The biaxial mechanics of thermally denaturing skin - Part II: Modeling

Abstract

Understanding the response of skin to superphysiological temperatures is critical to the diagnosis and prognosis of thermal injuries, and to the development of temperature-based medical therapeutics. Unfortunately, this understanding has been hindered by our incomplete knowledge about the nonlinear coupling between skin temperature and its mechanics. In Part I of this study we experimentally demonstrated a complex interdependence of time, temperature, direction, and load in skins response to superphysiological temperatures. In Part II of our study, we test two different models of skins thermo-mechanics to explain our observations. In both models we assume that skins response to superphysiological temperatures is governed by the denaturation of its highly collageneous microstructure. Thus, we capture skins native mechanics via a microstructurally-motivated strain energy function which includes probability distributions for collagen fiber orientation and waviness. In the first model, we capture skins response to superphysiological temperatures as a transition between two states that link the kinetics of collagen fiber denaturation to fiber coiling and to the transformation of each fibers constitutive behavior from purely elastic to viscoelastic. In the second model, we capture skins response to super-physiological temperatures instead via three states in which a sequence of two reactions link the kinetics of collagen fiber denaturation to fiber coiling, followed by a state of fiber damage. Given the success of both models in qualitatively capturing our observations, we expect that our work will provide guidance for future experiments that could probe each models assumptions toward a better understanding of skins coupled thermo-mechanics and that our work will be used to guide the engineering design of heat treatment therapies.