Dynamic viscoelastic properties of collagen gels in the presence and absence of collagen fibrils.

We measured the dynamic viscoelasticities of collagen gels prepared and modified by four different methods: i) collagen gels cross-linked by 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) after their preparation, ii) collagen gels cross-linked simultaneously with their preparation, iii) collagen gels irradiated with gamma rays after their preparation, and iv) collagen gels directly formed from an acidic collagen solution by gamma-cross-linking. Dynamic viscoelasticities of all samples were measured using a rheometer before and after heating for 30min at 80°C. The collagen gels sequentially cross-linked by 125mM EDC after preparation and then heated exhibited mechanically strong properties (storage modulus G', 7010Pa; loss modulus G″, 288Pa; Young's modulus E, 0.012 in the rapidly-increasing phase and 0.095 in the moderately-increasing phase; tensile strain, 5.29; tensile stress σ, 0.053). We generally conclude that the G' value decreases when gels without fibrils are heated. On the other hand, well cross-linked collagen gels with thick fibrils, such as gels sequentially cross-linked with 125mM EDC after preparation or gamma-cross-linked conventional gels irradiated at 40kGy, exhibit a distinct increase in G' value after heating. Those gels also have thick, twisted, or fused fibrils of collagen.