Shin Enosawa

Experimental study of active control of microbubbles in blood flow by forming their aggregations

We have experimented a method to control behavior of microbubbles in flow using an artificial blood vessel with multiple transducers to emit ultrasonic plane wave. Microbubbules are propelled in flow owing to a primary Bjerknes force, which is a physical phenomenon where an acoustic wave pushes an obstacle along its direction of propagation. Also they form aggregation when they are put into an ultrasound field because of secondary Bjerknes force, which acts as attractive or repulsive factor among neighboring microbubbles. Thus we consider that forming bubble aggregations is effective to be propelled before entering into an ultrasound field to receive greater primary Bjerknes force. We have investigated the phenomenon of bubble aggregations and observed behavior of aggregations with and without red blood cells in artificial blood vessels under various conditions of ultrasound exposure. As the results, when microbubble aggregations were formed, the efficiency was improved more than the condition without forming aggregation. The existence of red blood cells near microbubbles contributed to the increase in the size of aggregations propelled against the vessel wall.

Preliminary study on forming microbubble-surrounded cells as carriers for cellular therapy and evaluation of ultrasound controllability by fluorescence imaging

Although various cellular immune therapies have been proposed and developed, because the therapeutic cells disperse upon injection into blood flow, there is a limitation on the accumulation of the cells to the target area. We previously reported our attempts to actively control microbubbles in artificial blood vessels, and here we propose a new method of carrying therapeutic cells for cellular therapy using microbubbles and ultrasound. When microbubbles and their aggregations attach to the surface of therapeutic cells, the acoustic force needed to propel the cells is increased because of the size expansion and the boundary in acoustic impedance on the cell surface. We fabricated a cylindrical chamber including two ultrasound transducers to emit a suspension of microbubbles (TF-BLs, transferrin-bubble liposomes) on the cells (Colon-26) to enhance the adhesion of microbubbles on the cells. We found that the optimum conditions for producing BL-surrounded cells were a sound pressure of 100 kPa-pp, an exposure time of 30 s, and a TF-BL concentration of 0.33 mg lipid/mL, when the cell concentration was constant at 0.77 × 105/mL in phosphate-buffered saline. Using these BL-surrounded cells, we confirmed the controllability of the cells under ultrasound exposure, where the displacement increased in proportion to the sound pressure and was not confirmed with the original cells.

Active control of nanobubble-surrounded cells propelled by acoustic radiation force with verification of frequency dependence

Recently immunotherapy using therapeutic cells has attracted attention. Because the cells injected in human body disperse through the blood stream, there is a problem that small amount reaches to the diseased area. So we intend for in vivo cell delivery system, which is realized by attracting nanobubbles on the surface of cells to reduce its density for dynamic control of cells under ultrasound emission. The cells and bubbles were observed under fluorescent excitation to be observed their behavior in a thin channel. First we produced and observed bubble-surrounded cells using a fluorescence microscope and optimized the conditions of ultrasound exposure for production. Then we observed the controllability of the cells, which moved under exposure of ultrasound in the thin channel, where the motion was not confirmed in the cells without bubbles. Then we verified the motion of the bubble-surrounded cells according to the frequency of the ultrasound, where standing wave was priduced with the central frequency of 3-7 MHz, and maximum sound pressure of 150 kPa-pp. We found a remarkable difference in the frequencies, which will be useful for further active control of the cells.