Effect of processing and storage time of aqueous solutions on silk fibroin structure and methanol post-treatment of electrospun fibers

Silk fibroin (SF) is a natural and biocompatible polymer that is obtained from silk cocoons or from silk fibrous waste. In this study, aqueous solutions of SF were obtained from silk fibrous waste (SF W ) and subjected to concentration by reverse dialysis with polyethylene glycol (PEG) or freeze-drying with subsequent resolubilization prior to processing via electrospinning. The regenerated SF W was stored at 4 °C, and its stability during refrigeration was investigated in terms of structural changes using visual monitoring and attenuated total reflectance Fourier transform infrared spectroscopy (FTIR-ATR) until gelation occurred. The electrospun nonwovens produced through both concentration processes were post-treated using methanol (MeOH), by immersion in the liquid solvent and by exposure to a saturated atmosphere. The electrospun nonwovens before and after post-treatments were characterized using scanning electron microscopy (SEM), FTIR-ATR, and differential scanning calorimetric (DSC). SF W re-suspended after the PEG concentration remained liquid and retained its electrospinnability for 21 days, while the resolubilized SF W solution after freeze-drying formed a gel after only 7 days. During these processability windows, it was possible to obtain SF W homogeneous fibers. Increasing solution storage time was correlated with increased fiber diameter and reduced porosity of the electrospun nonwovens. The fibers treated with MeOH, both liquid and vapor, showed an increase in diameter and crystallinity, as well as a reduction in their apparent porosity. This work contributes new information about pre- and post-processing of SF W aqueous solutions for electrospun SF W nonwovens, expanding the potential of a by-product of silk production in value-added biomaterials production. Graphical abstract