Rheology of reconstituted type I collagen gel in confined compression

Collagen gels are used extensively for studying cell-matrix mechanical interactions and for making tissue equivalents, where these interactions lead to bulk deformation of the sparse network of long, highly entangled collagen fibrils and syneresis of the interstitial aqueous solution. We have used the confined compression test in conjunction with a biphasic theory to characterize collagen gel mechanics. A finite element method model based on our biphasic theory was used to analyze the experimental results. The results are qualitatively consistent with a viscoelastic collagen network, an inviscid interstitial solution, and significant factional drag. Using DASOPT, a differential-algebraic equation solver coupled with an optimizing algorithm, the aggregate modulus for the collagen gel was estimated as 6.32 Pa, its viscosity as 6.6 × 10⁴ Pa s, and its interphase drag coefficient as 6.4 × 10 Pa s m^-2 in long-time (5 h) creep. Analysis of short-time (2 min) constant strain rate tests gave a much higher modulus (318.3 Pa), indicating processes that generate high resistance at short time but relax too quickly to be significant on a longer time scale. This indication of a relaxation spectrum in compression is consistent with that characterized in shear based on creep and dynamic testing. While Maxwell fluid behavior of the collagen network is exhibited in shear as in compression, the modulus measured in shear was larger. This is hypothesized to be due to microstructural properties of the network. Furthermore, parameter estimates based on the constant strain rate data were used to predict accurately the stress response to sinusoidal strain up to 15% strain, defining the linear viscoelastic limit in compression.