Remodelling of continuously distributed collagen fibres in soft connective tissues

Extracellular matrix remodelling plays an essential role in tissue engineering of load-bearing structures. The goal of this study is to model changes in collagen fibre content and orientation in soft connective tissues due to mechanical stimuli. A theory is presented describing the mechanical condition within the tissue and accounting for the effects of collagen fibre alignment and changes in fibre content. A fibre orientation tensor is defined to represent the continuous distribution of collagen fibre directions. A constitutive model is introduced to relate the fibre configuration to the macroscopic stress within the material. The constitutive model is extended with a structural parameter, the fibre volume fraction, to account for the amount of fibres present within the material. It is hypothesised that collagen fibre reorientation is induced by macroscopic deformations and the amount of collagen fibres is assumed to increase with the mean fibre stretch. The capabilities of the model are demonstrated by considering remodelling within a biaxially stretched cube. The model is then applied to analyse remodelling within a closed stented aortic heart valve. The computed preferred fibre orientation runs from commissure to commissure and resembles the fibre directions in the native aortic valve.     

Age-Dependent Changes in Geometry,Tissue Composition and Mechanical Properties of Fetal to Adult Cryopreserved Human Heart Valves

There is limited information about age-specific structural and functional properties of human heart valves, while this information is key to the development and evaluation of living valve replacements for pediatric and adolescent patients. Here, we present an extended data set of structure-function properties of cryopreserved human pulmonary and aortic heart valves, providing age-specific information for living valve replacements. Tissue composition, morphology, mechanical properties, and maturation of leaflets from 16 pairs of structurally unaffected aortic and pulmonary valves of human donors (fetal-53 years) were analyzed. Interestingly, no major differences were observed between the aortic and pulmonary valves. Valve annulus and leaflet dimensions increase throughout life. The typical three-layered leaflet structure is present before birth, but becomes more distinct with age. After birth, cell numbers decrease rapidly, while remaining cells obtain a quiescent phenotype and reside in the ventricularis and spongiosa. With age and maturation–but more pronounced in aortic valves–the matrix shows an increasing amount of collagen and collagen cross-links and a reduction in glycosaminoglycans. These matrix changes correlate with increasing leaflet stiffness with age. Our data provide a new and comprehensive overview of the changes of structure-function properties of fetal to adult human semilunar heart valves that can be used to evaluate and optimize future therapies, such as tissue engineering of heart valves. Changing hemodynamic conditions with age can explain initial changes in matrix composition and consequent mechanical properties, but cannot explain the ongoing changes in valve dimensions and matrix composition at older age.     

Improved prediction of the collagen fiber architecture in the aortic heart valve

Living tissues show an adaptive response to mechanical loading by changing their internal structure and morphology. Understanding this response is essential for successful tissue engineering of load-bearing structures, such as the aortic valve. In this study, mechanically induced remodeling of the collagen architecture in the aortic valve was investigated. It was hypothesized that, in uniaxially loaded regions, the fibers aligned with the tensile principal stretch direction. For biaxial loading conditions, on the other hand, it was assumed that the collagen fibers aligned with directions situated between the principal stretch directions. This hypothesis has already been applied successfully to study collagen remodeling in arteries. The predicted fiber architecture represented a branching network and resembled the macroscopically visible collagen bundles in the native leaflet. In addition, the complex biaxial mechanical behavior of the native valve could be simulated qualitatively with the predicted fiber directions. The results of the present model might be used to gain further insight into the response of tissue engineered constructs during mechanical conditioning.     

Stress related collagen ultrastructure in human aortic valves--implications for tissue engineering

Understanding the response of tissue structures to mechanical stress is crucial for optimization of mechanical conditioning protocols in the field of heart valve tissue engineering. In heart valve tissue, it is unclear to what extent mechanical loading affects the collagen fibril morphology. To determine if local stress affects the collagen fibril morphology, in terms of fibril diameter, its distribution, and the fibril density, this was investigated in adult native human aortic valve leaflets. Transmission electron microscopy images of collagen fibrils were analyzed at three locations: the commissures, the belly, and the fixed edge of the leaflets. Subsequently, the mechanical behavior of human aortic valves was used in a computational model to predict the stress distribution in the valve leaflet during the diastolic phase of the cardiac cycle. The local stresses at the three locations were related to the collagen fibril morphology. The fibril diameter and density varied significantly between the measured locations, and appeared inversely related. The average fibril diameter increased from the fixed edge, to the belly, and to the commissures of the leaflets, while fibril density decreased. Interestingly, these differences corresponded well with the level of stress at the locations. The presented data showed that large tissue stress is associated with greater average fibril diameter, lower fibril density, and wider fibril size distribution compared with low stress locations in the leaflets. The findings here provide insight in the effect of mechanical loading on the collagen ultrastructure, and are valuable to improve conditioning protocols for tissue engineering.     

Quantitative histology of the hypertrophied human heart

Myocardial hypertrophy accompanies systemic hypertension and aortic stenosis, i.e., pressure overload. In man cardiac failure only appears after years of pressure overload, during which time cardiac function had been maintained. The structural correlates of cardiac failure have been a subject of much interest for many years. Several hypotheses relating alterations in muscle fiber alignment, capillary density, or collagen content have been offered. The application of morphometric techniques has provided essential quantitative information on the structural components of the normal and diseased heart. These data indicate that muscle fiber alignment remains normal in the pressure overloaded heart despite the presence of hypertrophy or the appearance of clinical failure. On the other hand, capillary density is decreased and collagen content is increased in hypertrophied hearts. Chemical studies on collagen concentration however have yielded inconsistent results. The relative contribution of the microcirculation and collagenous structure of the myocardium on its respective O2 availability, mechanical behavior, and deterioration in pump function will require further investigation.     

