Time-resolved high-speed fluorescence imaging of bubble-induced sonoporation.

The uptake of drugs through a cell membrane is enhanced by the use of bubbles and ultrasound. Little is known about the physical mechanisms underlying the uptake at short timescales. Here we study the bubble-assisted uptake of propidium iodide (PI) by endothelial cells at a millisecond timescale using high-speed fluorescence imaging. Single microbubbles were insonified at a driving frequency of 1MHz and at acoustic pressures varying from 200 to 1200 kPa for a duration of 10 and 100 cycles. At a pressure of 200 kPa and 10 cycles, 50% of the cells showed uptake of PI, and this percentage increased to 90% for a pressure of 400 kPa. At a pressure of 1200 kPa all cells showed uptake of PI. The high-speed fluorescence recordings revealed that a localized pore in the cell membrane is formed right at the position of the bubble. Uptake was observed within several milliseconds after insonation and the size of the induced pore was found to be dependent on the bubble radius. Furthermore, the inflow of PI is diffusion-driven. The pore is formed temporarily and closes within several seconds after the ultrasound exposure.