Structural and Morphological Characterization of Tussar and Bivoltine Silk Fibers

Technical Summary: Structural and Morphological Characterization of Tussar and Bivoltine Silk Fibers

Problem Statement
Silk fibers, composed primarily of fibroin and sericin, are valued for their unique combination of mechanical strength, flexibility, and biocompatibility. However, different silk varieties possess distinct physicochemical properties driven by their specific amino acid profiles and molecular organizations. While Tussar silk (wild, from Antheraea species) and Bivoltine silk (domesticated, from Bombyx mori) are both technologically significant, their fundamental differences in response to external physical stimuli, particularly electromagnetic irradiation, are not well understood at the microstructural level. Existing literature largely focuses on single silk types or chemically processed films, leaving a knowledge gap regarding the comparative response of natural Tussar and Bivoltine fibers under identical irradiation conditions. This study addresses the need to systematically compare these two varieties to optimize their selection for advanced textile engineering, biomedical scaffolding, and functional composite design.

Methodology
The study employed a comparative analysis of Tussar and Bivoltine silk fibers sourced from certified government institutions in India. The experimental design involved:

1. Sample Preparation: Fibers were cleaned, dried, and divided into control (untreated), 5-minute irradiated, and 10-minute irradiated groups.
2. Irradiation: Samples were subjected to controlled microwave irradiation (2.45 GHz, 1000W) for 5 and 10 minutes.
3. Characterization Techniques:
   • X-ray Diffraction (XRD): Used to evaluate crystallinity, identify $\beta$-sheet structures, and calculate crystallite size, lattice strain, dislocation density, and interplanar spacing using the Scherrer equation and Williamson-Hall method.
   • Fourier Transform Infrared (FTIR) Spectroscopy: Utilized to analyze chemical bonding, functional groups, and the preservation of the protein backbone (Amide I, II, and III bands).
   • Scanning Electron Microscopy (SEM): Employed to observe surface morphology, fibrillar alignment, and structural integrity.
   • Mechanical Testing: Breaking load, elongation, and tenacity were measured to assess structural integrity and load-bearing capacity.

Key Contributions and Results
The study reveals distinct structural and mechanical divergences between the two silk types upon microwave exposure:

• Crystallinity and XRD Analysis:

  • Tussar Silk: Exhibited a significant reduction in crystallinity and diffraction intensity after 5 minutes of irradiation, indicating chain scission and structural fragmentation. While a partial recovery in intensity and crystallite size was observed at 10 minutes, the crystallinity remained below the raw sample level, suggesting limited recrystallization and a tendency toward an amorphous state.
  • Bivoltine Silk: Demonstrated superior stability. While 5 minutes of irradiation caused a slight decrease in intensity, 10 minutes of exposure resulted in a significant increase in diffraction intensity and Crystallinity Index (CI), rising from 89% (raw) to 92%. This indicates irradiation-induced molecular reorganization and enhanced $\beta$-sheet alignment without significant lattice distortion.

• FTIR Spectroscopy:

  • Tussar Silk: Showed a reduction in peak intensities and band broadening after 5 minutes, suggesting partial disruption of hydrogen bonding and $\beta$-sheet ordering.
  • Bivoltine Silk: Displayed continuous intensification and sharpening of Amide I and II bands after 10 minutes, denoting improved chain alignment and strengthened intermolecular interactions.

• Mechanical Properties:

  • Tussar Silk: Mechanical performance degraded significantly after 5 minutes (reduced breaking load and tenacity) but showed a notable recovery at 10 minutes, slightly exceeding raw fiber values.
  • Bivoltine Silk: Exhibited a monotonic increase in mechanical properties with irradiation time. Breaking load, elongation, and tenacity improved progressively, with the 10-minute sample showing the highest performance (e.g., tenacity increasing from 2.19 to 2.857 g/den).

• Surface Morphology (SEM):

  • Tussar Silk: Raw fibers appeared rougher and more heterogeneous. Irradiation induced visible surface degradation, including roughness, fibril disruption, and microstructural discontinuities.
  • Bivoltine Silk: Raw fibers were smooth and compact. Post-irradiation, the fibers remained structurally intact with only minor surface irregularities, maintaining a uniform and compact morphology indicative of better molecular packing.

Significance and Claims
The paper concludes that Bivoltine silk possesses excellent irradiation tolerance and structural adaptability compared to Tussar silk. The authors claim that controlled microwave irradiation effectively controls the microstructure and performance of silk, particularly for Bivoltine varieties, by promoting the reorientation of fibroin chains and the stabilization of $\beta$-sheet domains.

The study asserts that these findings are crucial for the rational design of next-generation silk-based materials. Specifically, Bivoltine silk is identified as a more suitable candidate for applications requiring high structural stability and enhanced mechanical performance under irradiation, such as advanced textiles, load-bearing biomedical materials, and functional composites. Conversely, Tussar silk's susceptibility to thermal and structural degradation suggests it may be better suited for applications where natural porosity and aesthetic texture are prioritized over irradiation-induced structural reinforcement. The work emphasizes that understanding these differential responses is essential for selecting appropriate silk types and processing strategies in functional material design.