Biochemical and mechanical behavior of ostrich pericardium as a new biomaterial

Performance of xenogeneic pulmonary visceral pleura as bioprosthetic heart valve cusps in swine

Objective

Bovine pericardium is common biological material for bioprosthetic heart valve. There remains a significant need, however, to improve bioprosthetic valves for longer-term outcomes. This study aims to evaluate the chronic performance of bovine pulmonary visceral pleura (PVP) as bioprosthetic valve cusps.

Methods

The PVP was extracted from the bovine lung and fixed in 0.625% glutaraldehyde overnight at room temperature. The PVP valve cusps for the bioprosthetic valve were tailored using a laser cutter. Three leaflets were sewn onto a nitinol stent. Six PVP bioprosthetic valves were loaded into the test chamber of the heart valve tester to complete 100 million cycles. Six other PVP bioprosthetic valves were transcardially implanted to replace pulmonary artery valve of six pigs. Fluoroscopy and intracardiac echocardiography were used for in vivo assessments. Thrombosis, calcification, inflammation, and fibrosis were evaluated in the terminal study. Histologic analyses were used for evaluations of any degradation or calcification.

Results

All PVP bioprosthetic valves completed 100 million cycles without significant damage or tears. In vivo assessments showed bioprosthetic valve cusps open and coaptation at four months post-implant. No calcification and thrombotic deposits, inflammation, and fibrosis were observed in the heart or pulmonary artery. The histologic analyses showed complete and compact elastin and collagen fibers in the PVP valve cusps. Calcification-specific stains showed no calcific deposit in the PVP valve cusps.

Conclusions

The accelerated wear test demonstrates suitable mechanical strength of PVP cusps for heart valve. The swine model demonstrates that the PVP valve cusps are promising for valve replacement.

A comparative study of different tissue materials for bioprosthetic aortic valves using experimental assays and finite element analysis

BACKGROUND AND OBJECTIVE

Extracting the mechanical behaviors of bioprosthetic aortic valve leaflets is necessary for the appropriate design and manufacture of the prosthetic valves. The goal of this study was to opt a proper tissue for the valve leaflets by comparing the mechanical properties of the equine, porcine, and donkey pericardia with those of the bovine pericardium and human aortic valve leaflets.

METHODS

After tissue fixation in glutaraldehyde, the mechanical behaviors of the pericardial tissues were experimentally evaluated through computational methods. The relaxation tests were performed along the tissue fiber direction. The Mooney-Rivlin model was utilized to describe the hyperelastic behavior of the tissues at the ramp portion. The viscous behaviors at the hold portion were extracted using the Fung quasi-linear viscoelastic (QLV) model. Furthermore, the extracted parameters were used in the modeling of the bovine, equine, porcine, and donkey pericardia through finite element analysis (FEA).

RESULTS

Based on the results, relaxation percentages of the equine, donkey, and bovine pericardia were greater than that of the porcine pericardium and similar to the native human aortic valve leaflets. Indeed, the equine and donkey pericardia were found more viscous and less elastic than the porcine pericardium. Compared with the porcine pericardium, the mechanical properties of the equine and donkey pericardia were rather closer to those of the native human leaflets and bovine pericardium. The computational analysis demonstrated that the donkey pericardium is preferable over other types of pericardium due to the low stress on the leaflets during the systolic and diastolic phases and the large geometric orifice area (GOA).

CONCLUSION

The donkey pericardium might be a good candidate valve leaflet material for bioprosthetic aortic valves.

Aortic Leaflet Stresses Are Substantially Lower Using Pulmonary Visceral Pleura Than Pericardial Tissue

Background: Porcine heart and bovine pericardium valves, which are collagen-based with relatively little elastin, have been broadly utilized to construct bioprosthetic heart valves (BHVs). With a larger proportion of elastin, the pulmonary visceral pleura (PVP) has greater elasticity and could potentially serve as an advantageous biomaterial for the construction/repair of BHVs. The question of how the aortic valve’s performance is affected by its bending rigidity has not been well studied.

Methods: Based on the stress–strain relationships of the pericardium and PVP determined by planar uni-axial tests, a three-dimensional (3D) computational fluid–structure interaction (FSI) framework is employed to numerically investigate the aortic valve’s performance by considering three different cases with Young’s modulus as follows: E=375, 750, and 1500 kPa, respectively.

