Viscoelasticity of dynamically fixed bioprosthetic valves. II. Effect of glutaraldehyde concentration

Resistance to Tensile Stress of a Bioadhesive Utilized for Medical Purposes: Loctite 4011

Sutures are the materials presently employed to secure and give shape to the valve leaflets of cardiac bioprostheses. Their high resistance and low degree of elasticity in comparison with the calf pericardium of which the leaflets are made generates internal stresses that contribute to the failure of the bioprostheses.

Biological adhesives are bonding materials that have begun to be utilized in surgery, although there is a lack of experience in their use with inert tissues or bioprostheses.

We report our study of Loctite 4011, a biological glue composed of a cyano-acrylate that has been employed for medical purposes, in which samples of pericardium bonded with this adhesive were subjected to uniaxial tensile stress. The samples were glued in such a way as to leave an overlap of 1 cm2 between the surfaces of the tissue. The series included 83 samples: 12 tested 24 h after bonding, 17 after 45 days, 17 after 90 days, 19 after 106 days and 18 after 152 days. The samples subjected to deferred trials were preserved using three types of chemical substances: glutaraldehyde, glycerol or saline plus antibiotics. The mean resistance to rupture of the series tested 24 h after gluing was 0.15 MPa (1.47 machine kg). This resistance remained nearly unchanged, regardless of the preservation solution employed, for at least 152 days, the time at which the study ended. The stress strain curves demonstrated a high degree of elasticity throughout the 152 days, a finding that was not influenced by the preservation solution.

This adhesive showed a considerable resistance to tensile stress, although probably insufficient to replace sutures. However, it maintained a surprisingly high degree of elasticity in the samples. Perhaps the time has come to combine these two elements, sutures and adhesives, to improve the elasticity of the structure without a loss of resistance, and increase the durability of bioprostheses.

Resistance and Stability of A New Method for Bonding Biological Materials Using Sutures and Biological Adhesives

The valve leaflets of cardiac bioprostheses are secured and shaped by sutures which, given their high degree of resistance and poor elasticity, have been implicated in the generation of stresses within the leaflets, contributing to the failure of the bioprostheses. Bioadhesives are bonding materials that have begun to be utilized in surgery, although there is a lack of experience in their use with inert tissues or bioprostheses.

Tensile testing is performed until rupture in samples of calf pericardium, a biomaterial employed in the manufacture of bioprosthetic heart valve leaflets.

One hundred and thirty-two trials are carried out in three types of samples: intact or control tissue ( n = 12); samples transected and glued in an overlapping manner with a cyanoacrylate ( n = 60); and samples transected, sewn with a commercially available suture material and reinforced at the suture holes with the same cyanoacrylate ( n = 60). Seven days after their preparation, 12 samples from each group, including the controls, are subjected to tensile testing until rupture and the findings are compared.

In the stability study, groups of 12 each of the remaining 48 glued and 48 sutured and glued samples underwent tensile testing until rupture on days 30, 60, 90, and 120, after their preparation.

The results show that bonding with the adhesive provided a resistance ranging between 1.04 and 1.87 kg, probably insufficient for use in valve leaflets, but also afforded a high degree of elasticity. After 120 days, both the glued and the sutured and glued series show excellent elastic behavior, with no rigidity or hardening of the pericardium. These samples present reversible elongation, or strain, when they surpass their elastic limit at rupture. This finding may be due to a load concentration that is damaging to the pericardium, to the behavior of the tissue as an amorphous material, or perhaps to both circumstances.

These results need to be confirmed in future studies as they may be of value in the design and manufacture of cardiac bioprostheses.

Histological/Biological Characterization of Decellularized Bovine Jugular Vein

Several deficiencies in currently available right ventricular valved conduits make them problematic for use in infants and children. A solution would be to develop a tissue-engineered valved conduit containing autologous cells. A method was devised to produce a decellularized bovine matrix scaffold for developing a tissue-engineered right ventricular valved conduit. Fresh bovine jugular veins were treated with sodium deoxycholate and Triton X-100. The major structural proteins of the fresh and decellularized jugular venous valves and vessel walls were detected by histological methods. Thickness, water absorption rate, water maintenance rate, disruption strength, and extensibility were determined. Circumferential and radial specimens of valves and vessel walls were subjected to tensile testing. Histological analysis showed that no cell fragments were retained within the decellularized matrix scaffold and the major structural proteins had been retained intact. There were no significant differences in thickness, rates of absorption and maintenance of water, disruption strength, and extensibility between the decellularized and fresh veins. It was concluded that this treatment can successfully remove cellular components while maintaining the major structural components and the histological and biological properties of bovine jugular veins.

