Microbubble shell break-up and collapse during gas exchange

Lipid-coated microbubbles are used as contrast agents for ultrasound imaging and to augment ultrasound therapy. However, the response of a single-gas microbubble upon sudden immersion into a multi-gas environment is not well understood. The objective of our study was to experimentally measure the change in diameter of an SF₆ microbubble suddenly exposed to air-saturated media, in order to determine the effects of lipid composition and microbubble size. Microbubbles were expected to grow with the initial inrush of O₂ and N₂ and then dissolve with the efflux of SF₆. We report here on the anomalous growth and dissolution behavior exhibited by lipid-coated microbubbles. The lipid shell resisted expansion beyond the initial radius, and this resistance increased with lipid acyl chain length and decreasing microbubble size. The lipid shell also resisted collapse beyond the initial diameter. Our results demonstrate the effect of lipid shell mechanics on the stability of microbubbles exposed to different gas environments, which may inform our prediction of the in vivo fate of intravenously injected microbubbles.