Mechanical effects of increases in the load applied in uniaxial and biaxial tensile testing. Part II. Porcine pericardium

Evaluation of transcatheter heart valve biomaterials: Biomechanical characterization of bovine and porcine pericardium

OBJECTIVE

Bovine pericardium (BP) has been identified as a choice biomaterial for the development of surgical bioprosthetic heart valves (BHV) and transcatheter aortic valves (TAV). Porcine pericardium (PP) and younger BP have been suggested as candidates TAV leaflet biomaterials for smaller-profile devices due to their reduced thickness; however, their mechanical and structural properties remain to be fully characterized. This study characterized the material properties of chemically treated thick (PPK) and thin (PPN) PP, as well as fetal (FBP), calf (CBP) and adult (ABP) BP tissues in order to better understand their mechanical behavior.

METHODS

Planar biaxial testing and uniaxial failure testing methods were employed to quantify tissue mechanical responses and failure properties. Fiber characteristics were examined using histological analysis.

RESULTS

ABP and CBP tissues were significantly stiffer and stronger than the younger FBP tissues. Histological analysis revealed a significantly larger concentration of thin immature collagen fibers in the FBP tissues than in the ABP and CBP tissues. While PP tissues were thinnest, they were stiffer and less extensible than the BP tissues.

CONCLUSIONS

Due to comparable mechanical properties but significantly reduced thickness, CBP tissue may be a more suitable material for TAV manufacturing than ABP tissue. FBP tissue, despite its reduced thickness and higher flexibility, was weaker and should be studied in more detail. Although PP tissues are the thinnest, they were least extensible and failed at earlier strain than BP tissues. The differences between PP and BP tissues should be further investigated and suggest that they should not be used interchangeably in the manufacturing of TAV.

Characterization of mechanical properties of pericardium tissue using planar biaxial tension and flexural deformation

Flexure is an important mode of deformation for native and bioprosthetic heart valves. However, mechanical characterization of bioprosthetic leaflet materials has been done primarily through planar tensile testing. In this study, an integrated experimental and computational cantilever beam bending test was performed to characterize the flexural properties of glutaraldehyde-treated bovine and porcine pericardium of different thicknesses. A strain-invariant based structural constitutive model was used to model the pericardial mechanical behavior quantified through the bending tests of this study and the planar biaxial tests previously performed. The model parameters were optimized through an inverse finite element (FE) procedure in order to describe both sets of experimental data. The optimized material properties were implemented in FE simulations of transcatheter aortic valve (TAV) deformation. It was observed that porcine pericardium TAV leaflets experienced significantly more flexure than bovine when subjected to opening pressurization, and that the flexure may be overestimated using a constitutive model derived from purely planar tensile experimental data. Thus, modeling of a combination of flexural and biaxial tensile testing data may be necessary to more accurately describe the mechanical properties of pericardium, and to computationally investigate bioprosthetic leaflet function and design.

Age dependent differences in collagen alignment of glutaraldehyde fixed bovine pericardium

Bovine pericardium is used for heart valve leaflet replacement where the strength and thinness are critical properties. Pericardium from neonatal animals (4-7 days old) is advantageously thinner and is considered as an alternative to that from adult animals. Here, the structures of adult and neonatal bovine pericardium tissues fixed with glutaraldehyde are characterized by synchrotron-based small angle X-ray scattering (SAXS) and compared with the mechanical properties of these materials. Significant differences are observed between adult and neonatal tissue. The glutaraldehyde fixed neonatal tissue has a higher modulus of elasticity (83.7 MPa) than adult pericardium (33.5 MPa) and a higher normalised ultimate tensile strength (32.9 MPa) than adult pericardium (19.1 MPa). Measured edge on to the tissue, the collagen in neonatal pericardium is significantly more aligned (orientation index (OI) 0.78) than that in adult pericardium (OI 0.62). There is no difference in the fibril diameter between neonatal and adult pericardium. It is shown that high alignment in the plane of the tissue provides the mechanism for the increased strength of the neonatal material. The superior strength of neonatal compared with adult tissue supports the use of neonatal bovine pericardium in heterografts.

Age dependent differences in collagen alignment of glutaraldehyde fixed bovine pericardium

Bovine pericardium is used for heart valve leaflet replacement where the strength and thinness are critical properties. Pericardium from neonatal animals (4–7 days old) is advantageously thinner and is considered as an alternative to that from adult animals. Here, the structures of adult and neonatal bovine pericardium tissues fixed with glutaraldehyde are characterized by synchrotron-based small angle X-ray scattering (SAXS) and compared with the mechanical properties of these materials. Significant differences are observed between adult and neonatal tissue. The glutaraldehyde fixed neonatal tissue has a higher modulus of elasticity (83.7 MPa) than adult pericardium (33.5 MPa) and a higher normalised ultimate tensile strength (32.9 MPa) than adult pericardium (19.1 MPa). Measured edge on to the tissue, the collagen in neonatal pericardium is significantly more aligned (orientation index (OI) 0.78) than that in adult pericardium (OI 0.62). There is no difference in the fibril diameter between neonatal and adult pericardium. It is shown that high alignment in the plane of the tissue provides the mechanism for the increased strength of the neonatal material. The superior strength of neonatal compared with adult tissue supports the use of neonatal bovine pericardium in heterografts.

Supramolecular structure of human aortic valve and pericardial xenograft material: atomic force microscopy study

Pericardial tissue (bovine or porcine), chemically stabilized with glutaraldehyde (GA), is widely used in cardiovascular surgery in the form of bioprosthetic valves. GA reacts with tissue proteins and creates inter- and intra-molecular cross-links, resulting in improved durability. However, tissue calcification and mechanical damage are still unresolved problems. The purpose of this study was to examine the surface topography of normal human aortic valve and GA-stabilized porcine pericardium tissue in order to gain comparative insight into supramolecular structure of both tissues. The analysis was focused on morphologic evaluation of collagen constituents of the tissues. Atomic force microscopy working in the contact mode in air was employed in the study. Considerable diversity in the spatial orientation of collagen fibrils for the human aortic valve and pericardial tissue were observed. It was found that different forms of collagen fibril packing, i.e. dense and "in phase" or loose, could have an impact on the collagen D-banding pattern. Stabilization with GA introduced significant changes in the surface topography of collagen fibrils and in their spatial organization on the tissue surface. Strong disturbance in the fibril's D-spacing was observed. It was also suggested, that the observed structural changes at the supramolecular level might make an important contribution to the progressive damage and calcification of the tissue. The presented results demonstrate that the AFM method can be useful for non-destructive structural characterization of heart valves and bioprosthetic heart valve material.