The effect of different solvents on the structure of silk fibroin-PVA composite foams

Abstract

To be able to obtain silk fibroin in different forms (film, fiber, sponge etc.), fibroin should be dissolved in suitable solvents. Different solvents are used for this process. Very few of the studies in the literature were investigated on the effects of the solvent system on the structure of the final product. This work aimed to fulfil this deficiency. For this reason, the main purpose of this work is to investigate the effect of different solvents on the obtained structure of fibroin / Polyvinyl Alcohol (PVA) composite sponges. Since this study's final goal is to use the obtained composite sponges as a wound dressing, this presented work is the first part of the planned study that mainly consists of preliminary preparation steps. Fibroin / PVA composite sponges were obtained by using lyophilisation (freeze drying) method in this work. Silk fibroin based sponges with high porosity were produced by employing this technique. In this preliminary study, first the effects of different solvents that are used in the dissolution of silk fibroin on the general structure of the obtained material and the composite structures that are formed by mixing with PVA were investigated. Then, produced composite sponge structures were characterized by using Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FTIR), Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) methods. When the composite sponge structures, obtained in Calcium Chloride (CaCl<inf>2</inf>)-Ethanol-H<inf>2</inf>O, were examined, it was observed that the secondary structure was dominated mainly by ? sheet. The FTIR results showed that the Amide I and Amide II bands had changes in their location and values after mixing with PVA. These results demonstrated that the interaction between fibroin molecules and PVA molecules is quite strong. The silk fibroin, dissolved in Lithium Bromide (LiBr), mainly consisted the random coil structures and the structure had not transformed into a ? sheet conformation during the formation of the composites, which affected the ratio of the crystal structure to the amorphous structure and thermal stability of the composite sponges. The thermal properties of the material were analyzed using TGA and DSC. In summary, when the fibroin solution, obtained by dissolving fibroin in the solvent system of CaCl<inf>2</inf>-Ethanol-H<inf>2</inf>O, with the fibroin solutions, obtained by dissolving fibroin in the solvent system of LiBr are compared, it is found that their secondary structural conformation and morphologies were different and the fibroin structures that were obtained from CaCl<inf>2</inf>-Ethanol-H<inf>2</inf>O solvent system formed more stable and strong complexes with PVA. © 2019 Elsevier B.V., All rights reserved.