We develop a generic description of active visco-elastic gels that captures elastic stresses in the medium by a tensorial dynamic variable. In nonequilibrium situations with self-gererated flows, this reactive elastic stress persists at long times in a visco-elastic system. We show that these properties of the active gels are important for self-propulsion of gels and also govern nonequilibrium gell swelling. The dynamics of active materials that we discuss here are relevant to cell mechanics and the dynamics of the cell cytoskeleton.
When tissues form by the repeated division of cells, structures with well defined morphologies emerge. The mechanisms by which shape and size are controled are still largely unknown. By combining theory with quantitative experiments, we investigate the control of tissue growth during the development of the fly wing. We observe that the graded concentration profiles of the growth factor Dpp scales with tissue size. We propose a general mechanism of growth control by Dpp in which relative increases of the Dpp signal induce cell division. Our theoretical work demonstrates that self-organized tissue growth based on this principle can lead to spatially homogeneous growth that ends at a specific size. Our theory is consistent with experiments on different tissues and mutant conditions.
Active fluids generate internal stresses and spontaneous flows, driven by force generating processes on the molecular scale. The cell cytoskeleton is an example of an active fluid in which the action of motor molecules gives rise to forces and complex dynamic behaviors. Here we consider the situation where active stress is regulated by diffusing molecular species. In this case, patterns of flow fields and concentration profiles emerge from the interplay of active stress regulators with spontaneous flow fields. The paradigm presented in this work is a generalization of the concept of Turing patterns to include effects of mechanically active processes. Such a generalization was already alluded to in Alan Turings seminal work on morphogenesis (Turing A.M., Bull Math. Biol. 1990 (1952)).