Cell manipulation by radiation forces

Particles suspended in acoustic fields experience nonzero time averaged radiation forces that can cause particle movement, interparticle attraction or repulsion, and can exert a torque on suspended particles. In a standing wave, such field forces result in migration of the particles to positions separated by half an acoustic wavelength. The forces depend on some or all of the following properties: particle size and shape, sound frequency, the square of the sound-pressure amplitude, and the differences between the density and compressibility of the particles and those of the suspending phase. Threshold pressure for the vertical migration of particles to half-wavelength separations is derived. It is shown that appropriate choice of sound-pressure amplitude should lead either to the separation of particles of different densities and compressibilities or to the concentration of mixed samples essentially at the same location. Applications of radiation forces to bring cells together in suspension for cell electrofusion procedures and the use of ultrasound to support cells away from solid surfaces are described. Examples of cell concentration, cell sedimentation, cell alignment, and cell-cell interaction are also presented.