Figure 5.

Representative stress-strain curve seen in mechanical testing of valves in tension (blue line). The first linear portion of the bi-linear curve is called the toe region. It corresponds to the region where collagen is crimped and elastic fibers dominate the material properties of the valve. The second linear portion can be referred to as the collagen region because it corresponds to the region where collagen is fully uncrimped and is dominating the material properties. The slope of this region of the curve is defined as the Young’s modulus or stiffness of the material. One can determine a value for the extensibility by extrapolating the collagen region backwards using the Young’s modulus as the slope (dashed red line) to where it crosses the x-intercept. The extensibility corresponds to the degree of crimping of collagen in the valve. The curved area between these two regions is the transition region. It corresponds to the region in which collagen is actively uncrimping. The sharpness of this transition, defined by the radius of transition curvature, corresponds to the alignment and crosslinking present in the collagen in the tissue.