Chitosan is a naturally occurring biopolymer (found in mushrooms and marine shells) with bacteriostatic, fungistatic, and film-forming properties that confer the following benefits:  

Hemostatic 

Chitosan's hemostatic effect is independent of normal clotting cascades. The positive charge of chitosan allows it to interact with negatively charged red blood cells through electrostatic adhesion, triggering the hemagglutination process. While the exact hemostatic mechanism of chitosan is not fully understood, theories suggest involvement in plasma sorption, erythrocyte coagulation, and platelet activation and aggregation. The protonated amino groups of chitosan attract negatively charged red blood cells, contributing to hemostasis. Additionally, chitosan stimulates platelet aggregation and adhesion by adsorbing plasma proteins and signaling thrombin, a clotting promoter. This multifaceted interaction with blood components makes chitosan a promising material for promoting hemostasis in wound care and surgical applications. 

Bio-adhesive 

Chitosan is a spontaneous, self-forming polymer. When provided with the appropriate conditions, it can form large aggregates. In materials engineering, this fact is often used to create bio scaffolds, hydrogels, and protein films. However, it can also serve as a bio-adhesive film that envelopes a viral, bacterial, or mammalian cell target. The chitosan film disrupts the natural import and export of nutrients that can ultimately lead to cell death.  

Wound Healing 

A study by Caetano et al. investigated the healing properties of a chitosan–alginate membrane on cutaneous wounds in rats. The findings indicated that the wound was not infected, fibroplasia increased significantly, fibroblasts were better arranged in the newly formed tissue, and the quality of scar tissue improved. Additionally, on the 7th day of treatment, the chitosan–alginate membrane significantly reduced inflammatory infiltrate, leading to a decrease in neutrophils and CD4+ lymphocytes, suggesting improved regulation of the inflammatory stimulus. The chitosan–alginate complex also stimulated the migration of CD11B+ macrophages, essential for growth factor release and enzyme digestion of extracellular content, facilitating the transition to the second phase of the healing process. Chitosan was found to regulate granulation tissue formation and angiogenesis, ensuring correct collagen fiber deposition and enhancing proper repair of injured dermal tissue. The hemostatic and analgesic effects of chitosan were attributed to its positive charge, interacting with red blood cells and aiding in clotting. Howling et al. demonstrated that chitosan enhanced fibroblast proliferation, with the proliferative effect linked to the deacetylation degree of chitosan. High deacetylation degrees increased mitogenic activity in fibroblasts. Chitosan's poor solubility at physiological pH led to the development of water-soluble derivatives like O-carboxymethyl-N,N,N-trimethyl chitosan (CMTMC). Recent studies on carboxylated and trimethylated chitosan (CMTMC) formulations showed potential in wound-healing applications, promoting proliferation and migration of human dermal fibroblasts without toxicity. The positive charge of chitosan contributed to both hemostatic and analgesic features, interacting with red blood cells and exhibiting an analgesic effect in experimental studies on scalded rats. Notably, carboxymethyl chitosan specifically demonstrated an analgesic effect in contrast to chitosan. 

Anti-bacterial 

Gram-Positive Bacteria  

The distinction between Gram-positive and Gram-negative bacteria lies in their cell wall structure. Gram-positive bacteria feature teichoic acids, specifically wall teichoic acids and lipoteichoic acid anchored in the cell membrane. Teichoic acids play a vital role in pathogenesis and modulate susceptibility to cationic molecules. The resistance of Gram-positive bacteria to antimicrobial agents is attributed to teichoic acids and their substituents, regulating the bacterial cell's negative charge and preventing the binding of extracellular molecules. Chitosan, a positively charged substance, interacts with negatively charged teichoic acids, disrupting cell wall rigidity and allowing entry into the cell. The presence of d-alanine esters attached to teichoic acids influences bacterial surface charge, affecting susceptibility to glycopeptide antibiotics and cationic antimicrobial peptides. Studies indicate that the absence of d-alanyl esters increases the negative charge of the cell surface, enhancing sensitivity to such antimicrobials. 

Gram-Negative Bacteria  

Gram-negative bacteria possess a more intricate cell surface structure, comprising an outer lipid bilayer, a peptidoglycan cell wall, and a plasmatic membrane. The distinctive feature of Gram-negative bacteria is the outer lipid bilayer, containing phospholipids in the inner layer and lipopolysaccharides (LPSs) in the outer layer. This outer layer acts as a selective barrier, featuring porins that selectively diffuse certain hydrophilic molecules and a high negative charge conferred by LPS molecules. LPS consists of lipid A and the inner core, with negatively charged groups responsible for interactions with cationic polymers. The study by Davidova et al. demonstrated interactions between LPS and chitosan's amino groups, leading to displacement of negatively charged dye molecules from the complex formed with the cationic polymer. This interaction suggests a potential neutralization of LPS endotoxins by chitosan. The study also proposed that chitosan may disrupt nutrient exchange by forming a polymer layer on the bacterial cell surface, potentially leading to bacterial cell death. 

 

 Figure: Anti-microbial mechanism of action 

Anti-fungal 

Salic acids and other mannoproteins cause the outer surface of many fungal species to contain an overall net negative charge. Chitosan is a cationic molecule (positively charged) and therefore can form stable ionic interactions with the cell wall of these fungi. The resulting ionic interactions destabilize the overall structure fungal cell wall. Additionally, the change in net-charge at the surface of the cell wall causes changes in concentrations of important cell ions such as potassium (K+) and sodium (Na+). Altering the spatial patterning of these ions further disrupts cellular processes and disrupting fungal metabolism.  

Anti-viral 

While chitosan has shown promise in various biomedical applications, including wound healing, its antiviral properties are not as extensively studied or established. Some research suggests that chitosan may exhibit antiviral activity, potentially inhibiting the replication of certain viruses. The mechanisms proposed for this antiviral effect include interference with viral adsorption and entry into host cells, as well as the inhibition of viral replication processes. However, the specific antiviral properties of chitosan, including its effectiveness against different types of viruses and the underlying molecular mechanisms, require further investigation. It's essential to note that the antiviral nature of chitosan is an area of ongoing research, and more studies are needed to fully understand its potential in combating viral infections. 

References: 

Chitin, Chitosan, and Its Derivatives for Wound Healing: Old and New Materials 

Chitosan-Based Functional Materials for Skin Wound Repair: Mechanisms and Applications 

Chitosan as a Wound Dressing Starting Material: Antimicrobial Properties and Mode of Action 

Chitosan-alginate membranes accelerate wound healing