Mapping local interaction forces by AFM
Nanomechanics of cells
On a soft sample such as a cell the force curve looks different than the one presented in figure 1 which has been taken on a stiff sample. In contact the tip will not directly follow the movement of the sample, because it intends the sample somewhat. It turns out that the curve will be non-linear in fact. The difference between a linear force curve taken on a stiff sample and a curved force curve taken on a soft sample will be the sample indentation. By modelling the geometry and material properties of the sample and the tip, we can calculate the local elastic or Young's modulus. The most simple model is the so called Hertz-model (9) in its extension by Sneddon (10) which is approriate in many cases, but fails to analyze the data under some circumstances. Cases in which the Hertz model fails are very thin parts of the sample such that the underlying stiff substrate is "felt" by the indenting AFM tip (11) or highly structured sample areas such as stress fibres in cells (12).