Fine-tuning of material properties by catch bonds.

Semiflexible polymer networks are ubiquitous in biological systems, including a scaffolding structure within cells called the actin cytoskeleton. The polymers in these networks are interconnected by transient bonds. For example, actin filaments in the cytoskeleton are physically connected via cross-linker proteins. The mechanical and kinetic properties of the cross-linkers significantly affect the rheological properties of the actin cytoskeleton. Here, we employed an agent-based model to elucidate how the force-dependent behaviors of the cross-linkers determine the material properties of passive networks without molecular motors and the force generation of active networks with molecular motors. The cross-linkers are assumed to behave either as a slip bond, whose dissociation rate increases with forces, or as a catch-slip bond, whose dissociation rate decreases with forces at low force level but increases with forces at high force level. We found that catch-slip-bond cross-linkers can simultaneously increase both the stress and the strain at the yield point. Through a systematic variation in the force dependence of the catch-slip bonds, we identified the specific parameter regimes that enable network reinforcement and enhanced extensibility simultaneously. Specifically, we found that a sufficiently large force threshold for the catch-slip transition is essential for maintaining dynamic force-bearing elements that turnover continuously-a mechanism not achievable with slip bonds. Additionally, we demonstrate that such force-dependent redistribution of the catch-slip bonds substantially enhances internal contractile forces generated by a motor in active networks. STATEMENT OF SIGNIFICANCE: Polymer networks are ubiquitous in industrial and biological systems. The polymers in these networks are often interconnected by transient bonds. The transient bonds behave as a slip bond whose dissociation rate is proportional to forces or as a catch-slip bond whose dissociation rate decreases with increased force (catch) at low force level but increases with increased force (slip) at high force level. In this study, we computationally tested different types of catch-slip bonds to define how the material properties of polymer networks are fine-tuned by each property of molecular bonds. We found that catch-slip bonds can increase both stress and strain at a yield point, which is impossible to achieve without the catch-slip bonds.