Poly-L-lysine hydrochloride is widely used as a coating agent in cell culture to enhance the adhesion of cells to surfaces, particularly for difficult-to-attach cells like neurons. It is also employed in microscopy to improve the adherence of tissue sections to slides, ensuring better sample stability during staining and analysis. Additionally, it serves as a transfection reagent in molecular biology, facilitating the delivery of nucleic acids into cells. In diagnostics, it is utilized to immobilize biomolecules on solid surfaces for assays, improving sensitivity and accuracy. Its cationic nature also makes it useful in various biotechnological applications, including drug delivery systems and biosensors.