Comparison of natural crosslinking agents for the stabilization of xenogenic articular cartilage

Osteochondral xenografts are potentially inexpensive, widely available alternatives to fresh allografts. However, antigen removal from xenogenic cartilage may damage the extracellular matrix and reduce compressive stiffness. Non-crosslinked xenogenic cartilage may also undergo rapid enzymatic degradation in vivo. We hypothesized that natural crosslinking agents could be used in place of glutaraldehyde to improve the mechanical properties and enzymatic resistance of decellularized cartilage. This study compared the effects of genipin (GNP), proanthocyanidin (PA), and epigallocatechin gallate (EGCG), on the physical and mechanical properties of decellularized porcine cartilage. Glutaraldehyde (GA) served as a positive control. Porcine articular cartilage discs were decellularized in 2% sodium dodecyl sulfate and DNase I followed by fixation in 0.25% GNP, 0.25% PA, 0.25% EGCG, or 2.5% GA. Decellularization decreased DNA by 15% and GAG by 35%. For natural crosslinkers, the average degree of crosslinking ranged from approximately 50% (EGCG) to 78% (GNP), as compared to 83% for the GA control. Among the natural crosslinkers, only GNP significantly affected the disc diameter, and shrinkage was under 2%. GA fixation had no significant effect on disc diameter. Decellularization decreased aggregate modulus; GA and GNP, but not EGCG and PA, were able to restore it to its original level. GNP, PA, and GA conferred a similar, almost complete resistance to collagenase degradation. EGCG also conferred substantial resistance but to a lesser degree. Overall, the data support our hypothesis and suggest that natural crosslinkers may be suitable alternatives to glutaraldehyde for stabilization of decellularized cartilage. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1037-1046, 2016.

How preconditioning affects the measurement of poro-viscoelastic mechanical properties in biological tissues

It is known that initial loading curves of soft biological tissues are substantially different from subsequent loadings. The later loading curves are generally used for assessing the mechanical properties of a tissue, and the first loading cycles, referred to as preconditioning, are omitted. However, slow viscoelastic phenomena related to fluid flow or collagen viscoelasticity are initiated during these first preconditioning loading cycles and may persist during the actual data collection. When these data are subsequently used for fitting of material properties, the viscoelastic phenomena that occurred during the initial cycles are not accounted for. The aim of the present study is to explore whether the above phenomena are significant for articular cartilage, by evaluating the effect of such time-dependent phenomena by means of computational modeling. Results show that under indentation, collagen viscoelasticity dominates the time-dependent behavior. Under UC, fluid-dependent effects are more important. Interestingly, viscoelastic and poroelastic effects may act in opposite directions and may cancel each other out in a stress-strain curve. Therefore, equilibrium may be apparent in a stress-strain relationship, even though internally the tissue is not in equilibrium. Also, the time-dependent effects of viscoelasticity and poroelasticity may reinforce each other, resulting in a sustained effect that lasts longer than suggested by their individual effects. Finally, the results illustrate that data collected from a mechanical test may depend on the preconditioning protocol. In conclusion, preconditioning influences the mechanical response of articular cartilage significantly and therefore cannot be neglected when determining the mechanical properties. To determine the full viscoelastic and poroelastic properties of articular cartilage requires fitting to both preconditioning and post-preconditioned loading cycles.

