Significance: Collective cell motion is very important in many biological processes such as wound healing, embryogenesis, or cancer progression. Nevertheless, it is not clear which parameters control the transition from freely moving single cells to collective jammed motion. In this article, we uncover complex dynamics as a cell monolayer ages, where cell motion is shown to gradually slow down with time, while the distance over which cell displacements are correlated first increases drastically and then decreases. This change of behavior is not controlled by cell density but rather by a maturation of the cell−cell and cell−substrate contacts. By comparing experiments, analytic model, and detailed particle-based simulations, we shed light on this biological amorphous solidification process.

The remarkable deformability of the human red blood cell (RBC) results from the coupled dynamic response of the phospholipid bilayer and the spectrin molecular network. Here we present quantitative connections between spectrin morphology and membrane fluctuations of human RBCs by using dynamic full-field laser interferometry techniques. We present conclusive evidence that the presence of adenosine 5′-triphosphate (ATP) facilitates non-equilibrium dynamic fluctuations in the RBC membrane that are highly correlated with the biconcave shape of RBCs. Spatial analysis of the fluctuations reveals that these non-equilibrium membrane vibrations are enhanced at the scale of spectrin mesh size. Our results indicate that the dynamic remodeling of the coupled membranes powered by ATP results in non-equilibrium membrane fluctuations manifesting from both metabolic and thermal energies and also maintains the biconcave shape of RBCs.

The biological function of transmembrane proteins is closely related to their insertion, which has most often been studied through their lateral mobility. For >30 years, it has been thought that hardly any information on the size of the diffusing object can be extracted from such experiments. Indeed, the hydrodynamic model developed by Saffman and Delbrück predicts a weak, logarithmic dependence of the diffusion coefficient D with the radius R of the protein. Despite widespread use, its validity has never been thoroughly investigated. To check this model, we measured the diffusion coefficients of various peptides and transmembrane proteins, incorporated into giant unilamellar vesicles of 1-stearoyl-2-oleoyl- sn -glycero-3-phosphocholine (SOPC) or in model bilayers of tunable thickness. We show in this work that, for several integral proteins spanning a large range of sizes, the diffusion coefficient is strongly linked to the protein dimensions. A heuristic model results in a Stokes-like expression for D , ( D ∝ 1/ R ), which fits literature data as well as ours. Diffusion measurement is then a fast and fruitful method; it allows determining the oligomerization degree of proteins or studying lipid–protein and protein–protein interactions within bilayers.

Combining microcontact printing with particle image velocimetry, live cell imaging and numerical simulations, we have studied the role of cell density, intercel