The glutaraldehyde-stabilized porcine aortic valve xenograft. I. Tensile viscoelastic properties of the fresh leaflet material

Enhance Fracture Toughness and Fatigue Resistance of Hydrogels by Reversible Alignment of Nanofibers

Biological tissues, such as heart valve, tendon, etc., possess excellent mechanical properties, which arises from their inherent anisotropic arrangement of soft and hard phases. Inspired by the anisotropic structures, many methods have been developed to synthesize hydrogels that can achieve mechanical properties comparable to biological tissues. Here, we describe a new method to enhance fracture toughness and fatigue resistance of hydrogels by introducing nanofibers which can reversibly align with elastic deformation to form an anisotropic structure. As a demonstration, we introduce stiff, rod-like cellulose nanocrystals (CNCs) into a polyacrylamide (PAAm) network. CNCs aggregate into clusters to form hard phases and entangle with the PAAm network. The CNC/PAAm composite hydrogel is initially isotropic, becomes anisotropic upon loading, and recovers to be isotropic upon unloading. During the deformation, the aligned CNC clusters at the crack tip can transmit the stress over the size of the cluster, effectively resisting crack growth. We use photoelasticity and small-angle X-ray scattering (SAXS) tests to observe the change of microstructures associated with deformation. The fracture toughness of CNC/PAAm hydrogels with different sizes of CNCs can reach 1000 J/m². The fatigue threshold is about 100 J/m², an order of magnitude higher than that of PAAm hydrogel. This work provides a simple and general method to strengthen hydrogels under both monotonic and cyclic loads.

Soft Materials by Design: Unconventional Polymer Networks Give Extreme Properties

Hydrogels are polymer networks infiltrated with water. Many biological hydrogels in animal bodies such as muscles, heart valves, cartilages, and tendons possess extreme mechanical properties including being extremely tough, strong, resilient, adhesive, and fatigue-resistant. These mechanical properties are also critical for hydrogels' diverse applications ranging from drug delivery, tissue engineering, medical implants, wound dressings, and contact lenses to sensors, actuators, electronic devices, optical devices, batteries, water harvesters, and soft robots. Whereas numerous hydrogels have been developed over the last few decades, a set of general principles that can rationally guide the design of hydrogels using different materials and fabrication methods for various applications remain a central need in the field of soft materials. This review is aimed at synergistically reporting: (i) general design principles for hydrogels to achieve extreme mechanical and physical properties, (ii) implementation strategies for the design principles using unconventional polymer networks, and (iii) future directions for the orthogonal design of hydrogels to achieve multiple combined mechanical, physical, chemical, and biological properties. Because these design principles and implementation strategies are based on generic polymer networks, they are also applicable to other soft materials including elastomers and organogels. Overall, the review will not only provide comprehensive and systematic guidelines on the rational design of soft materials, but also provoke interdisciplinary discussions on a fundamental question: why does nature select soft materials with unconventional polymer networks to constitute the major parts of animal bodies?

Rate-Dependent and Relaxation Properties of Porcine Aortic Heart Valve Biomaterials

Objective: This work evaluates the rate-dependent and relaxation properties of native porcine heart valves, glutaraldehyde fixed porcine pericardium, and decellularized sterilized porcine pericardium. Methods: Biaxial tension testing was performed at strain-rates of 0.001 s⁻¹, 0.01 s⁻¹, 0.1 s⁻¹, and 1 s⁻¹. Finally, relaxation testing for 300 s was performed on all heart valve biomaterials. Results: No notable rate-dependent response was observed for any of the three biomaterials with few significant differences between any strain-rates. For relaxation testing, native tissues showed the most pronounced drop in stress and glutaraldehyde the lowest drop in stress although no tissues showed anisotropy in the relaxation. Conclusions: Increasing the strain-rate of the three biomaterials considered does not increase the stress within the tissue. This indicates that there will not be increased fatigue from accelerated wear testing compared to loading at physiological strain-rates as the increase strain-rates would likely not significantly alter the tissue stress.

Materials and manufacturing perspectives in engineering heart valves: a review

Valvular heart diseases (VHD) are a major health burden, affecting millions of people worldwide. The treatments for such diseases rely on medicine, valve repair, and artificial heart valves including mechanical and bioprosthetic valves. Yet, there are countless reports on possible alternatives noting long-term stability and biocompatibility issues and highlighting the need for fabrication of more durable and effective replacements. This review discusses the current and potential materials that can be used for developing such valves along with existing and developing fabrication methods. With this perspective, we quantitatively compare mechanical properties of various materials that are currently used or proposed for heart valves along with their fabrication processes to identify challenges we face in creating new materials and manufacturing techniques to better mimick ​the performance of native heart valves.

Mechanical considerations for polymeric heart valve development: Biomechanics, materials, design and manufacturing

The native human heart valve leaflet contains a layered microstructure comprising a hierarchical arrangement of collagen, elastin, proteoglycans and various cell types. Here, we review the various experimental methods that have been employed to probe this intricate microstructure and which attempt to elucidate the mechanisms that govern the leaflet's mechanical properties. These methods include uniaxial, biaxial, and flexural tests, coupled with microstructural characterization techniques such as small angle X-ray scattering (SAXS), small angle light scattering (SALS), and polarized light microscopy. These experiments have revealed complex elastic and viscoelastic mechanisms that are highly directional and dependent upon loading conditions and biochemistry. Of all engineering materials, polymers and polymer-based composites are best able to mimic the tissue-level mechanical behavior of the native leaflet. This similarity to native tissue permits the fabrication of polymeric valves with physiological flow patterns, reducing the risk of thrombosis compared to mechanical valves and in some cases surpassing the in vivo durability of bioprosthetic valves. Earlier work on polymeric valves simply assumed the mechanical properties of the polymer material to be linear elastic, while more recent studies have considered the full hyperelastic stress-strain response. These material models have been incorporated into computational models for the optimization of valve geometry, with the goal of minimizing internal stresses and improving durability. The latter portion of this review recounts these developments in polymeric heart valves, with a focus on mechanical testing of polymers, valve geometry, and manufacturing methods.

