| Abstract
| - We report on a fluorescence microscopy study of the monolayer collapse and shedding behavior due to shellcompression during the dissolution of air-filled, lipid-coated microbubbles in degassed media. The monolayer shellwas comprised of saturated diacyl phosphatidylcholine (C12:0 to C22:0) and an emulsifier, poly(ethylene glycol)-40stearate. The morphologies of monolayer collapse structures and shed particles were monitored as a function ofphospholipid acyl chain length (n) and temperature. The two components formed a single miscible phase when thephospholipid was near or above its main phase transition temperature, and collapse occurred via suboptical particlesto vesicles (both were shed) and tubes as chain length increased. Conversely, two-phase coexistence was observedwhen the lipid was below its main phase transition temperature. For these bubbles, a transition from primary collapseto secondary collapse was observed. Primary collapse was observed as a loss of expanded phase due to vesiculation.Secondary collapse involved the rapid propagation of monolayer folds and simultaneous deformation. For very rigidmonolayers, we observed substantial surface buckling with simultaneous nucleation and growth of folds. The foldsmerged at a single point or region, providing a conduit for the entire excess lipid to shed in a single event, and thebubble smoothed and became more spherical. These results are discussed in the context of general binary phospholipidcollapse behavior, microbubble dissolution behavior, medical applications, and the dissolution behavior of naturalmicrobubbles.
|
