Monofunctional polyethylene glycol (PEG) contains a single chemically reactive end and is commonly used for PEGylation, surface grafting, and nanoparticle coating. Monofunctional PEG is commonly used in bioconjugation, surface functionalization, and nanoparticle coating. 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.