  Actin is a key component of the cytoskeleton, which plays central roles in
cell motility, division, growth, and tensile strength. To enable this wide
range of transient mechanical processes and properties, networks of actin
filaments continuously disassemble and reassemble via active
de/re-polymerization. However, the question remains as to how
de/re-polymerization kinetics of individual actin filaments translate to
time-varying mechanics of dis/re-assembling networks. To address this question,
to ultimately elucidate the molecular mechanisms that enable cells to exhibit a
myriad of transitory mechanical properties, we couple time-resolved active
microrheology with microfluidics to measure the time-varying viscoelastic
moduli of entangled and crosslinked actin networks during chemically-triggered
network disassembly and reassembly. We also develop a corresponding
mathematical model that relates the time-evolution of filament length to the
resulting time-dependent storage moduli of evolving networks. During
disassembly, we find that the moduli exhibit two distinct exponential decays,
with experimental time constants of $\sim$169 and $\sim$47 min, which we show
arises from a phase transition from a rigid percolated network to a non-rigid
regime occurring after $\sim$90 min. During reassembly, measured moduli exhibit
linear increase with time for $\sim$90 min, after which steady-state values are
achieved. Our theoretical model shows that reassembly mechanics are dominated
by elongation kinetics, and that elongation rates are much slower than has been
recently assumed. Our measurements shed much needed light onto how
polymerization kinetics map to time-varying mechanics of actively evolving
cytoskeleton networks, and provide a powerful platform for studying other
active and dynamic systems currently under intense investigation.