High resolution three-dimensional strain mapping of bioprosthetic heart valves using digital image correlation

Transcatheter aortic valve replacement (TAVR) is a safe and effective treatment option for patients deemed at high and intermediate risk for surgical aortic valve replacement. Similar to surgical aortic valves (SAVs), transcatheter aortic valves (TAVs) undergo calcification and mechanical wear over time. However, to date, there have been limited publications on the long-term durability of TAV devices. To assess longevity and mechanical strength of TAVs in comparison to surgical bioprosthetic valves, three-dimensional deformation analysis and strain measurement of the leaflets become an inevitable part of the evaluation. The goal of this study was to measure and compare leaflet displacement and strain of two commonly used TAVs in a side-by-side comparison with a commonly used SAV using a high-resolution digital image correlation (DIC) system. 26-mm Edwards SAPIEN 3, 26-mm Medtronic CoreValve, and 25-mm Carpentier-Edwards PERIMOUNT Magna surgical bioprosthesis were examined in a custom-made valve testing apparatus. A time-varying, spatially uniform pressure was applied to the leaflets at different loading rates. GOM ARAMIS® software was used to map leaflet displacement and strain fields during loading and unloading. High displacement regions were found to be at the leaflet belly region of the three bioprosthetic valves. In addition, the frame of the surgical bioprosthesis was found to be remarkably flexible, in contrary to CoreValve and SAPIEN 3 in which the stent was nearly rigid under a similar loading condition. The experimental DIC measurements can be used to characterize the anisotropic materiel behavior of the bioprosthetic heart valve leaflets and validate heart valve computational simulations.

Consistent trilayer biomechanical modeling of aortic valve leaflet tissue

Aortic valve tissue exhibits highly nonlinear, anisotropic, and heterogeneous material behavior due to its complex microstructure. A thorough understanding of these characteristics permits us to develop numerical models that can shed insight on the function of the aortic valve in health and disease. Herein, we take a closer look at consistently capturing the observed physical response of aortic valve tissue in a continuum mechanics framework. Such a treatment is the first step in developing comprehensive multiscale and multiphysics models. We highlight two important aspects of aortic valve tissue behavior: the role of the collagen fiber microstructure and the native prestressing. We propose a model that captures these two features as well as the heterogeneous layer-scale topology of the tissue. We find the model can reproduce the experimentally observed multiscale mechanical behavior in a manner that provides intuition on the underlying mechanics.

Application of hydrogels in heart valve tissue engineering

With an increasing number of patients requiring valve replacements, there is heightened interest in advancing heart valve tissue engineering (HVTE) to provide solutions to the many limitations of current surgical treatments. A variety of materials have been developed as scaffolds for HVTE including natural polymers, synthetic polymers, and decellularized valvular matrices. Among them, biocompatible hydrogels are generating growing interest. Natural hydrogels, such as collagen and fibrin, generally show good bioactivity but poor mechanical durability. Synthetic hydrogels, on the other hand, have tunable mechanical properties; however, appropriate cell-matrix interactions are difficult to obtain. Moreover, hydrogels can be used as cell carriers when the cellular component is seeded into the polymer meshes or decellularized valve scaffolds. In this review, we discuss current research strategies for HVTE with an emphasis on hydrogel applications. The physicochemical properties and fabrication methods of these hydrogels, as well as their mechanical properties and bioactivities are described. Performance of some hydrogels including in vitro evaluation using bioreactors and in vivo tests in different animal models are also discussed. For future HVTE, it will be compelling to examine how hydrogels can be constructed from composite materials to replicate mechanical properties and mimic biological functions of the native heart valve.

Knitting for heart valve tissue engineering

Knitting is a versatile technology which offers a large portfolio of products and solutions of interest in heart valve (HV) tissue engineering (TE). One of the main advantages of knitting is its ability to construct complex shapes and structures by precisely assembling the yarns in the desired position. With this in mind, knitting could be employed to construct a HV scaffold that closely resembles the authentic valve. This has the potential to reproduce the anisotropic structure that is characteristic of the heart valve with the yarns, in particular the 3-layered architecture of the leaflets. These yarns can provide oriented growth of cells lengthwise and consequently enable the deposition of extracellular matrix (ECM) proteins in an oriented manner. This technique, therefore, has a potential to provide a functional knitted scaffold, but to achieve that textile engineers need to gain a basic understanding of structural and mechanical aspects of the heart valve and in addition, tissue engineers must acquire the knowledge of tools and capacities that are essential in knitting technology. The aim of this review is to provide a platform to consolidate these two fields as well as to enable an efficient communication and cooperation among these two research areas.

Electrospun Polyurethane and Hydrogel Composite Scaffolds as Biomechanical Mimics for Aortic Valve Tissue Engineering

In this study, a composite scaffold consisting of an electrospun polyurethane and poly(ethylene glycol) hydrogel was investigated for aortic valve tissue engineering. This multilayered approach permitted the fabrication of a scaffold that met the desired mechanical requirements while enabling the 3D culture of cells. The scaffold was tuned to mimic the tensile strength, anisotropy, and extensibility of the natural aortic valve through design of the electrospun polyurethane mesh layer. Valve interstitial cells were encapsulated inside the hydrogel portion of the scaffold around the electrospun mesh, creating a composite scaffold approximately 200 μm thick. The stiffness of the electrospun fibers caused the encapsulated cells to exhibit an activated phenotype that resulted in fibrotic remodeling of the scaffold in a heterogeneous manner. Remodeling was further explored by culturing the scaffolds in both a mechanically constrained state and in a bent state. The constrained scaffolds demonstrated strong fibrotic remodeling with cells aligning in the direction of the mechanical constraint. Bent scaffolds demonstrated that applied mechanical forces could influence cell behavior. Cells seeded on the outside curve of the bend exhibited an activated, fibrotic response, while cells seeded on the inside curve of the bend were a quiescent phenotype, demonstrating potential control over the fibrotic behavior of cells. Overall, these results indicate that this polyurethane/hydrogel scaffold mimics the structural and functional heterogeneity of native valves and warrants further investigation to be used as a model for understanding fibrotic valve disease.

