  Cytoskeletal networks have a self-healing property where networks can repair
defects to maintain structural integrity. However, both the mechanisms and
dynamics of healing remain largely unknown. Here we report an unexplored
healing mechanism in microtubule-motor networks by active crosslinking. We
directly generate network cracks using a light-controlled microtubule-motor
system, and observe that the cracks can self-heal. Combining theory and
experiment, we find that the networks must overcome internal elastic resistance
in order to heal cracks, giving rise to a bifurcation of dynamics dependent on
the initial opening angle of the crack: the crack heals below a critical angle
and opens up at larger angles. Simulation of a continuum model reproduces the
bifurcation dynamics, revealing the importance of a boundary layer where free
motors and microtubules can actively crosslink and thereby heal the crack. We
also formulate a simple elastic-rod model that can qualitatively predict the
critical angle, which is found to be tunable by two dimensionless geometric
parameters, the ratio of the boundary layer and network width, and the aspect
ratio of the network. Our results provide a new framework for understanding
healing in cytoskeletal networks and designing self-healable biomaterials.
