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 treate d bioprosthetic valve materials is lower than that of fresh tissues. T his finding appears paradoxic, because cross-linked tissues are usuall y stiffer than fresh ones. To investigate this phenomenon, the stiffne ss of 12 each of fresh and glutaraldehyde treated strips of porcine ao rtic valve cusps was measured at different levers of applied preload. To eliminate shrinkage effects, tissues were fixed white constrained a t 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 0.5 4 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 prel oad). It is concluded that the observed drop in elastic modulus at low loads is a phenomenon associated with cross-linking of the collagen f iber matrix. Because a crosslinked 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 crimpe d and the slope of the curve is lower. At high toads, however, the sti ffness of fixed tissues is equivalent to that of fresh. Measuring stif fness at a given stress or tension, rather than at a particular state of collagen fiber crimp, leads to the apparent lower stiffness of fixe d tissues.