(163b) Leveraging Kosmotropic Aggregation of Silk Fibroin to Promote Continuous Nano-Thin Coating Formation

Silk fibroin is a fiber-forming protein derived from the thread of Bombyx mori silkworm cocoons. This biocompatible protein, under the influence of kosmotropic agents such as potassium phosphate, can undergo self-assembly to transform from a water-soluble protein to a robust β-sheet rich elastomeric material. By leveraging the concurrent non-specific adsorption and self-assembly of silk fibroin, we demonstrate the growth of adherent and smooth nano-thin silk fibroin coatings on a variety of surfaces. These coatings demonstrate the ability to functionalize topographically complex surfaces without requiring chemical derivatization or specific adhesive interactions, showing potential as a versatile surface modification for biomedical applications. Here, we examine the effects of coating solution parameters such as protein concentration, pH, time, as well as kosmotropic and chaotropic salts from the Hofmeister series, on the solution-phase assembly and interfacial adsorption behavior of silk fibroin. Coating formation, measured using quartz crystal microbalance with dissipation, shows a strong dependence on pH, salt species, and salt concentration. At low-to-intermediate potassium phosphate and ammonium sulfate concentrations, these coatings display continual growth over time with no apparent saturation, which is a behavior not described by traditional protein adsorption models. In contrast, while citric acid and sodium chloride solutions increase the amount of silk fibroin adsorbed compared to silk fibroin without additional salts, they do not facilitate continual coating growth or coating behavior that depends on pH/salt concentration. These results suggest that attractive protein-protein interactions are required for sustained coating growth. In our system, surface adsorption and solution-phase protein assembly are likely inter-dependent phenomena. We therefore also investigated the solution-phase self-assembly of silk fibroin under various pH and ion concentrations that promoted or did not promote coating growth. Our results showed that at low-to-intermediate potassium phosphate and ammonium sulfate concentrations – where coatings grow continuously without saturation – silk fibroin forms small aggregates with narrow size distribution. At higher salt concentrations where coating growth is inhibited, silk fibroin assembles into large micron-sized aggregates. Citric acid and sodium chloride solutions initiate solution-phase assembly to a lesser degree and maintain a small and narrow size distribution, even at high concentrations. Thus, our findings suggest that the continuous growth of nanothin silk fibroin coatings by interfacial self-assembly requires a balance between inter-protein assembly and surface adsorption.