# Hyaluronic Acid-Based Wound Dressing with Antimicrobial Properties for Wound Healing Application
## 2 HA Antimicrobial Properties  
As well as serving as a barrier to protect wound tissues from external bacterial infections, substrates for wound dressing should possess an inherent antimicrobial ability to make them even more attractive, in order to prevent wound infections and promote an effective healing process [20]. The ability of HA to inhibit microbial attack during wound healing is associated with its proven ability to reduce bacterial adhesion and biofilm formation in different contexts; not being bactericidal but bacteriostatic, it delays or prevents the growth of bacteria without destroying them [21]. By comparing different biomaterials, both synthetic and natural with bacteriostatic properties, such as collagen, hydroxyapatite, HA and PLGA, it has been proved that HA  
Figure 1: Hyaluronic acid (HA) molecule representation. The two dimers N-acetyl-D-glucosamine and D-glucuron acid are highlighted. shows the greatest bacteriostatic effect against _S. epidermidis_ and _P. aeruginosa_. Therefore, it has been hypothesized that the HA molecules also form a random network of chains that can act as a sieve preventing the spread of bacteria [7]. In particular, a study about the bacteriostatic properties of three different concentrations (0.5, 1.0 and 2.0 mg/mL) of solution of HA at different molecular weights (MW), low (L), medium (M) and high (H) (respectively, 141, 757 and 1300 kDa) concluded that higher MWHA showed better effects in minimizing bacterial contamination when tested on oral and non-oral microorganisms in the planktonic phase [22]. When compared to PLGA, Collagen I and Hydroxyapatite, HA showed the most significant bacteriostatic properties on _P. Aeruginosa_, _S. aureus_, _S. epidermidis_ and _\(\beta\)-hemolytic Streptococcus_[7]. HA/carbon nanofibers composite films have been produced with enhanced mechanical properties. The antimicrobial activity of the HA derivatives was tested against _Staphylococcus aureus_, a Gram-positive bacterium, and results showed a good antibacterial effect (Figure 2) [11]. A bio-resorbable membrane made of HA and carboxymethylcellulose has been designed. A cellular function investigation has shown how higher concentrations of HA/CMC induced greater bacteriostatic effects on _S. aureus_ and _E. coli_. Thus, they concluded that HA/CMC might have dose-dependent bacteriostatic effects on _S. aureus_ and _E. coli_[23]. A Gantrez(r)S97/HA crosslinked hydrogel was fabricated and characterized. Synthesized hydrogels showed a significant antimicrobial activity against planktonic bacterial suspensions, indicating they are an ideal substrate for drug delivery devices or biomaterials, such as medicated wound dressings or anti-infective coating [24]. Then, two possible explanations can be considered to justify HA's bacteriostatic effect. First, due to the macromolecular nature of HA, the presence of long chains give rise to a high viscosity solution in which bacteria's motion and diffusion is hindered: a proof is that HMWHA-concentrated solutions showed potent bacteriostatic activity. Then, a second possible mechanism acts synergistically with the previous mechanism, so the excess of HA might cause the saturation of the bacterial hyaluron lyase. Therefore, bacteria seem to lose tissue penetration capability, allowing an efficient immune response by the host [7,25]. To further improve HA's inherent antibacterial activity, in the literature several modification methods have been proposed. As an example, polyurethane films were coated with a previously modified HA with thiol groups (HA-SH), using polydopamine as the binding agent. Ocatdecyl acrylate (C18) was subsequently used to bind thiol groups in order to attract albumin, allowing the system to selectively bind albumin, a protein responsible for bacterial adhesion, thus granting an effective antimicrobial activity. In order to make sure  
Figure 2: Different MW of HA and HA-CNF (carbon nanofibers) films showing antimicrobial activity against _S. aureus_.