Comparison and evaluation of biomechanical, electrical, and biological methods for assessment of damage to tissue collagen

In regard to evaluating tissue banking methods used to preserve or otherwise treat (process) soft allograft tissue, current tests may not be sufficiently sensitive to detect potential damage inflicted before, during, and after processing. Using controlled parameters, we aim to examine the sensitivity of specific biomechanical, electrical, and biological tests in detecting mild damage to collagen. Fresh porcine pulmonary heart valves were treated with an enzyme, collagenase, and incubated using various times. Controls received no incubation. All valves were cryopreserved and stored at -135 °C until being rewarmed for evaluation using biomechanical, permeability, and cell viability tests. Statistically significant time dependent changes in leaflet ultimate stress, (p = 0.006), permeability (p = 0.01), and viability (p ≤ 0.02, four different days of culture) were found between heart valves subjected to 0-15 min of collagenase treatment (ANOVA). However, no statistical significance was found between the tensile modulus of treated and untreated valves (p = 0.07). Furthermore, the trends of decreasing and increasing ultimate stress and viability, respectively, were somewhat inconsistent across treatment times. These results suggest that permeability tests may offer a sensitive, quantitative assay to complement traditional biomechanical and viability tests in evaluating processing methods used for soft tissue allografts, or when making changes to current validated methods. Multiple test evaluation may also offer insight into the mechanism of potential tissue damage such as, as is the case here, reduced collagen content and increased tissue porosity.

Anisotropic poly(ethylene glycol)/polycaprolactone hydrogel-fiber composites for heart valve tissue engineering

The recapitulation of the material properties and structure of the native aortic valve leaflet, specifically its anisotropy and laminate structure, is a major design goal for scaffolds for heart valve tissue engineering. Poly(ethylene glycol) (PEG) hydrogels are attractive scaffolds for this purpose as they are biocompatible, can be modified for their mechanical and biofunctional properties, and can be laminated. This study investigated augmenting PEG hydrogels with polycaprolactone (PCL) as an analog to the fibrosa to improve strength and introduce anisotropic mechanical behavior. However, due to its hydrophobicity, PCL must be modified prior to embedding within PEG hydrogels. In this study, PCL was electrospun (ePCL) and modified in three different ways, by protein adsorption (pPCL), alkali digestion (hPCL), and acrylation (aPCL). Modified PCL of all types maintained the anisotropic elastic moduli and yield strain of unmodified anisotropic ePCL. Composites of PEG and PCL (PPCs) maintained anisotropic elastic moduli, but aPCL and pPCL had isotropic yield strains. Overall, PPCs of all modifications had elastic moduli of 3.79±0.90 MPa and 0.46±0.21 MPa in the parallel and perpendicular directions, respectively. Valvular interstitial cells seeded atop anisotropic aPCL displayed an actin distribution aligned in the direction of the underlying fibers. The resulting scaffold combines the biocompatibility and tunable fabrication of PEG with the strength and anisotropy of ePCL to form a foundation for future engineered valve scaffolds.

Mechanics of the pulmonary valve in the aortic position

Mathematical models can provide valuable information to assess and evaluate the mechanical behavior and remodeling of native tissue. A relevant example when studying collagen remodeling is the Ross procedure because it involves placing the pulmonary autograft in the more demanding aortic valve mechanical environment. The objective of this study was therefore to assess and evaluate the mechanical differences between the aortic valve and pulmonary valve and the remodeling that may occur in the pulmonary valve when placed in the aortic position. The results from biaxial tensile tests of pairs of human aortic and pulmonary valves were compared and used to determine the parameters of a structurally based constitutive model. Finite element analyzes were then performed to simulate the mechanical response of both valves to the aortic diastolic load. Additionally, remodeling laws were applied to assess the remodeling of the pulmonary valve leaflet to the new environment. The pulmonary valve showed to be more extensible and less anisotropic than the aortic valve. When exposed to aortic pressure, the pulmonary leaflet appeared to remodel by increasing its thickness and reorganizing its collagen fibers, rotating them toward the circumferential direction.

The tensile and viscoelastic properties of aortic valve leaflets treated with a hyaluronidase gradient

Purpose

When diseased, aortic valves are typically replaced with bioprosthetic heart valves (BPHVs), porcine valves or bovine pericardium that are fixed in glutaraldehyde. These replacements fail within 10-15 years due to calcification and fatigue, and their failure coincides with a loss of glycosaminoglycans (GAGs). This study investigates this relationship between GAG concentration and the tensile and viscoelastic properties of aortic valve leaflets.

Methods

Aortic valve leaflets were dissected from porcine hearts and digested in hyaluronidase in concentrations ranging from 0-5 U/mL for 0-24 hours, yielding a spectrum of GAG concentrations that was measured using the uronic acid assay and confirmed by Alcian Blue staining. Digested leaflets with varying GAG concentrations were then tested in tension in the circumferential and radial directions with varying strain rate, as well as in stress relaxation.

Results

The GAG concentration of the leaflets was successfully reduced using hyaluronidase, although water content was not affected. Elastic modulus, the maximum stress, and hysteresis significantly increased with decreasing GAG concentration. Extensibility and the radius of transition curvature did not change with GAG concentration. The stress relaxation behavior and strain-rate independent nature of the leaflet did not change with GAG concentration.

Conclusions

These results suggest that GAGs in the spongiosa lubricate tissue motion and reduce stresses experienced by the leaflet. This study forms the basis for predictive models of BPHV mechanics based on GAG concentration, and guides the rational design of future heart valve replacements.

Anisotropic strain transfer through the aortic valve and its relevance to the cellular mechanical environment

Aortic valve interstitial cells are responsible for maintaining the valve in response to their local mechanical environment. However, the complex organization of the extracellular matrix means cell strains cannot be directly derived from gross strains, and knowledge of tissue structure-function correlations is fundamental towards understanding mechanotransduction. This study investigates strain transfer through the valve, hypothesizing that organization of the valve matrix leads to non-homogenous local strains. Radial and circumferential samples were cut from aortic valve leaflets and subjected to quasi-static mechanical characterization. Further samples were imaged using confocal microscopy, to determine local strains in the matrix. Mechanical data demonstrated that the valve was significantly stronger and stiffer when loaded circumferentially, comparable with previous studies. Micromechanical studies demonstrated that strain transfer through the matrix is anisotropic and indirect, with local strains consistently smaller than applied strains in both orientations. Under radial loading, strains were transferred linearly to cells. However, under circumferential loading, strains were only one-third of applied values, with a less direct relationship between applied and local strains. This may result from matrix reorganization, and be important for preventing cellular damage during normal valve function. These findings should be taken into account when investigating interstitial cell behaviours, such as cell metabolism and mechanotransduction.

