Ultrasound-Induced Calcein Release From eLiposomes

Ultrasound is explored as a method of inducing the release of encapsulated materials from eLiposomes, defined as liposomes containing emulsion droplets. Emulsions were formed using perfluorohexane and perfluoropentane. eLiposomes were formed by folding interdigitated lipid sheets into closed vesicles around the emulsion droplets. Cryogenic transmission electron microscopy was used to verify droplet encapsulation. Self-quenched calcein was also encapsulated inside the vesicles. A fluorometer was used to measure baseline fluorescence, calcein release after ultrasound exposure, and total release from the vesicles. eLiposome samples released 3 to 5 times more of the encapsulated calcein than did controls when exposed to 20-kHz ultrasound. Calcein release increased with exposure time and intensity of ultrasound. eLiposomes with large (400 nm) droplets produced more calcein release than small (100 nm) droplets. These observations suggest that the emulsions are vaporized by ultrasound and that the Laplace pressure in the emulsions has an effect on droplet vaporization.