The acoustic radiation force magnitude depends on several factors such as frequency, acoustic energy density, particle volume and the acoustic contrast factor Φ ac which describes the relationship between the density and compressibility of the suspended cells and the surrounding fluid. 14 These acoustic radiation forces and pressure fields have been proven safe for cells in several studies. 13 Due to non-linear scattering of the acoustic wave, the stationary pressure field exert acoustic radiation forces on small particles or cells suspended in the fluid filled cavity. The acoustophoresis principle in bulk acoustic wave applications is based on establishing an acoustic standing wave in a fluid containing cavity such as a microfluidic channel, 10 chamber 11,12 or well. 1 One approach to gently and non-invasively manipulate the position of cells and particles is acoustophoresis 2 which has been extensively used in concentration, 3 washing, 4 separation, 5 trapping 6,7 and acoustic streaming 8,9 applications. Introduction Microfluidic methods for separation, spatial control and analysis of cells are appreciated for their potential to substantially reduce time and reagents needed in clinical settings and basic biology research. The spatial separation between viable and dead cells along the channel width demonstrates a novel acoustophoresis approach for binary separation of viable and dead cells in a cell-size independent and robust manner. Using this information, we were able to calculate the effective acoustic impedance of viable K562 and MCF-7 cells. By investigating the trapping location of viable and dead K562, MCF-7 and A498 cells as a function of the suspension medium density, we observed that beyond a specific medium density the viable cells were driven to the pressure anti-node while the dead cells were retained in the pressure node. In this paper, we use microchannel acoustophoresis to show that the cell state within a cell population, in our case living and dead cells, influences the mechanical phenotype. The acousto-mechanical properties of a cell population are known to be heterogeneously distributed but are often assumed to be constant over time. The acoustic radiation force, originating from ultrasonic standing waves and utilized in numerous cell oriented acoustofluidic applications, is dependent on the acoustic contrast factor which describes the relationship between the acousto-mechanical properties of a particle and its surrounding medium.
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