Many essential phenomena in biology involve changes in cell shape. Cell deformation occurs in response to physical forces either coming from the external environment or intracellularly generated. In most tests of cell rheology, an external constraint is usually superimposed on an already mechanically active cell, thus the measurements may reflect both active motion and passive viscoelastic deformation. To show that active and passive processes could be distinguished on a time scale basis, we designed a novel piezo-controlled micromanipulation system to impose dynamic mechanical deformations on individual cells. Chick fibroblasts were seized between two glass microplates; one of the plates, more flexible, served as a sensor of the applied force. Controlled amounts of unidirectional compression and traction in the range of 10(−8)-10(−7) N were applied, using either step functions or sinusoidal signals at chosen frequencies. These tests allowed identification of three time scale dependent regimes. (1) A dominant elastic response, characterized by a linear stress-strain relationship, was especially apparent at short times (seconds); (2) A viscous behavior, characterized by force relaxation and irreversible cell deformation, was noticeable at intermediate times (minutes). Data from traction and oscillatory excitation tests were well fitted by a three-element Kelvin viscoelastic model, allowing the calculation of two elastic moduli in the range of 600–1,000 N/m2 and an apparent viscosity of about 10(4) Pa.s. (3) A contractile regime, in which actin-dependent traction forces were developed in response to uniaxial load was apparent at longer times (several tens of minutes). These forces were in the order of 4 × 10(−8) N above viscous relaxation. Thus we could distinguish, on a time scale basis, the specific contributions of passive viscoelasticity and active traction, and evaluate their mechanical characteristics within one experiment on a single cell.

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