Local nanomechanical properties of HeLa-cell surfaces

Using the Digital Pulsed Force Mode (DPFM) approach, the local mechanical properties of living HeLa cells have been examined. The cells were attached to a favorable glass-like substrate. The AFM used for the experiments was a commercial AFM setup by WITec. At every point of the image, approach and retract curves have been performed. The repetition rate of the cycle was 175 Hz. A total of about 500,000 curves has been recorded and completely evaluated for each experiment. The substrate served as an online reference material for calibration purposes. First, the force trajectories were corrected for the viscous drag force in the liquid environment. Second, the curves within the region of the substrate were phase corrected to compensate for the time lag of the signal in the setup assuming a purely elastic response of the reference material. Finally, all the force traces have been corrected by using this information and evaluated according to common continuum-elastic models. The resulting images allow the assignment of values of Young's modulus, local adhesion, hysteretic behavior, etc. at a high lateral resolution all over the cell body. In this paper, we describe the procedure of our measurement and the corresponding signal correction strategy of our automated data evaluation.