Shinya Miyazawa

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.

Active induction of bubble liposome at the bifurcation of in vivo blood vessel with optical measurement and robotic positioning

For active induction of microbubbles to apply for drug delivery, we have reported active control of microbubbles using acoustic radiation force. However, since there was no evidence that in vivo bubbles act as similar as in vitro experiments, we have examined an experiment with in vivo conditions. However, since the position determination to adjust the focal point of ultrasound was in manual, the effect of the active induction of microbubbles was unreliable. Thus we have updated the experimental setup by introducing an optical position sensor and a robotic positioning for active control of in vivo microbubbles. We have prepared and examined the property of bubble liposomes (BLs) to estimate the concentration using brightness variation on an echogram. First we have measured the destruction property of BLs using the experimental setup including a circulation path before applying an in vivo experiment. From the result we are able to estimate the amount of BLs destructed by continuous ultrasound exposure according to maximum sound pressure. Also, we have prepared the parallel link robot, which can hold an ultrasound transducer, with an optical position sensor to enhance the position accuracy. Since the maximum error using the system was 1.19 mm, which is less than the beam width of sound pressure distribution, we regarded the position accuracy using the system is satisfied with the determination of the focal point. In the experiment using a rabbit ear, we have indirectly confirmed the active induction of BLs, where a significant difference in brightness between two paths and the difference of concentration in the induced path was several times higher than that in the other path. We enhanced the accuracy of active induction of in vivo BLs using the robotic system with optical position tracking.

Experimental analysis of behavior in nanobubbles using echograms under ultrasound exposure

Although we have reported our attempts to actively control microbubbles in flow using acoustic radiation force for future drug delivery systems, the microbubbles we used are not applicable for in vivo experiments. Thus, we examined two types of nanobubble with a drug-retaining function. Because the nanobubbles are invisible in a conventional optical observation, we observed the behavior of nanobubbles using ultrasound images (echograms). First, we found the optimal settings of echography to guarantee the relationship between the brightness variation and lipid concentration of nanobubbles. Then, we derived the destructive coefficient using two types of path under continuous ultrasound exposure of 5 MHz. Results indicate that the controllability is related to the construction of nanobubbles and the spatial distribution of the ultrasound field. We realized that the design of the ultrasound field is important with Bubble A, whereas the frequency of ultrasound emission needs to be discussed with Bubble B.

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.