Cell Visco-Elasticity Measured with AFM and Optical Trapping at Sub-Micrometer Deformations

Abstract

The measurement of the elastic properties of cells is widely used as an indicator for cellular changes during differentiation, upon drug treatment, or resulting from the interaction with the supporting matrix. Elasticity is routinely quantified by indenting the cell with a probe of an AFM while applying nano-Newton forces. Because the resulting deformations are in the micrometer range, the measurements will be affected by the finite thickness of the cell, viscous effects and even cell damage induced by the experiment itself. Here, we have analyzed the response of single 3T3 fibroblasts that were indented with a micrometer-sized bead attached to an AFM cantilever at forces from 30–600 pN, resulting in indentations ranging from 0.2 to 1.2 micrometer. To investigate the cellular response at lower forces up to 10 pN, we developed an optical trap to indent the cell in vertical direction, normal to the plane of the coverslip. Deformations of up to two hundred nanometers achieved at forces of up to 30 pN showed a reversible, thus truly elastic response that was independent on the rate of deformation. We found that at such small deformations, the elastic modulus of 100 Pa is largely determined by the presence of the actin cortex. At higher indentations, viscous effects led to an increase of the apparent elastic modulus. This viscous contribution that followed a weak power law, increased at larger cell indentations. Both AFM and optical trapping indentation experiments give consistent results for the cell elasticity. Optical trapping has the benefit of a lower force noise, which allows a more accurate determination of the absolute indentation. The combination of both techniques allows the investigation of single cells at small and large indentations and enables the separation of their viscous and elastic components.

Dynamics Study of the Deformation of Red Blood Cell by Optical Tweezers

“…Therefore, the graph of the Figure 4(b) fits well with the Equation (15). It may be noted that there is no difference between the conventional diagram and rational diagram (Figure 4(a)) in the elastic range since the deformation is low.…”

On the role of the actin cytoskeleton and nucleus in the biomechanical response of spread cells

“…interpret MA experimental data [3,14]. However, such material models fail to consider the underlying biomechanisms of cell response to mechanical stimuli and a number of studies have demonstrated that passive material model parameters must be artificially altered if any experimental parameter is altered [15,16], i.e.…”

The influence of lateral forces on the cell stiffness measurement by optical tweezers vertical indentation

“…The force-indentation (F-Id) curve is then used to determine the elastic moduli, using the Hertz model [26] :…”

“…This highlights the fact that the cells are sensitive to the applied forces and the loading rates (measured in N/s) [24] because of their viscoelastic, inhomogeneous and anisotropic nature. Therefore, for characterization of different cell lines it is important to complement AFM vertical indentation measurements by OT measurements following the same scheme [25][26][27] .…”

“…We introduce here, the concept of total force which comprises both axial and lateral force components. Axial indentation in OT can be monitored using video tracking [28] or a photo diode QPD [26] . In both of these approaches, a trap bead is moved against the stationary cell and only the axial component of the force is monitored.…”