Synergistic Effects of Glycerol and Rosemary Essential Oil on the Mechanical and Optical Behavior of Calcium-Crosslinked Alginate Films

Sodium alginate (SA), a biopolymer valued for its absorbent and hemostatic characteristics, is commonly used in wound dressings and biodegradable films. In this study, we developed and characterized sodium alginate-based biofilms infused with Rosemary essential oil, focusing on their mechanical, optical properties. Films were plasticized with glycerol (SAG) and further reinforced with rosemary essential oil (SAG+E.OROSEMARY), both before and after ionic crosslinking with calcium chloride (CaCl₂). Mechanical testing of sodium alginate films reveals that plasticizing them with glycerol significantly reduces their stiffness (from ~2218 MPa to ~17 MPa) and tensile strength (from ~62 MPa to ~5 MPa), while increasing their flexibility (from ~11% to ~25%). Adding rosemary oil slightly enhances tensile strength and elongation. However, crosslinking greatly increases stiffness and strength (up to ~2600–2700 MPa and 59–66 MPa), while reducing elongation to 3–8%. This indicates that the structure is more rigid and less flexible. Optical analysis using UV-visible spectroscopy indicated a refractive index of approximately 1.45 and an optical band gap of around 5.52 eV, with a slight increase observed after CaCl₂ immersion. Chemical modifications induced by the incorporation of essential oils and crosslinking were confirmed through FTIR spectroscopy. These findings demonstrate that combining glycerol and rosemary essential oil effectively tailors the mechanical and functional properties of alginate-based films, and that calcium crosslinking further modulates their strength and ductility. Such tunable biofilms have potential applications in active packaging and wound healing, where a balance between flexibility and rigidity is required.