Effects of collagen orientation on the medium-term fatigue response of heart valve biomaterials
Worldwide 275,000 diseased heart valves are replaced annually and approximately 50% are bioprosthetic heart valves (BHV). BHV are fabricated from biologically derived tissues chemically modified to reduce immunogenecity and improve durability. BHV are nonviable, non-renewing biomaterials that undergo progressive degenerative changes in-vivo resulting in durability issues, which can be due to both calcific and non-calcific mechanisms. In-vitro durability testing of intact valves up to 200x106 cycles is used to assess BHV durability. In-vitro durability testing confounds characterization and modeling of fatigue. Thus there is a need for elucidation of the underlying mechanisms in the BHV response to repeated cyclic loading (RCL), independent of BHV design.
In this study, the effects of collagen orientation on the medium-term (up to 50x106 cycles) BHV RCL response was investigated. Glutaraldehyde treated bovine pericardium were subjected to cyclic tensile loading to stress levels of 500±50 kPa at a frequency of 22 Hz. Two specimen groups were examined, with the preferred collagen fiber direction parallel (PD) and perpendicular (XD) to the direction of loading. Small angle light scattering (SALS) was used to assess the degree of fiber reorientation of the BHV collagenous network after 0 and 50x106 cycles. After 0, 20x106 and 50x106 cycles, specimens were subjected to biaxial mechanical testing and Fourier transform IR spectroscopy (FT-IR) was performed to assess molecular level changes to collagen . In addition, and the collagen fiber crimp period was also measured.
Substantial permanent set effects were observed in both groups. In the perpendicular group, the areal stretch, which is a measure of overall tissue compliance, increased significantly while in the parallel group the areal stretch decreased significantly after 50x106 cycles. After 50x106 cycles, SALS measurements revealed that in the perpendicular group, the collagenous fibers became less aligned and in the parallel group, the collagen fibers became more highly aligned. The only significant changes in collagen crimp were an increase in collage crimp period from 23.46±1.39 mm (at 0 cycles) to 28.14±0.84 mm after 20x106 cycles in the parallel group. FT-IR spectra indicated that RCL of both of the groups lead to collagen conformational changes and early denaturation after 20x106 cycles.
The results of this study suggest that 1) collagen orientation plays a critical role in BHV fatigue response, and 2) chemical fixation technologies that allow greater fiber mobility under functional stresses yet without permanent set effects may yield more durable materials.
Advisor:Michael S. Sacks; Lar G. Gilbertson; William S. Slaughter
School:University of Pittsburgh
School Location:USA - Pennsylvania
Source Type:Master's Thesis
Date of Publication:05/14/2003