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Chitosan is a biopolymer produced through the deacetylation of chitin, a material found in the exoskeleton of crustaceans and the cell walls of fungi. It has found uses in a variety of industries including in cosmetics for delivery, hydration, film-forming and to modify viscosity. After learning of the antibacterial properties of chitosan, Mihaela Leonida, PhD, a professor of chemistry at Fairleigh Dickinson University, further investigated this property for cosmetic and pharmaceutical uses. She found that the material exhibited notable antimicrobial effects, which were enhanced after nano-sizing it. In addition, it acted synergistically with known antimicrobial metal ions. These properties suggested applications in wound healing and anti-aging skin care.
To investigate the uses for chitosan, Leonida’s team first deacetylated chitin. “[Deacetylation] is not a well-controlled process; [while] it deacetylates, it may and will also partially depolymerize,” said Leonida. The team obtained both longer and shorter chitosans. “The degree of deacytylation depends upon who is conducting the process,” explained Leonida. The next step involved producing bioavailable chitosan nanoparticles (CNPs). “There are a lot of active principles that do not go across membranes, including chitosan. We wanted to increase the uptake and enhance bioavailability, and nano-sized chitosan overcomes the lipophilic barrier and the mucosal walls, allowing for better penetration,” noted Leonida.
Chitosan is a linear polymer, which makes it desirable for cross-linking, according to Leonida. Her team therefore used ionic gelation to produce the CNPs by placing chitosan in a mild, acidic and aqueous solution to dissolve it. The team cross-linked the polymer with the negatively charged sodium tripolyphosphate. She explained, “Depending on the application, factors such as pH, the amount of chitosan and the amount of cross-linker can be varied.” After the process was complete, scanning and transmission electron microscopy, and infrared spectroscopy confirmed that CNPs were produced.
The CNPs showed a higher antimicrobial efficacy than chitosan alone against Stephylococcus saprophyticus and Escherichia coli. Leonida explained there are a number of theories as to why; one involves its function as a chelator. “[Chitosan] is able to bind ions, so some speculate that it chelates metal ions inside the microbes to prevent their growth. In the same way, when ionic gelation is [performed in the presence of metal ions], some of the metals are retained inside of the particles. The explanation may well be the chelating capacity of chitosan.”
The team conducted the same ionic gelation process using difficult-to-formulate metal ions with known antimicrobial effects. “Copper is a known antibacterial but is not desirable on its own.” She noted that metals pose a problem with toxicity and disposal, adding “Silver is a good antibacterial on its own, but it is not desirable in large concentrations.” The CNPs worked synergistically with the metal ions against S. saprophyticus and E. coli; most notably, ~7% copper delivered via CNPs enhanced its antibacterial effect by several orders of magnitude. “In the case of copper and Gram-positives, the [CNPs] had a huge impact, compared with copper alone.” For silver, there was not such an enhancement, however, its safety was improved.