Measuring the viscous and elastic properties of single cells using video particle tracking microrheology

We present a simple and \emph{non-invasive} experimental procedure to measure the linear viscoelastic properties of cells by passive video particle tracking microrheology. In order to do this, a generalised Langevin equation is adopted to relate the time-dependent thermal fluctuations of a bead, chemically bound to the cell's \emph{exterior}, to the frequency-dependent viscoelastic moduli of the cell. It is shown that these moduli are related to the cell's cytoskeletal structure, which in this work is changed by varying the solution osmolarity from iso- to hypo-osmotic conditions. At high frequencies, the viscoelastic moduli frequency dependence changes from $\propto \omega^{3/4}$ found in iso-osmotic solutions to $\propto \omega^{1/2}$ in hypo--osmotic solutions; the first situation is typical of bending modes in isotropic \textit{in vitro} reconstituted F--actin networks, and the second could indicate that the restructured cytoskeleton behaves as a gel with "\textit{dangling branches}". The insights gained from this form of rheological analysis could prove to be a valuable addition to studies that address cellular physiology and pathology.