Biaxial mechanical evaluation of planar biological materials

A fundamental goal in constitutive modeling is to predict the mechanical be havior of a material under a generalized loading state. To achieve this goa l, rigorous experimentation involving all relevant deformations is necessar y to obtain both the form and material constants of a strain-energy density function. For both natural biological tissues and tissue-derived soft biom aterials, there exist many physiological, surgical, and medical device appl ications where rigorous constitutive models are required. Since biological tissues are generally considered incompressible, planar biaxial testing all ows for a two-dimensional stress-state that can be used to characterize ful ly their mechanical properties. Application of biaxial testing to biologica l tissues initially developed as an extension of the techniques developed f or the investigation of rubber elasticity [43, 57]. However, whereas for ru bber-like materials the continuum scale is that of large polymer molecules, it is at the fiber-level (similar to1 mum) for soft biological tissues. Th is is underscored by the fact that the fibers that comprise biological tiss ues exhibit finite nonlinear stress-strain responses and undergo large stra ins and rotations, which together induce complex mechanical behaviors not e asily accounted for in classic constitutive models. Accounting for these be haviors by careful experimental evaluation and formulation of a constitutiv e model continues to be a challenging area in biomechanics. The focus of th is paper is to describe a history of the application of biaxial testing tec hniques to soft planar tissues, their relation to relevant modern biomechan ical constitutive theories, and important future trends.