Biophysical implications of bubble dynamics

Evidence is reviewed, from theory and experiment, that biological systems can be affected by ultrasound at low levels, if resonant gas bodies are present. In a suspension of cells or other particles a pulsating gas bubble causes the particles to migrate toward its surface via radiation force. This motion, in addition to acoustic microstreaming, transports particles into the bubble near-field where they are subjected to highly localized stress fields. In plant leaves containing gas-filled channels, the ultrasonic intensity required to produce cell death varies with frequency, showing minima in ranges corresponding roughly to calculated frequencies for resonance of the channels.