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Shape regulation generates elastic interaction between active force dipoles
Roman Golkov , Yair Shokef
School of Mechanical Engineering, Tel Aviv University
[email protected]
[email protected]
Recently published experimental results show that the rigidity of the extracellular matrix influences the interactions between living cells. On very soft gels (500 Pa) cells tend to touch and remain in contact, while on very stiff gels (33KPa) cells contact and migrate away from each other. We study a coarse grained model in which cells are represented by spherical force dipoles applying contractile forces or displacements on their surrounding linearlyelastic infinite medium The elastic interaction energy vanishes for dipoles of any shape that apply isotropic forces or displacements. In order to identify an interaction mechanism we distinguish between "passive" and "active'' force dipoles. In contrast to "passive" dipoles that apply constant isotropic forces or displacements on their surface, "active" dipoles alter the applied forces and displacements in accordance to the changes in their mechanical environment. We consider such regulatory behavior in which cells tend to preserve their spherical shape. Our general analytical solution for the case of two "active" spherical force dipoles includes terms that regulate the volume and others that regulate the rigid body motion of the dipoles due to the interaction. We may include or exclude these two terms from the solution and thus four different scenarios of mechanical regulation may be considered. We find that regulation of the cell volume affects the sign of interaction energyand that regulation of cell position determines the exponent in the algebraic decay of the interaction energy with the separation between cells. Namely when rigid body motion of the cells is actively regulated the exponent is − 4, and when it is not, the exponent is − 6. This is similar to van der Waals interactions between induced electric dipoles.