Comparison of tensile properties of xenopericardium from three animal species and finite element analysis for bioprosthetic heart valve tissue

Bioprosthetic heart valves still have poor long-term durability due to calcification and mechanical failure. The function and performance of bioprostheses is known to depend on the collagen architecture and mechanical behavior of the target tissue. So it is necessary to select an appropriate tissue for such prostheses. In this study, porcine, equine, and bovine pericardia were compared histologically and mechanically. The specimens were analyzed under light microscopy. The planar biaxial tests were performed on the tissue samples by applying synchronic loads along the axial (fiber direction) and perpendicular directions. The measured biaxial data were then fitted into both the modified Mooney-Rivlin model and the anisotropic four parameter Fung-type model. The modified Mooney-Rivlin model was applied to the modeling of the bovine, equine, and porcine pericardia using finite element analysis. The equine pericardium illustrated a wavy collagen bundle architecture similar to bovine pericardium, whereas the collagen bundles in the porcine pericardium were thinner and structured. Wavy pericardia may be preferable candidates for transcutaneous aortic valves because they are less likely to be delaminated during crimping. Based on the biaxial tensile test, the specimens indicated some degree of anisotropy; the anisotropy rates of the equine specimens were almost identical, and higher than the other two specimens. In general, porcine pericardium appeared stiffer, based on the greater strain energy magnitude and the average slope of the stress–stretch curves. Moreover, it was less distensible (due to lower areal strain) than the other two pericardial tissues. Furthermore, the porcine model induced localized high stress regions during the systolic and diastolic phases of the cardiac cycle. However, increased mechanical stress on the bioprosthetic leaflets may cause tissue degeneration and reduce the long-term durability of the valve. Based on our observations, the pericardial specimens behaved as anisotropic and nonlinear tissues—well-characterized by both the modified Mooney-Rivlin and the Fung-type models. The results indicate that, compared to bovine pericardium, equine tissue is mechanically and histologically more appropriate for manufacturing heart valve prostheses. The results of this study can be used in the design and manufacture of bioprosthetic heart valves.     

Quantification and Analysis of Leaflet Flutter on Biological Prosthetic Cardiac Valves

The use of porcine or bovine pericardium biological cardiac valves has as its main disadvantage a relatively short lifespan, with failures due to calcification and fatigue. Increasing these valves’ durability constitutes a great challenge. An understudied phenomenon is the effect of flutter, an oscillation of the leaflets that can cause regurgitation and accelerate calcification and fatigue. As a starting point to study how to reduce or prevent these oscillations, a method was developed to quantify the flutter frequencies occurring at the point of the valve's full opening. On a test bench that simulates the heart flow, the cusp behaviors of eight biological valves were filmed with a high speed camera at 2000 frames per second at different flow rates and motion capture software obtained the frequencies and amplitudes of the vibrations of each leaflet. Oscillations in the range of 200 Hz with average amplitudes of 0.4 mm were found; larger nominal diameter valves obtained lower values, and bovine pericardial valves had superior performance compared to porcine valves. A dimensionless analysis was performed to find a relationship between the geometric and mechanical properties of the valves with the critical speed of the onset of fluttering. This relationship inspired a method to predict whether flutter will occur in the bioprosthesis. This method is a new tool for the consideration of maximizing the life of prosthetic valves.     

Structural Model for Viscoelastic Properties of Pericardial Bioprosthetic Valves

The benefit of bioprosthetic aortic valve over mechanical valve replacements is the release of thromboembolism and digression of long‐term anticoagulation treatment. The function of bioprostheses and their efficiency is known to depend on the mechanical properties of the leaflet tissue. So it is necessary to select a suitable tissue for the bioprosthesis. The purpose of the present study is to clarify the viscoelastic behavior of bovine, equine, and porcine pericardium. In this study, pericardiums were compared mechanically from the viscoelastic aspect. After fixation of the tissues in glutaraldehyde, first uniaxial tests with different extension rates in the fiber direction were performed. Then, the stress relaxation tests in the fiber direction were done on these pericardial tissues by exerting 20, 30,40, and 50% strains. After evaluation of viscoelastic linearity, the Prony series, quasilinear viscoelastic (QLV) and modified superposition theory were applied to the stress relaxation data. Finally, the parameters of these constitutive models were extracted for each pericardium tissue. All three tissues exhibited a decrease in relaxation rate with elevating strain, indicating the nonlinear viscoelastic behavior of these tissues. The three‐term Prony model was selected for describing the linear viscoelasticity. Among different models, the QLV model was best able to capture the relaxation behavior of the pericardium tissues. More stiffness of porcine pericardium was observed in comparison to the two other pericardium tissues. The relaxation percentage of porcine pericardium was less than the two others. It can be concluded that porcine pericardium behaves more as an elastic and less like a viscous tissue in comparison to the bovine and equine pericardium.     

