Heterobifunctional polyethylene glycol (PEG) has different functional groups at each end of the PEG chain. These linkers allow selective conjugation between two different molecules or surfaces. It is widely used in bioconjugation reactions to connect different molecules or surfaces to achieve specific chemical and biological functions. Oligopolyethylene glycol (such as PEG-2, PEG-4) is mainly used as a solvent and penetration enhancer to promote the dissolution of active ingredients, improve stability and solubility, enhance the permeability and absorption of oral drugs, and thus improve bioavailability.