Ultrasound targeted microbubble destruction (UTMD) is a phenomenon based out of acoustic cavitation and microbubble oscillation.  Ultrasound (US) alone has been shown to increase the cellular uptake of proteins and small-molecule drugs but at dangerously high energy levels.  The increased uptake is caused by gas cavitation, or the formation of bubbles in liquid and subsequent oscillation from the acoustic input.  Microbubbles, on the other hand, oscillate at a much lower and safer US frequency while still increasing the uptake of small molecules on the cellular level.  Although there are competing theories to explain this phenomenon, the most prominent is that the bubbles oscillate at a specific US frequency and collapse, causing microstream swirling, microjetting and hydrodynamic shock waves.  The result is sonopores, 30-100 nanometers (nm) in diameter, in the cell membranes (Figure 1, Tinkov et. al) that allow proteins, plasmids, and small-molecule drugs to easily enter the cell [1, 7].  The sonopores last for several seconds or minutes while the cells use ATP and Ca2+ to repair their membranes.  Because of the viscous nature of the cytoplasm inside the cell the nucleus is not affected by these shock waves. [1]

UTMD can also effect the capillary vasculature, causing an increased capillary permeability.  Depending on the duration of the US treatment the vessels can even hemorrhage. [1]