Naoto Hosaka

Production and validation of acoustic field to enhance trapping efficiency of microbubbles by using a matrix array transducer

We have developed a new matrix array transducer for controlling the behavior of microbubbles, which is different from that for high-intensity focused ultrasound (HIFU) therapy, in order to emit continuous wave by designing an acoustic field including multiple focal points. In the experiment using a thin-channel model, a wider acoustic field has an advantage for trapping microbubbles. In the experiment using a straight-path model, we have confirmed that a higher concentration of acoustic energy does not result in more aggregates. The dispersion of acoustic energy is important because the trapping performance is affected by the relationship between the shape of the acoustic field and the concentration of the suspension.

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.

Quantitative measurement of acoustic radiation force on a thin catheter for use in endovascular therapy

In this study, the quantitative measurement of acoustic radiation force acting on a thin catheter has been carried out to develop endovascular therapy with microbubbles. First, it is elucidated that the force acting on a thin catheter made of perfluoroalkoxy (PFA) copolymer can be obtained from the cantilever equation in the effective range, where the displacement of the catheter divided by the cube of the length of the catheter is less than 1.0 × 10−5 mm−2. Next, on the basis of the cantilever theory, the force acting on the catheter (diameter 400 µm, material PFA) is measured to be 24 µN under the continuous ultrasound (frequency 2 MHz, sound pressure 300 kPa) irradiation. Furthermore, it is observed that the force depends on the ultrasound frequency. Finally, we conclude that the force is obtained under practical conditions for the realization of endovascular therapy and suggest that thin catheter navigation using ultrasound is fully promising.

Active induction of in vivo microbubbles by acoustic radiation force at the bifurcation of blood vessel and its evaluation.

Alhough the development of drug delivery system using microbubbles and ultrasound is expected, because microbubbles diffuse in bloodstream, we have so far reported our attempts for active control of the microbubbles in flow by acoustic radiation force in order to increase local concentration of the microbubbles. However, there was no evidence that in vivo microbubbles act as similar as in vitro experiments, because there were limitations for reproduction of in vivo conditions. In this study, we have elucidated the relationship between brightness variation and microbubbles concentration in the suspension to estimate the absolute concentration in an invisible condition considering in vivo experiment. Then we conducted an experiment of active induction of microbubbles in a Y-form bifurcation of artificial blood vessel, where experimental conditions were with focused ultrasound, the central frequency of 5 MHz, flow velocity of 30 mm/s, and maximum sound pressure of 300 kPa-pp, respectively. Then we applied the conditions for active induction of in vivo microbubbles to compare with in vitro experiments. We used a bifurcation of blood vessel in an ear of a rabbit because the bifurcation shape in its blood vessel is visible. As the results of the experiment, the microbubbles concentration in the induced path was almost two times higher than that in the other path, which agrees with the results from in vitro experiments.

Forming acoustic attraction force to concentrate microbubbles in flow using a matrix array transducer

We have reported our attempts for active path selection of microbubbles by acoustic radiation forces, where we have investigated to control microbubbles by forming multiple focal points of continuous wave using a matrix array transducer. However, because those focal points were located to sweep microbubbles along the slope of sound pressure, it was difficult to concentrate microbubbles against the direction of flow. To produce attractive force to concentrate microbubbles in flow, we formed time-shared acoustic field of two focal points with phase variation. We have succeeded to concentrate microbubbles in water flow utilizing two focal points with opposite phase, where streamline of microbubbles was clearly confirmed in a thin channel. Also we confirmed induction performance using an artificial blood vessel with Y-form bifurcation, where induction rate to a desired path was calculated and varied according to the emission pattern of the focal points in time-shared acoustic fields.

Production of acoustic field with multiple focal points to control high amount of microbubbles in flow using a 2D array transducer

We have newly developed a 2D array transducer to control the behavior of microbubbles, which is different from that for HIFU therapy, to emit continuous wave by designing acoustic field including multiple focal points. In the experiment using a straight path model, we have confirmed that higher concentration of acoustic energy does not result more aggregation. We also have confirmed the trapped areas of microbubbles are located not in the peak of the distribution of sound pressure, but in the middle range. The dispersion of acoustic energy is important because there was a relation in the trapping performance of microbubbles and the shape of acoustic field.

Feasibility of thin catheter manipulation in the capillary blood vessel using acoustic radiation force

This paper is to demonstrate possibility that a thin catheter less than 0.5 mm diameter could be introduced in a thin blood vessel like the capillary by using acoustic radiation force. In the experiment, a 0.4 mm diameter tube likened to the thin catheter could be controlled and introduced to the aimed flow channel of 2 mm width Y-shape bifurcation. Value of the radiation force was estimated by two methods that are cantilever method and calculation method from acoustic pressure. As a result, it was estimated that the catheter was pushed by the force of several dozens of micro Newton.

New discovery of thin catheter movement under acoustical field of focused transducer

This paper reports that the phenomenon that an acoustic force is generated on the thin catheter, whose diameter is less than 1 mm, in the direction perpendicular to acoustic radiation direction. We consider that it has a potential of catheter control to bend in arbitrary directions. In the phenomenon, the catheter is bent towards the focal point in the near acoustic field, whereas it is bent away from the focal point in the far acoustic field. Accordingly, by actively moving the direction of the sound source perpendicular to the direction of sound propagation, the displacement of the catheter depended on the initial position of the catheter including near and far from the focal point in the acoustic field. There is a possibility that the catheter can be introduced to move in the arbitrary directions through multiple bifurcations in blood vessel network.

Active induction of microbubbles in flow at T-form bifurcation through acoustic focal points with phase variation

We have ever reported the method to produce three-dimensional acoustic force field to prevent microbubbles dispersing in flow. However, because produced acoustic force worked only to propel microbubbles in the direction of propagation of ultrasound, there was a limitation in direction to affect the behavior of microbubbles. In this research we examined to produce attractive force toward the transducer by considering phase variation of acoustic field. We used a flat matrix array transducer including 64 PZT elements, which was specially developed to produce a continuous wave. We prepared a T-form bifurcation model as artificial blood vessel, which was difficult to control the course of microbubbles. We produced an acoustic field of two focal points with opposite phase, where the middle of the points covers the bifurcation. As the results, when microbubbles suspension (average diameter of 4 um, density of 2.35 μl/ml) was injected with velocity of 40 mm/s, we confirmed that microbubbles aggregations were produced before reaching the bifurcation point and entered the bifurcation to be propelled to the desired path, where the course of microbubbles corresponded to the middle of the two focal points.

Active control of microbubbles stream in multi-bifurcated flow by using 2D phased array ultrasound transducer

We have previously reported our attempt to propel microbbles in flow by a primary Bjerknes force, which is a physical phenomenon where an acoustic wave pushes an obstacle along its direction of propagation. However, when ultrasound was emitted from surface of the body, controlling bubbles in against flow was needed. It is unpractical to use multiple transducers to produce the same number of focal points because single element transducer cannot produce more than two focal points. In this study, we introduced a complex artificial blood vessel according to a capillary model and a 2D array transducer to produce multiple focal points for active control of microbubbles in against flow. Furthermore, we investigated bubble control in viscous fluid. As the results, we confirmed clearly path selection of MBs in viscous fluid as well as in water.