Nobuyuki Watarai

Production of Local Acoustic Radiation Force to Constrain Direction of Microcapsules in Flow

We have ever reported our attempt to control the direction of microcapsules in flow by acoustic radiation force. However, the diameter of capsules was too large to be applied in vivo. Furthermore, the acoustic radiation force affected only the focal area because focused ultrasound was used. Thus, we have improved our experiment by using microcapsules as small as blood cells and introducing a plane wave of ultrasound. We prepared an artificial blood vessel including a Y-form bifurcation established in two observation areas. Then, we newly defined the induction index to evaluate the difference in capsule density in two downstream paths. As a result, the optimum angle of ultrasound emission to induct to the desired path was derived. The induction index increased in proportion to the central frequency of ultrasound, which is affected by the aggregation of capsules to receive more acoustic radiation force.

Effect of Existence of Red Blood Cells in Trapping Performance of Microbubbles by Acoustic Radiation Force

We have proposed a method to control microbubbles by making use of acoustic radiation force, which is generated with acoustic propagation, to correspond to therapeutic applications of ultrasound. By preventing bubbles from passing through the desired target area, the local concentration of bubbles can be enhanced. However, we have never experimentally confirmed this phenomenon under in vivo conditions or close to those. Thus, we carried out an experiment to evaluate the trapping performance of bubbles using a suspension of red blood cells (RBCs) and an artificial blood vessel. By defining the trapping index to evaluate the amount of trapped microbubbles, we have confirmed that the trapping performance was enhanced according to the concentration of RBCs and the sound pressure, but not according to the central frequency of ultrasound. The results indicate that the existence of RBCs near microbubbles contributed to the increase in the size of aggregations propelled against the vessel wall.

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.

Dependence of aggregate formation of microbubbles upon ultrasound condition and exposure time

We have previously reported our attempts to control microbubbles (microcapsules) behavior in flow by primary Bjerknes force to increase the local concentration of the bubbles at a diseased part. However, there was a limitation in efficiency to propel bubbles of μm-order size. Thus we consider that forming aggregates of bubbles is effective to be propelled before entering into an ultrasound field by making use of secondary Bjerknes force under continuous ultrasound exposure. In this study, we observed the phenomena of aggregates formation by confirming variation of diameter and density of aggregates under various conditions of ultrasound exposure. Then we elucidated frequency dependence of the size of aggregates of micro-bubbles.

Observation of flow variation in capillaries of artificial blood vessel by producing microbubble aggregations

Microbubbles form their aggregations between the neighboring microbubbles by the effect of secondary Bjerknes force under ultrasound exposure. However, because of the difficulty to reproduce a capillary-mimicking artificial blood vessel, the behavior of aggregations in a capillary has not been predicted. Thus we prepared artificial blood vessels including a capillary model, which was made of poly(vinyl alcohol) (PVA) by grayscale lithography method, with minimum diameter of the path of 0.5 mm. By using this model we investigated the possibility of artificial embolization, where the microbubble aggregations might block entire vessels not to penetrate flow in downstream. Confirming that the sizes of flown aggregation were greater than the section area of the minimum path in the capillary model, we investigated the probability of path block in it. As the results we confirmed the probability increased in proportion to sound pressure and inversely to flow velocity. We are going to investigate with more kinds of parameters to enhance the possibility of artificial embolization.

Study to prevent the density of microcapsules from diffusing in blood vessel by local acoustic radiation force

We have already reported our attempt to constrain direction of microcapsules in flow owing to an acoustic radiation force. However, the diameter of capsules was too large not to be applied in vivo. Furthermore, acoustic radiation force affected only in focal area because focused ultrasound was used. Thus we have improved our experiment by using microcapsules as small as blood cells and introducing a plane wave of ultrasound. We prepared an artificial blood vessel including a Y-form bifurcation established two observation areas. Then we newly defined the induction index to evaluate the difference of capsule density in two paths of downstream. As the result, optimum angle of ultrasound emission to induce to desired path was derived. And the induction index increased in proportion to the central frequency of ultrasound, which is affected by forming aggregation of capsules to receive more radiation force.

