Antibiotic-free antibacterial hydrogels for wound healing applications

Over the past decades, bacterial cellulose (BC) has attracted great interest as a wound dressing thanks to its innate hydrogel-like structure and high biocompatibility. However, the lack of antibacterial features of cellulose poses some limitations to its use, especially in the context of increasing antimicrobial resistance. In this work, various strategies were explored for the development of novel BC-based wound healing devices that can efficiently inhibit bacterial proliferation. First, the production of BC by fermentation of Gluconacetobacter xylinus and its purification from the biomass were optimised. The material was characterised with respect to its morphological, chemical and mechanical properties. The introduction of antibacterial functional groups was then investigated by ring-opening reaction of two epoxides with the hydroxyl groups of glucose in wet basic conditions. Biological tests evidenced that the functionalisation caused a decrease in the bacterial cell count of about 50% for Staphylococcus aureus and Escherichia coli in the first 24 hours, whereas cell viability of over 90% was observed for keratinocytes for up to 6 days. An alternative method for the modification of bacterial cellulose was also studied to improve the antibacterial efficiency. The functionalisation was carried out in anhydrous conditions in two steps, i.e. acrylation by Schotten-Baumann reaction and thiol-ene Michael type addition. Once again, the material was chemically and mechanically characterised, and biological studies were carried out: >99% reduction in the bacterial cell count was observed for both S. aureus and E. coli, with high cytocompatibility towards keratinocytes at all time-points. Finally, the fabrication of Cu-chitosan/BC composites was investigated. SEM images showed no evident phase separation between the polymers, whereas mechanical assays evidenced an increase in the tensile strength and elastic modulus as compared to plain chitosan. An optimal copper concentration threshold was then identified that ensures low cytotoxicity towards keratinocytes and antibacterial activity upon direct contact of over 90% for both S. aureus and E. coli. Overall, the materials developed showed promising results in terms of biological performance to tackle the ever-growing problem of antimicrobial resistance.