Donkey pericardium compares favorably with commercial xenopericardia used in the manufacture of transcatheter heart valves

Transcatheter aortic valve implantation (TAVI) has gained considerable acceptance in the past decade due to its lower risks than conventional open‐heart surgery. However, the deformation and delamination of the leaflets during the crimping procedure have raised questions about the durability and long‐term serviceability of the pericardium tissue from which the leaflets are made. The collagen architecture, wall thickness and mechanical properties of donkey pericardium were investigated to assess its suitability as an alternative material for the manufacture of heart valves. Coupons sampled from different locations of donkey pericardium were investigated. Bovine, equine, and porcine pericardium specimens served as controls. The donkey pericardium had a similar surface morphology to that of the control pericardia except for the wavy topology on both the fibrous and serous sides. The average thickness of donkey pericardium (ca. 120 µm) was significantly lower than that from bovine (375 µm) and equine (410 µm), but slightly higher than that from porcine (99 µm) specimens. The interlaced wavy collagen bundles in the pericardium were composed of collagen fibers about 100 nm in diameter. This unique structure ensures that the donkey pericardium has a comparable ultimate tensile strength (UTS) and a much higher failure strain than the commercial pericardia used for the manufacture of heart valves. The donkey pericardium has an organized wavy collagen bundle architecture similar to that of bovine pericardium and has a satisfactory UTS and high failure strain. The thin and strong donkey pericardium might be a good candidate valve leaflet material for TAVI.     

Prospects in Lyophilization of Bovine Pericardium

Almost 30 years after the introduction of heart valve prostheses patients worldwide are benefiting from the implant of these devices. Among the various types of heart valves, the ones made of treated bovine pericardium have become a frequently used replacement of the heart's native valve. Lyophilization, also known as freeze-drying, is an extremely useful technique for tissue storage for surgical applications. This article gives a brief overview on the current bovine pericardium lyophilization development, including the chemical modification to improve physical–chemical characteristics and the advanced technologies used to guarantee a high-quality product. It was shown that lyophilization process can be successfully applied as a method of bovine pericardium preservation and also as a technological tool to prepare new materials obtained by chemical modification of native tissues.       

光氧化处理脱细胞牛心包构建组织工程心肌补片的实验研究

目的 探讨经染料介导光氧化处理的脱细胞牛心包构建组织工程心肌补片的可行性。方法 新鲜牛心包先脱细胞,再经光氧化法处理,消毒后种植雄性SD骨髓间充质干细胞(MSCs)。结扎雌性大鼠左冠状动脉前降支,制作心肌梗死模型,1周后将符合心肌梗死标准的大鼠随机分成3组,心肌梗死对照组(MI)、补片组(P)、种细胞补片组(P+C)分别进行干预。4周后,超声评价心功能;2周、4周取材行组织学和免疫组化检查。结果 脱细胞处理完全去除了牛心包组织中的细胞,光氧化处理使组织结构致密;种植的细胞在组织表面形成连续的细胞层。P+C组牛心包心肌补片降解程度、微血管密度在2周、4周时均较P组大。超声评价补片4周后大鼠心功能,P+C组左室射血分数(LVEF)、左室缩短分数(LVFS)均较MI组和P组大,与MI组的差异有统计学意义。结论 光氧化处理脱细胞牛心包构建的组织工程心肌补片可以延缓心功能恶化,该处理方法具有良好的应用潜力。     