Active Control of Microbubbles in a Capillary Flow by Producing Multiple Acoustic Radiation Forces

We have ever reported our attempt to control the direction of microcbubbles in the flow of an artificial blood vessel by acoustic radiation force. However, the shape of the blood vessel was Y-form therefore too simple to be considered in vivo. Furthermore, the acoustic radiation force affected wide focal area because plane ultrasound was used. To set multiple transducers accurately, we need to position them using a very special configuration. Thus, we have improved our experiment by using a complex artificial blood vessel model and constructed an experimental system with multiple transducer fixtures including xyz-stage to set their respective focal points. In addition, we introduced focused waves of ultrasound for fine bubble control. Thus we prepared artificial blood vessels according to a capillary model, which were made of poly (vinyl alcohol) (PVA) by grayscale lithography method, with a minimum diameter of 0.5 mm. Then, we prepared two transducers to orient bubbles toward one desired path out of four originating from consecutive two bifurcations. Two acoustic fields were targeted at the consecutive bifurcations. We evaluated brightness of the four paths past the two bifurcations, which is decreased with bubbles existence. As a result, observed brightness of the desired path was significantly decreased compared with those of other paths. Difference between observed brightnesses increased with the number of ultrasound sources, which affect the amount of acoustic radiation force received by bubbles. From these results, microbubbles could be oriented to one desired path of multistep capillary. For further analysis, we are aiming at active control of bubbles in vivo.

Production and active control of microbubbles aggregations in artificial capillary with multiple sound sources

We have previously reported our attempts for active control of microbubble aggregations, by making use of acoustic force, which acts to propel microbubbles and to adjust the size of aggregations. However, because we have used simple shape of artificial blood vessels, the behavior of aggregations in a capillary, e.g., probability to obstruct in bloodstream, possibility of embolization, has not been predicted. Thus we designed and fabricated a capillary-mimicking artificial blood vessel to apply to production and active control of microbubble aggregations with multiple sound sources. Then we have set two kinds of transducers to produce aggregations and to propel aggregations, respectively. First we measured the size of aggregation, which increases according to the exposure time of ultrasound emission. When ultrasound was stopped, the aggregation suddenly flaked off the vessel wall and flew to downstream, propelled to the desired path and finally caught at a narrower path. We verified the same experiment under similar parameters to calculate the probability of path block. When the flow velocity was 20 mm/s, almost 50% of aggregations were induced to the desired path and 80% of them blocked the narrowest path in downstream.

Evaluation of local density enhancement of microcapsules in artificial blood vessel during exposure to focused ultrasound

We have proposed a physical DDS (Drug Delivery System) which makes use of microcapsules of μm size, which may contain a specified drug and also are easily affected by ultrasound exposure near their resonant frequency, to release various kinds of medications. These capsules are easily detected and actuated by ultrasound. However, because of the diffusion of capsules after injection into human body, it was difficult to enhance the efficiency of drug delivery. Thus we have considered a method for controlling the density of capsules in flow which uses acoustic radiation force, which moves the capsules to balance flow resistance. We have experimented with trapping microcapsules or microbubbles in flow of an artificial blood vessel. We have evaluated the effect of radiation force by measuring the trapped area of capsules or bubbles for various frequencies, sound pressures, and exposure times of sinusoidal ultrasound. The trapped area of capsules or bubbles increased with sound pressure and exposure time, and decreased with frequency. From those results, we have derived optimal conditions for trapping the capsules or bubbles.We have proposed a physical DDS (Drug Delivery System) which makes use of microcapsules of μm size, which may contain a specified drug and also are easily affected by ultrasound exposure near their resonant frequency, to release various kinds of medications. These capsules are easily detected and actuated by ultrasound. However, because of the diffusion of capsules after injection into human body, it was difficult to enhance the efficiency of drug delivery. Thus we have considered a method for controlling the density of capsules in flow which uses acoustic radiation force, which moves the capsules to balance flow resistance. We have experimented with trapping microcapsules or microbubbles in flow of an artificial blood vessel. We have evaluated the effect of radiation force by measuring the trapped area of capsules or bubbles for various frequencies, sound pressures, and exposure times of sinusoidal ultrasound. The trapped area of capsules or bubbles increased with sound pressure and exposure time, and de...