By Samudrapom DamReviewed by Lexie CornerOct 17 2024
A recent paper published in Scientific Reports presents a novel silk fibroin-based covering material for vascular tissue engineering, optimizing its mechanical properties.
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Challenges of Polymer Coverings
Vascular stents, which consist of a metal structure and a polymer covering, play a critical therapeutic role in cardiovascular treatment. The polymer acts as a physical barrier between blood and the vessel wall, isolating hemangiomas and preventing excessive tissue proliferation and dissection formation.
Commercial covering materials primarily include polytetrafluoroethylene (PTFE) and polyethylene terephthalate (PET). While these synthetic materials offer excellent biostability and mechanical strength, they can lead to complications such as luminal surface fibrosis and stent migration due to their biological inertness, poor compliance, and inability to endothelialize effectively.
Long-term follow-up studies have reported a high prevalence of postoperative re-interventions for both PTFE- and PET-covered stents, highlighting the need for biodegradable advanced biomedical materials that align with developments in tissue regenerative medicine.
Importance of Silk Fibroin
Silk fibroin possesses satisfactory tissue compatibility and hemocompatibility as a biomaterial. It has been investigated extensively in tissue engineering applications. Silk fibers display exceptional mechanical properties as a natural biomacromolecule, including a 4 to 26 % breaking strain and a 300 to 740 MPa breaking strength.
Vascular grafts made from braided silk fibers demonstrate rapid endothelialization, long-term patency without intimal hyperplasia, and strong mechanical properties. In this study, researchers fabricated a silk film by coating regenerated silk fibroin onto a silk braiding fabric using layer-by-layer self-assembly and rotary drying.
This silk film was then employed as a covering material for stent construction, showing no cytotoxicity and excellent water leakage resistance, indicating its potential application.
The Proposed Approach
In this research, a silk film for covered stents was developed using a layer-by-layer self-assembly strategy with regenerated silk fibroin on silk braiding fabric. The study investigated how these self-assembly and fabric parameters affected the silk films' mechanical properties.
The objective was to improve compressive resistance and compliance by optimizing the self-assembly and braiding parameters, addressing the poor compliance of commercially available covering materials. The materials used included Bombyx mori raw silk, sodium carbonate, lithium bromide, and polyethylene glycol diglycidyl ether (PEG-DE).
Silk Film Preparation: Raw silk was degummed with sodium carbonate and dissolved in a 9.3 M lithium bromide solution. The resulting silk fibroin aqueous solution was dialyzed using deionized (DI) water, concentrated to 80 mg/mL, and cross-linked with PEG-DE at a silk fibroin mass ratio of 1.0:0.8.
Silk tubular fabrics were braided by varying the number of axial yarns (100, 60, or 40 threads/10 cm along the axial direction), silk yarn sizes (3×2 or 1×2 silk yarn), and braiding angles (65°, 25°, or 90°). Both 3×2 and 1×2 silk yarns were degummed using sodium carbonate.
The braided silk fabric was then fully immersed for 30 seconds in the cross-linked silk fibroin solution and dried on a rotary drying device for 20 minutes at 60 °C, forming a self-assembled layer of silk fibroin.
A 10 mm tubular silk film was obtained after 1 to 6 immersing-drying cycles. These silk films were designated based on their silk yarn size, braiding angle, axial yarn number, and number of self-assembled layers, such as 1×2/90°/60/SF6 film. Researchers performed tensile tests, burst strength tests, compressive resistance tests, and compliance tests on the fabricated silk films.
Study Significance
The results revealed a significant correlation between layer-by-layer self-assembly, fabric parameters, and both mechanical strength and compliance. Increasing the braiding angle led to a decrease in radial breaking strength while significantly increasing breaking elongation.
For instance, the 1×2/90°/100/SF6 film's radial breaking strength decreased by 32 %, and the breaking elongation increased by 57 % when compared to the 1×2/25°/100/SF6 film. Similarly, silk films with a greater number of axial yarns exhibited higher radial breaking strength and elongation at the same braiding angle.
Additionally, increasing the number of self-assembly layers in 1×2/90°/60 silk films significantly enhanced radial breaking strength. However, breaking elongation initially increased before declining with more self-assembly layers. The 1 × 2/90°/60/SF6 film demonstrated excellent compliance of 2.6 %/100 mmHg, which matched the clinical gold standard's compliance/human saphenous veins (0.7-2.6 %/100 mmHg).
In summary, this work presents a novel material and approach to advancing vascular-covered stents that align with the biomechanics of blood vessels and enhance bioactivity.
Journal Reference
Yu, Y. et al. (2024). Tailoring silk-based covering material with matched mechanical properties for vascular tissue engineering. Scientific Reports. DOI: 10.1038/s41598-024-75343-y, https://www.nature.com/articles/s41598-024-75343-y
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