A percolation transition in multicellular fibroblast networks drives compaction of collagen disks.

The mechanical interactions between cells and the extracellular matrix (ECM) governs a wide range of physiological processes— ranging from wound closure to stem cell fate determination and embryonic development to cancer metastasis. However the role of inter-cell interactions and collective effects in modulating ECM compaction is less well understood. A common biophysical assay to understand cell-matrix mechanical interactions is to measure the compaction of ECM or synthetic substrates by the action of cellular traction forces. Collagen disks seeded with fibroblasts exhibit a sharp, phase transition-like behavior in compaction as a function of cell density switching from small to high compaction above a critical cell density. Previous models that account for the compaction due to local cellular traction forces but do not consider cell-cell interactions explicitly, fail to account for such a transition. We develop a simulation framework that explicitly simulates the underlying matrix as a mechanical fiber network and the cells as nodes that apply traction on the matrix, while also being able to connect to and exert forces on other nearby cells. We show explicitly that a percolation transition in the multicellular network as a function of increasing cell density can drive the observed phase transition in gel compaction.