Distinct Factors Influencing Cell's Elasticity
The use of the AFM to determine Young’s modulus has several limitations that, in fact, made this method to be extremely difficult in the determination of the modulus absolute value. The most important factors influencing cells elasticity can be grouped into several classes related to
(a) the applied calibration methodology linked with the uncertainties in the determination of cantilever spring constant and photodetector sensitivity;
- (b) the variability of cell-related factors such as culture conditions (buffer composition), density of cells, (if they are touching each other or not), confluence on a substrate, the number of passages, the day of measurement after the passage, etc.;
- (c) the variability of the experimental conditions provided by the AFM device, such as load speed, place of poking, number of force curves recorded at one place, location of the measurements on cell surface, the presence of the stiff substrate below the investigated cells, etc.;
- (d) the applied contact mechanics model used to describe cellular elastic properties (typically, the Hertz contact mechanics with Sneddon’s modifications and cone approximation, etc.);
- (e) the way of data analysis, especially the determination of the point of contact between the indenting AFM tip and cell surface, range of indentation depth or load force;
- (f) the physical and chemical substrate properties influencing cell behavior.
All these sources of errors may raise doubts in usefulness of the AFM technique for measurements and analysis of the mechanical properties of living cells. However, first of all, the exact knowledge of the absolute Young’s modulus is not always needed. Very often, either the relative changes or observation of the tendency can be sufficient to bring the valuable information on the state of biological material (under condition that a reference sample is measured together with the investigated ones in the same experimental conditions). It should be stressed here that multiple research papers prove that despite the various uncertainty sources mentioned above, the relation between healthy and cancerous cells is preserved. This has been shown for various cancers, including, for example, bladder , breast [54, 55], prostate [54, 56], ovarian , and thyroid  cancers.