Results: The stroke volumes are 112, 99.6, and 91.4 ml as Young’s modulus increases from 375 to 750 and 1500 kPa, respectively. Peak geometric opening area (GOA) values are 2.3, 2.2, and 2.0 cm² for E=375, 750, and 1500 kPa, respectively. The maximum value of the aortic leaflet stress is about 271 kPa for E=375 kPa, and it increases to about 383 and 540 kPa for E=750 and 1500 kPa in the belly region at the peak systole, while it reduces from 550 kPa to 450 and 400 kPa for E=375, 750, and 1500 kPa, respectively, at the instant of peak “water-hammer”.

Conclusion: A more compliant PVP aortic leaflet valve with a smaller Young’s modulus, E, has a higher cardiac output, larger GOA, and lower hemodynamic resistance. Most importantly, the aortic leaflet stresses are substantially lower in the belly region within the higher compliance PVP aortic valve tissue during the systole phase, even though some stress increase is also found during the fast-closing phase due to the “water-hammer” effect similar to that in the pericardial tissue. Future clinical studies will be conducted to test the hypothesis that the PVP-based valve leaflets with higher compliance will have lower fatigue or calcification rates due to the overall lower stress.

Pulmonary Visceral Pleura Biomaterial: Elastin- and Collagen-Based Extracellular Matrix

Objective: The goal of the study is to determine the structural characteristics, mechanical properties, cytotoxicity, and biocompatibility of the pulmonary visceral pleura (PVP).

Background: Collagen and elastin are the major components of the extracellular matrix. The PVP has an abundance of elastin and collagen that can serve as a potential biomaterial for clinical repair and reconstructions.

Methods: The PVP was processed from swine and bovine lungs. Chemical analyses were used to determine collagen and elastin contents in the PVPs. Immunofluorescence microscopy was used to analyze the structure of the PVP. The stress–strain relationships and stress relaxation were determined by using the planar uniaxial test. The cytotoxicity of the PVP was tested in cultured cells. In in vivo evaluations, the PVP was implanted in the sciatic nerve and skin of rats.

Results: Collagen and elastin contents are abundant in the PVP with larger proportions of elastin than in the bovine pericardium and porcine small intestinal submucosa. A microstructural analysis revealed that the elastin fibers were distributed throughout the PVP and the collagen was distributed mainly in the mesothelial basal lamina. The incremental moduli in stress–strain curves and relaxation moduli in the Maxwell–Wiechert model of PVP were approximately one-tenth of the bovine pericardium and small intestinal submucosa. The minimal cytotoxicity of the PVP was demonstrated. The axons proliferated in the PVP conduit guidance from proximal to distal sciatic nerves of rats. The neo-skin regenerated under the PVP skin substitute within 4 weeks.

Conclusions: The PVP is composed of abundant collagen and elastin. The structural characteristics and mechanical compliance of the PVP render a suitable biological material for repair/reconstruction.

Novel Biomaterial for Artery Patch in Swine Model With High-Fat Diet

Objective

We evaluated swine and bovine pulmonary visceral pleura (PVP) in artery patch-angioplasty in swine model of high-fat diet.

Background

Arterial patch-angioplasty is frequently used for repair or reconstruction of arteries. An autologous patch is often limited by the number and dimension of donor tissue and can result in donor complications. Furthermore, mechanical mismatch is a cause of poor performance of vascular reconstruction. Here, we introduce a readily available patch biomaterial with similar compliance as native arteries.

Methods

The PVP was peeled from swine and bovine lungs by hydro-dissection. The swine and bovine PVPs were crosslinked with glutaraldehyde and then sterilized. The swine PVP (sPVP) patches were implanted in the carotid and femoral arteries of six Yorkshire pigs that were fed a regular diet and euthanized at 2 and 4 months postoperative. The bovine PVP (bPVP) patches were implanted in the carotid artery of six Yucatan pigs that were fed a high-fat diet and euthanized at 4 months postoperative. Patency was evaluated by ultrasound and angiography. Neo-endothelium and media were evaluated by histologic examination.

