Effect of pre-trapping of micro bubbles on mechanical damage enhancement in bubble cavitation

It is considered that sonopolation, which is a method to make small pores through cell membrane by bubble cavitation, is a sophisticated means in order to improve efficacy in ultrasonic wave irradiated drug delivery system. However, neither precise mechanism of sonoporation nor optimum sequence of the ultrasonic wave has been clarified. In this paper, effects of pre-trapping of micro bubbles to the target wall, which is carried out by pumping ultrasonic wave before insonation by high intensity ultrasonic wave, is evaluated by N-isopropylacrylamide (NIPA) gel flow channel phantom. Both trapped bubbles on the wall after pre-trapping and micro hollows produced on the wall after insonation by high intensity ultrasonic wave are evaluated simultaneously. Results show that micro bubbles aggregate and they make bubble mass of about a few tens microns in diameter by pre-trapping of bubbles, however, these bubbles produce micron-sized hollows on the wall after irradiating high intensity ultrasonic wave. Fixed frequency method and frequency sweep method are compared as pre-trapping methods of micro bubbles. If frequency sweep method is adopted, it is found that micro hollows of about twice the amount are produced than the fixed frequency method.