Shear Stress‐Induced Total Blood Trauma in Multiple Species

The common complications in heart failure patients with implanted ventricular assist devices (VADs) include hemolysis, thrombosis, and bleeding. These are linked to shear stress‐induced trauma to erythrocytes, platelets, and von Willebrand factor (vWF). Novel device designs are being developed to reduce the blood trauma, which will need to undergo in vitro and in vivo preclinical testing in large animal models such as cattle, sheep, and pig. To fully understand the impact of device design and enable translation of preclinical results, it is important to identify any potential species‐specific differences in the VAD‐associated common complications. Therefore, the purpose of this study was to evaluate the effects of shear stress on cells and proteins in bovine, ovine, and porcine blood compared to human. Blood from different species was subjected to various shear rates (0–8000/s) using a rheometer. It was then analyzed for complete blood counts, hemolysis by the Harboe assay, platelet activation by flow cytometry, vWF structure by immunoblotting, and function by collagen binding activity ELISA (vWF : CBA). Overall, increasing shear rate caused increased total blood trauma in all tested species. This analysis revealed species‐specific differences in shear‐induced hemolysis, platelet activation, and vWF structure and function. Compared to human blood, porcine blood was the most resilient and showed less hemolysis, similar blood counts, but less platelet activation and less vWF damage in response to shear. Compared to human blood, sheared bovine blood showed less hemolysis, similar blood cell counts, greater platelet activation, and similar degradation of vWF structure, but less impact on its activity in response to shear. The shear‐induced effect on ovine blood depended on whether the blood was collected via gravity at the abattoir or by venepuncture from live sheep. Overall, ovine abattoir blood was the least resilient in response to shear and bovine blood was the most similar to human blood. These results lay the foundations for developing blood trauma evaluation standards to enable the extrapolation of in vitro and in vivo animal data to predict safety and biocompatibility of blood‐handling medical devices in humans. We advise using ovine venepuncture blood instead of ovine abattoir blood due to the greater overall damage in the latter. We propose using bovine blood for total blood damage in vitro device evaluation but multiple species could be used to create a full understanding of the complication risk profile of new devices. Further, this study highlights that choice of antibody clone for evaluating platelet activation in bovine blood can influence the interpretation of results from different studies.     

Stentless porcine and pericardial valve in aortic position.

Fifty-seven patients underwent aortic valve replacement with a stentless glutaraldehyde-fixed bioprosthesis; 27 received a porcine aortic valve and 30 had a bovine pericardial valve. Two groups of 30 patients each who had aortic valve replacement with a tilting-disc mechanical valve or a stented porcine bioprosthesis served as controls. There were no differences in sex, body surface area, valve lesion, and valve size among the four groups. Results were assessed on a Doppler-based determination of maximum velocity across the valve, aortic valve area, and degree of valve regurgitation. Velocity across the valve was significantly less with stentless pericardial valves than with stentless porcine valves, stented bioprostheses, and mechanical valves. Stentless valves had a significantly larger aortic valve area when compared with stented valves. Mild central aortic insufficiency was detected more often with stentless pericardial than with stentless porcine bioprostheses (p = 0.04). Stentless valves showed a higher incidence of complete atrioventricular block when compared with stented valves (p = 0.04). Long-term studies are now warranted to assess the durability of both types of stentless valves.     

Study on decellularized porcine aortic valve/poly (3-hydroxybutyrate-co-3-hydroxyhexanoate) hybrid heart valve in sheep model

To overcome shortcomings of current heart valve prostheses, novel hybrid valves were fabricated from decellularized porcine aortic valves coated with poly (3-hydroxybutyrate-co-3-hydroxyhexanoate [PHBHHx]). In the mechanical test in vitro, the biomechanical performance of hybrid valve was investigated. In an in vivo study, hybrid valve conduits were implanted in pulmonary position in sheep without cardiopulmonary bypass. Uncoated grafts were used as control. The valves were explanted and examined histologically and biochemically 16 weeks after surgery. The hybrid valve conduits maintained original shapes, were covered by a confluent layer of cells, and had less calcification than uncoated control. The mechanical test in vitro revealed that PHBHHx coating improved tensile strength. The results in vivo indicated that PHBHHx coating reduced calcification and promoted the repopulation of hybrid valve with the recipient's cells resembling native valve tissue. The hybrid valve may provide superior valve replacement with current techniques.     

