Effect of the surfactant pluronic on the stability of lipid-stabilized perfluorocarbon nanobubbles

Microbubble (MB) contrast agents are widely used in diagnostic and therapeutic ultrasound (US) applications. However, due to their 1-10 μm size range, MBs are constrained to the vasculature and thus this limits applications in cancer diagnosis and therapy. To expand contrast enhanced US capabilities, we have developed sub-micron contrast agents, nanobubbles (NB), stabilized with Pluronic, a nonionic triblock copolymer surfactant, to the phospholipid shell stabilizing perfluoropropane (C3F8) gas. NBs, with diameter of ∼200-400 nm are small enough to extravasate from leaky tumor vasculature and accumulate in tumors through the EPR effect. Prior work has shown that bubble echogenicity and stability are, in part, dependent on the surface tension of the stabilizing shell. In this study, we evaluate the effect of Pluronic on surface tension of the lipid films and how its presence in the shell affects NB size, yield, echogenicity and signal decay at clinically-relevant imaging frequencies. To do so, the surface tension of lipid films with varying compositions was measured using pendant drop tensiomentry. Bubble size and concentration were determined using a particle metrology system. Results show a significant decrease in NB surface tension through the incorporation of Pluronic, especially at a molar ratio of 0.2, where this value decreased by 27% (p < 0.0001). At this concentration, a significant decrease in the signal decay over time resulting in a stability increase of 39% (p < 0.0001) was observed. The Pluronic had little impact on size; NBs had an average diameter of 165 ± 7 nm. Pluronic also had negligible effect on NB concentration.