Fabrication and Characterization of Citric Acid Modified Chitosan Hydrogel

Chitosan (CS), a natural biopolymer, has been widely used to construct tissue engineering scaffolds. However, the poor mechanical properties of CS in wet state limit its application and efficacy in repairing tissues. Citric acid (CA), as a kind of organic acid existed in the human metabolic cycle, was proposed to modify CS for the purpose of improving the wet mechanical properties of CS. CS-CA hydrogel was formed after a freeze-gelation process with the formulated CS-CA solutions. Emphasis was placed on the effects of m(CA):m(CS-CA) and w(CS-CA) on the properties of formed CS-CA hydrogels. FT-IR analysis showed that CS and CA formed physical crosslinking in the form of ionic bonds between NH₃⁺ existed in protonated CS and the carboxylic acid sites in CA. SEM showed interconnected porous structure of the CS-CA hydrogel after freeze-gelation treatment. Water absorption kinetics, porosity, and mechanical tests showed that optimal performance could be attained when m(CA):m(CS-CA) was 0.3 and w(CS-CA) was 3%. Namely, the optimized hydrogel could absorb water quickly within 1 min to reach saturation. The swelling ratio was up to 1 134%. The ultimate tensile strength could reach 103 kPa. The compress stress was as high as 18 kPa. After 20 cycles of compression test, the hydrogel could restore its original height and shape without the presence of obvious cracks and deformation. The newly developed CS-based hydrogel with improved wet mechanical properties could be applied in wound dressings or as scaffolds for engineering different tissues (e.g., skin and cartilage).