Cell Viscoelasticity as a Function of Substrate Stiffness

While it is known that cells respond to the mechanical properties of their environment, limited information is available regarding mechanosensing and the means by which such stimuli are transduced. The cytoskeleton, a critical component used by cells to sense substrate stiffness, determines the viscoelasticity of cells. Information on cell viscoelasticity as a function of substrate stiffness and vimentin levels can lead to a larger composite model of the mechanism through which cells respond to external mechanical stimuli.3T3 fibroblasts were grown on collagen-coated polyacrylamide gels, of which the stiffness was controlled by varying both the concentrations of acrylamide and bisacrylamide. An atomic force microscope (AFM) was used to perform dynamic micromechanical tests. The force-mapping mode was employed to obtain 24 by 24 maps of force curves. For each force curve, the AFM first indents a cell with a 700 pN force and then applies a 10 Hz sinusoidal strain to the cell. The storage, E', and loss modulus, E'', were calculated by fitting the sinusoidal portion of the applied force and resulting indentation to obtain the amplitude and phase offset.Both E' and E'' increase with substrate stiffness. While the increase in E' is consistent with literature, we found that the ratio of E'' to E' decreases as substrate stiffness increases. This agrees with our hypothesis that cells become more solid-like as the elastic modulus of the extracellular matrix increases. Additionally, the value of E' was higher in vimentin null cells, suggesting that cells change their cytoskeletal composition to adapt to the loss of vimentin. This may involve an increase in the concentration of actin filaments, which contribute more to the elasticity than vimentin. Further experiments will investigate the potential link between vimentin expression and the mechanosensing ability of the cells.