**7. Poly(ethylene) glycol (PEG)**

Chemically, poly(ethylene) glycol (PEG) (C2*<sup>n</sup>* H4*<sup>n</sup>*+2O*<sup>n</sup>*+1) is a polyether or polymer of ethylene oxide.

The physical properties of PEG vary with respect to its chain length, whereas its chemical properties are almost the same. It is available in different molecular weights and different geometries such as branched PEG, star PEG and comb PEG. PEG is non-toxic and excreted through kidney. Degradation of the drug can be protected due to its surface modification property, and it has been extensively used as liposomal targeting by liposomal coating. The liposomes have longer circulation time in blood, reduced macrophage uptake, higher gene transfection efficiencies, larger available concentration and bioavailability [94, 115–120].
