micelle core upon the application of ultrasound this is confirmed by the above mentioned lower release of rb in comparison to dox at higher local drug concentrations in micelle cores drug ppo hydrophobic interactions are replaced by weaker drug drug interactions which facilitates drug release in parallel with measuring drug release the threshold for transient cavitation was measured by trapping radicals that were produced upon the collapse of cavitation bubbles cavitation threshold increased with increasing ultrasound frequency at 20 khz radicals were observed even at a power density as low as 0 01 w cm 2 which is consistent with the relatively high efficiency of 20 khz ultrasound for drug release from micelles at 67 khz no radicals and no drug release were observed below a power density of 1 0 w cm 2 these data are summarized in table 5 the data suggest that transient cavitation plays an important role in triggering drug release from micelles it is hypothesized that shock waves produced by transient cavitation events disrupt micelles and release drug into aqueous environment during the u201c ultrasound off u201d phase the micelles are restored and drug is re encapsulated which takes less than 1 s at 37 u00b0 c the findings described above allow to formulate a new concept of a localized drug delivery based on drug encapsulating in polymeric micelles to prevent unwanted interactions with normal cells in combination with focusing ultrasound on the tumor.