Results

All arteries in patch-angioplasties remained patent with no adhesions, inflammation, or aneurysms. Biomarkers of endothelial cells (e.g., Factor VIII and eNOS) were detected in the neo-endothelial cells. We observed endothelial cell–cell junctions in the confluent neo-endothelium in the PVP patches. Neo-media composed of vascular smooth muscle developed similar as native arteries. In the hypercholesterolemic model, we observed the accumulation of cholesterol in both arterial tissues and in the neo-vascular tissues in the PVP patches. Protein expressions of lipid transport and metabolism (e.g., APOE-1, ABCA, and PACK9) were also observed in both arterial and neo-vascular tissues.

Conclusion

The PVP patch-angioplasty overcomes the pitfalls of compliance mismatch of synthetic patches and has a non-thrombogenic surface. The proliferation of vascular cells assembled to generate the neo-endothelium and media in the patch-angioplasties to support long-term patency. The neo-vascular tissue in PVP patch-angioplasty also developed similar cellular functions for lipid transport and metabolism compared with native arteries in hypercholesterolemia.

Decellularized xenografts in regenerative medicine: From processing to clinical application

Decellularized xenografts are an inherent component of regenerative medicine. Their preserved structure, mechanical integrity and biofunctional composition have well established them in reparative medicine for a diverse range of clinical indications. Nonetheless, their performance is highly influenced by their source (ie species, age, tissue) and processing (ie decellularization, crosslinking, sterilization and preservation), which govern their final characteristics and determine their success or failure for a specific clinical target. In this review, we provide an overview of the different sources and processing methods used in decellularized xenografts fabrication and discuss their effect on the clinical performance of commercially available decellularized xenografts.

Homologous and heterologous assessment of a novel biomaterial for venous patch

OBJECTIVE

This study evaluated swine and bovine pulmonary visceral pleura (PVP) as a vascular patch. Venous patches are frequently used in surgery for repair or reconstruction of veins. Autologous patches are often limited by the number and dimension of donor tissue and can result in donor complications. Bovine pericardium is the most common heterologous patch used by vascular surgeons. Researchers, however, are continually seeking to improve heterologous and synthetic patches for improved outcome.

METHODS

The PVP was peeled from swine and bovine lungs and cross-linked with glutaraldehyde. After sterilization and rinsing, the PVP patches were implanted in the jugular vein (10 × 35 mm) of pigs and dogs. Patency was evaluated by ultrasound, and animals were euthanized at 2 and 4 months. Neoendothelium and neomedia were evaluated by histologic analysis.

RESULTS

The jugular vein patched by PVP in pigs and dogs remained patent at 2 and 4 months with no adhesions, inflammation, or aneurysm in the patches. The biomarkers of endothelial cells-factor VIII, platelet/endothelial cell adhesion molecule 1, and endothelial nitric oxide synthase-were detected in the neoendothelial cells. The expression of vascular smooth muscle cell (VSMC) α-actin was robust in the neomedia at 2 and 4 months. Neomedia composed of VSMCs developed to nearly double the thickness of adjacent jugular vein. The circumferential orientation of VSMCs in neomedia further increased in the 4-month group.

CONCLUSIONS

The cross-linked swine and bovine PVP patch has a nonthrombogenic surface that maintains patency. The PVP patch may overcome the pitfall of compliance mismatch of synthetic patches. The proliferation of vascular cells assembled in the neoendothelium and neomedia in the patches may support long-term patency.

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.

Processing of collagen based biomaterials and the resulting materials properties

Collagen, the most abundant extracellular matrix protein in animal kingdom belongs to a family of fibrous proteins, which transfer load in tissues and which provide a highly biocompatible environment for cells. This high biocompatibility makes collagen a perfect biomaterial for implantable medical products and scaffolds for in vitro testing systems. To manufacture collagen based solutions, porous sponges, membranes and threads for surgical and dental purposes or cell culture matrices, collagen rich tissues as skin and tendon of mammals are intensively processed by physical and chemical means. Other tissues such as pericardium and intestine are more gently decellularized while maintaining their complex collagenous architectures. Tissue processing technologies are organized as a series of steps, which are combined in different ways to manufacture structurally versatile materials with varying properties in strength, stability against temperature and enzymatic degradation and cellular response. Complex structures are achieved by combined technologies. Different drying techniques are performed with sterilisation steps and the preparation of porous structures simultaneously. Chemical crosslinking is combined with casting steps as spinning, moulding or additive manufacturing techniques. Important progress is expected by using collagen based bio-inks, which can be formed into 3D structures and combined with live cells. This review will give an overview of the technological principles of processing collagen rich tissues down to collagen hydrolysates and the methods to rebuild differently shaped products. The effects of the processing steps on the final materials properties are discussed especially with regard to the thermal and the physical properties and the susceptibility to enzymatic degradation. These properties are key features for biological and clinical application, handling and metabolization.

