We investigated the viability of vascular endothelial cells with the existence of lipid bubbles under ultrasound exposure. First, we estimated the various situations of bubbles on the cells including either adhesion, floating, or both of them using not only image analysis but also an experiment to retain the cells in flow. Then we examined the viability measurement of the cells using the ultrasound conditions with the frequency of 3 MHz, a maximum sound pressure of 400 kPa-pp, and a maximum irradiation time of 60 s. We found that the floating bubbles caused more damage on the cells rather than the adhered bubbles. Because insufficient adhesion of bubbles might cause damage by floating bubbles, we consider that the adhered bubbles were protective of cells against floating bubbles. However, excessive bubbles with a higher concentration than the saturation also might cause damage by destructing both of floating and adhered bubbles.

We confirmed that bubble-surrounded cells (BSCs) contained in flow were retained on the walls of an artificial blood vessel by forming an acoustic field with multiple focal points using tempo-spatial division emission. In order to realize the cell delivery system, we investigated the relationship between the concentration of T-cells and brightness in the microscopic images. Next, we defined the applied acoustic intensity, derived from the sound pressure distribution of every type of acoustic field. We studied the retention performance of BSCs versus various flow velocities, number and spatial intervals of the focal points, and maximum sound pressure. From the results, the optimal acoustic field to retain the cells depends on both acoustic intensity and flow velocity, where multiple focal points with an acoustic intensity of 50-120 mW/cm2 were more effective than the single focal point with 180 mW/cm2 in the range of a flow velocity of 10-20 mm/s.

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