Anisotropic elasticity of bovine pericardial tissues

Influence of inflammation and cardiac hypertrophy on mechanical properties of human pericardium

Different devices for mechanical circulatory support (MCS) have been developed for the treatment of refractory cardiogenic shock. However, all of them are associated with direct blood contact, the need for anticoagulation and bleeding complications. To overcome these limitations the pericardial sac got into the focus as a promising implantation site for MCS. For this purpose, further knowledge about the mechanical properties of human pericardium is required. In this prospective, monocentric, experimental pilot study 56 samples of human pericardium were extracted postmortem from 13 critically ill patients. After preparation of test specimens uniaxial tensile tests were performed. The primary end points were load at fracture per sample width and strain at fracture. Acute inflammation was assessed by blood levels of C-reactive protein, white blood count and procalcitonin measured at several times during hospital stay. Inflammatory load was estimated by area under the inflammatory curves. Correlation and regression analysis were used to assess the relationship of primary end points to inflammation, comorbidities and postmortem time to preparation. Human pericardium showed a load at fracture per sample width of 1.95 [1.38–2.94] N/mm (median [inter quartile range]) and a strain at fracture of 89.29 [73.84–135.23] %. Markers of acute inflammation and cardiac hypertrophy did not correlate to load or strain at fracture. However, strain at fracture increased with higher body mass index and an increasing number of postmortem days. In contrast, higher patient age was associated with a lower strain at fracture. Inflammation and cardiac hypertrophy did not influence mechanical properties of human pericardium.

Effect of Substrate Stiffness on Mechanical Coupling and Force Propagation at the Infarct Boundary

Heterogeneous intercellular coupling plays a significant role in mechanical and electrical signal transmission in the heart. Although many studies have investigated the electrical signal conduction between myocytes and nonmyocytes within the heart muscle tissue, there are not many that have looked into the mechanical counterpart. This study aims to investigate the effect of substrate stiffness and the presence of cardiac myofibroblasts (CMFs) on mechanical force propagation across cardiomyocytes (CMs) and CMFs in healthy and heart-attack-mimicking matrix stiffness conditions. The contractile forces generated by the CMs and their propagation across the CMFs were measured using a bio-nanoindenter integrated with fluorescence microscopy for fast calcium imaging. Our results showed that softer substrates facilitated stronger and further signal transmission. Interestingly, the presence of the CMFs attenuated the signal propagation in a stiffness-dependent manner. Stiffer substrates with CMFs present attenuated the signal ∼24-32% more compared to soft substrates with CMFs, indicating a synergistic detrimental effect of increased matrix stiffness and increased CMF numbers after myocardial infarction on myocardial function. Furthermore, the beating pattern of the CMF movement at the CM-CMF boundary also depended on the substrate stiffness, thereby influencing the waveform of the propagation of CM-generated contractile forces. We performed computer simulations to further understand the occurrence of different force transmission patterns and showed that cell-matrix focal adhesions assembled at the CM-CMF interfaces, which differs depending on the substrates stiffness, play important roles in determining the efficiency and mechanism of signal transmission. In conclusion, in addition to substrate stiffness, the degree and type of cell-cell and cell-matrix interactions, affected by the substrate stiffness, influence mechanical signal conduction between myocytes and nonmyocytes in the heart muscle tissue.

Native Bovine and Porcine Pericardia Respond to Load With Additive Recruitment of Collagen Fibers

Bovine and porcine pericardia are currently used for manufacturing prosthetic heart valves: their design has become an increasingly important area of investigation in parallel with progressively expanding indications for the transcutaneous approach to heart valves replacement. Before being cut and shaped, pericardial tissues are expected to be properly characterized. Actually, the mechanical assessment of these biomaterials lacks standardized protocols. In particular, the role of preconditioning for achieving a constant mechanical response of tissue samples is still controversial. In the present work, the mechanical response to uniaxial load of native bovine and porcine pericardia, with and without preconditioning was assessed; moreover, the mechanical behavior of pericardia was investigated and explained. It was demonstrated that: (i) pericardial tissue samples hold memory of the loading history but just within the extent of the deformation applied; (ii) the behavior of native bovine and porcine pericardia in response to load is explained by a mechanism based on the additive recruitment of collagen fibers; (iii) the current concept that plasticity is absent in pericardium has to be at least in part reconsidered.

