Microobject manipulation using ultrasonic waves is expected to play important roles in constructing future drug or gene delivery systems. The acoustic radiation force, which is applied to microobjects, traps the objects at the desired position. A microjet, which is produced by bubble explosion under high-intensity ultrasonic waves, creates microholes through the cell membrane (sonoporation), which is considered as a sophisticated method of improving the doses of drugs or genes injected into a tissue. Aiming at increasing the trapping force in micro bubble manipulation using ultrasonic waves, we have proposed a novel method based on the self-organization of microbubbles. This method uses seed bubbles in order to trap the target bubbles. In this study, the proposed method is applied to yeast cell trapping using ultrasonic waves. An ultrasonic wave contrast agent (Levovist; Shering A.G., Germany) is used as a seed bubble. It is shown that the number of trapped yeast cells depends on the preparation of the yeast cells. In order to evaluate the result, two additional experiments are carried out by changing the internal gas of the seed bubbles and by using bubbles with a polymer shell.

Microbubble manipulation by acoustic radiation force may play an important role in future drug delivery systems, because the required bubble manipulations, such as bubble trapping at the desired position and payload release by bubble destruction using a high-intensity ultrasonic wave, are controlled by ultrasonic waves. In this paper, a novel method of microbubble trapping by bubble nonlinear oscillation is proposed. Two ultrasonic waves, which have a harmonic frequency relation, are used in order to generate the force for trapping bubbles. The first wave is a pumping wave, which has a relatively high sound pressure. This wave is used for inducing the nonlinear oscillation of bubbles. The second wave is a control wave, whose frequency is set to be the harmonic frequency of the nonlinear bubble oscillation. The pressure gradient of the control wave in conjunction with the harmonic component of nonlinear oscillation generates the Bjerknes force applied to bubbles. This force forms multiple traps with a narrow separation for bubbles, which flow into the cross area of the two ultrasonic waves.