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Other drugs commonly applied to alter the cytoskeleton are latrunculin, jasplakinolide, blebbistatin, and ML.Their efcient functioning is based on their ability to pass the cell membrane.This renders them poisonous at high doses but generally makes their use simple at lower doses.The drugs are widely used to investigate cell functions such as cell migration. One major drawback of most cytoskeletal drugs is the unspecic side effects in living cells because they might interfere with a whole set of signaling pathways that are not yet known.Modern genetic engineering provides an alternative way of manipulating cellular systems and in principle allows to specically add or remove single cytoskeleton components.RNA interference, for instance, hinders the expression of certain proteins and is, therefore, able to more specically target the expression of single cytoskeletal elements.A remaining difculty in cells is to overcome possible redundancy effects which is a characteristic and inalienable property of life.Nevertheless, these genetic approaches would allow more controlled and more versatile parameter changes, but they still have not found their way into the biological physicists standard repertoire.Differing cell responses can be expected already from cell shape variations between adherent and suspended state, as well as between resting and migrating cells.Moreover, the different rheological approaches either probe local cellular features or entire cells.Figure shows typical measurement curves for a selection of four different methods.With the optical stretcher, it is possible to perform a variety of pulling experiments; most prominently a step stress experiment where a constant stress is applied and a creep curve of the cellular deformation is monitored.Cells are held at trap power for s, then stretched with. W for sand subsequently monitored at trap power for another s.Comparison of different archetypes of response to applied step stress.Viscoelastic behavior is characterized by combining both elastic and viscous properties.After the stretch, a fast recovery to a remaining deformation is seen in comparison to its Nebivolol hydrochloride original state.This behavior was rst interpreted in terms of a simple springdashpot model and a single relaxation time. From these experiments, a complex elastic a broad frequency range from modulus g T gcan be derived with a complex torque T being torque per bead volume and the complex bead displacement d. From g, the traditional storage modulus G and nite element calculations. Since both moduli obey the same powerlaw exponent, their ratio remains constant over frequency.This behavior is known as structural damping or simply powerlaw behavior and was seen before in biological tissue. The frequency dependence of the storage modulus G was determined for the human alveolar epithelial A cell line at different oscillation frequencies.This curve form resembles data measured with magnetic twisting rheology.Figure shows mean square displacement curves obtained with the passive microrheology.The displacement curves for single particles display a linear relationship between time and mean square displacement as one would expect for ordinary diffusion.Crosscorrelating the motion of two particles, however, reveals superdiffusion behavior with an exponent greater than, indicating the Sofosbuvir presence of nonthermal uctuations.In this respect, the large quantitative spread of cellular stiffness is already a very important and robust nding which indicates the ability of single cells to easily vary with respect to their own mechanical properties.

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