Figure 10.

Reverse Sonoporation Mechanisms: Sonoporation traditionally involves microbubbles in close proximity to cells. However, sonoporation is also known to occur with liposomes. With the exception of nesting microbubbles inside the aqueous core of phospholipid vesicles, the putative mechanisms of Figure 9 are not applicable here; another mechanism or mechanisms must account for ultrasound-induced leakage from liposomes. One possibility is nucleation of a gas bubble in the hydrophobic region of the phospholipid bilayer; subsequent growth, expansion, and/or translocation of this bubble creates the pore for transport (D).⁸² Another possibility is that inertial cavitation of bubbles (generated either inside the liposome core, within the liposomal bilayer, or in the bulk aqueous phase) catastrophically disrupts the liposome (indeed, this is believed to be the mechanism by which sonication creates small, uni-lamellar vesicles from giant, multi-lamellar vesicles) (E). Lastly, we propose a new mechanism, which recognizes that negative ultrasound pressure causes the entire liposome to expand, leading to dilation and increased separation between phospholipids (F). Such dilation would be enhanced by formation of a bubble - e.g., via nucleation of dissolved gases or vaporization of water - (or by nesting of one or more microbubbles) inside the aqueous core of the liposome and subsequent expansion of this bubble with negative ultrasound pressure. Although mechanism (F) would not create holes per se, it would account for the increased membrane permeability observed upon application of ultrasound.