Elastic fibers in the aortic valve spongiosa: a fresh perspective on its structure and role in overall tissue function

This study characterizes the elastic fiber structure within the aortic valve spongiosa, the middle layer of the tri-laminate leaflet. The layer is rich in glycosaminoglycans and proteoglycans, through which it resists compression and lubricates shear between the outer layers. Elastin in this layer forms a fine, interweaving structure, yet it is unclear how this particular structure, which uses elasticity to preload the leaflet, assists spongiosa function. In this study, immunohistochemistry (IHC) and scanning electron microscopy (SEM) are used to characterize spongiosa elastin, as well as investigate regional differences in structure. IHC for elastin highlights an intermediate structure which varies in thickness and density between regions. In particular, the spongiosa elastin is thicker in the hinge and coaptation region than in the belly. SEM of NaOH-digested leaflets shows a rectilinear pattern of elastic fibers in the hinge and coaptation region, as opposed to a radially oriented stripe pattern in the belly. In conclusion, elastic fibers in the spongiosa connect the two outer layers and vary regionally in structure, while possibly playing a role in responding to regionally specific loading patterns.

Aortic valve disease and treatment: the need for naturally engineered solutions

The aortic valve regulates unidirectional flow of oxygenated blood to the myocardium and arterial system. The natural anatomical geometry and microstructural complexity ensures biomechanically and hemodynamically efficient function. The compliant cusps are populated with unique cell phenotypes that continually remodel tissue for long-term durability within an extremely demanding mechanical environment. Alteration from normal valve homeostasis arises from genetic and microenvironmental (mechanical) sources, which lead to congenital and/or premature structural degeneration. Aortic valve stenosis pathobiology shares some features of atherosclerosis, but its final calcification endpoint is distinct. Despite its broad and significant clinical significance, very little is known about the mechanisms of normal valve mechanobiology and mechanisms of disease. This is reflected in the paucity of predictive diagnostic tools, early stage interventional strategies, and stagnation in regenerative medicine innovation. Tissue engineering has unique potential for aortic valve disease therapy, but overcoming current design pitfalls will require even more multidisciplinary effort. This review summarizes the latest advancements in aortic valve research and highlights important future directions.

Guidance for removal of fetal bovine serum from cryopreserved heart valve processing

Bovine serum is commonly used in cryopreservation of allogeneic heart valves; however, bovine serum carries a risk of product adulteration by contamination with bovine-derived infectious agents. In this study, we compared fresh and cryopreserved porcine valves that were processed by 1 of 4 cryopreservation formulations, 3 of which were serum-free and 1 that utilized bovine serum with 1.4 M dimethylsulfoxide. In the first serum-free group, bovine serum was simply removed from the cryopreservation formulation. The second serum-free formulation had a higher cryoprotectant concentration, i.e. 2 M dimethylsulfoxide, in combination with a serum-free solution. A colloid, dextran 40, was added to the third serum-free group with 2 M dimethylsulfoxide due to theoretical concerns that removal of serum might increase the incidence of tissue cracking. Upon rewarming, the valves were inspected and subjected to a battery of tests. Gross pathology revealed conduit cracking in 1 of 98 frozen heart valves. Viability data for the cryopreserved groups versus the fresh group demonstrated a loss of viability in half of the comparisons (p < 0.05). No significant differences were observed between any of the cryopreserved groups, with or without bovine serum. Neither routine histology, autofluorescence-based multiphoton imaging nor semiquantitative second-harmonic generation microscopy of extracellular matrix components revealed any statistically significant differences. Biomechanics analyses also revealed no significant differences. Our results demonstrate that bovine serum can be safely removed from heart valve processing and that a colloid to prevent cracking was not required. This study provides guidance for the assessment of changes in cryopreservation procedures for tissues.

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.

On the biomechanics of heart valve function

Heart valves (HVs) are fluidic control components of the heart that ensure unidirectional blood flow during the cardiac cycle. However, this description does not adequately describe the biomechanical ramifications of their function in that their mechanics are multi-modal. Moreover, they must replicate their cyclic function over an entire lifetime, with an estimated total functional demand of least 3x10(9) cycles. The focus of the present review is on the functional biomechanics of heart valves. Thus, the focus of the present review is on functional biomechanics, referring primarily to biosolid as well as several key biofluid mechanical aspects underlying heart valve physiological function. Specifically, we refer to the mechanical behaviors of the extracellular matrix structural proteins, underlying cellular function, and their integrated relation to the major aspects of valvular hemodynamic function. While we focus on the work from the author's laboratories, relevant works of other investigators have been included whenever appropriate. We conclude with a summary of important future trends.

Heart valve function: a biomechanical perspective

Heart valves (HVs) are cardiac structures whose physiological function is to ensure directed blood flow through the heart over the cardiac cycle. While primarily passive structures that are driven by forces exerted by the surrounding blood and heart, this description does not adequately describe their elegant and complex biomechanical function. Moreover, they must replicate their cyclic function over an entire lifetime, with an estimated total functional demand of least 3x10(9) cycles. As in many physiological systems, one can approach HV biomechanics from a multi-length-scale approach, since mechanical stimuli occur and have biological impact at the organ, tissue and cellular scales. The present review focuses on the functional biomechanics of HVs. Specifically, we refer to the unique aspects of valvular function, and how the mechanical and mechanobiological behaviours of the constituent biological materials (e.g. extracellular matrix proteins and cells) achieve this remarkable feat. While we focus on the work from the authors' respective laboratories, the works of most investigators known to the authors have been included whenever appropriate. We conclude with a summary and underscore important future trends.

Planar biaxial creep and stress relaxation of the mitral valve anterior leaflet

A fundamental assumption in mitral valve (MV) therapies is that a repaired or replaced valve should mimic the functionality of the native valve as closely as possible. Thus, improvements in valvular treatments are dependent on the establishment of a complete understanding of the function and mechanical properties of the native normal MV. In a recent study [Grashow et al. ABME 34(2), 2006] we demonstrated that the planar biaxial stress-strain relationship of the MV anterior leaflet (MVAL) exhibited minimal hysteresis and a stress-strain response independent of strain rate, suggesting that MVAL could be modeled as a "quasi-elastic" material. The objective of our current study was to expand these results to provide a more complete picture of the time-dependent mechanical properties of the MVAL. To accomplish this, biaxial stress-relaxation and creep studies were performed on porcine MVAL specimens. Our primary finding was that while the MVAL leaflet exhibited significant stress relaxation, it exhibited negligible creep over the 3-h test. These results furthered our assertion that the MVAL functionally behaves not as a linear or non-linear viscoelastic material, but as an anisotropic quasi-elastic material. These results appear to be unique in the soft tissue literature; suggesting that valvular tissues are unequalled in their ability to withstand significant loading without time-dependent material effects. Moreover, insight into these specialized characteristics can help guide and inform efforts directed toward surgical repair and engineered valvular tissue replacements.

