 The only problem to using liposomes "in vivo" is their immediate uptake and clearance by the RES system and their relatively low stability in vitro.To combat this, polyethylene glycol (PEG) can be added to the surface of the liposomes.Increasing the mole percent of PEG on the surface of the liposomes by 4-10% significantly increased circulation time "in vivo" from 200 to 1000 minutes.