Dynamic behavior of prosthetic aortic tissue valves as viewed by high-speed cinematography.

Using a valve testing apparatus of our own design and with a high-speed (600 to 800 frames per second) 16 mm movie camera, films were made of Hancock porcine, Carpentier-Edwards porcine, and Ionescu-Shiley bovine pericardial valves mounted in the aortic position and cycled under physiological conditions at 72 to 100 beats per minute. Fresh and explanted valves were observed using saline or 36.5% glycerol as the pumping solution. When fresh valves were studied using saline solution as the pumpint fluid, the Hancock and Carpentier-Edwards porcine valves showed high-frequency leaflet vibration, which increased in frequency with higher cycling rates. Abnormal leaflet motion was decreased when glycerol was used as the blood analogue. The Ionescu-Shiley bovine pericardial valve did not show abnormal leaflet motion under these conditions. Conclusions drawn from tissue valve testing studies that use excessively high pulsing rates and pressures (accelerated testing) and saline or water as pumping solutions cannot be transposed to predict the fate of tissue valves in a clinical setting.     

Bioprosthetic heart valve rupture associated with trauma

Disruption of a bovine pericardial bioprosthetic aortic heart valve occurred in a motor vehicle accident, and was treated by valve replacement for progressive aortic insufficiency. Leaflet rupture was through areas of noninflammatory tissue degeneration, corresponding to regions of repeated mechanical stress and trauma that occur during the normal function of tissue valves. Patients with bioprosthetic heart valves may be predisposed to traumatic valve injury. Early diagnosis and replacement of these disrupted valves should be accomplished to avoid sudden, unpredictable heart failure.     

In vivo hemodynamic comparison of porcine and pericardial valves

The bovine pericardial valve and the SupraAnnular valve have been developed to improve the hemodynamic function of tissue valves. Hemodynamic performances of the standard Carpentier-Edwards porcine valve, the Carpentier-Edwards SupraAnnular valve, and the Carpentier-Edwards bovine pericardial valve were compared in the aortic position. One hundred patients undergoing aortic valve replacement were studied intraoperatively. Mean gradient across the valve decreased for standard and pericardial valves as valve size increased. At the same flow rate, the 23 mm pericardial valve had larger valve orifice areas, higher performance indices, and lower gradients than the 23 mm SupraAnnular valve. The SupraAnnular valve is hemodynamically superior to the standard Carpentier-Edwards porcine bioprosthesis. The Carpentier-Edwards pericardial valve, however, is less obstructive in the aortic position than either of the porcine valves.     

On Stress Reduction in Bioprosthetic Heart Valve Leaflets by the Use of a Flexible Stent

It has been postulated that flexible stent posts can reduce tensile stress at the commissures of tissue heart valves by about 90% when compared with the same valve mounted on a rigid stent. We have used a detailed computer model to investigate the role of flexible stent posts in reducing stress in the leaflets of three types of bioprosthetic heart valves: the bovine pericardial and the high- and zero-pressure fixed porcine valves. The models use stress/strain data from biaxial experiments to characterize the tissue properties and are subjected to a back pressure of 120 mmHg. We found that strain was reduced linearly with stent post deflection and that this was a purely static process--it did not require the load to be applied impulsively. This finding was in close agreement with earlier experimental studies, which measured the same strain reduction whether the valve was loaded quasi-statically or at physiological rates. In addition, we found that for this mechanism to be effective the valve must have good coaptation at the center and the tissue should be stiff; in other cases, the advantages of strain reduction through the use of a flexible stent are considerably diminished.     

