Manipulation of cells in ultrasound standing waves

Abstract

The acoustic radiation force operating on a cell in a 0.5-MPa pressure amplitude 3-MHz standing wave field equals that experienced in a 250-g gravitational field. Consequently, cells may be driven rapidly to preferred points in the sound field. Acoustic radiation pressure can drive bacteria, flowing by an acoustic reflector (prepared as an immunosensor surface), onto that surface where they are then captured. Animal cells have been driven onto the fabric of wide-gauge filters to form, a basis for a supported bioreactor. Real standing wave fields do not have the ideal plane-wave form, particularly when the transducer diameter is much larger than a wavelength (typical transducer diameter of 20 mm, wavelength of 0.5 mm at 3 MHz in water). Pressure variations within a node plane then give rise to regions towards which cells migrate and may be held against flow in potential traps. These traps are exploited commercially to retain cells in fermentors in the pharmaceutical industry. Most recently, ultrasound traps have been employed to concentrate animal cells in the focal plane of a fluorescence microscope so that cell interactions and their consequences at the molecular level can be monitored.