Resolving Filament Level Mechanics in Collagen Networks using Activity Microscopy

Collagen is the most abundant protein in humans and the primary component of the extracellular matrix, a meshwork of biopolymer networks, which provides structure and integrity to tissues. Its mechanical properties profoundly influence the fate of cells. The cell-matrix interaction, however, is not well understood due to a lack of experimental techniques to study the mechanical interplay between cells and their local environment. Here we introduce Activity Microscopy, a new way to visualize local network mechanics with single filament resolution. Using collagen I networks in vitro, we localize fibril positions in two-dimensional slices through the network with nanometer precision and quantify the fibrils transverse thermal fluctuations with megahertz bandwidth. Using a fibrils thermal fluctuations as an indicator for its tension, we find a heterogeneous stress distribution, where \"cold\" fibrils with small thermal fluctuations surround regions of highly fluctuating \"hot\" fibrils. We seed HeLa cells into collagen networks and quantify the anisotropy in the propagation of their forces.