A Combination of Constitutive Damage Model and Artificial Neural Networks to Characterize the Mechanical Properties of the Healthy and Atherosclerotic Human Coronary Arteries

It has been indicated that the content and structure of the elastin and collagen of the arterial wall can subject to a significant alteration due to the atherosclerosis. Consequently, a high tissue stiffness, stress, and even damage/rupture are triggered in the arterial wall. Although many studies so far have been conducted to quantify the mechanical properties of the coronary arteries, none of them consider the role of collagen damage of the healthy and atherosclerotic human coronary arterial walls. Recently, a fiber family‐based constitutive equation was proposed to capture the anisotropic mechanical response of the healthy and atherosclerotic human coronary arteries via both the histostructural and uniaxial data. In this study, experimental mechanical measurements along with histological data of the healthy and atherosclerotic arterial walls were employed to determine the constitutive damage parameters and remodeling of the collagen fibers. To do this, the preconditioned arterial tissues were excised from human cadavers within 5‐h postmortem, and the mean angle of their collagen fibers was precisely determined. Thereafter, a group of quasistatic axial and circumferential loadings were applied to the arterial walls, and the constrained nonlinear minimization method was employed to identify the arterial parameters according to the axial and circumferential extension data. The remodeling of the collagen fibers during the tensile test was also predicted via Artificial Neural Networks algorithm. Regardless of loading direction, the results presented a noteworthy load‐bearing capability and stiffness of the atherosclerotic arteries compared to the healthy ones (P < 0.005). Theoretical fiber angles were found to be consistent with the experimental histological data with less than 2 and 5° difference for the healthy and atherosclerotic arterial walls, respectively. The pseudoelastic damage model data were also compared with that of the experimental data, and interestingly, the arterial mechanical behavior for both the primary loading (up to the elastic region) and the discontinuous softening (up to the ultimate stress) was well addressed. The proposed model predicted well the mechanical response of the arterial tissue considering the damage of collagen fibers for both the healthy and atherosclerotic arterial walls.     

Pathological Effects of Processed Bovine Pericardial Scaffolds—A Comparative In Vivo Evaluation

The objective of the present study was to assess the biocompatibility and regenerative potential of decellularized bovine pericardial scaffold in comparison with glutaraldehyde‐treated and fresh bovine pericardial implants using short‐term intramuscular implantation testing in a rat model. The inflammatory and immune responses were assessed using histopathological examination, special stains for connective tissue, histomorphometric evaluation, and immunohistochemistry. The decellularized pericardium showed an active tissue remodeling response with complete cellular invasion, minimum connective tissue encapsulation, extensive fibrovascular tissue formation, and collagen deposition. On the contrary, the glutaraldehyde‐treated pericardial implants showed incomplete degradation and cellular invasion, while the fresh pericardial implants elicited a severe foreign body reaction. The results of immunohistochemical staining revealed a minimum T helper (CD4+) lymphocyte response in decellularized pericardial implants compared with its glutaraldehyde‐treated and fresh counterparts. The decellularized bovine pericardium was better accepted as a prosthetic scaffold, which permitted maximum collagen deposition and active tissue remodeling by invading host cells and showed good tissue integration in vivo compared with glutaraldehyde‐treated and fresh/untreated pericardium.     

脱细胞天然支架生物力学性能变化及预处理改善组织相容性的实验研究

目的观察脱细胞猪主动脉瓣的生物力学性能变化,探讨不同预处理改善天然支架组织相容性的效果。方法新鲜猪主动脉瓣经酶加去污剂法去除细胞,力学测试仪检测其最大负荷、最大应力、最大应变和弹性模量的变化,苏木精-伊红（HE）染色、Ⅰ型胶原免疫组化染色和扫描电镜观察其病理形态学变化;将脱细胞瓣膜分别予磷酸缓冲液、多聚赖氨酸和未灭活胎牛血清包被处理,然后种植大鼠主动脉肌成纤维细胞,甲基噻唑基四唑试验检测细胞黏附率,HE染色和扫描电镜观察形态学变化。结果酶加去污剂法能完全脱去瓣膜细胞,基本维持胶原纤维的空间结构,但其最大负荷、最大应力及弹性模量下降,最大应变上升（P〈0．05）;胎牛血清预处理去细胞瓣能显著提高肌成纤维细胞的黏附率,促进细胞生长、分化和增殖,并在瓣膜表面形成连续的细胞层（F值=129．26,P=0．000）。结论酶加去污剂法可较完全去除猪主动脉瓣膜细胞并保持细胞外基质的三维结构,但其生物力学性能有所下降;胎牛血清预处理能改善脱细胞瓣天然支架的细胞黏附、生长和繁殖。     

