HO-PEG-COOH is widely used in bioconjugation and drug delivery systems due to its biocompatibility and ability to modify the surface properties of molecules. It is often employed to improve the solubility and stability of therapeutic agents, particularly in the development of PEGylated drugs. The hydroxyl (-OH) and carboxyl (-COOH) functional groups allow for easy attachment to biomolecules such as proteins, peptides, and nanoparticles, enhancing their circulation time in the body and reducing immunogenicity. Additionally, it is utilized in the synthesis of hydrogels and other biomaterials for tissue engineering and regenerative medicine applications. Its amphiphilic nature also makes it suitable for use in micelle formation for targeted drug delivery.