Dependence of endothelial drug delivery on monodisperse microbubble dynamics

Ultrasound insonification of microbubbles has been shown to increase vascular permeability to locally enhance drug delivery. For more effective and controllable therapeutic outcomes, uniform acoustic responses from microbubbles and a deeper understanding of the biophysical mechanisms of drug delivery are critical. In this study, we investigated the impact of monodisperse microbubble size and their dynamics on cellular responses and drug delivery outcomes in endothelial cells. Monodisperse microbubbles with radii of 1.5, 2.2, 2.7, and 2.9 μm were produced using a microfluidic flow-focusing device. Upon insonification (2 MHz, 220 kPa peak negative pressure, 10 cycles), the microbubble oscillation was captured in real-time at 10 million frames per second using ultra-high-speed imaging, while confocal microscopy was employed to observe cellular responses in both 2D and 3D. For the 65 microbubbles studied, the 2.2 μm microbubbles, i.e., corresponding to the resonant radius at 2 MHz ultrasound, exhibited the highest sonoporation rate (75 %), induced the largest membrane perforations (a median value of 78 μm²) and highest intracellular drug uptake. The 1.5 μm microbubbles achieved a comparable sonoporation rate (73 %), yet with significantly smaller membrane perforations (a median value of 20 μm²), lower intracellular drug uptake, and highest occurrence of transendothelial drug delivery pathways (64 %). Mechanistically, microbubble-generated shear stress was identified as the significant factor driving sonoporation, while normal stress did not show significance. In conclusion, our study highlights the importance of carefully selecting the microbubbles size to maximize microbubble-mediated drug delivery outcomes and facilitate safe translation of monodisperse microbubbles into clinical practice.