Functionalization of chitosan with a polycarboxylic macrocycle yields injectable hydrogel with pH and salts responsiveness

The development of injectable hydrogels that respond to physiological stimuli represents a promising strategy for a range of biomedical applications, although the precise tuning of gelation kinetics, mechanical stability, and biocompatibility remains a significant challenge. This study presents a pH- and osmolarity-responsive injectable hydrogel, formulated from a combination of chitosan and chitosan functionalized with a macrocyclic polycarboxylate. The functionalization of chitosan significantly modifies the electrostatic charges along the polymer backbone, enabling fast gelation under physiological conditions. The gelation process is driven by pH neutralization and osmolarity increase, where electrostatic interactions between the zwitterionic chitosan and unmodified chitosan generate a dynamic, entangled network strengthened by both electrostatic crosslinking and hydrophobic interactions. Rheological and structural analyses reveal the possibility of fine-tuning the gelation kinetics and mechanical properties by altering the ratio of zwitterionic chitosan to conventional chitosan in the formulation. Here, the optimized 67:33 % ratio achieved favorable compromise between rapid gelation and stability in physiological media. Different microscopy experiments, including conventional scanning electronic microscopy and live imaging, have confirmed the porous architecture of the hydrogels. In vivo experiments confirmed the injectability, biocompatibility, and biodegradability of the hydrogels, with gradual degradation observed by magnetic resonance imaging and fluorescence imaging over time in healthy mice after subcutaneous administration. Additionally, preclinical safety assessments in rabbits demonstrated good local tolerance to both single and repeated subcutaneous injections, with no systemic toxicity observed. These findings support the potential for broad biomedical future applications, including drug delivery, wound healing, local metal uptake and tissue regeneration.