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.
Highlights
Understanding the mechanics of cells has become increasingly important, since many cellular processes have been found to be regulated by, or linked to changes in the mechanical properties of the cell
Cells Respond Elastic at Small Deformations and Viscous at Larger Deformations We first investigated the cellular response at different forces by performing indentation measurements with a 1.98 mm diameter bead that was attached to an atomic force microscopy (AFM) cantilever (Fig. 1A)
To study the cell mechanical responses at very small deformations we constructed a vertically operating optical trap that allowed the application of forces of less than 10 pN, which will minimally disturb the physiological condition of the cell
Summary
Understanding the mechanics of cells has become increasingly important, since many cellular processes have been found to be regulated by, or linked to changes in the mechanical properties of the cell. Distinct mechanical properties have been measured for different cell types, which can be related to their specific roles in a tissue [3]. This relation can be employed to distinguish for example cancer cells from their healthy counterparts [4,5]. Cells respond to the composition and stiffness of the surface which they are cultured on, and show a reduced stiffness when grown on soft substrates [6,7] These findings indicate that both mechanical and biochemical signals act in a concerted way to define the cellular response upon stimuli
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