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Quantification of the properties of nonlinear media using dual-probe atomic force microscopy

Saltuk Bugra Aksu, University of Nebraska - Lincoln

Abstract

The mechanical response of soft materials like certain polymers and biological media can not be described using simple linear elastic deformation theories. These materials have a response that is more complex and often nonlinear. Measurement of the mechanical response is important, because cells for example, have a strong correlation between mechanical response and internal changes and processes occurring within the cell. In other words the state of health of the cell is related to this mechanical response. Therefore, quantification of the mechanical properties of the live cell membranes is of great scientific interest. Despite the importance, until now a nondestructive experiment to achieve this goal was not available. ^ The properties of cell membranes have been investigated using a variety of methods. Chemical analyses of cells include a great risk of changing the very properties of the living system that are being probed during the time of investigation. Mechanical techniques used to quantify the physical parameters include the micropipette technique, optical tweezers, microindentation or so-called "cell poker" technique, magnetic bead rheometry and atomic force microscopy (AFM). Among these technologies for precise and controlled mechanical loading of cells, AFM with a calibrated cantilever offers the greatest potential for gaining new insight into properties of biological samples at sub-micron scales, since AFM can provide quantitative results. However, current analyses of experimental data are often based on simple linear material models such that the results are qualitative at best. ^ In order to determine the mechanical properties of such nonlinear media, results from experiments must be accompanied with a consistent mathematical model. Therefore, a nonlinear constitutive model is used here to emulate the material response to a point load applied with an AFM tip. The main drawback of such models is that knowledge of the entire deformed shape of the sample under load is required. Currently it is not possible to acquire such information using AFM. AFM technology uses a single probe which performs only one task at a time—AFM can be used either to extract qualitatively local stiffness, to image surface topography, or to perform mechanical manipulation at the sub-micron scale. Integration of another probe to an already existing AFM would allow nanomanipulation and/or force application on the sample during topography imaging simultaneously. Thus, a new experimental technology is proposed—a dual-probe AFM. ^ In this dissertation, the technology for a dual-probe AFM is described in detail and is used on example materials. The nonlinear constitutive model is then used to analyze measured data for extracting nonlinear constitutive properties. Results show that the theoretical model used here gives reasonable and consistent results for the samples examined. The work presented here should expand the understanding of mechanical properties of nonlinear media. ^

Subject Area

Engineering, Mechanical

Recommended Citation

Aksu, Saltuk Bugra, "Quantification of the properties of nonlinear media using dual-probe atomic force microscopy" (2007). ETD collection for University of Nebraska - Lincoln. AAI3295233.
http://digitalcommons.unl.edu/dissertations/AAI3295233

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