Intracellular uptake of liposomes. (A) Flow cytometric assessment of the intracellular delivery of liposomes to tumor cells. (B) Assessment of the intracellular uptake of liposomes by A549 tumor cells using fluorescence microscopy. PEI-1, PEI-2, PEI-3, and PEI-4 represent PEI contents of 10%, 40%, 70%, and 100% (w/w total lipid) in liposomal formulations, respectively. Error bar represents mean ± SD (n = 3); *p < 0.001.