Fig. 3.

Stress relaxation is associated with a weak cross-linking, axial dependent reorganization, and rebinding of fibers to the networks. Stress relaxation tests for (A) collagen, and (B) fibrin with and without covalent cross-linking. For cross-linking, collagen was treated by GTA and fibrin containing factor XIII was used. Data are shown as mean ± SD; n = 5. (C) Results for two sequential stress relaxation tests with identical positive strains. (D) The polar distribution of angles of collagen fibers in the network initially (Left), under shear (Center), and following removal of the imposed shear (Right). (E) A comparison of the averaged orientation of collagen fibers in the network, corresponding to the conditions in D. Data are shown as mean ± SEM; n = 387–439. (F) Multiple-stepwise stress relaxation test with a positive strain followed immediately by a test with a negative strain of the same magnitude, and (G) a stress relaxation test with a positive strain followed by a test with a negative strain of the same magnitude following an equilibration time. (H) A comparison of the recovery of the elastic modulus in a negative strain stress relaxation test following a positive stress relaxation test with or without an equilibration time. G₁ and G₂ represent the initial elastic modulus starting with stress relaxation. Data are shown as mean ± SD; n = 5. *P < 0.05; **P < 0.01; ****P < 0.0001 (Student’s t test). Insets are all schematics that describe the procedure for multiple stress relaxation tests. Red lines represent the first step of stress relaxation, and blue ones represent the successive stress relaxation.