Synthesis and characterization of novel Spirulina protein isolate (SPI)-based hydrogels through dual-crosslinking with genipin/Zn2+

Spirulina protein isolate (SPI) is a bio-polymer that can be used in the design and synthesis of hydrogels due to its biodegradability and non-toxicity. Herein, dual-crosslinked hydrogels were synthesized based on spirulina protein isolate through covalent crosslinking with genipin and metal coordination with Zn2+, and used as delivery systems. From the analysis of colorimetric indices, a high correlation between color changes and the reaction of cross-linkers with amine group structure was observed. By increasing genipin concentration in the range of 5–15 mM, the degree of cross-linking increased up to 83 %. Cross-linked hydrogels, depending on the type and concentration of cross-linkers, had a smooth to rough and dense structure with low porosity. In the FTIR spectra, significant shifts were observed in amide bonds, which are related to the formation of secondary amides after genipin crosslinking. The maximum water absorption capacity (381 %) was obtained at pH 7.4, which was significantly reduced by increasing the concentration of cross-linkers or decreasing the pH of the environment. The release rate of vitamin B6 (model) at pH 2 (45 %) was much lower than at pH 7.4 (65 %) which makes hydrogels suitable for intestinal delivery. Vitamin B6 release data indicated the best fit to the Peppas–Sahlin model in all the cases (98–99 %) and showed that the release process occurs through quasi-Fickian diffusion. The finding reveals that spirulina protein-based hydrogels can potentially be used in the design of practical carriers for the targeted delivery of bioactive compounds.