The Origins of Strain Stiffening in Stiff Biopolymer Networks

Abstract

The extracellular matrix proteins fibrin and collagen form biopolymer networks which are major constituents of tissues, tendons and blood clots. Their mechanical properties are important for proper function in the body. Previous work using atomic force microscopy (AFM) has characterized the mechanical properties of individual collagen and fibrin fibers, but there is currently no model which can predict bulk material properties from its constituent properties. Here, we study fibrin and collagen networks undergoing shear on a confocal microscope and compare this to bulk rheological measurements. We track individual fiber branchpoints as function of strain. We characterize the non-affinity of the motion and show that the low strain, linear regime corresponds to highly non-affine motion while the high strain, linear regime corresponds to affine motion. We also characterize individual fiber strain as a function of overall system strain and show that linear elastic beams are sufficient to describe the overall strain stiffening response measured in rheology.