## 3 Chemical Modification of Hyaluronan
### Amidation of Hyaluronic Acid
Figure 5: The synthetic route of derivatives using a double type of chemical modification of HA for crosslinking: (**a**) amidation of HA with 3,3’-dithiobis (propanoic dihydrazide) (TPH) was mediated by EDC/NHS in MES buffer (above); (**b**) The second pathway involved the oxidation of HA [67]. In recent years, the application of bio-adhesive substrates as the potential wound dressing has captured wide interest. These materials are inspired by mussels, a marine organism that secretes a considerable amount of 3,4-dihydroxyproxyphenylalanine (DOPA). The di-hydroxxyl moieties help mussels achieve wet adhesion [70]. Mussel bio-adhesion materials included tannic acid [71], dopamine [72,73], or tyramine [74]. HA has been modified for adhesive hydrogel fabrication. [PERSON] prepared polydopamine-based NPs (124 \(\pm\) 8 nm) to load dimethyloxylglyglycine (DMOG), a proangiogenic drug. The NPs were coated by HA-cysteine attached to HA via EDC/NHS. polydopamine enhanced the gel strength from G' = 98.6 \(\pm\) 1.6 Pa to 162.70 \(\pm\) 2.2 Pa. Cells cultivated in the hydrogel enhanced the production of capillary tubes [73]. Hydrogels made of extracellular matrix such as collagen and HA could be used for wound healing. For example, [PERSON] et al. prepared a hydrogel via in situ crosslinking of chemically modified collagen I with p-hydroxybenzoic acid and tyramine-modified HA. To reach this goal, HA (2 \(\times\) 10\({}^{5}\) g/mol) was chemically modified by EDC/NHS with tyramine (DS = 14.2%) in water. The crosslinking reaction of the two polymers was performed via enzymatic polymerization using horseradish peroxidase. The hydrogels were soft and viscous with high adhesive properties [74]. [PERSON] and coworkers prepared a hydrogel consisting of collagen grafted with gallic acid/HA-tyramine/\(\gamma\)-poly (glutamic acid) grafted with 3-APB. Intracellular ROS scavenging measurements were performed using cells treated with H\({}_{2}\)O\({}_{2}\) based on the determination of 2, 7-dichlorodifluorofluor-escin diacetate. The hydrogel showed a reduced ROS effect. The hydrogel was tested on a wound full-thickness skin mice model defect. On day 14 th, a wound closure rate of approximately 95% was observed [75]. [PERSON] and collaborators prepared an HA-based hydrogel after double chemical modification. An excess of maleic anhydride was reacted with HA (1.28 \(\times\) 10\({}^{6}\) g/mol). The DS of acrylate groups was determined as 75%. Secondly, dopamine was attached via EDC-mediated activation, with a substitution degree determined as 87% and oxidized in a second step. The photo-crosslinking of the methacrylated substituents was mediated with lithium phenyl (2,4,6-trimethylbenzoyl) phosphininate (LAP). The adhesion strength of the hydrogels was determined to be 179.9 \(\pm\) 39.4 kPa by lap-shear test using gelatin as a skin tissue model. The compressive stress was determined to be 600 kPa, and it can withstand compressive strain up to 90%. The hydrogel stays in the wound site up to 6 days with a 30% loss of weight [76]. The high substitution of dopamine groups produced a fast hemostasis, but the reaction conditions were degradative as the Mw decreased to 2.8 \(\times\) 10\({}^{5}\). A disadvantage of this type of material is the structural inhomogeneity in the hydrogel after oxidation of catechol by using NaIO\({}_{4}\). Platelet-rich plasma (PRP), because of its richness in several growth factors, has shown significant therapeutic potential in the treatment of diabetic wounds. A recent study based on bio-functionalized scaffolds made of HA combined with PRP concerning HA without PRP was utilized for chronic wounds. The platelet growth factors supported the three phases of wound healing (inflammation, proliferation, remodeling) and repair cascade [77]. However, PRP is rapidly cleared from the wound site, posing a challenge for achieving a sustained therapeutic effect [78]. To address this problem, [PERSON] et al. prepared adhesive hydrogels by combining DOPA-modified HA and PRP.