Basic studies on sonoporation with size- and position-controlled microbubbles adjacent to cells

We have been studying a sonoporation technique that uses one-shot exposure of short-pulsed ultrasound to cells with attached microbubbles. Here, we introduce a new observation system developed for elucidation of sonoporation mechanisms. An inverted-type microscope was equipped with optical tweezers that enable position control of a microbubble. A Laguerre-Gaussian (LG) beam, a Gaussian beam with a dark spot at the center, was used to trap a microbubble with a refractive index lower than that of the surrounding water. The LG beam was produced using a phase-only spatial light modulator and 1,065-nm laser light. Results of basic experiments showed that the tweezers can trap bubbles of 1-30 μm in diameter suspended in water, and the minimum optical power for trapping bubbles of 5 μm in diameter was around 10 mW. Sonoporation experiments were carried out using size- and position-controlled bubbles placed on cell membranes or placed at separated positions from cells, and the results indicated the importance of the newly developed system for elucidation of sonoporation mechanisms.