| Suspensions of lipid-coated microbubbles are currently being developed for use as ultrasound contrast agents, drug delivery vehicles and blood substitutes. In our study, first, we examined the effect of lipid acyl chain length and the cooling rate on the microbubble surface domain morphology. The average domain size decreased with increasing cooling rate for all acyl chain lengths. The shape factor increased with chain length for the highest cooling rate. Second, we investigated the effect of microstructure on molecular oxygen permeation through condensed phospholipid monolayers. Oxygen permeability was shown to increase linearly with domain boundary density at a constant phospholipid acyl chain length and, accordingly, was shown to decrease exponentially with increasing chain length at a constant domain boundary density. Modification of the energy barrier theory to account for microstructural effects, in terms of the domain boundary density, provides a general equation to model passive transport through polycrystalline monolayer films. Last, we demonstrated that the phase conditions and microstructure of the shell were critical to determine the dissolution behaviors of the lipid-coated microbubble. For these two-phase coexistence bubbles, a transition from primary collapse, as loss of expanded phase due to vesiculation, to secondary collapse, as the rapid propagation of monolayer folds and simultaneous deformation, was observed. For very rigid monolayers, we observed substantial surface buckling with simultaneous nucleation and growth of folds. |
