Ultrafast dynamics of acoustic vaporization and payload release of phase-shift emulsions

Abstract

Acoustic droplet vaporization (ADV) is the phase-transitioning of perfluorocarbon emulsions via focused ultrasound. ADV has been utilized in many biomedical applications. For drug delivery and tissue regeneration, we developed composite hydrogel scaffolds, termed acoustically responsive scaffolds (ARSs), which here contained fluorescently labeled monodispersed emulsions (Ø: 12.3 ± 0.8 μm). Using ultra-high speed microscopy, we investigated the dynamics of ADV (2.5 MHz, P-: 6 MPa, pulse length: 6μs) in ARSs containing emulsions with perfluoropentane, perfluorohexane, and perfluorooctane. Vaporization and growth dynamics were studied in micro- to millisecond time scales at frame rates up to 10 Mfps. During ADV, the generated bubbles expanded 4.8-, 2.8-, and 2-fold greater than the initial diameter of the perfluoropentane, perfluorohexane, and perfluorooctane emulsions, respectively. Stable bubbles formed from ADV in perfluoropentane and perfluorohexane, though growth rates post-ADV were significantly different. Bubbles recondensed within perfluorooctane emulsions, which exhibited a 20% decrease in diameter following five repeated ADV events. Stable or transient bubble formation was further compared with classical nucleation theory predictions. Additionally, we compared the release dynamics of fluorescently labeled payloads from the emulsions. The results provide physical insight enabling the modulation of bubble dynamics with ADV and hence release kinetics, which can be used for therapeutic applications.