HA structure and sites of modification. The disaccharide repeat units of HA are shown with the primary sites for chemical modifications generally used for tissue engineering applications. The hydroxyl groups of HA form covalent linkages such as ether, esters, and hemiacetal bonds. The carboxyl groups can form ether, anhydride, and carbamide bonds, while Schiff bases can originate from HA dialdehyde reacting with amino groups. Some HA adducts commonly employed in regenerative medicine are also shown.