The effects of decellularization and cross-linking techniques on the fatigue life and calcification of mitral valve chordae tendineae

Moisture adsorption by decellularized bovine pericardium collagen matrices studied by terahertz pulsed spectroscopy and solid immersion microscopy

In this paper, terahertz (THz) pulsed spectroscopy and solid immersion microscopy were applied to study interactions between water vapor and tissue scaffolds-the decellularized bovine pericardium (DBP) collagen matrices, in intact form, cross-linked with the glutaraldehyde or treated by plasma. The water-absorbing properties of biomaterials are prognostic for future cell-mediated reactions of the recipient tissue with the scaffold. Complex dielectric permittivity of DBPs was measured in the 0.4-2.0 THz frequency range, while the samples were first dehydrated and then exposed to water vapor atmosphere with 80.0 ± 5.0% relative humidity. These THz dielectric measurements of DBPs and the results of their weighting allowed to estimate the adsorption time constants, an increase of tissue mass, as well as dispersion of these parameters. During the adsorption process, changes in the DBPs' dielectric permittivity feature an exponential character, with the typical time constant of =8-10 min, the transient process saturation at =30 min, and the tissue mass improvement by =1-3%. No statistically-relevant differences between the measured properties of the intact and treated DBPs were observed. Then, contact angles of wettability were measured for the considered DBPs using a recumbent drop method, while the observed results showed that treatments of DBP somewhat affects their surface energies, polarity, and hydrophilicity. Thus, our studies revealed that glutaraldehyde and plasma treatment overall impact the DBP-water interactions, but the resultant effects appear to be quite complex and comparable to the natural variability of the tissue properties. Such a variability was attributed to the natural heterogeneity of tissues, which was confirmed by the THz microscopy data. Our findings are important for further optimization of the scaffolds' preparation and treatment technologies. They pave the way for THz technology use as a non-invasive diagnosis tool in tissue engineering and regenerative medicine.

Collagen fibre-mediated mechanical damage increases calcification of bovine pericardium for use in bioprosthetic heart valves

In cases of aortic stenosis, bioprosthetic heart valves (BHVs), with glutaraldehyde-fixed bovine pericardium leaflets (GLBP), are often implanted to replace the native diseased valve. Widespread use of BHVs, however, is restricted due to inadequate long-term durability, owing specifically to premature leaflet failure. Mechanical fatigue damage and calcification remain the primary leaflet failure modes, where glutaraldehyde treatment is known to accelerate calcification. The literature in this area is limited, with some studies suggesting mechanical damage increases calcification and others that they are independent degenerative mechanisms. In this study, specimens which were non-destructively pre-sorted according to collagen fibre architecture and uniaxially cyclically loaded until failure or 1 million cycles, were placed in an in vitro calcification solution. The weakest specimen group (those with fibres aligned perpendicular to the load) had statistically significantly higher volumes of calcification when compared to those with a high fatigue life. Moreover, SEM imaging revealed that ruptured and damaged fibres presented calcium binding sites; resulting in 4 times more calcification in fractured samples in comparison to those which did not fail by fatigue. To the authors' knowledge, this study quantifies for the first time, that mechanical damage drives calcification in commercial-grade GLBP and that calcification varies spatially according to localised damage levels. These findings illustrate that not only is calcification of GLBP exacerbated by fatigue damage, but that both failure phenomena are underpinned by the collagen fibre organisation. Consequently, controlling for GLBP collagen fibre architecture in leaflets could minimise the progression of these primary failure modes in patient BHVs. STATEMENT OF SIGNIFICANCE: Mechanical damage and calcification are the primary premature failure modes of glutaraldehyde-fixed bovine pericardial (GLBP) leaflets in bioprosthetic heart valves. In this study, commercial-grade GLBP specimens which were uniaxially cyclically loaded to failure or 1 million cycles, were placed in an in vitro calcification solution. MicroCT and SEM analysis showed that localised calcification levels varied spatially according to damage, where ruptured fibres offered additional calcium binding sites. Furthermore, specimens with a statistically significant lower fatigue life were associated with statistically significant higher calcification. This study revealed that mechanical damage drives calcification of GLBP. Non-destructive pre-screening of collagen fibres demonstrated that both the fatigue life and calcification potential of commercial-grade GLBP, are underpinned by the collagen fibre architecture.

