The wavy nature of collagen fibrils deduced from the dispersion of their second-order nonlinear optical anisotropy parameters ρ

From P-SHG experiments, second-order nonlinear optical anisotropy parameters ρ = χZZZ/χZXX of collagen tissues are calculated assuming the same model of supercoiled collagen fibril characterized by a variable angle θ. Dispersion of experimental ρ values is converted into distribution of θ values based on the wavy nature of collagen fibrils deduced from EM studies. For tendon, the results show that the dispersion of experimental ρ values is mainly due to Poisson photonic shot noise assuming a slight fibrillar undulation with θ = 2.2° ± 1.8°. However for skin and vessels, the dispersion of experimental ρ values is mainly due to a stronger fibrillar undulation with θ = 16.2° ± 1.3°. The results highlight that this undulation is reduced during the development of liver fibrosis therefore, contributing to the rigidity of the tissue.