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 interacti ons lead to bulk deformation of the sparse network of long, highly ent angled collagen fibrils and syneresis of the interstitial aqueous solu tion. We have used the confined compression test in conjunction with a biphasic theory to characterize collagen gel mechanics. A finite elem ent 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 frictional drag. Using DASOPT, a differential-algebraic eq uation solver coupled with an optimizing algorithm, the aggregate modu lus for the collagen gel was estimated as 6.32 Pa, its viscosity as 6. 6 x 10(4) Pa s, and its interphase drag coefficient as 6.4 x 10(9) Pa s m(-2) in long-time (5 h) creep. Analysis of short-time (2 min) const ant strain rate tests gave a much higher modulus (318.3 Pa), indicatin g 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 characteri zed in shear based on creep and dynamic testing. While Maxwell fluid b ehavior of the collagen network is exhibited in shear as in compressio n, the modulus measured in shear was larger. This is hypothesized to b e due to microstructural properties of the network. Furthermore, param eter estimates based on the constant strain rate data were used to pre dict accurately the stress response to sinusoidal strain up to 15% str ain, defining the linear viscoelastic limit in compression. (C) 1997 T he Society of Rheology.