Mechanical testing of glutaraldehyde cross-linked mitral valves. Part two: Elastic and viscoelastic properties of chordae tendineae

The aim of this study was to assess whether the mechanical properties of mitral valve chordae tendineae are sensitive to being cross-linked under load. A total 64 chordae were extracted from eight porcine hearts. Two chordae (posterior basal) from each heart were subjected to uniaxial ramp testing and six chordae (two strut, two anterior basal and two posterior basal) were subjected to dynamic mechanical analysis over frequencies between 0.5 and 10 Hz. Chordae were either cross-linked in tension or cross-linked in the absence of loading. Chordae cross-linked under load transitioned from high to low extension at a lower strain than cross-linked unloaded chordae (0.07 cf. 0.22), with greater pre-transitional (30.8 MPa cf. 5.78 MPa) and post-transitional (139 MPa cf. 74.1 MPa) moduli. The mean storage modulus of anterior strut chordae ranged from 48 to 54 MPa for cross-linked unloaded chordae, as compared to 53-61 MPa cross-linked loaded chordae. The mean loss modulus of anterior strut chordae ranged from 2.3 to 2.9 MPa for cross-linked unloaded chordae, as compared to 3.8-4.8 MPa cross-linked loaded chordae. The elastic and viscoelastic properties of chordae following glutaraldehyde cross-linking are dependent on the inclusion/exclusion of loading during the cross-linking process; with loading increasing the magnitude of the material properties measured.

Mechanical testing of glutaraldehyde cross-linked mitral valves. Part one: In vitro mechanical behaviour

The aim of this study was to perform an initial assessment, in vitro, of the feasibility of using a glutaraldehyde cross-linked porcine mitral valve to retain acute functionality, focusing on assessing mitral regurgitation. Six porcine hearts were tested using an in vitro simulator. Testing was repeated following cross-linking of mitral valves; where cross-linking was achieved by placing them in a glutaraldehyde solution. The simulator enabled systolic pressure on the ventricular side of the valve to be mimicked. Following testing, mitral valve leaflets underwent Scanning Electron Microscopy of the ventricular surface of both the anterior and posterior leaflets (1 cm² samples). The peak pressure withstood by cross-linked valves was significantly lower than for untreated valves (108 mmHg cf. 128 mmHg for untreated valves; p  < 0.05). The peak pressure was typically reached 0.5 s later than for the untreated valve. While both cross-linked and untreated valves exhibited endothelium denudation, the unfixed valve had less endothelial loss. Glutaraldehyde cross-linking of porcine mitral valves may be of potential value in assessing improved bioprosthetic mitral valve replacements. However, a more immobile valve exhibiting endothelial denudation (i.e. sclerosis) was a possible concerns identified following in vitro acute assessment.

Decellularized tissues as platforms for in vitro modeling of healthy and diseased tissues

