# Contributed Talk - Friday, 17 September I 10:40 AM (CEST)

Martin Handelshauser: "Viscoelastic Behavior of Collagen Fibrils Investigated by a Nonlinear Rheological Model"

TU Wien, Austria

The arcitecture of biological tissues spans from atoms, proteins up to the organ level. Also, tissues show viscoelastic behavior when forces act on them. Because of this and the complex architecture the question arises: how does viscoelasticity extends to the smallest structural components of tissues, i.e. to collagen fibrils? Some published methods for the determination of viscoelastic material properties of collagen fibrils are elaborate and complicated by design. Because of that, it is desirable to find a model that can describe the viscoelastic behaviour of collagen fibrils. Nonlinear rheological models are a promising approach. Here, we use a nonlinear rheological Maxwell model (composed of a non-linear spring, a, and a non-linear dashpot, η) and its constitutive equation for the assessment of tensile viscoelastic properties of collagen fibrils. The nonlinear Maxwell model shows a high quality of fit (R^2=0.99) and the values for the apparent tensile modulus are in the same range as previously published results. The spring constant a is increasing with the tensile modulus due to the viscous nature of the model. The viscosity is in a range of 〖10〗^10 MPa.s, which resembles bitumen or molten glass. With the nonlinear Maxwell model, we can describe the viscoelastic behavior of collagen fibrils during tension. The results for the tensile modulus, spring constant, viscosity and relaxation time are dependent on the displacement speed. Per the applicability of the Maxwell model the results also suggest collagen fibrils to be a highly viscous. Other rheological models, like Kelvin-Voigt, were also tested but did not show good qualities of fit or the results for the apparent tensile modulus were not comparable to previously published results. But the nonlinear Maxwell model is a promising first step towards a simpler rheological model capable of describing the viscoelastic properties of collagen fibrils. In future studies, further validation will be done through creep or stress relaxation experiments.