Preparation and characterization of an acellular bovine pericardium intended for manufacture of valve bioprostheses

Major problems with biological heart valves post-implantation are associated with progressive structural deterioration and calcification attributed to glutaraldehyde processing, dead cells, and cell fragments present in the native tissue. In spite of these problems, glutaraldehyde still is the reagent of choice. The results with acellular matrix xenograft usually prepared by detergent treatment in association with enzymes are rather conflicting because while preserving mechanical properties, tissue morphology and collagen structure are process dependent. This work describes a chemical approach for the preparation of an acellular bovine pericardium matrix intended for the manufacture of heart valve bioprostheses. Cell removal was performed by an alkaline extraction in the presence of calcium salts for periods ranging from 6 to 48 h. The results showed that cell removal was achieved after 12 h, with swelling and negative charge increasing with processing time. Nevertheless, collagen fibril structure, ability to form fibrils, and stability to collagenase were progressive after 24-h processing. There was no denaturation of the collagen matrix. A process is described for the preparation of acellular bovine pericardium matrices with preserved fibril structure and morphology for the manufacture of cardiac valve bioprostheses and may be used in other applications for tissue reconstruction.     

The behavior of pericardial versus porcine valve xenografts in the growing sheep model

The comparative long-term behavior of the pericardial versus the porcine bioprostheses is not yet known. The need for long follow-up times to answer this question makes the growing sheep model an attractive alternative, given its ability to induce early valve degeneration. Sixty-three sheep, 12 to 16 weeks old, were operated on and received 39 porcine (11 Xenomedica, 10 Carpentier-Edwards S, nine Hancock I standard, and nine Hancock I T6-treated) and 24 pericardial (14 Mitroflow and 10 Ionescu-Shiley low profile) prostheses of clinical quality in the tricuspid position. Of the 52 operative survivors (32 received porcine valves and 20 received pericardial bioprostheses), six animals (five pericardial and one porcine) were eliminated because of bioprosthetic infection. Late sudden death before the scheduled killing occurred significantly more often (p less than 0.0001) in the pericardial (8/15 or 53%) than in the porcine group (1/31 or 3%). Calcium content of the explanted valves was significantly correlated with time in the pericardial group and the Xenomedica porcine prostheses (p less than 0.05) but not in the Hancock I and Carpentier-Edwards S valves, where it was only marginally significant (0.1 greater than p greater than 0.05). Linear regression analysis of tissue calcium content showed a similar slope for the pericardial group and Xenomedica porcine valves, in comparison with the remaining porcine valves. Comparison between the two lines using covariance analysis demonstrated a statistically significant difference between them, which shows that the pericardial and Xenomedica porcine valves appear to reach higher levels of calcification in a shorter follow-up time than the Hancock I, standard and T6-treated, and the Carpentier-Edwards S valve in this animal model.     

Cytotoxicity and endothelial cell adhesion of lyophilized and irradiated bovine pericardium modified with silk fibroin and chitosan

Grafts of biological tissues have been used since the 1960s as an alternative to the mechanical heart prostheses. Nowadays, the most consolidated treatment to bovine pericardial (BP) bioprostheses is the crosslinking with glutaraldehyde (GA), although GA may induce calcification in vivo. In previous work, our group demonstrated that electron beam irradiation applied to lyophilized BP in the absence of oxygen promoted crosslinks among collagen fibers of BP tissue. In this work, the incorporation of silk fibroin (SF) and chitosan (CHIT) in the BP not treated with GA was studied. The samples were irradiated and then analyzed for their cytotoxicity and the ability of adhesion and growth of endothelial cells. Initially, all samples showed cytotoxicity. However, after a few washing cycles, the cytotoxicity due to acetic acid and ethanol residues was removed from the biomaterial making it suitable for the biofunctional test. The samples modified with SF/CHIT and electron beam irradiated favored the adhesion and growth of endothelial cells throughout the tissue.