Effect of alternative crosslinking methods on the low strain rate viscoelastic properties of bovine pericardial bioprosthetic material

Early failures of bovine pericardial heart valves have been due to leaflet perforation/tearing and calcification. Since glutaraldehyde fixation has been shown to produce marked changes in leaflet mechanics and has been linked to the development of calcification, alternative crosslinking techniques have been suggested as means to overcome these obstacles. We have examined the low strain rate viscoelastic behavior of bovine pericardium: (1) fresh; (2) chemically treated with glutaraldehyde, cyanimide, or polyglycidyl ether; or (3) physically treated by freeze-drying or heat-drying. Shrinkage temperature tests were conducted to assess intrahelical crosslinking. Polyglycidyl ether and glutaraldehyde both produced substantial crosslinking, with the shrinkage temperature rising above 80 degrees C. Mechanical changes were nearly equivalent, both showing decreased stress relaxation and increased extensibility consistent with intrahelical crosslinking and shrinkage during fixation. Cyanimide, known to crosslink pure collagen materials, showed no evidence of crosslinking intact tissue. Heat-drying, also effective in pure collagen preparations, produced an increase in UTS and tissue modulus, but otherwise left the tissue unchanged. Freeze-drying had no mechanical effect, and therefore provides an attractive means for the storage of connective tissues for later mechanical testing.