Design and experimental study of the mode-I and mixed-mode fracture toughness of Polyacrylamide-Amylopectin hydrogel with tunable chitosan topohesion

Hydrogels are soft and water-rich polymer networks with tunable adhesive properties, that are extensively utilized in biomedical engineering applications. Due to their excellent bonding characteristics, hydrogels can adhere to various surfaces, including skin and organ tissues. However, the fracture properties of soft-soft (e.g. gel-gel and gel-tissue) interfaces have not been thoroughly explored, and typical peel-test configurations have limitations. Hence, in this work, two fracture test setups are designed, built, and tested to accommodate the mode-I and mixed-mode (I/II) fracture characteristics of interfacially bonded hydrogels. A single network hydrogel composed of Polyacrylamide and Amylopectin was selected to evaluate the proposed test setups. The hydrogels were photocured into T-shaped cross-section adherents and bonded together to form test specimens. The interface of the adherents are topologically connected with 200 µL of chitosan solution consisting of average molecular weights of 1.5 kDa, 15 kDa, 120 kDa, 250 kDa, 343 kDa, and a control group with no chitosan solution. The study investigates the effect of different chitosan molecular weights and pH (ranging from 2.5 to 4.5) on the interfacial fracture toughness. The mode-I and mixed-mode (50% mixity) fracture initiation toughness are evaluated using nonlinear J-integral fracture mechanics. The chitosan with the highest molecular weight and pH resulted in a 200% increase in mode-I fracture toughness compared to the control with no chitosan topological connections, which is attributed to the robust interfacial chemistry and crack tip blunting phenomena. In the mixed-mode loading conditions, the toughness is lower than the mode-I toughness for all configurations, and the highest molecular weight and pH lead to an 64% increase in the mixed-mode toughness. These results are supplemented with analysis by Digital Image Correlation (DIC), which is utilized to compare the strain distribution around the fracture process zone at initiation and crack growth resistance curves (R-curves) to assess the propagation fracture mechanics.