A Design of Experiment Rational Optimization of the Degumming Process and Its Impact on the Silk Fibroin Properties

Abstract

Silk fibroin is a protein with a unique combination of properties and is widely studied for biomedical applications. The extraction of fibroin (degumming) from the silk filament impacts the properties of the outcoming material. The degumming can be conducted with different procedures. Among them, the most used and studied procedure in the research field is the alkali degumming with sodium carbonate (Na₂CO₃). In this study, by the use of a statistical method, namely, design of experiment (DOE), we characterized the Na₂CO₃ degumming, taking into consideration the main process factors involved and changing them within a selected range of values. We considered the process temperature and time, the salt concentration, and the number of baths used, testing the impact of these variables on the fibroin properties by building empirical models. These models not only took into consideration the direct effect of the process factors but also their combined effect, which are not conventionally detectable with other methods. The weight loss and the amount of sericin removed in the process were determined and used as a measure of the effectiveness of the process. The secondary structure, the molecular weight, the diameter of fibers, and their morphology and mechanical properties were studied with the intent to correlate the macroscopical properties with the structural changes. We report, for the first time, the possibility to effectively remove all sericin from the silk fibroin using Na₂CO₃, using a process that requires less salt, water, and energy, in comparison with the standard alkali protocol, making this technique overall more environmentally sustainable; in addition, we have demonstrated the possibility to tune the material properties by varying the degumming conditions and even to optimize them with empirical statistically based equations that allow one to directly set the optimal process parameters. The major effect on the macroscopical properties (such as the ultimate strength and Young's modulus) has been proved to be correlated with the removal of sericin instead of the microstructural variations. Finally, a ready-to-use table with a set of optimized degumming procedures to maximize or minimize the studied properties was provided.

Keywords: design of experiment; mechanical properties; molecular weight; secondary structure; silkworm silk; statistical methods.