and distinct strength and elastic modulus. The secondary structure
terminal regions. The core further divides into GAGAGS, and hierarchical arrangement of SF ultimately define the
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tyrosine-rich (Y), GAAS, and non-repetitive domains. properties of the resulting biomaterials. In particular,
GAGAGS and Y domains form β-sheet nanocrystals hydrophobic sections composed of repetitive amino acid
with unique arrangements, while GAAS sequences create sequences aggregate to form β-folded nanocrystals.
tetrapeptide β-turns that interrupt (GX)n crystalline
domains. Interestingly, the hydrophilic non-repetitive 3. Properties of FS
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segments fold into a distorted Ω-shape, enhancing protein FS represents a distinctive class of structural proteins,
chain flexibility and facilitating 180° reversals that promote renowned for its excellent biocompatibility, tunable
antiparallel β-sheet formation, ultimately strengthening degradability, and unparalleled mechanical characteristics.
the stability and mechanical properties of B. mori silk. 39 These unique characteristics render FS highly suitable for a
Species of the genus Antheraea (family Saturniidae), diverse array of processing techniques, encompassing both
including Antheraea pernyi and Antheraea yamamai, are aqueous and organic solvent-based methods. Moreover,
essential for tussah silk production. Tussah silk, similar its chemical modifiability allows adaptation to a broad
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in amino acid composition to Nophila silk, is characterized spectrum of biomedical applications. The following
by elongated poly(A) repeats. This region alternates sections elaborate on these superior properties.
between (A)n sequences (where n = 11 – 13) and glycine- 3.1. Mechanical properties
rich domains, containing characteristic motifs such as
GGYG, GSGA, and GGAG. This extended poly(A) region FS exhibits remarkable stiffness and superior tensile
enhances the crystallinity of Tunnah silk relative to Nophila strength along its longitudinal axis, while maintaining a
silk, though both remain less crystalline than mulberry considerable level of ductility. FS displays a well-balanced
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silk. In terms of β-sheet domain content, the crystallinity combination of modulus, elongation at break, and tensile
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hierarchy is mulberry silk > tussah silk > Nophila silk. strength, conferring outstanding ductility and toughness.
Spider silk’s enormous molecular weight (~350 kDa) To better contextualize the superior mechanical
endows its secondary structure with remarkable properties of FS compared to other materials, Table 1
complexity. Research indicates that over 30,000 spider and Figure 1 present a comparison of the mechanical
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species possess the ability to produce silk, with each properties of various fibrous materials, including both

Volume 4 Issue 2 (2025) 3 doi: 10.36922/MSAM025130020