Shear-driven solidification and nonlinear elasticity in epithelial tissues.

Biological processes, from morphogenesis to tumor invasion, spontaneously generate shear stresses inside living tissue. The mechanisms that govern the transmission of mechanical forces in epithelia and the collective response of the tissue to bulk shear deformations remain, however, poorly understood. Using a minimal cell-based computational model, we investigate the constitutive relation of confluent tissues under simple shear deformation. We show that an undeformed fluid-like tissue acquires finite rigidity above a critical applied strain. This is akin to the shear-driven rigidity observed in other soft matter systems. Interestingly, shear-driven rigidity in tissue is a first-order transition and can be understood as arising from the second order critical point that governs the liquid-solid transition of the undeformed system. We further show that a solid-like tissue responds linearly only to infinitesimally small strains and rapidly switches to a nonlinear response, with substantial stress stiffening. Finally, we propose a mean-field formulation for cells under shear that offers a simple physical explanation of shear-driven rigidity and nonlinear response in a tissue.