Starch Hydrogel Films with Dual Cross-Linking: Structural and Functional Characterization

Hydrogels consist of three-dimensional polymeric networks with hydrophilic functional groups. This study evaluated the influence of chemical cross-linking (Cc) using citric acid (CA) and physical cross-linking (Pc) based on cooling and heating cycles as an eco-friendly alternative to conventional cross-linking techniques for starch hydrogel films. The samples were subjected to both methods to assess the potential synergistic effects of a dual cross-linking approach. Cc hydrogel films were produced using 0, 0.5, 1.5, 2.5% CA concentrations and subjected to two Pc cycles. Structural, mechanical, thermal, and morphological analyses were performed while also assessing hydration and barrier properties. Results indicated that Cc promoted the formation of cross-links and significantly stabilized the matrix. At a 0.5% CA concentration, chemical cross-linking was more effective than cooling and heating cycles, reducing solubility by up to 11% when compared to Pc sample. Kinetic swelling studies conducted for 24 h showed that CA at 2.5% achieved a stable equilibrium with a significant decrease in water uptake compared to the control, also showing a denser and more rigid network. The 1.5% CA decreased the water vapor transmission rate by approximately 190% compared to the control group in both treatments (Cc and Pc). These findings highlight that the combination cross-linking has the potential to customize the physicochemical properties of starch/xanthan hydrogels for biomedical and controlled-release applications.