Probing elasticity and adhesion of live cells by atomic force microscopy indentation

Abstract

Atomic force microscopy (AFM) indentation has become an important technique for quantifying the mechanical properties of live cells at nanoscale. However, determination of cell elasticity modulus from the force-displacement curves measured in the AFM indentations is not a trivial task. The present work shows that these force-displacement curves are affected by indenter-cell adhesion force, while the use of an appropriate indentation model may provide information on the cell elasticity and the work of adhesion of the cell membrane to the surface of the AFM probes. A recently proposed indentation model (Sirghi, Rossi in Appl Phys Lett 89:243118, 2006), which accounts for the effect of the adhesion force in nanoscale indentation, is applied to the AFM indentation experiments performed on live cells with pyramidal indenters. The model considers that the indentation force equilibrates the elastic force of the cell cytoskeleton and the adhesion force of the cell membrane. It is assumed that the indenter-cell contact area and the adhesion force decrease continuously during the unloading part of the indentation (peeling model). Force-displacement curves measured in indentation experiments performed with silicon nitride AFM probes with pyramidal tips on live cells (mouse fibroblast Balb/c3T3 clone A31-1-1) in physiological medium at 37 degrees C agree well with the theoretical prediction and are used to determine the cell elasticity modulus and indenter-cell work of adhesion.

Determination of work of adhesion of biological cell under AFM bead indentation

“…Consequently, only the retraction stage, as shown in red lines in Fig. 5(a, b), can be used to facilitate a direct fitting by JKR model even though theoretically speaking both loading and unloading parts are influenced by adhesion force (Sirghi et al, 2008). It can be seen that the JKR model fits very well with most of the unloading curve data except for the beginning part.…”

“…When an AFM indenter approaches a cell, the molecules (atoms) of the two surfaces interact mainly by Van Der Waals forces and hence the adhesion force is inevitable in cell-indenter interaction. This adhesion force has been studied by many researchers (Zhang and Zhang, 2008, Sirghi et al, 2008, Maciaszek et al, 2014. Therefore, it is desirable to take cell adhesion into consideration for the determination of its mechanical properties in cell AFM indentation.…”

“…1 (a, b), respectively. In this study, the cell is firstly treated as homogeneous, incompressible and linear elastic sphere with a typical diameter of 25µm, where the pre-stress tension in cyto-membrane can be neglected (Sirghi et al, 2008) (assumption of pure elasticity of the cell will be justified in the next section). Here, the assumption of homogeneity is given and thus a global equivalent elastic model of the cell can be studied (Ladjal et al, 2009).…”

“…The previous studies have found the value of is about 500Pa by using Hertz contact theory to fit the F-D curve of an AFM indented MIN6 cell (Siamantouras et al, 2014b). The work of adhesion ∆γ ranges from 20 to 100 μJ/m 2 , and here we assume ∆γ to be 50 μJ/m 2 (Sirghi et al, 2008). A typical value for 0 is 0.5 nm while cell is treated as incompressible which means its Poisson's ratio = 0.5.…”

“…As a result, adhesion forces are inevitably associated with performance in atomic force microscopy (AFM) indentation which in turn provides a method to quantitatively study cell adhesion (Alessandrini and Facci, 2005;Maciaszek et al, 2014). The work of adhesion (regarded as magnitude of energy needed per unit area for new surface) has been used to characterize cell adhesion (Sirghi et al, 2008).…”