UC San Diego

Skin is the outermost layer of the body and acts as a primary protective barrier against external agents such as heat, light, infection, and injury. Additionally, skin regulates a broad range of physiological parameters and hosts several vital components. In order to fulfill these functions throughout life, skin must be able to withstand and recover from significant deformation as well as mitigate tear propagation that can occur during growth, movement, and injuries affecting its integrity. Hence, characterizing the mechanical behavior of skin and understanding the underlying mechanisms of deformation at different spatial scales is essential in a large spectrum of applications such as surgery, cosmetics, forensics, biomimetics and engineering of protective gear or artificial grafts.

In this dissertation, a comprehensive list of experimental techniques that have been developed over the years to test skin’s nonlinear elastic, viscoelastic, and dissipative properties are reviewed. To identify parameters affecting its behavior, a significant number of models have been developed, some of which are detailed here. The principal structural elements within the dermis, and especially the arrangement and orientation of the collagen fibrils and fibers, are presented; their incorporation into the constitutive models is discussed.

We conduct a detailed investigation of the evolution of the collagen architecture of the dermis as a function of deformation, which reveals new aspects that extend our understanding of the mechanical response of porcine skin. The dermis is found to have a tridimensional woven structure of collagen fibers, which evolves with deformation. After failure, we observe that the fibers have straightened and aligned in the direction of tension. Time-dependent and dissipative effects are quantitatively established. Digital image correlation techniques were implemented to quantify skin’s anisotropy; measurements of the Poisson ratio and their evolution are reported for the first time. Based on new observations, we propose that fiber braiding is at least partially responsible for the monotonic increase of the tangent modulus of skin with strain, as well as its dissipative response to cyclical loads.

We incorporate these findings in a constitutive framework incorporating fibril stiffness, interfibrillar frictional sliding, and the effect of lateral fibers on the extension of a primary fiber, using analytical and coarse-grained modeling approaches. The representation of these important physical processes that occur during deformation of the dermis represents an advance in our understanding of these phenomena.

Finally, we estimate the toughness of porcine skin by conduct two types of experiment on pre-notched specimens, placing the tissue under shear Mode III and opening Mode I. We obtain two distinct toughness values of J_IIIc≈20.4 kJ/m^2 and J_Ic=30.4 kJ/m^2, indicating notable differences between these two modes of crack propagation. Digital image correlation is used to plot strain profiles around the tip of the crack, from which a strain-based criterion for crack propagation is established. The evaluation of the structure at the crack tip and regions undergoing more uniform states of deformation is conducted by ex situ transmission electron microscopy and in situ environmental scanning electron microscopy.