Plasticization and Antibacterial Activity of Nanofibrillated Silk Fibroin Films Using Glycerol and/or Propylene Glycol

Abstract

Nanofibrillation is a sustainable physical method for producing ultrafine fibroin nanofiber (FNF) films with remarkable ultimate tensile strength and great thermal stability. However, the brittleness and poor flexibility of these films limit their possible applications. Herein, two hydrophilic plasticizers, including glycerol (Glyc) and propylene glycol (PG), were used to improve FNF flexibility. Fibroin nanofibers were blended with glycerol or propylene glycol at concentrations of approximately 23% and 50% w/w (based on the FNF weight), and their individual effects on the chemical, structural, mechanical, and thermal properties of the obtained FNF films were evaluated. Glycerol exhibited a stronger plasticizing effect than propylene glycol. As the concentration of glycerol increased, the flexibility of FNF increased proportionally, although water solubility also increased. In contrast, no plasticization was observed at low PG concentrations, whereas at higher concentrations, a minimal plasticization effect was observed. Moreover, PG-plasticized films exhibited higher water absorption capacity and maintained good stability in water. Both glycerol and propylene glycol exhibited antibacterial activity against Escherichia coli under the tested acidic conditions, with inhibition zones of around 7 and 9 mm, respectively. Unexpectedly, only films of plasticized propylene glycol composites exhibited minimal antibacterial activity, likely due to reduced concentration or entrapment within the polymer matrix. These findings provide new insights into using hydrophilic plasticizers to tune the structure–property relationships of fibroin nanofibers and highlight their potential as flexible, sustainable biomaterials for biomedical applications.

Graphical Abstract

Illustration of the impact of glycerol and propylene glycol on the secondary structure of nanofibrillated fibroin nanofibers (FNFs)