On-chip measurements of cell compressibility via acoustic radiation

Measurements of mechanical properties of biological cells are of great importance because changes in these properties can be strongly associated with the progression of cell differentiation and cell diseases. Although state of the art methods, such as atomic force microscopy, optical tweezers and micropipette aspiration, have been widely used to measure the mechanical properties of biological cells, all these methods involve direct contact with the cell and the measurements could be affected by the contact or any local deformation. In addition, all these methods typically deduced the Young's modulus of the cells based on their measurements. Herein, we report a new method for fast and direct measurement of the compressibility or bulk modulus of various cell lines on a microchip. In this method, the whole cell is exposed to acoustic radiation force without any direct contact. The method exploits the formation of an acoustic standing wave within a straight microchannel. When the polystyrene beads and cells are introduced into the channel, the acoustic radiation force moves them to the acoustic pressure node and the movement speed is dependent on the compressibility. By fitting the experimental and theoretical trajectories of the beads and the cells, the compressibility of the cells can be obtained. We find that the compressibility of various cancer cells (MCF-7: 4.22 +/- 0.19 x 10(-10) Pa-1, HEPG2: 4.28 +/- 0.12 x 10(-10) Pa-1, HT-29: 4.04 +/- 0.16 x 10(-10) Pa-1) is higher than that of normal breast cells (3.77 +/- 0.09 x 10(-10) Pa-1) and fibroblast cells (3.78 +/- 0.17 x 10(-10) Pa-1). This work demonstrates a novel acoustic-based method for on-chip measurements of cell compressibility, complementing existing methods for measuring the mechanical properties of biological cells.