liposome

A liposome is a microscopic bubble whose wall is built from the same kind of fatty molecules that make up cell membranes. These phospholipid molecules have a water-loving head and water-hating tails, so in water they spontaneously line up into a bilayer — a double sheet of lipid — that curls into a closed sphere with watery space inside. The result is a tiny sealed sac with a lipid shell and an aqueous core.

This architecture lets a liposome carry two kinds of drug at once. Water-soluble drugs can be dissolved in the inner aqueous core, while fat-soluble drugs can tuck into the oily middle of the bilayer. The lipid shell protects the cargo, reduces irritation, and can be engineered (for example by adding cholesterol for rigidity or polyethylene glycol for longer circulation) to control how fast the drug is released and where it goes.

Liposomes were among the first nanocarriers to reach the clinic, with products such as liposomal doxorubicin and liposomal amphotericin B. Their limitations are real: they can leak their contents, fuse or aggregate on storage, and are often physically and chemically less stable than solid carriers, which is why many are sold as freeze-dried powders to be reconstituted before use.