  While most chemical bonds weaken under the action of mechanical force (called
slip bond behavior), nature has developed bonds that do the opposite: their
lifetime increases as force is applied. While such catch bonds have been
studied quite extensively at the single molecule level and in adhesive
contacts, recent work has shown that they are also abundantly present as
crosslinkers in the actin cytoskeleton. However, their role and the mechanism
by which they operate in these networks have remained unclear. Here, we present
computer simulations that show how polymer networks crosslinked with either
slip or catch bonds respond to mechanical stress. Our results reveal that catch
bonding may be required to protect dynamic networks against fracture, in
particular for mobile linkers that can diffuse freely after unbinding. While
mobile slip bonds lead to networks that are very weak at high stresses, mobile
catch bonds accumulate in high stress regions and thereby stabilize cracks,
leading to a more ductile fracture behavior. This allows cells to combine
structural adaptivity at low stresses with mechanical stability at high
stresses.
