Modelling of cross-linked fiber networks: from micro to macro models

In this talk, we will study a complex network composed of fibers having the ability to cross-link or unlink each other and to align with each other at the cross links. This model aims to describe networks of collagen fibers in a fibrous tissue. We will first present a microscopic model which features the following basic rules: We assume the existence of a fiber unit element (or monomer) modeled as a line segment of fixed length. We suppose that two fiber elements that cross each other may form a link, thereby creating a longer fiber. The fibers have the ability to branch off and to achieve complex network topologies. We include fiber resistance to bending by assuming the existence of a torque which, in the absence of any other force, makes the two linked fiber elements align with each other. Fibers are also subject to random positional and orientational noise and to external positional and orientational potential forces. Finally, cross-links may also be removed to model possible fiber breakage or depolymerization. We then formally derive a kinetic model for the fiber and cross-links distribution functions, and consider the fast linking/unlinking regime in which the model can be reduced to the fiber distribution function only. Then, we investigate its diffusion limit. The resulting macroscopic model consists of a system of nonlinear diffusion equations for the fiber density and mean orientation. In the case of a homogeneous fiber density, we show that the model is elliptic. We finally will present simulation results which show the good correspondence between the microscopic and the macroscopic models.