Strain-enhanced stress relaxation impacts nonlinear elasticity in collagen gels

Significance The extracellular matrix is a complex assembly of structural proteins that provides physical support and biochemical signaling to cells within our tissues. One of the key structural components of the extracellular matrix is collagen, and matrices of collagen exhibit remarkable mechanical properties. Their resistance to deformation is enhanced as deformation is increased over short timescales, a behavior termed strain stiffening, yet they exhibit some characteristics of viscous fluids at longer timescales. Strikingly, we show that the strain stiffening of collagen matrices is coupled with their liquid-like behavior: at greater deformations, these matrices become stiffer but then flow more rapidly to relax this increase in stiffness. These complex mechanical behaviors are likely to be relevant to cellular interactions with these matrices. The extracellular matrix (ECM) is a complex assembly of structural proteins that provides physical support and biochemical signaling to cells in tissues. The mechanical properties of the ECM have been found to play a key role in regulating cell behaviors such as differentiation and malignancy. Gels formed from ECM protein biopolymers such as collagen or fibrin are commonly used for 3D cell culture models of tissue. One of the most striking features of these gels is that they exhibit nonlinear elasticity, undergoing strain stiffening. However, these gels are also viscoelastic and exhibit stress relaxation, with the resistance of the gel to a deformation relaxing over time. Recent studies have suggested that cells sense and respond to both nonlinear elasticity and viscoelasticity of ECM, yet little is known about the connection between nonlinear elasticity and viscoelasticity. Here, we report that, as strain is increased, not only do biopolymer gels stiffen but they also exhibit faster stress relaxation, reducing the timescale over which elastic energy is dissipated. This effect is not universal to all biological gels and is mediated through weak cross-links. Mechanistically, computational modeling and atomic force microscopy (AFM) indicate that strain-enhanced stress relaxation of collagen gels arises from force-dependent unbinding of weak bonds between collagen fibers. The broader effect of strain-enhanced stress relaxation is to rapidly diminish strain stiffening over time. These results reveal the interplay between nonlinear elasticity and viscoelasticity in collagen gels, and highlight the complexity of the ECM mechanics that are likely sensed through cellular mechanotransduction.