Fig. 1. Adjusting self-assembly degree of collagen hydrogels modulates viscoelastic properties independent of initial elastic modulus.

(A) Schematic showing the formation of hydrogels from collagen fibers with different self-assembly degrees under various low-temperature incubation times. Drawing not to scale. (B) Confocal microscopy images of hydrated collagen hydrogels. Scale bars, 10 μm. (C) Number of collagen fibers (n = 3; P = 0.7938; data are shown as means ± SD). (D and E) Length (D) and width (E) of individual collagen fibers (n = 3; *P < 0.05). (F) Initial elastic modulus as determined by compression testing of collagen hydrogels (P = 0.7195). Data are shown as means ± SD; one representative experiment is shown; three independent experiments; n = 4. (G) Representative stress relaxation curves of collagen hydrogels at a strain of 10%, which was normalized to the initial stress. (H) Time scale of stress relaxation, τ_1/e, for different collagen hydrogels. *P < 0.05. Data are shown as means ± SD; one representative experiment is shown; three independent experiments; n = 5. (I) Representative creep curves of collagen hydrogels at a stress of 10 Pa, which was normalized to the initial strain. (J) Time scale of creep response, τ_15%, for the different collagen hydrogels. *P < 0.05. Data are shown as means ± SD; one representative experiment is shown; three independent experiments; n = 3.