Biaxial mechanical/structural effects of equibiaxial strain during crosslinking of bovine pericardial xenograft materials

We have investigated the effect of biaxial constraint during glutaraldehyde crosslinking on the equibiaxial mechanical properties of bovine pericardiu m. Crosslinking of cruciate samples was carried out with: (i) no applied lo ad, (ii) an initial 25 g (similar to 30 kPa) equibiaxial load, or (iii) an initial 200 g (similar to 250 kPa) equibiaxial load. All loading during cro sslinking was done under a defined initial equibiaxial load and subsequentl y fixed biaxial strain. Load changes during crosslinking were monitored. Me chanical testing and constraint during crosslinking were carried out in a c ustom-built biaxial servo-hydraulic testing system incorporating four actua tors with phase-controlled waveform synthesis, high frame-rate video dimens ion analysis, and computer-interfaced data acquisition The paired biaxial s tress-strain responses under equibiaxial loading at 1 Hz (before and after treatment) were evaluated for changes in anisotropic extensibility by calcu lation of an anisotropy index. Scanning electron microscopy (SEM) was perfo rmed on freeze-fractured samples to relate collagen crimp morphology to con straint during crosslinking. Fresh tissue was markedly anisotropic with the base-to-apex direction of the pericardium being less extensible and stiffe r than the circumferential direction. After unconstrained crosslinking, the extensibility in the circumferential direction, the stiffness in the base- to-apex direction, and the tissue's anisotropy were all reduced. Anisotropy was preserved in the tissue treated with an applied 25 g load; however, ti ssue treated with an applied 200 g load became extremely stiff and nearly i sotropic. SEM micrographs correlated well with observed extensibility in th at the collagen fibre morphology changed from very crimped (unconstrained c rosslinking), to straight (200 g applied load). Biaxial stress-fixation may allow engineering of bioprosthetic materials for specific medical applicat ions. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.