Results on the rheological behavior of novel CHO cell suspensions in a large range of concentrations are reported. The concentration-dependent yield stress and elastic plateau modulus are formalized in the context of fractal aggregates under shear, and quite different exponents are found as compared to the case of red blood cell suspensions. This is explained in terms of intrinsic microscopic parameters such as the cell-cell adhesion energy and cell elasticity but also the cell's individual dynamic properties, found to correlate well with viscoelastic data at large concentrations (φ ≥ 0.5).The rheology of complex fluids has been studied extensively over the past decades [1] and has revealed very intriguing behaviors, in particular properties of suspensions, either micronic or colloidal, are still a subject of interest [2,3,4,5]. Classical behaviors of suspensions reveal shear-thinning effects usually, but other unusual ones like shear-thickening [6] (i.e. viscosity increase with shear rate) or yield stress have been observed [2,5]. The yield stress is the critical value of the shear stress needed to induce flow for a given fluid. It is closely related to the internal structure of the fluid therefore its ability to form (or break) particle clusters under flow. In this respect many studies have focused on solid sphere suspensions.On the other hand, there are much less works dedicated to suspensions of deformable particles, such as biological cell suspensions. The main works can be found in the field of blood rheology. Suspensions of Red Blood Cells (RBC) within plasma were first investigated by Chien [7,8] and revealed a shear-thinning behavior, but a more detailed inspection of the viscosity-shear rate diagrams showed that at low shear rates, the stress level is close to a constant σ s (Pa), called the yield stress. The well-known Casson's model [9], √ σ = √ σ s + √ µγ, relating the shear stress σ to the shear rateγ (µ being a constant viscosity) can be used to determine the yield stress. Chien and coauthors obtained σ s for a large range of hematocrit (H), i.e. the RBC volume concentration [10]. They showed a relationship of the type σ s ∼ (H −b)3 (b being a constant hematocrit) as also observed in a recent work [11].It is still not known yet whether this type of behavior is universal, or rather it could depend on cell type, cell shape or other biological effects such as cell adhesion or cell elasticity. In particular, one proposed explanation of the yield stress in RBCs suspensions is based on the existence of "rouleaux" which build due to cell interactions and exhibit large shape aspect ratios [8] and a fractal dimension D. Therefore it is necessary to apply strong enough stresses in order to break such aggregates, in close relation with the yield stress.In this letter we propose to investigate the rheology of a new cell suspension, consisting of CHO cells (Chinese Hamster Ovary cells) in a large range of concentrations. Such cells are commonly used in biology, easy to culture, and can be genetically modified ...

Mechanical stretch plays an important role in regulating shape and orientation of the vascular endothelial cell. This morphological response to stretch is basic to angiogenesis, neovascularization, and vascular homeostasis, but mechanism remains unclear. To elucidate mechanisms, we used cell mapping rheometry to measure traction forces in primary human umbilical vein endothelial cells subjected to periodic uniaxial stretches. Onset of periodic stretch of 10% strain amplitude caused a fluidization response typified by attenuation of traction forces almost to zero. As periodic stretch continued, the prompt fluidization response was followed by a slow resolidification response typified by recovery of the traction forces, but now aligned along the axis perpendicular to the imposed stretch. Reorientation of the cell body lagged reorientation of the traction forces, however. Together, these observations demonstrate that cellular reorientation in response to periodic stretch is preceded by traction attenuation by means of cytoskeletal fluidization and subsequent traction recovery transverse to the stretch direction by means of cytoskeletal resolidification.

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