A series of in vitro biaxial stress-strain experiments will be performed on rectangular sheets of unembalmed human skin. The purpose is to quantify the anisotropic, viscoelastic response of the dermis to time varying strains applied simultaneously in orthogonal axes. The stress relaxation and creep characteristics of skin in a biaxial loading environment will also be studied. A governing biaxial constitutive equation for the dermis will be developed based on a material strain energy formulation. These tasks will be addressed for tissue from various anatomic regions to quantify differences in response associated with physiologic function. A second primary goal is to identify a material property of the dermis, the energy of tearing. This constant describes the tensile failure characteristics of crack or tear propogation in thin sheets of material undergoing large deformations. The tearing energy constant will be quantified from measurements of applied force, displacement, and geometry of simple test specimen configurations. Both the biaxial stress-strain and failure studies will quantify fundamental material properties and response characteristics of normal tissue which are not available from uniaxial test protocols. These data are necessary for rational planning of elective surgical incisions and finite element analyses of proposed closure techniques. In addition, the intelligent design of dermal prostheses require a knowledge of the skin's mechanical stress-strain behavior in complex loading environments present in vivo.
