Indentation quantification for in-liquid nanomechanical measurement of soft material using an atomic force microscope: Rate-dependent elastic modulus of live cells

摘要

In this paper, a control-based approach to replace the conventional method to achieve accurate indentationnquantification is proposed for nanomechanical measurement of live cells using atomic force microscope. Accuratenindentation quantification is central to probe-based nanomechanical property measurement. The conventionalnmethod for in-liquid nanomechanical measurement of live cells, however, fails to accurately quantify thenindentation as effects of the relative probe acceleration and the hydrodynamic force are not addressed. As anresult, significant errors and uncertainties are induced in the nanomechanical properties measured. In this paper,na control-based approach is proposed to account for these adverse effects by tracking the same excitation forcenprofile on both a live cell and a hard reference sample through the use of an advanced control technique, and bynquantifying the indentation from the difference of the cantilever base displacement in these two measurements.nThe proposed control-based approach not only eliminates the relative probe acceleration effect with no neednto calibrate the parameters involved, but it also reduces the hydrodynamic force effect significantly when thenforce load rate becomes high. We further hypothesize that, by using the proposed control-based approach, thenrate-dependent elastic modulus of live human epithelial cells under different stress conditions can be reliablynquantified to predict the elasticity evolution of cellmembranes, and hence can be used to predict cellular behaviors.nBy implementing the proposed approach, the elastic modulus of HeLa cells before and after the stress processnwere quantified as the force load rate was changed over three orders of magnitude from 0.1 to 100 Hz, where thenamplitude of the applied force and the indentation were at 0.4–2 nN and 250–450 nm, respectively. The measurednelastic modulus of HeLa cells showed a clear power-law dependence on the load rate, both before and after thenstress process. Moreover, the elastic modulus of HeLa cells was substantially reduced by two to five times due tonthe stress process. Thus, our measurements demonstrate that the control-based protocol is effective in quantifyingnand characterizing the evolution of nanomechanical properties during the stress process of live cells.