Both Polyethylene glycol (PEG) and Monomethoxypolyethyleneglycol (MPEG) have been used since many years as non toxic auxiliary material in cosmetics, foodstuffs and pharmaceuticals (FDA accepted).
The conjugation of PEGs or MPEGs with lipids leads to heterobifunctional PEG Lipids resp. MPEG Lipids.
MPEG derived lipids are used to produce PEGylated liposomes. Due to their enhanced stability against blood plasma they are also called Long Circulating Liposomes.
Lipids derived from difunctional PEGs are usually bearing a reactive group on the other end of the PEG chain. This reactive group is used to attach Peptides, Antibodies etc. to the Liposome surface. Such Immunoliposomes are used for cell specific targeting or in vaccines.