Viscoelasticity of entangled actin networks studied by long-pulse magnetic bead microrheometry

Abstract

We studied the viscoelastic response of entangled actin networks using embedded microbeads driven by force pulses with amplitudes in the range from 3 to 120 pN and durations up to 60 s. We distinguished three regimes in the time dependence of the compliance J(t) of the network. These were characterized by specific power laws J(t) approximately t(alpha)(i) (i=1, 2, 3). In the short-time regime (i=1), we observed the exponent alpha1 approximately 0.75. In the long-time regime (i=3), we find that alpha3 approximately 1. For the intermediate-time interval (i=2), we observed a novel dynamic regime: for all actin concentrations and all applied forces, it was characterized by the exponent alpha3 approximately 0.5. In both regimes i=2 and i=3, the compliance depended upon the actin concentration c, such as J approximately c(-gamma)(i) with gamma2 approximately 1.1 and gamma 3 approximately 1.4. Using these results, we calculated the shear modulus in the frequency domain and found that the intermediate-time regime in the t domain corresponds to its plateau behavior.

Common variation in the C-terminal region of the fibrinogen β-chain: effects on fibrin structure, fibrinolysis and clot rigidity

“…A custom-built magnetic tweezer device was used to exert 100 pN force pulses of 5 seconds' duration every 30 seconds for either 60 or 180 minutes. This technique has been applied previously to actin networks, 20 where the particle is free to flow through the sample. In the case of fibrin, however, the particle is restricted by the fibrous network of the fibrin sample.…”

Entanglement Density Tunes Microscale Nonlinear Response of Entangled Actin

“…20 This unique micro-viscosity scaling is suggested to arise from a steady-state build up of filaments in front of the probe and depletion behind the probe, rather than shear friction dissipation as in macrorheology measurements. 20 In contrast, for the higher concentrations, the viscosity not only deviates from the 7/5 scaling, but also decreases with increasing speed, indicating the onset of shear-thinning. Namely, viscosity is no longer an intrinsic property of the network, rather the network viscosity is rate-dependent, becoming less viscous with increasing speed.…”

“…Several macro-and microrheology studies have reported agreement with the c 7/5 scaling, 23,34,44 and recent microrheology measurements of the microscale creep compliance of entangled actin reported that viscosity scales as η ∼ c 7/5 . 20 On the other hand, other recent microrheology measurements 8,44,45 reported scaling of G ∼ c while still others [46][47][48][49] showed G ∼ c 11/5 and G ∼ c 9/16 . Further, the majority of experimental and theoretical studies have reported stress softening of entangled actin rather than stiffening in the nonlinear regime, understood to be due to the available non-affine bending modes not present in crosslinked or rigid rod networks.…”

“…[15][16][17] Nonetheless, flows induced by microrheology are in fact non-uniform, and can lead to polymer buildup at the leading edge and depletion at the trailing edge of a moving probe leading to unique microscale visoelastic regimes not present at the macroscale, 18,19 leading to unique microscale viscoelastic regimes not present at the macroscale. 19,20 However, it is these discrepancies between the two techniques that can actually shed light on the microscale structures of biopolymer networks and varying lengthscale-dependent mechanical regimes. 18,21 Despite these technological advances, most of the previous rheological studies on actin networks have focused on the near-equilibrium linear regime (accessible to passive microrheology techniques) and mechanics of cross-linked networks.…”

Altered Actin Centripetal Retrograde Flow in Physically Restricted Immunological Synapses

“…We find that actin flow velocity distribution is significantly decreased in proximity of confined TCR clusters and then recovers to normal levels after traversing. The localized reduction of actin flow velocity by TCR clusters would be expected from a dissipative coupling [18] between TCR clusters and the viscoelastic actin network [22], [23], [24], [25], [26]. In contrast, a more static association between TCR and actin, as would be predicted if strong TCR-adapter-actin binding interactions existed, would be expected to result in trapped actin at fixed TCR clusters and flow patterns around the fixed obstacles.…”