Substrate effect on Dynamic Indentation Measurement of Biological Cell Properties
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Substrate effect on Dynamic Indentation Measurement of Biological Cell Properties Guoxin Cao and Namas Chandra Department of Engineering Mechanics, University of Nebraska-Lincoln, Lincoln, NE 68588 ABSTRACT Viscoelastic mechanical properties of biological cells are commonly measured using atomic force microscope (AFM) dynamic indentation method with spherical tips. Storage and loss modulii of cells are then computed from the indentation force-displacement response under dynamic loading conditions. It is shown in current numerical simulations that those modulii computed based on existing analysis can not reflect the true values due to the substrate effect. This effect can alter the indentation modulus by changing the geometric relations between the indentation displacement and the contact area. Typically, the cell modulii are significantly overestimated in the existing indentation analysis. INTRODUCTION It is believed that AFM is one of the fastest, cheapest and most convenient ways to measure the cell properties [1-9]. In this method, the biological cell attached onto the substrate is indented using the spherical indenter with either the force (or the displacement) specified. The indentation response of the cell is obtained by measuring the P-δ relationship of the indenter tip. Using this relationship, the mechanical properties of the cell can be computed. The substrate can significantly affect the cell indentation behavior especially in the thin region of the cell with deep indentation. Typically, the basic mechanism of the substrate effect is considered as the stress stiffen effect since the substrate is orders of magnitude stiffer than the cell [5, 6]. In present study, it is found that the substrate can also change the relationship between the indentation contact area and the indentation displacement in cell indentation. Due to this change, the cell modulus will be significantly overestimated based on the existing indentation analysis. In order to obtain the true cell properties, the true relationship between the contact radius and the indentation displacement is required. In present work, the effect of substrate on the contact area is identified and a new relationship between indentation displacement and contact area is determined based on the numerical simulations. THEORETICAL BACKGROUND AND COMPUTATIONAL METHODS The linear elastic quasi-static indentation behavior can be described by Hertz contact theory. For a rigid spherical indenter, the indentation force is given by P=
4 E a3 , 3 1 −ν 2 R
(1)
where a is the contact radius, R the spherical indenter tip radius, ν the Poisson’s ratio, E the elastic modulus. The schematic of spherical indentation using AFM is shown in Figure 1. If the strain is within the elastic limit when δδ can be essentially met and the substrate effect is also weak. There is a good agreement between ã and a. which shows that Hertz solution is accurate when R>>δ, without substrate effect. 1 Fitting Curve a% R = (δ R )
12
+ kδ R
Hertz Solution a R = (δ R )
Normalized C
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