Structural mechanism for alteration of collagen gel mechanics by glutaraldehyde crosslinking

Soft collagenous tissues that are loaded in vivo undergo crosslinking during aging and wound healing. Bioprosthetic tissues implanted in vivo are also commonly crosslinked with glutaraldehyde (GA). While crosslinking changes the mechanical properties of the tissue, the nature of the mechanical changes and the underlying microstructural mechanism are poorly understood. In this study, a combined mechanical, biochemical and simulation approach was employed to identify the microstructural mechanism by which crosslinking alters mechanical properties. The model collagenous tissue used was an anisotropic cell-compacted collagen gel, and the model crosslinking agent was monomeric GA. The collagen gels were incrementally crosslinked by either increasing the GA concentration or increasing the crosslinking time. In biaxial loading experiments, increased crosslinking produced (1) decreased strain response to a small equibiaxial preload, with little change in response to subsequent loading and (2) decreased coupling between the fiber and cross-fiber direction. The mechanical trend was found to be better described by the lysine consumption data than by the shrinkage temperature. The biaxial loading of incrementally crosslinked collagen gels was simulated computationally with a previously published network model. Crosslinking was represented by increased fibril stiffness or by increased resistance to fibril rotation. Only the latter produced mechanical trends similar to that observed experimentally. Representing crosslinking as increased fibril stiffness did not reproduce the decreased coupling between the fiber and cross-fiber directions. The study concludes that the mechanical changes in crosslinked collagen gels are caused by the microstructural mechanism of increased resistance to fibril rotation.

Age-related changes in material behavior of porcine mitral and aortic valves and correlation to matrix composition

Recent studies showing significant changes in valvular matrix composition with age offer design criteria for age-specific tissue-engineered heart valves. However, knowledge regarding aging-related changes in valvular material properties is limited. Therefore, 6-week, 6-month, and 6-year-old porcine aortic valves (AV) and mitral valves (MV) were subjected to uniaxial tensile testing. In addition to standard material parameters, the radius of transition curvature (RTC) was measured to assess the acuteness of the transition region of the tension-strain curve. Radially, the MV had greater stiffness and a smaller RTC compared with the AV. Circumferentially, the center of the MV anterior leaflet (MVAC) had the highest stiffness (MVAC > AV > MV free edge [MVF]), greater stress relaxation (MVAC > MVF/AV), lowest extensibility (MVAC < AV < MVF), and smaller RTC compared with MVF (AV < MVAC < MVF). AV and MV radial strips had a larger RTC compared with circumferential strips. Aging elevated stiffness for MV and AV radial and circumferential strips, elevated stress relaxation in AV and MVF circumferential strips, and increased RTC for MV radial and MVF circumferential strips. In conclusion, there are significant age-related differences in the material properties of heart valves, which parallel differences in tissue composition and structure, likely impact valve function, and highlight the need for age-specific design goals for tissue-engineered heart valves.

The telescoping suture--Part 1: Does this technique improve the mechanical behavior of a biomaterial?: Calf pericardium

The authors study the mechanical behavior of calf pericardium employed in the construction of cardiac valve leaflets when subjected to telescoping suture, followed by tensile stress until rupture. One hundred twenty pericardial tissue samples were employed, 60 cut from root-to-apex and another 60 cut in transverse direction. Each of these two groups consisted of 12 control samples that were left unsutured and four sets of 12 samples each that were rejoined by telescoping suture using silk, Prolene, nylon or Gore-Tex., and subjected to tensile stress. At the rupture of the sutured tissues, the tensile stress of the suture materials ranged between 57.54 MPa for the series sewn lengthwise with Gore-tex and 114.08 MPa for the series sewn crosswise with silk. At these levels of stress, the deformation of the suture thread was much less marked than that of the calf pericardium, and internal stresses were produced that were difficult for the biomaterial to absorb. There was a loss of real load in all the sutured series when the observed resistance to rupture, expressed in kilograms, was compared with the estimated value. This loss of resistance did not invalidate the telescoping suture technique since the resistance to rupture was still much greater than that associated with suturing the two edges of the cut pericardium together. This report confirms the deleterious role of the shear force generated in the pericardium by the suture.

Assessment of pericardium in cardiac bioprostheses. A review

Cardiac valve bioprostheses are assessed in terms of their present and future clinical utility. The problems concerning durability basically involve early failure due to tears in the valve leaflets and late failure mainly associated with calcification of the biological tissue. New strategies for selection and chemical treatment of the biomaterials employed are analyzed, and the available knowledge regarding their mechanical behavior is reviewed. It is concluded that the durability of these devices, and thus their successful use in the future, depends on the knowledge of the interactions among the different biomaterials of which they are composed, the development of new materials, and the engineering design applied in their construction.