Multifrequency Optomechanical Stiffness Measurement of Single Adherent Cells on a Solid Substrate with High Throughput

Simultaneous time-varying viscosity, elasticity, and mass measurements of single adherent cancer cells across cell cycle

Biophysical studies on single cells have linked cell mechanics to physiology, functionality and disease. Evaluation of mass and viscoelasticity versus cell cycle can provide further insights into cell cycle progression and the uncontrolled proliferation of cancer. Using our pedestal microelectromechanical systems resonant sensors, we have developed a non-contact interferometric measurement technique that simultaneously tracks the dynamic changes in the viscoelastic moduli and mass of adherent colon (HT-29) and breast cancer (MCF-7) cells from the interphase through mitosis and then to the cytokinesis stages of their growth cycle. We show that by combining three optomechanical parameters in an optical path length equation and a two-degree-of-freedom model, we can simultaneously extract the viscoelasticity and mass as a function of the nano-scaled membrane fluctuation of each adherent cell. Our measurements are able to discern between soft and stiff cells across the cell cycle and demonstrated sharp viscoelastic changes due to cortical stiffening around mitosis. Cell rounding before division can be detected by measurement of mechanical coupling between the cells and the sensors. Our measurement device and method can provide for new insights into the mechanics of single adherent cells versus time.

Optomechanical microrheology of single adherent cancer cells

There is a close relationship between the mechanical properties of cells and their physiological function. Non-invasive measurements of the physical properties of cells, especially of adherent cells, are challenging to perform. Through a non-contact optical interferometric technique, we measure and combine the phase, amplitude, and frequency of vibrating silicon pedestal micromechanical resonant sensors to quantify the "loss tangent" of individual adherent human colon cancer cells (HT-29). The loss tangent, a dimensionless ratio of viscoelastic energy loss and energy storage - a measure of the viscoelasticity of soft materials, obtained through an optical path length model, was found to be 1.88  ±  0.08 for live cells and 4.32  ±  0.13 for fixed cells, revealing significant changes (p < 0.001) in mechanical properties associated with estimated nanoscale cell membrane fluctuations of 3.86  ±  0.2 nm for live cells and 2.87  ±  0.1 nm for fixed cells. By combining these values with the corresponding two-degree-of-freedom Kelvin-Voigt model, we obtain the elastic stiffness and viscous loss associated with each individual cell rather than estimations from a population. The technique is unique as it decouples the heterogeneity of individual cells in our population and further refines the viscoelastic solution space.