The Red Kangaroo pericardium as a material source for the manufacture of percutaneous heart valves

BACKGROUND

The kangaroo pericardium might be considered to be a good candidate material for use in the manufacture of the leaflets of percutaneous heart valves based upon the unique lifestyle. The diet consists of herbs, forbs and strubs. The kangaroo pericardium holds an undulated structure of collagen.

MATERIAL AND METHOD

A Red Kangaroo was obtained after a traffic fatality and the pericardium was dissected. Four compasses were cut from four different sites: auricular (AUR), atrial (ATR), sternoperitoneal (SPL) and phrenopericardial (PPL). They were investigated by means of scanning electron microscopy, light microscopy and transmission electron microscopy.

RESULTS

All the samples showed dense and wavy collagen bundles without vascularisation from both the epicardium and the parietal pericardium. The AUR and the ATR were 150±25μm thick whereas the SPL and the PPL were thinner at 120±20μm. The surface of the epicardium was smooth and glistening. The filaments of collagen were well individualized without any aggregation, but the banding was poorly defined and somewhat blurry.

CONCLUSION

This detailed morphological analysis of the kangaroo pericardium illustrated a surface resistant to thrombosis and physical characteristics resistant to fatigue. The morphological characteristics of the kangaroo pericardium indicate that it represents an outstanding alternative to the current sources e.g., bovine and porcine. However, procurement of tissues from the wild raises supply and sanitary issues. Health concerns based upon sanitary uncertainty and reliability of supply of wild animals remain real problems.

Biomechanical Behavior of Bioprosthetic Heart Valve Heterograft Tissues: Characterization, Simulation, and Performance

The use of replacement heart valves continues to grow due to the increased prevalence of valvular heart disease resulting from an ageing population. Since bioprosthetic heart valves (BHVs) continue to be the preferred replacement valve, there continues to be a strong need to develop better and more reliable BHVs through and improved the general understanding of BHV failure mechanisms. The major technological hurdle for the lifespan of the BHV implant continues to be the durability of the constituent leaflet biomaterials, which if improved can lead to substantial clinical impact. In order to develop improved solutions for BHV biomaterials, it is critical to have a better understanding of the inherent biomechanical behaviors of the leaflet biomaterials, including chemical treatment technologies, the impact of repetitive mechanical loading, and the inherent failure modes. This review seeks to provide a comprehensive overview of these issues, with a focus on developing insight on the mechanisms of BHV function and failure. Additionally, this review provides a detailed summary of the computational biomechanical simulations that have been used to inform and develop a higher level of understanding of BHV tissues and their failure modes. Collectively, this information should serve as a tool not only to infer reliable and dependable prosthesis function, but also to instigate and facilitate the design of future bioprosthetic valves and clinically impact cardiology.

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.

The effect of different treatment modalities on the calcification potential and cross-linking stability of bovine pericardium

Porcine heart valves and bovine pericardium exhibit suitable properties for use as substitutes in cardiothoracic surgery, but must meet several requirements to be safe and efficient. Treatment with glutaraldehyde (GA) render some of these requirements, but calcification and degradation post-implant remain a problem. This study aimed to identify additional biochemical treatments that will minimize calcification potential without compromising the physical properties of pericardium. Pericardium treated with GA calcified severely after 8 weeks in the subcutaneous rat model, compared to tissue treated with higher concentrations of glycosaminoglycans (GAG) and commercial Glycar patches. GA, lower concentrations GAG and Glycar pericardium had high denaturation temperatures due to enhanced cross-linking. Tensile strength of GA tissue was significantly lower than GAG-treated or Glycar tissues, due to lower water content with resultant lower flexibility and suppleness. Pericardium treated with 0.01 M GAG gave acceptable denaturation temperatures, tensile strength and reduced calcification potential. All tissue treatments evoked comparable host immune responses, and no significant difference in resistance to enzymatic degradation. Ineffective stabilization and fixation of cross-links following GAG treatment, as well as limited penetration into the pericardium, resulted in GAG leaching out into the surrounding host tissue or storage medium, and prohibits safe clinical use of such tissue.

