High-frequency affine mechanics and nonaffine relaxation in a model cytoskeleton

Abstract

The cytoskeleton is a network of crosslinked, semiflexible filaments, and it has been suggested that it has properties of a glassy state. Here we employ optical-trap-based microrheology to apply forces to a model cytoskeleton and measure the high-bandwidth response at an anterior point. Simulating the highly nonlinear and anisotropic stress-strain propagation assuming affinity, we found that theoretical predictions for the quasistatic response of semiflexible polymers are only realized at high frequencies inaccessible to conventional rheometers. We give a theoretical basis for determining the frequency when both affinity and quasistaticity are valid, and we discuss with experimental evidence that the relaxations at lower frequencies can be characterized by the experimentally obtained nonaffinity parameter.

Local mechanical response in semiflexible polymer networks subjected to an axisymmetric prestress

“…However, this estimate is likely too conservative as it ignores strain stiffening. Furthermore, recent experiments on vimentin networks have shown how the range of validity of these calculations can be extended to give quantitative agreement with the data, as explained in detail elsewhere [43]. Further investigation is required to rigorously delineate the limits of these predictions, possibly via microscopic numerical modeling.…”

“…6 and 7 respectively, for material parameters representative of the vimentin networks of [43]. For the smaller force, the displacement field is symmetric behind and in front of the force and indeed can be shown to obey the linear solution except near the point of force application.…”

“…For simplicity we consider here just the case of a force monopole that can be generated, e.g., by perturbing a probe particle with an optical or magnetic trap and focus attention on points directly behind and in front of the force, for which the axisymmetric theory developed here should be valid. This protocol has been recently applied to vimentin networks [43]. The methodology was outlined in Sec.…”

“…The predictions of this model have been compared to cross-linked vimentin networks driven by a colloidal particle in an optical trap [43], in particular the response curves (Fig. 9).…”

“…Interestingly, this factor was constant over the assayed range and applying the same value consistently agreed with data for independent metrics, suggesting nonaffinity may be amenable to this model with minimal modifications. Both the data and the corresponding model predictions are described in full elsewhere [43].…”