Spherical cellular aggregates are in vitro systems to study the physical and biophysical properties of tissues. We present a novel approach to characterize the mechanical properties of cellular aggregates using micropipette aspiration technique. We observe an aspiration in two distinct regimes, a fast elastic deformation followed by a viscous flow. We develop a model based on this viscoelastic behavior to deduce the surface tension, viscosity, and elastic modulus. A major result is the increase of the surface tension with the applied force, interpreted as an effect of cellular mechanosensing.Embryonic morphogenesis, wound healing, cancer growth and metastasis are a few examples where the physical laws play an important role along with genetic cues in the functioning of a tissue. An aggregate of living cells, used as a model tissue, behaves like a viscoelastic liquid. Spreading and sorting are signatures of liquid-like behavior of embryonic tissues [1,2]. Moreover, cellular aggregates in solution round up to form "spheroids" in order to minimize their surface energy, similar to oil drops in water. This is a manifestation of surface tension, which has been related to intercellular adhesion energy [3]. In the past, the simple analogy between liquids and tissues has lead to valuable findings about the mechanics of embryonic mutual envelopment [4], tissue spreading [5], and cancer propagation [6]. A knowledge of the surface tension of tissues has also been essential for organ printing in tissue engineering [7].To measure the surface tension of cellular aggregates and investigate the role of surface tension in cell sorting, Steinberg and coworkers [2] introduced the parallel plate compression apparatus, which has since been used by other groups [8,9]. In this method, an aggregate is subjected to an imposed deformation and the surface tension is inferred from the relaxation force, while the viscosity of the tissue is obtained from the shape relaxation [10]. Difficulties in the evaluation of the principal radii of a compressed aggregate and the contact angle between the aggregate and the plate make this technique rather delicate. Deformation of aggregates under centrifugal forces is an alternative way that has been used to classify aggregates of various cell types [11]. Recently this technique has been combined with Axisymmetric drop shape analysis (ASDA) for measuring the surface tension of embryonic tissue [12].In this letter, we propose the use of micropipette aspiration technique to study the surface tension and the mechanical properties of cellular aggregates. This technique has previously been used to evaluate the viscoelastic properties of single cells [13,14] and the stiffness of * Electronic address: brochard@curie.fr tissues [15][16][17] at small deformations. For a Newtonian fluid, the aspiration dynamics is governed by the Washburn law, L(t) ∼ t 1/2 , where L(t) is the advancement of the liquid inside the pipette [18]. For a tissue, a completely different behavior is observed due to its viscoelastic properti...

To investigate the link between discrete small-scale and continuous large scale mechanical properties of a foam, we observe its two-dimensional flow in a channel, around an elliptical obstacle. We measure the drag, lift, and torque acting on the ellipse versus the angle between its major axis and the flow direction. The drag increases with the spanwise dimension, in marked contrast with a square obstacle. The lift passes through a smooth extremum at an angle close to, but smaller than 45 degrees. The torque peaks at a significantly smaller angle 26 degrees. No existing model can reproduce the observed viscous, elastic, plastic behavior. We propose a microscopic visco-elasto-plastic model which agrees qualitatively with the data.