Mechanics of biopolymer gels with implications for cell mechanics

Biopolymer gels comprise cross-linked network of stiff or semiflexible filaments (e.g. actin, collagen, and fibrin) and small solvent molecules. They are important constituents of both cellular cytoskeleton and extracellular matrix of tissues. Unlike most synthetic gels comprising of flexible polymers, biopolymer gels often exhibit highly nonlinear elastic responses to applied tensile or compressive forces over some small critical strain, e.g., fibrin gels stiffen at small shear strains above 10% and soften upon very small compression. The strain-stiffening nonlinearity of biopolymer gels has attracted much attention in the past decade and many studies have aimed to understand its physical origin. However, relatively less (both experimental and theoretical) attention has been paid to the elastic responses of biopolymer gels under compression. In the first part of the talk, I will report our recent theory on the compressive-softening in biopolymer gels. We propose two competing mechanisms that contribute to the elasticity of compressed biopolymer gels: (i) Stiffening, due to polymer densification by out-going solvent flow. (ii) Softening, due to the softening of single polymers after buckling. I will also address the importance of the length polydispersity of biopolymer gels. In the second part, I will explain the implications of the nonlinear elasticity of biopolymer gels for cell mechanics. I will show that the nonlinear (asymmetric) responses of biopolymer gels to both tension and compression can together significantly increase the range of force transmission and are hence critical for the long-range cell-cell and cell-matrix communication.