## 3 Chemical Modification of Hyaluronan
### Exterification of HA at the Hydroxyl  
The esterification of HA is generally carried out on carboxyl moieties. However, the esterification of HA at the hydroxyl moieties with high molecular weight polymers and oligomers for physical crosslinking is also performed with significant biological compatibility and good physicochemical properties (Figure 8). The formation of amphiphilic HA esters by esterification with oligomers was reported by [PERSON] et al. that conjugated DL-lactic acid oligomers (OLA) to HA. The reaction was performed in DMSO with the reaction of the cetyltrimethylammonium hyaluronate salt of HA (HA-CTA) with carboxyl chloride of PLGA mediated by thionyl chloride [94]. [PERSON] and collaborators reported that the grafting of poly (hydroxyalkanoaates) to HA was performed after activation of the carboxyl units with CDI/DMAP [95]. Both derivatives can be used for the encapsulation of hydrophobic drugs, which can be potentially used for wound healing. [PERSON] et al. prepared an amphiphilic derivative using poly (lactic-co-glycolic acid) grafted to HA (1.5 to 1.8 \(\times\) 10\({}^{6}\) g/mol). The activation was performed by EDC in DMSO [96]. [PERSON] et al. prepared a similar derivative for osteoarthritis by using HA (Mw\(\sim\) 2.1 to 4 \(\times\) 10\({}^{4}\) g/mol). The copolymer reacted with HA after activation by DCC/DMAP [97]. Figure 7: The alkylation reaction of HA with polyethylene glycol diglycidyl ether (PEG) for self-crosslinking [92]. On the other hand, the esterification reaction of HA at its hydroxyl moieties with low and reactive substituents is resumed on Figure 9 (reactions in water) and Figure 10 (reactions in DMSO). The esterification of HA with short hydrophobic moieties can be used for crosslinking via enzymatic esterification. [PERSON] et al. prepared a vinyl ester of HA via enzymatic esterification mediated by lipase acrylic resin from Candida Antarctica B (CAL-B) and hydroquinone. In this study, LMW-HA (1.14 \(\times\) 10\({}^{5}\), D \(\sim\) 1.6) was prepared through acidic degradation of HMW-HA. Furthermore, the reaction of divinyl adipate with HA was performed using a high excess of the reagent and prolonged reaction time (24, 48, 72, and 96 h, respectively) to reach a broad range of DS [98]. The reaction of HA with short-chain symmetric anhydrides such as methacrylic anhydride (MA) is usually performed by several groups. However, MA hydrolyzes in water fast, yielding methacrylic acid, which does not react with HA [99]. Thus, the esterification reaction is usually carried out using a high molar excess of reagents. The hydrolysis is pH and temperature dependent, making it difficult to achieve controlled DS and regioselectivity. Similar reactions were performed with glycidyl methacrylate or maleic anhydride due to easy control of side products and higher DS [100]. Vinyl substituents enable the photo-crosslinking of HA under several reagents and reaction conditions. [PERSON] and collaborators described the preparation of HA derivatives for crosslinking by reaction with either methacrylic (ME) or maleic anhydride (MA). While the synthesis of methacrylated HA was similarly as reported by [PERSON] et al. (4 \({}^{\circ}\)C, pH 8-9, 24 h) [100]. The reaction of HA (3.4 \(\times\) 10\({}^{5}\) g/mol) was performed with maleic anhydride  
Figure 8: Esterification of hydroxyl moiety of HA (abbreviated as HA-OH) with high molecular weight polymers such as (**a**) PLA [94]; (**b**) Polyhydroxyalkanoateates [95], and (**c**) PLGA [96]. after dissolution of the polysaccharide in nitromethane at 50 \({}^{\circ}\)C under nitrogen for 24 h. After dissolution in formamide, maleic anhydride reacted at 40 \({}^{\circ}\)C under inert atmosphere for 24 h. Both hydrogels were mineralized with CaPO4 by using in situ sol-gel synthesis. A degree of substitution of 51.1 \(\pm\) 4.6% and 79.9 \(\pm\) 2.5% was achieved using 20 and 30 molar excesses of methacrylic anhydride.