The cocktail method: influence of microbubble shell homogeneity on acoustic behavior and stability

Abstract

The influence of lipid shell organization on the acoustic behavior of microbubbles (MBs) has become a focal point of ultrasound research. Recent studies have demonstrated that even monodisperse MBs from the same batch can exhibit profoundly different acoustic responses. As high-resolution ultrasound imaging and MB-assisted drug delivery continue to advance, this heterogeneity may compromise performance, causing artifacts and reducing localization accuracy. This study investigates phospholipid organization on the MB surface during both formation and dynamic volumetric changes. Using a panel of membrane probes and labeled lipids in combination with high-resolution confocal microscopy, we characterize lipid surface dynamics, phase behavior, and micro-viscosity. We introduce the 'cocktail method', a straightforward thermal procedure designed to produce seemingly domainless MBs and evaluate how these structural modifications influence acoustic behavior. Our results identify distinct characteristics among individual lipid components during shell formation and provide a qualitative assessment of viscosity within specific lipid phases during expansion and compression. Collectively, these findings reveal that lipid organization impacts shell elasticity and acoustic behavior. Furthermore, we show that the intrinsic physicochemical properties of the lipids DSPC and DSPE-PEG5000 drive an inevitable degree of phase separation that persists despite thermal quenching. This study aims to improve our understanding of the relationship between microbubble lipid architecture and its impact on shell viscoelasticity, stability, and acoustic behavior, ultimately aiding the development of predictable microbubbles for advanced medical applications.