962376.fig.001
Figure 1: Schematic diagram of star PEG covalent binding reaction. Either one or two N-hydroxyl succinamide (NHS) terminal groups of star PEG react directly with primary amines through ester-amine reactions to form stable amide bonds; the remaining terminal groups do not participate in coupling, due to steric constraints [22] and hydrolyze to carboxylic acid groups during the reaction.