Uncovering the dominant role of ionic strength in chitosan-genipin and chitosan-genipin-apatite wollastonite gelation kinetics and resulting hydrogel properties

The study addresses the limited knowledge of the role of ionic strength in synthesis of chitosan-based hydrogels, specifically chitosan-genipin hydrogels, and their composites with apatite wollastonite (AW). We report how varying ionic strength, under constant pH, influences the gelation, swelling behaviour, and mechanical properties of hydrogels. Chitosan solutions were prepared in acetate buffers varied in ionic strength, and crosslinked with genipin in the presence or absence of AW bioactive glass powder. Findings indicate that increasing the ionic strength of chitosan stock solution from 0.31 to 0.65 M reduces the time required to reach the gelation point of formed hydrogels by 34 %. Based on fluorescence intensity measurements carried throughout the gelation process, the hydrogels formed using chitosan stock solution prepared in 1 v/v% acetic acid (ionic strength 0.09 M) exhibited the lowest crosslinking rate in the first 6 h (one-third to one-half compared to when buffered chitosan stock solutions are used). The mass change ratio of the formed hydrogels immersed in PBS showed that hydrogels prepared using chitosan solution of 0.31 M ionic strength had a 23.44 % greater weight change and 48.31 % higher compressive stress compared to the hydrogels prepared using 0.65 M ionic strength chitosan stock solutions.