Biomedical engineers are at the forefront of developing novel treatments to improve human health, however, many products fail to translate to clinical implementation. In vivo pre-clinical animal models, although the current best approximation of complex disease conditions, are limited by reproducibility, ethical concerns, and poor accurate prediction of human response. Hence, there is a need to develop physiologically relevant, low cost, scalable, and reproducible in vitro platforms to provide reliable means for testing drugs, biomaterials, and tissue engineered products for successful clinical translation. One emerging approach of developing physiologically relevant in vitro models utilizes decellularized tissues/organs as biomaterial platforms for 2D and 3D models of healthy and diseased tissue. Decellularization is a process that removes cellular content and produces tissue-specific extracellular matrix scaffolds that can more accurately recapitulate an organ/tissue's native microenvironment compared to other natural or synthetic materials. Decellularized tissues hold enormous potential for in vitro modeling of various disease phenotypes and tissue responses to drugs or external conditions such as aging, toxin exposure, or even implantation. In this review, we highlight the need for in vitro models, the advantages and limitations of implementing decellularized tissues, and considerations of the decellularization process. We discuss current research efforts towards applying decellularized tissues as platforms to generate in vitro models of healthy and diseased tissues, and where we foresee the field progressing. A variety of organs/tissues are discussed, including brain, heart, kidney, large intestine, liver, lung, skeletal muscle, skin, and tongue. STATEMENT OF SIGNIFICANCE: Many biomedical products fail to reach clinical translation due to animal model limitations. Development of physiologically relevant in vitro models can provide a more economic, scalable, and reproducible means of testing drugs/therapeutics for successful clinical translation. The use of decellularized tissues as platforms for in vitro models holds promise, as these scaffolds can effectively replicate native tissue complexity, but is not widely explored. This review discusses the need for in vitro models, the promise of decellularized tissues as biomaterial substrates, and the current research applying decellularized tissues towards the creation of in vitro models. Further, this review provides insights into the current limitations and future of such in vitro models.

Mechanics and Microstructure of the Atrioventricular Heart Valve Chordae Tendineae: A Review

The atrioventricular heart valves (AHVs) are responsible for directing unidirectional blood flow through the heart by properly opening and closing the valve leaflets, which are supported in their function by the chordae tendineae and the papillary muscles. Specifically, the chordae tendineae are critical to distributing forces during systolic closure from the leaflets to the papillary muscles, preventing leaflet prolapse and consequent regurgitation. Current therapies for chordae failure have issues of disease recurrence or suboptimal treatment outcomes. To improve those therapies, researchers have sought to better understand the mechanics and microstructure of the chordae tendineae of the AHVs. The intricate structures of the chordae tendineae have become of increasing interest in recent literature, and there are several key findings that have not been comprehensively summarized in one review. Therefore, in this review paper, we will provide a summary of the current state of biomechanical and microstructural characterizations of the chordae tendineae, and also discuss perspectives for future studies that will aid in a better understanding of the tissue mechanics-microstructure linking of the AHVs' chordae tendineae, and thereby improve the therapeutics for heart valve diseases caused by chordae failures.

Tissue-Engineered Grafts from Human Decellularized Extracellular Matrices: A Systematic Review and Future Perspectives

Tissue engineering and regenerative medicine involve many different artificial and biologic materials, frequently integrated in composite scaffolds, which can be repopulated with various cell types. One of the most promising scaffolds is decellularized allogeneic extracellular matrix (ECM) then recellularized by autologous or stem cells, in order to develop fully personalized clinical approaches. Decellularization protocols have to efficiently remove immunogenic cellular materials, maintaining the nonimmunogenic ECM, which is endowed with specific inductive/differentiating actions due to its architecture and bioactive factors. In the present paper, we review the available literature about the development of grafts from decellularized human tissues/organs. Human tissues may be obtained not only from surgery but also from cadavers, suggesting possible development of Human Tissue BioBanks from body donation programs. Many human tissues/organs have been decellularized for tissue engineering purposes, such as cartilage, bone, skeletal muscle, tendons, adipose tissue, heart, vessels, lung, dental pulp, intestine, liver, pancreas, kidney, gonads, uterus, childbirth products, cornea, and peripheral nerves. In vitro recellularizations have been reported with various cell types and procedures (seeding, injection, and perfusion). Conversely, studies about in vivo behaviour are poorly represented. Actually, the future challenge will be the development of human grafts to be implanted fully restored in all their structural/functional aspects.

Effect of glutaraldehyde based cross-linking on the viscoelasticity of mitral valve basal chordae tendineae

BACKGROUND

Mitral valve failure can require repair or replacement. Replacement bioprosthetic valves are treated with glutaraldehyde prior to implantation. The aim of this study was to determine the changes in mechanical properties following glutaraldehyde fixation of mitral valve chordae.

