Molecular motors robustly drive active gels to a critically connected state

TL;DR

This study demonstrates that molecular motors can induce a critically connected, scale-free contraction in active gels across a wide range of conditions, revealing a robust mechanism for network criticality driven by motor activity.

Contribution

The paper provides a quantitative model explaining how molecular motors induce critical connectivity and contraction in active gels, highlighting the importance of network restructuring and motor activity levels.

Findings

Motors cause scale-free clustering in crosslinked actin networks.

Critical behavior occurs over a broad range of crosslink concentrations.

Low motor activity levels promote global network contraction.

Abstract

Living systems often exhibit internal driving: active, molecular processes drive nonequilibrium phenomena such as metabolism or migration. Active gels constitute a fascinating class of internally driven matter, where molecular motors exert localized stresses inside polymer networks. There is evidence that network crosslinking is required to allow motors to induce macroscopic contraction. Yet a quantitative understanding of how network connectivity enables contraction is lacking. Here we show experimentally that myosin motors contract crosslinked actin polymer networks to clusters with a scale-free size distribution. This critical behavior occurs over an unexpectedly broad range of crosslink concentrations. To understand this robustness, we develop a quantitative model of contractile networks that takes into account network restructuring: motors reduce connectivity by forcing crosslinks to unbind. Paradoxically, to coordinate global contractions, motor activity should be low. Otherwise, motors drive initially well-connected networks to a critical state where ruptures form across the entire network.