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Bridging the molecular and hydrodynamic scales in active gels

 

In the last decade, biological systems such as the cell cortex, the mitotic spindle, or cellular tissues have been described as active gels. Hydrodynamic equations for such active materials were initially derived in the framework of nonequilibrium thermodynamics based on the seminal work of Lars Onsager. Such an approach introduces a number of unknown transport coefficients relating flows to forces that cause them, such as the viscosity relates fluid flows to pressure differences. Despite flows in biological systems are chemically regulated by proteins, the connection between their molecular kinetics and the transport coefficients of active gels remained obscure. In our paper, we bridge this gap by deriving the hydrodynamic equations of active gels from the dynamics of the crosslinker proteins of the gel. This yields explicit expressions for all the transport coefficients in terms of molecular parameters, including quantities that characterize the departure from equilibrium at the molecular scale. For instance, for actomyosin gels, we unveil a decrease of viscosity with activity (active thinning) that could explain some puzzling experimental results on cell cortex rheology. Therefore, by bridging the molecular and hydrodynamic scales, our results may shed light on the connection between macroscopic properties and underlying molecular processes in biological active gels.