Local Forces in Actin Networks with Gradients

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Over the last two decades, experimental results have shown the response of uniform entangled actin networks to local stresses and strains is nonlinear. One theory proposes that nonlinear stresses associated with large network strains arise from gradients in polymer density in the network that can exert a net entropic forces on micron sized beads embedded in the network. In order to better understand the physics of actin polymer gradients, we probe the theory in reverse. We use a microfluidics device and photo-uncaging of salt and ATP to generate a gradient in actin filament density. We track the motion of neutrally buoyant beads embedded in the network. We do not observe a bias in the beads motion down the gradient as suggested by the entropic force model, however we do measure an overall enhancement of the bead mobility in the gradient compared with isotropic actin networks at the same concentration. We compare these results with a dissipative particle dynamics implementation of the experiment and polymer theory.