A mechanism for the decrease in stiffness of bioprosthetic heart valve tissues after cross-linking.

It has been reported that the elastic modulus of glutaraldehyde treated bioprosthetic valve materials is lower than that of fresh tissues. This finding appears paradoxic, because cross-linked tissues are usually stiffer than fresh ones. To investigate this phenomenon, the stiffness of 12 each of fresh and glutaraldehyde treated strips of porcine aortic valve cusps was measured at different levels of applied preload. To eliminate shrinkage effects, tissues were fixed while constrained at the gauge length. From paired comparisons of fresh versus fixed, it was found that glutaraldehyde fixed materials had a consistently lower stiffness than fresh tissue at low loads (0.39 N/m for treated vs 0.54 N/m for fresh at a 150 N/m preload), but no difference was found at high loads (1.09 N/m for treated vs 1.07 N/m for fresh at 600 N/ m preload). It is concluded that the observed drop in elastic modulus at low loads is a phenomenon associated with cross-linking of the collagen fiber matrix. Because a cross-linked matrix offers greater resistance to stretch, fixed tissue generates higher tensions at lower strains, at a point on the loading curve where the collagen fibers are more crimped and the slope of the curve is lower. At high loads, however, the stiffness of fixed tissues is equivalent to that of fresh. Measuring stiffness at a given stress or tension, rather than at a particular state of collagen fiber crimp, leads to the apparent lower stiffness of fixed tissues.