Collagen fibre orientation and dispersion govern ultimate tensile strength, stiffness and the fatigue performance of bovine pericardium

Abstract

The durability of bovine pericardium leaflets employed in bioprosthetic heart valves (BHVs) can significantly limit the longevity of heart valve prostheses. Collagen fibres are the dominant load bearing component of bovine pericardium, however fibre architecture within leaflet geometries is not explicitly controlled in the manufacture of commercial devices. Thus, the purpose of this study was to ascertain the influence of pre-determined collagen fibre orientation and dispersion on the mechanical performance of bovine pericardium. Three tissue groups were tested in uniaxial tension: cross-fibre tissue (XD); highly dispersed fibre-orientations (HD); or preferred-fibre tissue (PD). Both the XD and PD tissue were tested under cyclic loading at 1.5 Hz and a stress range of 2.7 MPa. The results of the static tensile experiments illustrated that collagen fibre orientation and degree of alignment significantly influenced the material's response. Whereby, there was a statistically significant decrease in material properties between the XD groups and both the PD and HD groups for ultimate tensile strength and stiffness (p<0.01). Furthermore, HD tissue had a stiffness of approximately 58% of the PD group, and XD tissue had a stiffness of approximately 18% of the PD group. The dynamic behaviour of the XD and PD groups was extremely distinct; for example a Weibull analysis indicated that the 50% probability of failure in specimens with fibres orientated perpendicular (XD) to the loading direction occurred at 375 cycles. Due to this failure, XD specimens survived less than 20% of the cycles completed by those in which fibres were aligned along the loading direction (PD). The results from this study indicate that fibre architecture is a significant factor in determining static strength and fatigue life in bovine pericardium, and thus must be incorporated in the design process to improve future device durability.

Mechanical Roles in Formation of Oriented Collagen Fibers

“…68 The structure and mechanical properties of the fiber make the tissue have a certain bearing capacity, but the specific mechanical property of the tissue is highly dependent on the orientation of the collagen fibers. 54,[69][70][71][72] In vivo, all kinds of tissues are subjected to mechanical stimulation in various forms and at various amplitudes. With the effect of mechanics, the diversity of fiber orientation satisfies the physiological requirements of tissues.…”

An investigation into the critical role of fibre orientation in the ultimate tensile strength and stiffness of human carotid plaque caps

“…Ultimate tensile stress and ultimate tensile strain values were then extracted at the point of failure of the sample. Stiffness (E) was also calculated for each sample by taking 10 data points and calculating the slope of the final linear region observed in the stress-strain curves but ending before the final 20% of the curve to ensure consistency across samples [48].…”

“…Whilst imaging collagen fibre patterns in vivo is still beyond the state-of-the-art in cardiovascular imaging, it is possible to determine such fibre arrangements ex-vivo, albeit destructively in the majority of cases such as histology or microscopy [14,47]. In contrast, small angle light scattering (SALS) analysis is a non-destructive technique that can be used to pre-screen fibre structures in thin biological tissue samples ex-vivo [48,49] whereby the incident laser light angle is scattered orthogonally to the central axis of the sample's constituent fibres, providing detail on the sample structure.…”

Understanding the deformation gradient in Abaqus and key guidelines for anisotropic hyperelastic user material subroutines (UMATs)

“…This class of constitutive model has been used to simulate soft tissues such as: arterial (Creane et al, 2012;Famaey et al, 2013;Ghasemi et al, 2018;Nolan and McGarry, 2015), tendon (Khayyeri et al, 2016;Shearer, 2015) , cartilage (Nagel and Kelly, 2010), skin (Annaidh et al, 2012), myocardium (McEvoy et al, 2018, annulus fibrosis (Eberlein et al, 2004) to name but a few. These constitutive models are based on fibre vectors which indicate the direction in which structurally important fibres are orientated, for example collagen or elastin fibres in arterial tissues (Gaul et al, 2017;Whelan et al, 2019). This offers an attractive, intuitive method to define the anisotropy of a material.…”