The effects of anticalcification treatments and hydration on the molecular dynamics of bovine pericardium collagen as revealed by 13C solid-state NMR

This article describes a solid-state NMR (SSNMR) investigation of the influence of hydration and chemical cross-linking on the molecular dynamics of the constituents of the bovine pericardium (BP) tissues and its relation to the mechanical properties of the tissue. Samples of natural phenethylamine-diepoxide (DE)- and glutaraldehyde (GL)-fixed BP were investigated by (13)C cross-polarization SSNMR to probe the dynamics of the collagen, and the results were correlated to the mechanical properties of the tissues, probed by dynamical mechanical analysis. For samples of natural BP, the NMR results show that the higher the hydration level the more pronounced the molecular dynamics of the collagen backbone and sidechains, decreasing the tissue's elastic modulus. In contrast, in DE- and GL-treated samples, the collagen molecules are more rigid, and the hydration seems to be less effective in increasing the collagen molecular dynamics and reducing the mechanical strength of the samples. This is mostly attributed to the presence of cross-links between the collagen plates, which renders the collagen mobility less dependent on the water absorption in chemically treated samples.

Protein extraction and 2-DE of water- and lipid-soluble proteins from bovine pericardium, a low-cellularity tissue

Bovine pericardium (BP) is an important biomaterial used in the production of glutaraldehyde-fixed heart valves and tissue-engineering applications. The ability to perform proteomic analysis on BP is useful for a range of studies, including investigation of immune rejection after implantation. However, proteomic analysis of fibrous tissues such as BP is challenging due to their relative low-cellularity and abundance of extracellular matrix. A variety of methods for tissue treatment, protein extraction, and fractionation were investigated with the aim of producing high-quality 2-DE gels for both water- and lipid-soluble BP proteins. Extraction of water-soluble proteins with 3-(benzyldimethylammonio)-propanesulfonate followed by n-dodecyl beta-D-maltoside extraction and ethanol precipitation for lipid-soluble proteins provided the best combination of yield, spot number, and resolution on 2-DE gels (Protocol E2). ESI-quadrupole/ion trap or MALDI-TOF/TOF MS protein identifications were performed to confirm bovine origin and appropriate subcellular prefractionation of resolved proteins. Twenty-five unique, predominantly cytoplasmic bovine proteins were identified from the water-soluble fraction. Thirty-two unique, predominantly membrane bovine proteins were identified from the lipid-soluble fraction. These results demonstrated that the final protocol produced high-quality proteomic data from this important tissue for both cytoplasmic and membrane proteins.

Fixation of vascular grafts with increased glutaraldehyde concentration enhances mechanical properties without increasing calcification

Our objective was to study the effect of glutaraldehyde (GLU) concentration, heat, and photooxidation on mechanical properties and calcification of bovine pericardium grafts in an in vivo model. Fresh pericardia were treated as follows: 0.625% GLU for 7 days (standard); 0.625%, 1%, and 3% GLU at 4 degrees C for 20 days and 50 degrees C for additional 20 days; irradiation in cross-linking medium with metilene blue at 0 degrees C for 8 hours. Tissues were subjected to tensile mechanical tests (n = 76). Fixed patches were subcutaneously implanted in mice for 50 days (n = 16 per treatment). Calcification was assessed by atomic absorption spectrophotometry (n = 55) and von Kossa staining (n = 28). Analysis of variance and Tukey's test were used for statistical analysis. The 3% GLU and 3% GLU + heat treatments showed an enhancement of the mechanical properties above standard treatment. No significant difference was found in calcification between treatments. The 3% GLU treatment enhances the mechanical properties of the tissue above standard treatment without increasing calcification and without applying heat; therefore it is recommended for high-strength applications. Supplementary treatments to decrease calcification could be combined with this methodology to obtain a high-strength-low-calcification biomaterial for manufacturing of long-term cardiovascular grafts.