Physical and Antimicrobial Properties of Chitosan/Silver Nanoparticle Composite Hydrogels: Role of the Crosslinker

Chitosan hydrogels have widespread industrial applications due to their versatility and antimicrobial potential. However, their applicability can be limited by poor mechanical properties or because their fabrication requires the use of toxic compounds which can leach into their environment. Additionally, their poor water solubility under neutral conditions restricts their fabrication and applications to low pHs. Here, we synthesized a modified derivative [N-(2-hydroxy-3-trimethylammonium)propyl] chitosan chloride (HTCC), which is soluble and antimicrobial at neutral pH, and used it to compare the effect of three crosslinking agents on the formation of industrially relevant hydrogels. The crosslinkers sodium tripolyphosphate (TPP), glutaraldehyde (GA), and citric acid (CA) were compared in terms of their impact on the swelling potential, hydrophobicity, and mechanical properties. Swelling degrees ranging from 350 to 2350% for GA and TPP, respectively, were observed. Silver nanoparticles (Ag NPs) were synthesized in situ, leading to improved mechanical properties as evidenced by an increase in the Young modulus from 10.3 MPa for TPP-crosslinked systems to 87.4 MPa for TPP-crosslinked/Ag NP composites. Ag ion release rather than Ag NP leaching was determined to be the dominant strategy for antimicrobial action against Bacillus subtilis, Pseudomonas aeruginosa, Escherichia coli, Staphylococcus aureus, and Acinetobacter baumanii, causing significant increases (p < 0.05) in clearance ratios and biofilm shape factors, pointing to a synergism between the crosslinked HTCC and Ag NPs. The exceptional broad-spectrum antimicrobial/antifouling properties of these materials regardless of the crosslinking method allow for selection of different preparation techniques to tune desired traits for diverse industrial applications.