Stress relaxation of HepG2 cells measured by atomic force microscopy

Stress relaxation of HepG2 cells was examined with an atomic force microscope (AFM). In the measurement, a loading force was applied to the cell by an AFM tip, and a time series of the cantilever deflection signal was measured at a fixed position of the cantilever base displacement. The relaxation of the loading force was clearly observed on the HepG2 cells, and was well fitted to a stretched exponential function known as the Kohlrausch-Williams-Watts (KWW) function, which is empirically employed to represent dispersion processes of the system. The relaxation time and the stretching exponent parameter were estimated to be ∼0.5 s and 0.4-0.6, respectively. The latter indicated that the relaxation observed in HepG2 cells consisted of multiple relaxation processes. Moreover, it was found that the characteristic feature of the relaxation process was not strongly correlated with the elastic properties of the cells.