Stiffening of semiflexible biopolymers and cross-linked networks

TL;DR

This paper investigates the mechanical stiffening of 2D cross-linked semiflexible biopolymer networks under shear, combining analytical modeling with finite-element simulations to understand the contributions of filament mechanics and network reorganization.

Contribution

It introduces an analytical model for biopolymer network stiffening under shear, incorporating filament mechanics and network reorientation, validated by finite-element simulations.

Findings

Filament stiffening is enhanced when undulation dynamics are considered.

Stretching dominates the mechanical response of 2D filaments.

Network stiffening results from filament stiffening and topographical changes.

Abstract

We study the mechanical stiffening behavior in two-dimensional (2D) cross-linked networks of semiflexible biopolymer filaments under simple shear. Filamental constituents immersed in a fluid undergo thermally excited bending motions. Pulling out these undulations results in an increase in the axial stiffness. We analyze this stiffening behavior of 2D semiflexible filaments in detail: we first investigate the average, {static} force-extension relation by considering the initially present undulated configuration that is pulled straight under a tensile force, and compare this result with the average response in which undulation dynamics is allowed during pulling, as derived earlier by MacKintosh and coworkers. We will show that the resulting mechanical behavior is rather similar, but with the axial stiffness being a factor 2 to 4 larger in the dynamic model. Furthermore, we study the stretching contribution in case of extensible filaments and show that, for 2D filaments, the mechanical response is dominated by {enthalpic} stretching. Based on the single-filament mechanics, we develop a 2D analytical model describing the mechanical behavior of biopolymer networks under simple shear, adopting the affine deformation assumption. These results are compared with discrete, finite-element (FE) calculations of a network consisting of semiflexible filaments. The FE calculations show that local, nonaffine filament reorientations occur that induce a transition from a bending-dominated response at small strains to a stretching-dominated response at larger strains. Stiffening in biopolymer networks thus results from a combination of stiffening in individual filaments and changes in the network topography.