Effect of natural cross-linker on swelling and structural stability of kappa-carrageenan/hydroxyethyl cellulose pH-sensitive hydrogels

Genipin cross-linked kappa-carrageenan/hydroxyethyl cellulose hydrogels were prepared and the effect of cross-linking on hydrogels characteristics was investigated. Swelling and transform mechanisms of both native and cross-linked gels in different pH were also studied. We found that the concentration of genipin affects the physical stability of gels. The optimum concentration that a cross-linked hydrogel molecular structure can be in its most stable form is also discussed. Native hydrogels exhibited more swelling in alkaline medium than acidic and neutral; however, by increasing the cross-linker concentration, the swelling ability in neutral medium increased so that genipin cross-linked hydrogels could swell in pH 7 more than in pH 1.2 and 12. Fourier transform infrared spectroscopy (FTIR) was applied to study the formation of new bonding due to genipin reactions and explain hydrogels' stability in various concentrations. Differential scanning calorimetry (DSC) measurements reveal that by increasing genipin, the gel ability to hold water increases to some point and then decreases due to less structural stability. X-ray diffraction (XRD) and field emission scanning electron microscope (FESEM) tests were performed to study the crystallinity changes and microstructure of hydrogels. Finally, the power law model was applied to study the transform mechanism of hydrogels.