| Summary: | Single cell biomechanics is concerned with the viscoelastic properties of biological organisms on the length scales of whole cells and the macromolecules that comprise them (tens of microns to nanometres). This is an area of increasing research interest, with biomechanics being found to affect many healthy and diseased states of the cell. Many cell types are sensitive to the mechanical properties of their surroundings, which affects not only their own mechanical properties, but also such diverse things as mitotic cell division and stem cell differentiation. In probing cell mechanics, the cytoskeleton is of particular interest due to its multifaceted role; it is a prestressed network of proteinaceous filaments and cross-linkers which has been described as the source of order in the cell. The cytoskeleton is crucial to mechanosensing, force transduction, cell motility and division, and to regulation of the mechanical properties of cells. Many diseases are pathologies of the cytoskeleton, and it is a common target for anti-cancer drugs.
Here are included novel measurements using Brillouin light scattering including those of live cells demonstrating the potential for live cell study. Finally, the results from different techniques are compared, contrasted and discussed. Actin disruption in live cells by 1 μM latrunculin B was found by external passive microrheology to reduce geometric stiffness by 4 N/m after 40 minutes (median from 11 cells); a time course experiment with phonon microscopy found longitudinal elasticity reduced by 3.6% after 26 minutes with continued slow softening over 2 hours. To my knowledge, no prior publications report a time course of cell elasticity during cytoskeletal disruption. Microrheology experiments without drug treatment revealed cells stiffening 2 N/m and while cytoskeletal active strain rate decreased, the stress rate increased; this is the first such observation of changes in cell stress rate.
This thesis is concerned with the measurement of cellular viscoelasticity, and bringing together information from two measurement techniques which probe vastly different aspects of cell mechanics. The bulk of the thesis focusses on microrheology of living cells, with an analogy developed between a bead bound to the cell and an optically trapped bead: first a theoretical framework is built for data interpretation; next microrheological analyses are developed for measurements with optical tweezers; then results obtained from non-invasive measurements of live cells are presented and discussed. Phonon microscopy, based upon picosecond laser ultrasound, is detailed and applied to cell measurements; study of cytoskeletal dynamics within live cells is demonstrated.
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