  High-intensity ultrasound excites pre-existing bubbles in tissue-like
material, and the subsequent bubble activity may lead to damage. To investigate
such damage mechanisms, agar tissue-mimicking phantoms were subjected to
multiple pressure wave bursts of the kind being considered specifically for
burst wave lithotripsy (BWL). A notable feature of these experiments was the
formation of distinct linear defects, as might expected for a crack. We propose
a physical mechanism for how a single bubble, excited by ultrasound, may evolve
to from such features. It entails fracture, bubble stability and rectified mass
diffusion. These observed linear defects are a potential mechanism for spread
of ultrasound-induced bubbles and the subsequent damage.
