Bubble-Like Response of Living Blood Cells and Microparticles in an Ultrasound Field

The bilayer sonophore model suggests that ultrasound induces a pulsating structure in the intra-membrane hydrophobic space between the two lipid monolayer leaflets of the cell membrane, assembled by dissolved gas of the surrounding area, which absorbs acoustic energy and transforms it by creating intra-cellular structural changes. This void has been referred to as a bilayer sonophore. The bilayer sonophore inflates and deflates periodically when exposed to ultrasound and may itself radiate acoustic pressure pulses in the surrounding medium in the same way a gas bubble does: once exposed to ultrasound the bilayer sonophore becomes a mechanical oscillator and a source of intracellular cavitation activity. In this paper, we describe observations of the clustering behaviour of living cells and several other particles in a standing sound field generated inside a ring transducer. Upon sonication, blood cells and monodisperse polystyrene particles were observed to have been trapped in the same locations, corresponding to nodes of the ultrasound field. Because polystyrene is hydrophobic, it behaves like a particle trapped inside a thin gas shell. In fact, the sonophore model treats biological cells in a similar way. Microbubbles that form the ultrasound contrast agent Quantison™ behave differently, however. These microbubbles accumulated in circles faster than blood cells and polystyrene particles. In addition, they form tightly packed clusters at the nodes, indicating very strong secondary Bjerknes forces. Cluster formation is not to be expected in cells with predicted sonophore sizes on the order of 10-100 nm.