We first describe the biomechanics of multicellular aggregates, a model system for tissues and tumors. We characterize the tissue mechanical properties (surface tension, elasticity, viscosity) by a new pipette aspiration technique. We observe aggregate reinforcement with pressure. We interpret this reinforcement as a mechanosensitive active response of the acto-myosin cortex. Such an active behavior has previously been found to cause tissue pulsation during dorsal closure of Drosophila embryo.

We then describe the spreading of aggregates on decorated glass substrates. We find both partial and complete wetting regimes. For the dynamics, we find a universal spreading law at short time, analogous to that of a viscoelastic drop. At long time, we observe, for strong substrate adhesion, a precursor film spreading around the aggregate. The transition from “liquid” to “gas state” appears also to be present in the progression of a tumor from noninvasive to metastatic, known as the epithelial-mesenchymal transition.

Finally, we describe the effect of the substrate rigidity on the phase diagram of wetting. On soft gels decorated with fibronectin and strongly cohesive aggregates, we have observed a wetting transition induced by the substrate rigidity: on ultra soft gels, below an elastic modulus Ec the aggregates do not spread, whereas above Ec we observe a precursor film expending with a diffusive law. The diffusion coefficient D(E) present a maximum for E=Em. Near Em, we observe a new phenomenon: a cell monolayer expends outwards from the aggregate I apparently under tension. In this tense monolayer, holes nucleate, and lead to a symmetry breaking as the entire aggregate starts to move in a similar fashion as a “giant” keratocyte.