Microplates-based rheometer for a single living cell

We developed a new versatile micron-scale rheometer allowing us to measure the creep or the relaxation function (time analysis), as well as to determine the dynamical complex modulus (frequency analysis) of a single living cell. In this setup, a microscopic sample can be stretched or compressed uniaxially between two parallel microplates: one rigid, the other flexible. The flexible microplate is used as a nanonewton force sensor of calibrated stiffness, the force being simply proportional to the plate deflection. An original design of the microplates allows us to achieve an efficient feedback control of either strain or stress applied to the cell. Controlling the flexible plate deflection with a typical precision of less than 200 nm, we are able to apply stresses ranging from a few pascals to thousands of pascals with a precision better than 2%. The control of the flexible plate deflexion is achieved by direct imaging of the plate tip on a photosensitive detector mounted on the phototube of an inverted microscope. Thus, the detection principle is suitable to all usual microscopes and very easy to set up. Beyond the creep function, already analyzed in detail in a previous work, we report here the first measurement of the relaxation function, as well as of the storage and the loss dynamic moduli [G(')(f) and G(')(f), f ranging from 0.02 to 10 Hz] for an isolated living cell. Eventually, the rheometer we built is not limited to cell stretching. It should also be a powerful tool to study the rheology of micron sized samples such as microgels or vesicles, as well as to perform shear experiments. (c) 2006 American Institute of Physics.