Biaixal stress-stretch behavior of the mitral valve anterior leaflet at physiologic strain rates

Characterization of the mechanical properties of the native mitral valve leaflets at physiological strain rates is a critical step in improving our understanding of MV function and providing experimental data for dynamic constitutive models. We explored, for the first time, the effects of strain rate (from quasi-static to physiologic) on the biaxial mechanical properties of the native mitral valve anterior leaflet (MVAL). A novel high-speed biaxial testing device was developed, capable of achieving in vitro strain rates reported for the MVAL (Sacks et al., Ann. Biomed. Eng. 30(10):1280-1290, 2002). Porcine MVAL specimens were loaded to physiological load levels with cycle periods of 15, 1, 0.5, 0.1, and 0.05 s. The resulting loading stress-strain responses were found to be remarkably independent of strain rate. The hysteresis, defined as the fraction of the membrane strain energy between the loading and unloading curves tension-areal stretch curves, was low (approximately 12%) and did not vary with strain rate. The results of the present work indicated that MVAL tissues exhibit complete strain rate insensitivity at and below physiological strain rates under physiological loading conditions. These novel results suggest that experimental tests utilizing quasi-static strain rates are appropriate for constitutive model development for mitral valve tissues. The mechanisms underlying this quasi-elastic behavior are as yet unknown, but are likely an important functional aspect of native mitral valve tissues and clearly warrant further study.

Heart Valve Tissue Engineering

Tissue-engineered heart valves have been proposed by physicians and scientists alike to be the ultimate solution for treating valvular heart disease. Rather than replacing a diseased or defective native valve with a mechanical or animal tissue-derived artificial valve, a tissue-engineered valve would be a living organ, able to respond to growth and physiological forces in the same way that the native aortic valve does. Two main approaches have been attempted over the past 10 to 15 years: regeneration and repopulation. Regeneration involves the implantation of a resorbable matrix that is expected to remodel in vivo and yield a functional valve composed of the cells and connective tissue proteins of the patient. Repopulation involves implanting a whole porcine aortic valve that has been previously cleaned of all pig cells, leaving an intact, mechanically sound connective tissue matrix. The cells of the patients are expected to repopulate and revitalize the acellular matrix, creating living tissue that already has the complex microstructure necessary for proper function and durability. Regrettably, neither of the 2 approaches has fared well in animal experiments, and the only clinical experience with tissue-engineered valves resulted in a number of early failures and patient death. This article reviews the technological details of the 2 main approaches, their rationale, their strengths and weaknesses, and the likely mechanisms for their failure. Alternative approaches to valvular tissue engineering, as well as the role of industry in shaping this field in the future, are also reviewed.

Multiaxial mechanical behavior of biological materials

For native and engineered biological tissues, there exist many physiological, surgical, and medical device applications where multiaxial material characterization and modeling is required. Because biological tissues and many biocompatible elastomers are incompressible, planar biaxial testing allows for a two-dimensional (2-D) stress-state that can be used to fully characterize their three-dimensional (3-D) mechanical properties. Biological tissues exhibit complex mechanical behaviors not easily accounted for in classic elastomeric constitutive models. Accounting for these behaviors by careful experimental evaluation and formulation of constitutive models continues to be a challenging area in biomechanical modeling and simulation. The focus of this review is to describe the application of multiaxial testing techniques to soft tissues and their relation to modern biomechanical constitutive theories.

Effect of altered hydration on the internal shear properties of porcine aortic valve cusps

BACKGROUND

Dehydration of tissue due to glutaraldehyde fixation has been reported and was examined in this study of porcine aortic valve cusps. The effect of altered hydration on cusp internal shear properties was also examined.

METHODS

Hydration level was assessed by wet mass measurement of cusps stored in solutions for times up to 1000 minutes. Solutions used in this study included Hanks solution, porcine blood, 0.5% glutaraldehyde, and several dextran solutions. Shear testing was performed on physiologically hydrated, superhydrated, and dehydrated cusps.

RESULTS

There was very little difference between the physiologic and superhydrated leaflets; however, dehydration caused significant stiffening with increased hysteresis and stress relaxation.

CONCLUSIONS

Glutaraldehyde has been shown to increase shear stiffness of valve cusps. Tissue dehydration also increased shear stiffness but increased stress relaxation and hysteresis, which was contrary to observations reported after glutaraldehyde fixation. The significant effect of dehydration on cusp mechanical properties does not account for the effects observed after glutaraldehyde fixation, but it demonstrates that hydration level is an important factor that affects internal shear properties of valve cusps.

Biaxial mechanical properties of the native and glutaraldehyde-treated aortic valve cusp: Part II--A structural constitutive model

We have formulated the first constitutive model to describe the complete measured planar biaxial stress-strain relationship of the native and glutaraldehyde-treated aortic valve cusp using a structurally guided approach. When applied to native, zero-pressure fixed, and low-pressure fixed cusps, only three parameters were needed to simulate fully the highly anisotropic, and nonlinear in-plane biaxial mechanical behavior. Differences in the behavior of the native and zero- and low-pressure fixed cusps were found to be primarily due to changes in the effective fiber stress-strain behavior. Further, the model was able to account for the effects of small (< 10 deg) misalignments in the cuspal specimens with respect to the biaxial test axes that increased the accuracy of the model material parameters. Although based upon a simplified cuspal structure, the model underscored the role of the angular orientation of the fibers that completely accounted for extreme mechanical anisotropy and pronounced axial coupling. Knowledge of the mechanics of the aortic cusp derived from this model may aid in the understanding of fatigue damage in bioprosthetic heart valves and, potentially, lay the groundwork for the design of tissue-engineered scaffolds for replacement heart valves.

