Investigation of spatio-temporal inertial cavitation activity for optimization of needle-free ultrasound-enhanced vaccine delivery

Abstract

Ultrasound-induced cavitation is a promising mechanism for pain-free delivery of vaccines without needles. However, the relationships between cavitation energy and the various bioeffects involved in transdermal vaccination, including skin permeabilization, convective drug transport, reversible and irreversible sonoporation, and immune-stimulation, are not well understood. Previous transdermal ultrasound experiments have demonstrated inhomogeneous delivery across the exposed surface, which remains poorly understood. In this work, we first seek to fully characterize and quantify spatio-temporal cavitation activity across the skin surface, to identify a local cavitation dose that is optimal for all four key bioeffects. We have designed a new in vitro experimental setup that exposes several potential skin models to 265 kHz focused ultrasound, a new generation of protein-based cavitation nuclei (pCaNs), and a fluorescently labelled vaccine analogue, while simultaneously imaging cavitation activity parallel to the skin surface by Passive Acoustic Mapping (PAM). Subsequent staining and multi-photon microscopy of the skin models allows a direct comparison between the PAM-derived spatiotemporal inertial cavitation dose and locations where particular bioeffects were maximized. We will discuss the results of this investigation, how they relate to our recent in vivo study, and whether these findings enable enhancements of the spatial homogeneity and reproducibility of needle-free ultrasound vaccination.