PEGylation is the process of covalent and non-covalent attachment or merging of polymer chains with molecules and macrostructures (such as drugs, therapeutic proteins, or vesicles). PEG polymers have many inherent good biological properties, including high water solubility and lack of toxicity, immunogenicity and antigenicity. Therefore, in many applications, the chemical modification of bioactive compounds, such as peptides, antibody fragments, enzymes or small molecules with polyethylene glycol chain, so called PEGylation, usually leads to improved pharmacokinetics and biological function.