Biaxial mechanical properties of the natural and glutaraldehyde treated aortic valve cusp--Part I: Experimental results

To date, there are no constitutive models for either the natural or bioprosthetic aortic valve (AV), in part due to experimental complications related to the AV's small size and heterogeneous fibrous structure. In this study, we developed specialized biaxial testing techniques for the AV cusp, including a method to determine the local structure-strain relationship to assess the effects of boundary tethering forces. Natural and glutaraldehyde (GL) treated cusps were subjected to an extensive biaxial testing protocol in which the ratios of the axial tensions were held at constant values. Results indicated that the local fiber architecture clearly dominated cuspal deformation, and that the tethering effects at the specimen boundaries were negligible. Due to unique aspects of cuspal fiber architecture, the most uniform region of deformation was found at the lower portion as opposed to the center of the cuspal specimen. In general, the circumferential strains were much smaller than the radial strains, indicating a profound degree of mechanical anisotropy, and that natural cusps were significantly more extensible than the GL treated cusps. Strong mechanical coupling between biaxial stretch axes produced negative circumferential strains under equibiaxial tension. Further, the large radial strains observed could not be explained by uncrimping of the collagen fibers, but may be due to large rotations of the highly aligned, circumferential-oriented collagen fibers in the fibrosa. In conclusion, this study provides new insights into the AV cusp's structure-function relationship in addition to requisite data for constitutive modeling.

Mechanical properties of a porcine aortic valve fixed with a naturally occurring crosslinking agent

The study investigates the mechanical properties of porcine aortic valve leaflets fixed with a naturally occurring crosslinking agent, genipin, at distinct pressure heads. Fresh and the glutaraldehyde-fixed counterparts were used as controls. Subsequent to fixation, the changes in leaflet collagen crimps and its surface morphology were investigated by light microscopy and scanning electron microscopy (SEM). Also, the crosslinking characteristics of each studied group were determined by measuring its fixation index and denaturation temperature. In the mechanical testing, tissue strips made from each studied group were examined in both the circumferential and radial directions. Histological and SEM comparisons between fresh porcine aortic valve leaflet and those fixed at medium or high pressure revealed that the following changes may occur: elimination of the natural collagen crimping, and extensive loss of the endothelial layer. The denaturation temperatures of the glutaraldehyde-fixed leaflets were significantly greater than the genipin-fixed leaflets; however, their fixation indices were comparable. Generally, fixation pressure did not affect the crosslinking characteristics of the genipin- and glutaraldehyde-fixed leaflets. It was found that fixation of porcine aortic valves in genipin or glutaraldehyde did not alter the mechanical anisotropy observed in fresh valve leaflets. This indicated that the intramolecular and intermolecular crosslinks introduced into the collagen fibrils during fixation is of secondary importance to the presence of structural and mechanical anisotropy in fresh leaflet. Tissue fixation in genipin or glutaraldehyde may produce distinct crosslinking structures. However, the difference in crosslinking structure between the genipin- and glutaraldehyde-fixed leaflets did not seem to cause any significant discrepancies in their mechanical properties when compared at the same fixation pressure. Nevertheless, regardless of the crosslinking agent used, changes in mechanical properties and ruptured patterns were observed when the valve leaflets were fixed at distinct pressures.

Quasi-Linear Viscoelastic theory applied to internal shearing of porcine aortic valve leaflets

The elements of Quasi-Linear Viscoelastic (QLV) theory have been applied to model the internal shear mechanics of fresh and glutaraldehyde-fixed porcine aortic valve leaflets. A novel function estimation method was used to extract the material functions from experimental shear data obtained at one strain rate, and the model was used to predict the material response at different strain rates. In general, experiments and predictions were in good agreement, the larger discrepancies being in the prediction of peak stresses and hysteresis in cyclic shear. In shear, fixed tissues are stiffer (mean initial shear modulus, 13 kPa versus 427 Pa), take longer to relax to steady state (mean tau 2 4,736 s versus 1,764 s) with a slower initial relaxation rate (mean magnitude of G(0), 1 s-1 versus 5 s-1), and relax to a lesser extent than fresh tissues (mean percentage stress remaining after relaxation, 60 versus 45 percent). All differences were significant at p = 0.04 or less, except for the initial relaxation slope. We conclude that shear experiments can complement traditional tensile and biaxial experiments toward providing a complete mechanical description of soft biomaterials, particularly when evaluating alternative chemical fixation techniques.

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.

Application of dynamic computed tomography for measurements of local aortic elastic modulus

A novel computed tomographic (CT) technique used for the instantaneous measurement of the dynamic elastic modulus of intact excised porcine aortic vessels subjected to physiological pressure waveforms is described. This system was comprised of a high resolution X-ray image intensifier based computed tomographic system with limiting spatial resolution of 3.2 mm-1 (for a 40 mm field of view) and a computer-controlled flow simulator. Utilising cardiac gating and computer control, a time-resolved sequence of 1 mm thick axial tomographic slices was obtained for porcine aortic specimens during one simulated cardiac cycle. With an image acquisition sampling interval of 16.5 ms, the time sequences of CT slices were able to quantify the expansion and contraction of the aortic wall during each phase of the cardiac cycle. Through superficial tagging of the adventitial surface of the specimens with wire markers, measurement of wall strain in specific circumferential sectors and subsequent calculations of localised dynamic elastic modulus were possible. The precision of circumferential measurements made from the CT images utilising a cluster-growing segmentation technique was approximately +/- 0.25 mm and allowed determination of the dynamic elastic modulus E(dyn) with a precision of +/- 8 kPa. Dynamic elastic modulus was resolved as a function of the harmonics of the physiological pressure waveform and as a function of the angular position around the vessel circumference. Application of this dynamic CT (DCT) technique to seven porcine thoracic aortic specimens produced a circumferential average (over all frequency components) E(dyn) of 373 +/- 29 kPa. This value was not statistically different (p < 0.05) from the values of 430 +/- 77 and 390 +/- 47 kPa obtained by uniaxial tensile testing and volumetric measurements respectively.

The role of elastin in aortic valve mechanics

Recent morphologic observations of elastin structures in aortic valves suggest that elastin is mechanically coupled to collagen. Since the mechanical stiffness of elastin is considerably lower than that of collagen, and aortic valves contain relatively little elastin, the mechanical importance of elastin in heart valve function is not clear. We have hypothesized that elastin acts to return the collagen fiber structure back to a resting configuration between loading cycles. The objectives of this research were therefore to elucidate the mechanical relationship between elastin and collagen structures within the aortic valve. To isolate elastin in a morphologically intact state, whole porcine aortic valve leaflets were digested in 0.1 N sodium hydroxide solution (NaOH) at a temperature of 75 degrees C for 45 min. Elastin structures from the fibrosa and ventricularis were tested mechanically, and their loading curves compared to those of the original leaflet layers and to whole cusps. The elastin structures generated very low forces, having an elastic modulus only 0.05% that of the whole tissue. The contribution of elastin to tissue mechanics was significant at low strains and differed between the fibrosa and the ventricularis. Elastin tended to dominate the distensibility curves of the radial ventricularis, but participated very little in the fibrosa. The low but significant tensions produced by the elastin structures of the aortic valve, together with previously observed elastin morphology as well as the measurable preload of elastin, suggest that the purpose of elastin in the aortic valve leaflet is to maintain a specific collagen fiber configuration and return the fibers to this state, once external forces have been released.

