Evaluating hydrophobic recovery of N2 and H2O(g) plasma modified silk fibroin films aged at ambient and elevated temperatures

Silk fibroin is a naturally derived polymer with great potential for biomedical use due to its strength, lack of immune response, and ability to biodegrade. The relatively hydrophobic nature of silk, however, can cause challenges with cell adhesion in vivo. Therefore, modification must be performed to improve the surface hydrophilicity, enhancing silk utility in the biomedical space. Low-temperature plasma (LTP) treatment is an established method for polymer modification and has the benefits of being a solvent-free, adaptable process. N2 and H2O(g) LTP treatments are both well-documented as strategies to enhance polar functional groups on a polymer's surface. However, many polymers tend to revert to their original hydrophobic state upon aging, reversing the effects of LTP modification. The hydrophobic recovery of N2 and H2O(g) LTP-modified silk has not been previously studied but has important implications for the uses and longevity of silk substrates in biomedical contexts. The goal of this study was to systematically evaluate the hydrophobic recovery of N2 and H2O(g) LTP-treated silk films. Films were LTP-modified using optimized plasma parameters (applied power, pressure, treatment time) and aged under both ambient and elevated temperature conditions up to 6 weeks after the initial treatment. Silk film surface properties were evaluated immediately after treatment and throughout the aging process using both water contact angle goniometry and x-ray photoelectron spectroscopy. LTP-treated silk films demonstrated a significant decrease in hydrophobicity compared to the untreated controls. Remarkably, both N2 and H2O(g) LTP modifications resulted in surfaces that retained hydrophilic properties over the 6 week aging period. Our findings represent a departure from what has been previously demonstrated in most LTP-modified synthetic polymers, suggesting that the secondary structure of silk fibroin plays a critical role in resisting hydrophobic recovery.