polymeric micelle

A polymeric micelle forms in the same way soap cleans grease, but built from larger, custom-designed molecules. Block copolymers — chains with one water-loving (hydrophilic) section joined to one water-hating (hydrophobic) section — are dropped into water. The water-hating ends huddle together to hide from the water, creating a tiny oily core surrounded by a water-friendly shell. A poorly soluble drug hides in that oily core just as oily dirt hides inside a soap micelle.

This makes polymeric micelles excellent solubilisers for drugs that barely dissolve in water, letting such drugs be given by injection. The hydrophilic shell, often made of polyethylene glycol, keeps the particle dispersed and helps it evade rapid clearance. Compared with ordinary surfactant micelles, polymeric ones are larger, hold more drug, and stay together at much lower concentrations.

The catch is dilution. A micelle only exists above a threshold concentration called the critical micelle concentration; when an injected dose is diluted by the bloodstream below that threshold, the micelles can fall apart and dump their drug prematurely. Designers fight this by choosing copolymers with a very low critical micelle concentration or by cross-linking the core for extra stability.