FIGURE 4.

Effect of piezo extension rate on apparent stiffness of HL60 cells. (A) At increasing piezo extension rates, the viscosity of the HL60 causes increased cantilever deflection for the same piezo position. All legend values are in nanometers/second. At rates up to 415 nm/s, the deflection curves overlay each other. This indicates that the indentation rate was slow enough for viscosity not to be a factor. (B) HL60 apparent stiffness determined by Hertzian mechanics remains constant at low rates. Data in panel B are from the same cell as panel A. (C) The apparent stiffnesses of eight HL60 cells at low piezo extension rates were normalized and averaged. Normalization was performed by averaging the stiffness of each cell across the experimental extension rates and then dividing the stiffness at each rate by this average. At rates of 415 nm/s and below, apparent stiffness remained constant. Error bars represent the standard deviation.