Electrically measuring viscoelastic parameters of adherent cell layers under controlled magnetic forces

Electrical cell-substrate impedance sensing (ECIS) was used to measure the time-dependence and frequency-dependence of impedance for current flowing underneath and between cells. Osteosarcoma cells with a topology similar to a short cylinder (coin-like) surmounted by a dome were used in this study. Application of a small step increase in net vertical stress to the cells (4 and 7 dyn/cm2), via magnetic beads bound to the dorsal (upper) surface, causes an increase in cell body height and an increase in cell-cell separation, as well as stretching of the cell-substrate adhesion bonds. This results in a fast drop in measured resistance (less than 2 s), followed by a slower change with a time constant of 60-150 s. This time constant is about 1.5 times longer at 22 degrees C than that at 37 degrees C; it also increases with applied stress. Our frequency scan data, as well as our data for the time course of resistance and capacitance, show that the fast change is associated with both the under-the-cells and between-the-cells resistance. The slower change in resistance mainly reflects the between-the-cells resistance. To obtain viscoelastic parameters from our data we use a simple viscoelastic model comprising viscous and elastic elements (i.e., dashpot and two springs) for the cell body, and an elastic element (a spring) for the cell-substrate adhesion system. Our results show that the spring constants and the viscosity of the cell body components of this viscoelastic model decrease as the temperature increases, whereas the elastic modulus of cell-substrate adhesion increases with temperature. At 37 degrees C, for the cell body we obtain a value of about 10(5) P for the viscous element of the viscoelastic model, and a spring constant expressed in units of an elastic modulus of about 10(4) dyn/cm2 for the spring in series with the viscous element, with another spring with a modulus of about 2 x 10(3) dyn/cm2 in parallel with these. In comparable units, we have a modulus for the cell-substrate adhesion system of about 3 x 10(3) dyn/cm2.