Viscoelastic properties of self-assembled type I collagen fibers: molecular basis of elastic and viscous behaviors.

We have studied the strain rate dependence of incremental stress-strain curves of self-assembled type I collagen fibers in an effort to understand the molecular phenomena that contribute to the macroscopic mechanical behavior of tendons. Results of viscoelastic tests at strain rates between 10% and 1000% per min suggest that the slope of the elastic stress-strain curve is to a first approximation independent of strain rate while the slope of the viscous stress-strain curve increases with increased strain rate. After correction of the slope of the viscous stress-strain curve for the changes in strain rate, it is observed that the apparent viscosity decreases with increased strain rate. It is concluded that the approximate strain rate independence of the elastic spring constant of collagen is consistent with the spring-like behavior of the 12 flexible regions that make up the collagen D-period. These regions are poor in the rigid amino acid residues proline and hydroxyproline. In contrast, the thixotropy of collagen is consistent with the slippage of subfibrillar subunits during tensile deformation. It is hypothesized that at high strain rates subfibrillar subunits appear to "hydroplane" by each other on a layer of loosely bound water.