(88e) Phospholipid Encapsulation Properties and Effects On Microbubble Stability and Dynamics

Natural microbubbles are important mediators for a variety of environmental processes, and synthetic microbubbles are being used for a broad array of applications in the medical, food and biotech industries.  Current theoretical models of the stability and dynamics of lipid monolayer-coated microbubbles do not capture many of the salient features that govern size and persistence in multi-gas media.  The goal of our research was to elucidate those features through the observation of microbubbles of 2-20 µm diameter, and a homologous series of lipid encapsulations, growing and dissolving during gas exchange.  Theoretical analysis was also applied to give further insight on the mechanics of the lipid monolayer shell.  This methodology revealed several novel and subtle effects of this remarkable nano-film.  We observed that microbubble growth was severely inhibited for more solid encapsulations below a threshold size (~10 µm diameter).  Following growth, most microbubbles rapidly dissolved back to their original size, indicating the “self-healing” capability of the lipid monolayer shell.  The microbubbles then experienced a lag before spontaneous dissolution, and the lag and dissolution times were highly variable and, surprisingly, were more highly correlated to the reduced temperature of the encapsulation than the initial microbubble size.  Most of the microbubbles stabilized again at a diameter of 1-2 µm, and this “stable diameter” appeared to be universal and not correlated with either the initial microbubble size or the rigidness of the encapsulation.  These empirical observations illustrate the importance of the lipid monolayer mechanics and may be useful in establishing more robust theoretical models for microbubble stability and dynamics.