Stress relaxation measurement of fibroblast cells with atomic force microscopy

We measured the stress relaxation of mouse fibroblast NIH3T3 cells with an atomic force microscope (AFM) using a sharp silicon tip and a silica bead with a radius of ∼1 μm as an indenter. The decay of loading force was clearly observed in NIH3T3 cells at a small initial loading force of ∼0.4 nN and was well fitted to the stretched exponential function rather than to a single exponential function. The stretching exponent parameter was ∼0.5 for both indenters, indicating that the stress relaxation observed in NIH3T3 cells consisted of multiple relaxation processes. The time-domain AFM technique described in this report allows us to measure directly the relaxation process of living cells in a range from milliseconds to seconds.