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

OBJECTIVE

We have previously shown the benefits of dynamic fixation over conventional static fixation of bioprosthetic valves. In an attempt to increase the durability of bioprosthetic heart valves, we explored the benefit of low-concentration glutaraldehyde dynamic fixation.

METHODS

Pig aortic valves obtained fresh from the abattoir and excised with the entire root were dynamically fixed in glutaraldehyde phosphate buffer solutions varying in concentration from 0.05% to 2.5%. Denaturation temperatures were measured and mechanical testing was performed at low (3 mm/sec) to high physiologic rates (30 mm/sec) at 37 degrees C in isotonic modified Hanks solution.

RESULTS

When fixed dynamically in 0.05% glutaraldehyde solution for 24 hours, the tissue reached a degree of cross-linking (denaturation temperature = 82.8 degrees +/- 0.6 degree C) significantly higher than that obtained for 0.05% static fixation (denaturation temperature = 79.3 degrees +/- 0.9 degree C) (p < 0.05) but similar to that for conventional static fixation in 0.5% glutaraldehyde solution (denaturation temperature = 83.5 degrees +/- 0.3 degree C). After fixation in low-concentration glutaraldehyde (0.05%), final relaxation slopes and moduli in the circumferential direction were significantly higher than those for the statically fixed tissue but similar to those for the fresh tissue. However, both dynamic and static fixation had the effect of increasing tissue extensibility to similar extents in both directions, irrespective of glutaraldehyde concentration.

CONCLUSIONS

Dynamic glutaraldehyde fixation of a porcine aortic valve at lower concentrations resulted in a better degree of cross-linking and a material with biomechanical properties that more closely mimic those of natural heart valve tissue.

Dynamic glutaraldehyde fixation of a porcine aortic valve xenograft. I. Effect of fixation conditions on the final tissue viscoelastic properties

Sixty porcine aortic valves were fixed under dynamic conditions at specific durations, pressures and vibration rates in a 0.5% glutaraldehyde phosphate buffer (pH 7.4, 0.2 M). Tensile relaxation tests were performed at low through high extension rates (0.3, 3 and 30 mm s-1) and tissue denaturation temperatures were determined by the hydrothermal isometric tension method. Conventional statically fixed valves and fresh valves were used as controls. No differences between dynamic and static treatment were observed at pulsation rates above those expected in the physiological range (i.e. above 1.2 Hz) or at higher pressures such as 30 mmHg. However, differences in both stress relaxation rates and denaturation temperatures were delineated in milder fixation conditions, i.e. at low pressures (< 4 mmHg) and low vibration rates similar to that of the normal heart beat (approximately 1.2 Hz). In these conditions the relaxation rate of the dynamically fixed tissue (-7.4 +/- 0.7% of stress remaining per log(s)) was similar to that of the fresh tissue (-6.7 +/- 1.2% log(s-1)) and significantly higher than the statically treated tissue (-3.9 +/- 1.7% log(s-1)). The rates of stress relaxation appeared to be strain rate dependent in both radial and circumferential directions when the tissues were strained at physiological rates during testing (> approximately 15000% min-1). Dynamically treated valves showed higher denaturation temperatures (mean +/- SD) (89.4 +/- 0.5 degree C) compared with the statically fixed (82.7 +/- 1.4 degrees C) or untreated (fresh) valves (65.5 +/- 0.8 degree C). The results suggest a higher degree of internal cross-linking owing possibly to enhanced penetration of the glutaraldehyde reagent and a greater accessability of reactive cross-linking sites on the collagen molecules. Better stress relaxation rates are likely associated with an increase in potential shearing between adjacent collagen fibres thus preserving the natural stress-reducing mechanism of the fresh, untreated valves. The dynamically treated valves therefore possess characteristics that may enable them to better resist long-term mechanical fatigue and in vivo degradation.

Biaxial strain analysis of the porcine aortic valve

The function of a bioprosthetic heart valve is determined largely by the material properties of the valve cusps. The mechanics of natural and bioprosthetic valve cusps have been studied extensively using uniaxial tensile testing. This type of testing, however, does not duplicate the natural biaxial loading condition. Whole-valve biaxial testing therefore is preferred. The objective of the present study was to investigate the heterogeneity of the valve cusps by mapping out the regional variability of the biaxial strain versus pressure relationship. Whole porcine aortic valves were mounted horizontally, submerged in physiologic saline solution at 37 degrees C, and pressurized in the range of 0 to 130 mm Hg of pressure. The ventricular side of the cusps were marked with black dots and the three-dimensional position of these dots was recorded together with the aortic pressure. By calculating the distance between the dots in the radial and circumferential directions in different regions, the local strain versus pressure relationship was determined. The results showed that the valve cusp material strained by 23% +/- 0.8% in the radial direction and 10.0% +/- 0.5% in the circumferential direction before lock-up. It was also found that while the valve cusp was highly anisotropic in the central region, the basal region was relatively isotropic, and the cusp as a whole was asymmetrical in its distensibility.

Description of the mathematical law that defines the relaxation of bovine pericardium subjected to stress

A material subjected to traction stress increases in length; if we maintain the elongation constant, the stress varies over a period of time. This phenomenon has been referred to as relaxation. The purpose of this study was to define a mathematical law that relates the variation in stress to time when elongation remains constant in bovine pericardium. The mathematical function obtained after assaying 34 samples to the point of relaxation, subjected to initial stresses ranging from 0.17-10.07 MPa, responds to the following equation: y = -0.0252 + 0.953 alpha - (0.0165 + 0.015 alpha)lnt, where y is the stress withstood at an instant in time, t, after initial stress alpha. A normogram, validated by assays of up to 6,340 min duration (4.40 days), is presented for graphic calculation, permitting the computation of the loss of stress due to relaxation of this biomaterial, with initial stresses ranging from 1-10 MPa.

