Mechanical and Civil Engineering Seminar
Abstract: The physiology of walled cells is dramatically different from that of human cells, but the biomechanics of walled cells are far less studied. The presence of a strong cell wall in most bacterial, fungal, and plant cells allows them to withstand large hydrostatic pressures in the cytoplasm, called turgor. Turgor pressure conflates the mechanics of a number of different subcellular components and complicates the mechanical characterization of the cell. The reported material properties of walled cells in the literature span orders of magnitude. In this dissertation, I explore and introduce new models for single cells in indentation experiments to investigate the multiscale mechanics of walled cells in compression.
The subcellular components investigated range in size from a few nanometers to several micrometers, and the experimental forces applied to the cells also range in scale from a few nano-Newtons to several micro-Newtons. I will present the mechanical characterization of transgenic Arabidopsis thaliana and Nicotiana tabacum. A discrete spring model is introduced to evaluate the relative stiffness contributions of the cell walls and cytoplasm. By applying a generative statistics approach, the stiffness contributions from the cell wall, cytoplasm, and the actin filaments and microtubules of the cytoskeleton are quantified. The results of the multiscale biomechanical assay and analysis of the discrete spring model support that (i) the cytoskeleton contributes significantly to the mechanics of a cell in compression, which is a newly emerging discovery in the field of plant cell biomechanics, (ii) turgor pressure dominates the mechanical response of a turgid cell in compression, and (iii) turgor pressure increases the stiffness of the cell wall through stress.
Finally, I will present a finite-element model for the nano-indentation of an Escherichia coli bacteria cell in an atomic-force microscope. A sensitivity study demonstrates that the cell wall elastic modulus and turgor pressure dominate the variance of the resulting measurement. An inverse analysis and validation study prove that an ordinary least squares regression, the most widely used parameter fitting method, cannot confidently evaluate the cell wall elasticity and turgor pressure from a single indentation measurement. However, a newly proposed objective function is shown to improve the prediction significantly.
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