Statistics of Single Cell Mechanics Investigated by Atomic Force Microscopy
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Statistics of Single Cell Mechanics Investigated by Atomic Force Microscopy Shinichiro Hiratsuka, Yusuke Mizutani, PingGen Cai, Masahiro Tsuchiya, Hiroshi Tokumoto and Koichi Kawahara, Takaharu Okajima* Graduate School of Information Science and Technology, Hokkaido University, Sapporo, 0600814, Japan ABSTRACT We have developed the atomic force microsocpy (AFM) to measure the complex shear modulus, G*, of a large number of cells. In the AFM technique, live cells were arranged in a micro-fabricated glass substrate under the physiological conditions, and the AFM force measurement was examined in many different cells automatically. The results shown in the previous studies revealed that the frequency-dependent G* was well fitted to the so-called structural damping model, which consists of a single power-law function with a Newtonian viscous effect. However, the detail relationship has not been understood. The aim of this study was to verify the relationship between the storage and loss moduli. As results, we found that the relation between the hysteresivity (the ratio of the storage and loss moduli) and the power-law exponent was in good agreement with the structural damping model, and the result was the same as that observed in magnetic twisting cytometry (MTC), in which cells were cultured on flat substrates. This result indicated that the AFM technique presented here becomes a useful technique for precisely measuring the statistical behavior of single cell rheology. INTRODUCTION Rheological properties of living cells are important to understand various cell functions. Several techniques for measuring cell rheology at a single cell level have been developed. Among the techniques, magnetic twisting cytometry (MTC) [1, 2] and atomic force microscopy (AFM) [3, 4] are active methods based on the observation of deformation of the cell surface in response to an applied force. AFM was employed to measure single cell rheology in frequency [3, 4] and time domains [5-7]. It has the advantage of measuring the mechanical properties of cells at any region of the surface by controlling the position of AFM probe without cell surface modification. Moreover, the contact geometry between the AFM probe and cell surface is defined with a contact elastic model such as a Hertz model [9, 19], in which the cells are assumed to be homogeneous with flat surface [9], and controlled by changing the indentation of the probe to the surface. Cells exhibit not only temporal fluctuations but also individual differences in the same surrounding conditions. Therefore, statistical estimation is crucial in quantitatively understanding single cell rheology. Recently, we developed a new method of combining AFM force measurements with a microarray technique [8, 9]. The method involves placing cultured cells in the wells of a microarray. Subsequently, the force measurements of the cells are automatically examined at the centers of the wells without confirming the positions of all the cells. We observed that the distribution of the complex shear
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