Viscoelastic behavior of collagen matrices in the frequency and time domain: An experimental and modeling study

Collagen is the most abundant protein in the body. It plays critical roles in many supporting and connecting tissues. Collagen matrices prepared from commercially available collagen solution have been broadly used as a biomaterial in tissue engineering, drug delivery, and wound healing for its biocompatibility, low toxicity, and well-documented physical, chemical, and immunological properties. Collagen matrices are also used as three-dimensional model systems of extracellular matrix (ECM) in numerous studies of cell-ECM interactions under physiological and pathological conditions. As a biphasic material, collagen matrices contain a solid phase representing by collagen network and an interstitial fluid phase. This special structure makes collagen a viscoelastic material. The overall research goal of this thesis is to characterize the macroscopic elastic and viscoelastic properties of collagen matrices in both the frequency and time domain and to understand the deformation mechanisms using a coupled experimental and modeling approach. Hydrated collagen gel and dehydrated collagen thin film are exploited as two different hydration levels of collagen matrices. Genipin solution is used to induce crosslinking in collagen matrices.;Biaxial tensile stress relaxation results show that the rate of stress relaxation of both hydrated and dehydrated collagen matrices shows a linear initial stress level dependency. Increased crosslinking reduces viscosity in collagen gel, but the effect is negligible for thin film. Relaxation time distribution spectrum was obtained from the stress relaxation data by inverse Laplace transform. For most of the collagen matrices, three peaks at the short (0.3s ∼ 1s), medium (3s ∼ 90s), and long relaxation time (>200s) were observed in the continuous spectrum, which corresponds with relaxation mechanisms involve fiber, inter-fibril, and fibril sliding. The intensity of the long-term peaks increases with higher initial stress levels indicates the engagement of collagen fibrils at higher levels of tissue strain. Splitting of the middle peak was observed at higher initial stress levels suggesting increased structural heterogeneity at the fibril level with mechanical loading. A viscoelastic constitutive model combining hyperelastic and generalized Maxwell model was established with viscous material parameters obtained directly from analysis of the relaxation time spectrum.;Rheological shear relaxation and dynamic rheological tests were performed on collagen gel. Crossover of storage and loss modulus was observed from frequency sweep tests. The crossover frequency shows both strain amplitude and crosslinking dependency. Both dynamic moduli and shear relaxation modulus demonstrate strain-softening behavior. Conversion from frequency domain measurements to time domain properties was achieved through relaxation spectrum obtained by Tikhonov regularization method. The relaxation spectrum shows two obvious peaks between 0.01s∼0.1s and between 10s∼40s, which indicate the existence of fast and slow dominant relaxation processes. The peak between 0.01∼0.1s correspond is likely due to the collagen fiber-interstitial fluid sliding. In shear deformation both collagen fiber network and interstitial fluid play important roles, which contribute to the differences in relaxation mechanisms between rheological shear and tensile relaxation tests.