METHODS

To investigate the impact of glutaraldehyde on mitral valve chordae, 24 basal chordae were dissected from four porcine hearts. Anterior and posterior basal (including strut) chordae were used. All 24 chordae were subjected to a sinusoidally varying load (mean level 2N, dynamic amplitude 2N) over a frequency range of 0.5-10 Hz before and after glutaraldehyde treatment.

RESULTS

The storage and loss modulus of all chordal types decreased following glutaraldehyde fixation. The storage modulus ranged from: 108 to 119 MPa before fixation and 67.3-87.4 MPa following fixation for basal chordae; 52.3-58.4 MPa before fixation and 47.9-53.5 MPa following fixation for strut chordae. Similarly, the loss modulus ranged from: 5.47 to 6.25 MPa before fixation and 3.63-4.94 MPa following fixation for basal chordae; 2.60-2.97 MPa before fixation and 2.31-2.93 MPa following fixation for strut chordae.

CONCLUSION

The viscoelastic properties of mitral valve chordae are affected by glutaraldehyde fixation; in particular, the reduction in storage moduli decreased with an increase in chordal diameter.

Suitability of EGCG as a Means of Stabilizing a Porcine Osteochondral Xenograft

As a non-crosslinked osteochondral xenograft would be mechanically inferior to native cartilage and vulnerable to premature degradation, we seek a safe and effective method of xenograft stabilization. The purpose of this study was to evaluate the capacity for epigallocatechin gallate (EGCG) to stabilize a decellularized porcine osteochondral xenograft through collagen crosslinking. Our objectives were to assess the effects of EGCG on the degree of crosslinking, mechanical properties, collagenase resistance, cytotoxicity, and in vitro biocompatibility. EGCG is a green tea polyphenol that acts as a collagen crosslinker. Porcine osteochondral plugs were decellularized and then crosslinked by soaking in EGCG. The degree of crosslinking, cartilage compressive stiffness, cartilage-bone interface strength, coefficient of friction, and residual mass after collagenase exposure all increased with an increasing EGCG concentration. With the exception of the coefficient of friction, EGCG treatment could restore mechanical properties to levels equal to, or exceeding those, of native cartilage. EGCG treatment profoundly increased the enzymatic resistance, and 1% EGCG provided protection equivalent to 1% glutaraldehyde. EGCG up to 0.5 mM was essentially not cytotoxic to chondrocytes embedded in alginate, and autologous chondrocytes attached to decellularized, EGCG-fixed cartilage were all viable five days after seeding. Results demonstrate that EGCG has many beneficial effects on a decellularized osteochondral xenograft, and may be suitable for use in stabilizing such a graft prior to implantation for the repair of a defect.

Accelerating effects of genipin-crosslinked small intestinal submucosa for defected gastric mucosa repair

Slow healing of gastric mucosa defects caused by endoscopic surgery is a common but severe clinical problem for lack of an effective treatment. Small intestinal submucosa (SIS) is a bio-derived extracellular matrix scaffold with remarkable repairing ability for soft tissue, but its rapid degradation and poor mechanical properties in the stomach environment limit its application for gastric mucosa regeneration. Herein, we modified SIS by genipin, a natural crosslinking agent, to improve its resistance against degradation in gastric juice and to promote the healing of gastric mucosa defects. The crosslinking characteristics of genipin-crosslinked SIS (GP-CR SIS) were evaluated by crosslinking degree, swelling ratio and FITR, respectively. GP-CR SIS was highly resistant to gastric juice digestion and had a great improvement in mechanical properties. Additionally, GP-CR SIS maintained excellent biocompatibility according to a cytotoxicity test, hemolysis test, and rat subcutaneous implant assay. In an in vivo study, we treated defected gastric mucosa with GP-CR SIS in a rabbit endoscopic submucosal dissection (ESD)-related ulcer model. After two weeks of surgical treatment, GP-CR SIS significantly expedited wound closure and ameliorated newly constructed tissue by providing a protective microenvironment for rapid granulation tissue formation and accelerating angiogenesis/re-epithelialization. In conclusion, this study demonstrates the huge therapeutic potential of GP-CR SIS scaffolds for accelerating defected gastric mucosa regeneration.