A microstructural-based model for soft biological tissues was adapted to analyze the non-linear response of the collagen network in skin. Quasi-static uniaxial tensile tests were conducted on narrow and wide dorsal skin specimens of rats aged 1-4 months. Specimens were taken longitudinally and transversely to the spine and tested at 1.5 %/s. A simplified analysis in which the fibers for the narrow specimens were assumed aligned showed the effective stiffness to increase during maturation and to be greater laterally than longitudinally. It also indicated the migration of the "heel" portion of the response curve toward the origin during maturation to be the result of less crimping in the fibers. These results were supported by microscopical observations.

Based on the assumption that the difference in stiffness with specimen direction was caused by a preferential fiber orientation rather than different fiber material properties, a spatial distribution function to describe fiber orientation was added to the model. The fiber distribution was assumed elliptical with the major axis of the ellipse coinciding with the direction of greater stiffness. A collagen fiber stiffness and the major and minor radii values of the ellipse were determined from the effective values for the average narrow transverse and longitudinal specimens. These specimens were assumed to give the approximate response of a specimen for which all the fibers were oriented along the respective axes of the ellipse. The spatial distribution of fibers was found to change from a uniform (circular) distribution to a more elliptical distribution for which the preferential fiber orientation was in the transverse direction.

The model incorporating the spatial distribution function was used to predict the wide specimen response. The model predicted the actual response better than the continuum-based prediction. When fiber stiffness was normalized based on a total content of collagen the resulting fiber modulus was shown to decrease during a 1 to 2 month age period.

The model was found to be suited for response curves exhibiting a well-defined "heel" region. For curves exhibiting a more flat featureless "heel" region (typical of 1 month longitudinal specimens) the model was not adequate. The results of this study indicate that this simple fiveparameter model is a:​ useful tool in analyzing the microstructure of collagenous tissue. It also provides a means of comparing the mechanical and geometrical properties of the microstructure of specimens of different size, shape and orientation. Such a means has been unavailable.