Effect of suturing on the mechanical properties of bovine pericardium--implications for cardiac valve bioprosthesis

In the fabrication of a bioprosthetic heart valve from bovine pericardial tissues, the tissues are subjected to suturing. The stress-strain response of sutured as well as unsutured strips of this tissue were examined. The stress-strain response was determined using a tensile-testing machine. It was found that suturing weakens the tissue in that sutured strips are more extensible, exhibit a lower stress at rupture and a lower final elastic modulus. In addition, it was also found that the bigger the suture/needle size used the greater the decrease in tissue strength. In all, tissue strength was observed to decrease by 22 to 59% in this study. The weakening of the tissue is attributed to the puncture holes created by the surgeon's needle which create regions of weakness. This response of bovine pericardial tissue to suturing should be given due consideration in the fabrication of a bioprosthetic heart valve using this tissue.

Mapping of glutaraldehyde-treated bovine pericardium and tissue selection for bioprosthetic heart valves

Glutaraldehyde-crosslinked bovine pericardium is widely used in bioprosthetic heart valve fabrication. In an attempt to set a scientific basis for more reproducible tissue selection, we produced and analyzed topographical maps of glutaraldehyde-treated bovine pericardium. Whole pericardia were divided into specific anatomical areas and their thickness was measured and mapped on templates. In each area, the suture holding power was determined in both parallel and perpendicular (to the base-apex line) directions; analyses of the tearing patterns in each fragment were used to evaluate predominant fiber orientation, and observations were confirmed by polarized light microscopy. Complete maps were superimposed graphically to aid in the selection of certain areas that would have known fiber orientation, high suture holding power, and suitable thickness. Our results describe regional heterogeneity of bovine pericardial structure and mechanical properties, specifically demonstrating variations in thickness, suture holding power, and collagen fiber orientation. Two areas of choice (representing about 35% of the total) were described as suitable for use in bioprosthetic heart valve fabrication.

Mechanical and optical anisotropy of bovine pericardium

A Moiré fringe method was used to determine the shape of inflated disks of fresh bovine pericardium. The results suggested that the tissue was anisotropic and allowed the identification of the directions of the axes of elastic symmetry. Quantitative biaxial inflation and simple tension tests confirmed the anisotropy. In the circumferential direction the tissue was more extensible and had greater mean strength (18 MPa against 2.5 MPa) and mean terminal modulus (46 MPa against 14 MPa) than in the root-to-apex direction. The tissue was also optically anisotropic when viewed by transmitted polarised light and the directions of mechanical and optical anisotropy were related.

The bovine pericardial xenograft: I. Effect of fixation in aldehydes without constraint on the tensile viscoelastic properties of bovine pericardium

Tensile testing of tissue strips has been used to examine the effect of simple fixation in glutaraldehyde and formaldehyde on the viscoelastic properties of bovine pericardium. To assess tissue anisotropy, tissue strips were cut at 0 degree, 30 degrees, 60 degrees, and 90 degrees relative to the base-to-apex direction. Fresh anterior pericardium was modestly anisotropic, being least extensible in the base-to-apex direction; however, fixation removed this anisotropy. Fixation also produced a marked change in the response of the material to initial cyclic loading during preconditioning. Overall extensibility of the fixed material was significantly greater than that for the fresh tissue, consistent with a 10.7% shrinkage in aldehydes calculated from strain at fracture data. Reductions in stress relaxation and creep after fixation were noted as well, consistent with intrafibrillar crosslinking. Cyclic hysteresis and ultimate tensile strength were unaffected. Since the observed changes in the stress-strain response were largely attributable to shrinkage, control of shrinkage by physical means would allow for engineering modification of bovine pericardial mechanics for controlled anisotropy.