Biomechanical and structural properties of the explanted bioprosthetic valve leaflets

Porcine bioprosthesis were treated with 0.625% glutaraldehyde and stabilized under changing pressure from 4 to 30 mmHg. Bovine pericardium and 12 biovalves (of age between 14 days and 80 months) after implantation in the human body were investigated (7 porcine PB and 5 pericardial bioprosthesis--PCB). Circumferential and radial strips from porcine aortic valve leaflets, bovine pericardium and bioprosthetic leaflets were studied in light, transmitting and scanning electron microscopy. Uniaxial load tests were carried out to examine the deformability and strength of these tissues. Microscopic examination of the biovalves revealed that the PB and PCB tissue retained its original architecture, but with alterations in detailed structure. The collagen bundles stuck together with vacuolization between them. There were some areas of the collagen structure fragmentation which could lead to complete necrosis. Eighty months after implantation in patients, the PCB became more extensible and its ultimate strain increases 2.5 times. Ultimate stress decreases in the radial direction from 9.43 to 2.88 MPa, and in the circumferential direction from 9.43 to 6.44 MPa. Forty-eight months after implantation, PB tissue's ultimate stress decreases in the circumferential direction from 4.06 to 1.99 MPa. At the same time ultimate strain increases from 13 to 22%. This study is to improve the methods of tissue stabilization in 0.625% glutaraldehyde solution for the first 48 h at cyclic, changing construction of biovalves soft supporting stent after 48 h.

Anisotropic elasticity and strength of glutaraldehyde fixed bovine pericardium for use in pericardial bioprosthetic valves

Uniaxial tensile tests were performed on glutaraldehyde fixed bovine pericardial strips prepared from chemically modified pericardial samples. These samples originated from an area which demonstrated anisotropic mechanical properties in the native material and which is suitable for the construction of leaflets for pericardial bioprostheses. After glutaraldehyde fixation the tissue had retained its anisotropicity in stiffness and strength in two orthogonal directions. In the range of the functional stresses for a heart valve leaflet (< 1 MPa) the unconstrained fixation regime had modified the initial anisotropic elastic behavior into a more isotropic one. The implications of these findings are that leaflets manufactured from bovine pericardium can be made to resemble, to a degree, the well known anisotropy found in two orthogonal directions in natural human heart valve leaflets, or porcine bioprosthetic heart valve leaflets.

Natural preload of aortic valve leaflet components during glutaraldehyde fixation: effects on tissue mechanics

The mechanics of glutaraldehyde-fixed aortic valve leaflets depend largely on the amount of stress present during fixation. Our previous work has suggested that even when the aortic valve is flaccid, the leaflet components are preloaded. We have, therefore, hypothesized that fixing valve leaflets in this naturally preloaded state will affect the function of their components, the fibrosa and the ventricularis. We have compared the elastic response of fibrosa and ventricularis fixed under 'low' and 'zero' tensile and compressive preload by testing 120 of these layers: (i) fresh, (ii) glutaraldehyde-fixed, and (iii) isolated from whole porcine aortic valve leaflets fixed while intact. In both the radial and circumferential directions, the fibrosa from intact-fixed valves was more extensible than the fresh (39.2 vs 29.2% strain to high modulus phase at p < 0.0122, and 12.7 vs 8.1% strain, at p < 0.0003, respectively). The ventricularis from intact-fixed valves, however, was less extensible than when fresh (35.4 vs 63.7% strain, at p < 0.00001 in the radial direction). The fibrosa must have, therefore, been fixed under compression and the ventricularis under tension, when fixed together in the intact aortic valve cusp. The tensile stresses in the intact-fixed ventricularis produced a greater circumferential elastic modulus than in separately fixed tissue (9.62 vs 4.65 MPa, at p < 0.00001), likely through a fibre recruitment process. Compressive stresses in the fibrosa produced a decrease in the elastic modulus both radially and circumferentially (from 3.79 to 2.26 MPa at p < 0.0023, and from 9.55 to 4.65 MPa at p < 0.00001, respectively). Fixing porcine aortic valves at even minimal tensile and compressive preload, such as that which occurs naturally, significantly alters both the extensibility and the elastic modulus of the valve leaflet components.

Effect of pretreatment with epoxy compounds on the mechanical properties of bovine pericardial bioprosthetic materials

Early failures of bovine pericardial heart valves are due to leaflet perforation, tearing and calcification. Since glutaraldehyde fixation has been shown to produce marked changes in leaflet mechanics and has been linked to development of calcification, bovine pericardium fixed with the four hydrophilic epoxy formulations and their mechanical properties are studied in this paper. We measured the thicknesses, shrinkage temperatures, stress relaxations and stress-strain curves of bovine pericardiums after different treatments with (1) non-treatment (fresh), (2) glutaraldehyde (GA), (3) epoxy compounds followed by the posttreatment with GA (EP 1#, EP 2#), and (4) epoxy compounds (EP 3# and EP 4#). Results of this study showed that the hydrophilic epoxy compounds are good crosslinking agents. There are no significant differences of shrinkage temperature and ultimate tensile stress among all tissue samples pretreated with GA, EP 1# and EP 2#. However, the stress relaxations of tissue-samples pretreated with epoxy compounds followed by the posttreatment with GA (EP 1# and EP 2#) are significantly slower than that pretreated with GA, and the strains at fracture of EP 1# and EP 2# are also significantly larger than that of GA or epoxy compounds. These facts show that the bovine pericardium pretreated with the epoxy compound followed by the posttreatment with GA (EP 1# and EP 2#) possesses greater tenacity and potential durability in dynamic stress.

Micromechanics and mathematical modeling: an inside look at bioprosthetic valve function

A major contributing factor in the degeneration of glutaraldehyde-treated porcine xenograft bioprostheses is tearing of the valve cusps near their commissural attachment to the supporting stent. We have been examining aortic valves at the micromechanical level, and have developed several sensitive techniques to evaluate the biomechanical changes produced by the glutaraldehyde fixation process. Additionally, we have developed a mathematical modeling technique that stimulates valve function during the entire cardiac cycle. Our micromechanical tests have shown that compressive buckling is common to all fixed tissues, occurs at physiological bending curvatures, and is likely to be the primary mode of mechanical failure of bioprosthetic valves. We have also shown that existing glutaraldehyde fixation techniques inhibit the natural internal shearing of the valve cusps, and disable the interaction of the fibrosa and the ventricularis. With our modeling technique, we have shown that flexural stresses are indeed concentrated near the valve commissures, and that appropriate modifications of the supporting stent can reduce flexural deformations. With these new, more revealing techniques at hand, prospective valve designs can be better evaluated prior to large scale animals and clinical testing.