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. (C) 2016 The Japan Society of Applied Physics

目的：超音波と微小気泡との併用で超音波治療の効率向上が期待されているが，体内に注入後の微小気泡は血流と共に体内へ拡散してしまう．我々はこれまで，任意の箇所での微小気泡の濃度を高めることを目的として，流水中で超音波照射による微小気泡の誘導を行ってきた．この方法を，生体投与可能で様々な薬剤を包含できる微小気泡（バブルリポソーム，BLs）に応用する必要に迫られている一方，血流中での微小気泡の制御可能性については未確認であった．しかもBLsの平均直径は0.5 μmと小さく，従来の光学顕微鏡ではBLsの濃度を計測できない．対象と方法：そのため，超音波画像中の輝度変化からBLsの濃度を計測する校正法を確立し，生理食塩水とブタ血液の2種類の媒質を用いて実験を行った．まず超音波画像そのものによるBLsの破壊効果を調査し，BLsの濃度計測に影響を与えない超音波画像のMI値を検証した．さらにY字分岐を有する人工血管において，BLsを特定の経路に押し出す誘導実験を行った．ここでの実験条件は中心周波数5 MHz，最大音圧300 kPa-ppの集束波，流速30 mm/sとした．結果と考察：誘導実験の結果として，生理食塩水の場合に比べて，血流中ではBLsの破壊が軽減されることが今回の実験により初めて確認された．さらに誘導用音波の照射位置は，人工血管の分岐点で押し出すのでは無く，数mm離れた位置に設定した方が，誘導性能が向上することが分かった．結論：本研究の結果より，生体中におけるBLsの誘導の可能性を更に発展できる足がかりを確認できた．

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.

Bubble liposomes (BLs), which are gas-encapsulated liposomes several hundred nanometers in diameter, are expected to be developed as a novel tool for gene and drug delivery using ultrasound acoustic radiation force. However, since BLs are several hundred nanometers in diameter, difficulties exist in controlling their behaviors in blood flow under ultrasound exposure, since acoustic radiation forces have less effect on these small bubbles. In this study, we investigated the feasibility of active control of BLs in an artificial blood vessel under ultrasound exposure and attempted to evaluate the controllability. Then, we investigated the appropriate ultrasound conditions for active path selection of BLs in a bifurcated flow by applying acoustic radiation force. We prepared a single transducer to orient BLs toward one desired path. Two other transducers were targeted at the two paths after the bifurcation. We evaluated the areas of trapped BLs in the two paths after the bifurcation, to determine which path had increased BLs. The result showed a significant increase in area of trapped BLs in the desired path compared to the other path. Then, we defined the induction index of BLs by evaluating the area of trapped BLs, and changed the ultrasound conditions for active path selection of BLs by varying the sound pressure and frequency. We found that more BLs could be oriented to a desired path at higher sound pressure. For further study, we are aiming at active control of BLs in vivo.

We previously reported our attempt to propel microbubbles in a 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 the surface of the body, controlling bubbles in an against-flow was necessary. It is unpractical to use multiple transducers to produce the same number of focal points because single-element transducers cannot produce more than two focal points. In this study, we introduced a complex artificial blood vessel according to a capillary model and a two-dimensional (2D) array transducer to produce multiple focal points for the active control of microbubbles in an against-flow. From the results, about 15% more microbubbles were led to the desired path with multiple focal points of ultrasound relative to the no-emission case. (C) 2013 The Japan Society of Applied Physics

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.