Coronary artery anomalies and aortic valve morphology in the Syrian hamster

In the Syrian hamster, anomalies in the origin of the left coronary artery are significantly associated with the bicuspid condition of the aortic valve. In this species, bicuspid aortic valves are expressions of a trait, the variation of which takes the form of a phenotypic continuum, ranging from a tricuspid aortic valve with no commissural fusion to a bicuspid aortic valve with the aortic sinuses located in ventrodorsal orientation and devoid of any raphe. The intermediate stages of the continuum are represented by tricuspid aortic valves with a more or less extensive fusion of the ventral commissure and bicuspid aortic valves with a more or less developed raphe located in the ventral aortic sinus. The present study was designed to decide whether there is a gap between tricuspid and bicuspid aortic valves regarding the incidence of coronary artery anomalies, or whether this incidence varies according to the different tricuspid and bicuspid morphotypes of the continuum. The study was carried out in Syrian hamsters belonging to a single inbred family with a high incidence of tricuspid aortic valves with fusion of the ventral commissure, bicuspid aortic valves, and anomalies in the origin of the left coronary artery, i.e. single right coronary artery ostium in aorta, anomalous origin of the left coronary artery from the pulmonary artery, and anomalous origin of the left coronary artery from the dorsal aortic sinus. The specimens were examined by means of a stereomicroscope and, in several cases, scanning electron microscopy was also used. The relationships between anomalous coronary artery patterns and aortic valve morphologies were tested using a logistic regression model. The results obtained indicate that there is no discontinuity between tricuspid and bicuspid aortic valves regarding the incidence of coronary artery anomalies. The probability of occurrence of anomalous coronary artery patterns increases continuously according to the deviation degree of the aortic valve from its normal (tricuspid) design. The present findings suggest that in the Syrian hamster, the morphogenetic mechanisms involved in the formation of congenital anomalous aortic valves and anomalies in the origin of the left coronary artery, respectively, are strongly related from an aetiological viewpoint.     

Structural integrity of collagen and elastin in SynerGraft® decellularized-cryopreserved human heart valves

SynerGraft® (SG) decellularized–cryopreserved cardiac valve allografts have been developed to provide a valve replacement option that has reduced antigenicity, retained structural integrity, and the ability to be stored long-term until needed for implantation. However, it is critical to ensure that both the SG processing and cryopreservation of these allografts do not detrimentally affect the extracellular matrix architecture within the tissue. This study evaluates the effects of SG decellularization and subsequent cryopreservation on the extracellular matrix integrity of allograft heart valves. Human aortic and pulmonary valves were trisected, with one-third of each either left fresh (no further processing after dissection), decellularized, or decellularized and cryopreserved. Two-photon laser scanning confocal microscopy was used to visualize collagen and elastin in leaflets and conduits. The optimized percent laser transmission (OPLT) required for full dynamic range imaging of each site was determined, and changes in OPLT were used to infer changes in collagen and elastin signal intensity. Collagen fiber crimp period and collagen and elastin fiber diameter were measured in leaflet tissue. Statistically significant differences in OPLT and the dimensional characteristics of collagen and elastin in study groups were determined through single factor ANOVA. The majority of donor-aggregated average OPLT observations showed no statistically significant differences among all groups, indicating no difference in collagen or elastin signal strength. Morphometric analysis of collagen and elastin fibers revealed no significant alterations in treated leaflet tissues relative to fresh tissues. Collagen and elastin structural integrity within allograft heart valves are maintained through SynerGraft® decellularization and subsequent cryopreservation.     

Comparison of first and second heart sounds after mechanical heart valve replacement

In this article, the spectral features of first heart sounds (S1) and second heart sounds (S2), which comprise the mechanical heart valve sounds obtained after aortic valve replacement (AVR) and mitral valve replacement (MVR), are compared to find out the effect of mechanical heart valve replacement and recording area on S1 and S2. For this aim, the Welch method and the autoregressive (AR) method are applied on the S1 and S2 taken from 66 recordings of 8 patients with AVR and 98 recordings from 11 patients with MVR, thereby yielding power spectrum of the heart sounds. Three features relating to frequency of heart sounds and three features relating to energy of heart sounds are obtained. Results show that in comparison to natural heart valves, mechanical heart valves contain higher frequency components and energy, and energy and frequency components do not show common behaviour for either AVR or MVR depending on the recording areas. Aside from the frequency content and energy of the sound generated by mechanical heart valves being affected by the structure of the lungs–thorax and the recording areas, the pressure across the valve incurred during AVR or MVR is a significant factor in determining the frequency and energy levels of the valve sound produced. Though studies on native heart sounds as a non-invasive diagnostic method has been done for many years, it is observed that studies on mechanical heart valves sounds are limited. The results of this paper will contribute to other studies on using a non-invasive method for assessing the mechanical heart valve sounds.     

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.     

Atomic force microscopy and FT-IR spectroscopy investigations of human heart valves

Human aortic, mitral, tricuspid and pulmonary heart valves were investigated by the contact mode atomic force microscopy (AFM) in air, and using FT-IR spectroscopy in the frequency range 950-4000 cm(-1). Heart valves were collected post mortem from 65-78 years old patients who died from non-cardiac diseases. All of the examined valves showed considerable heterogeneity in the surface topography of collagen fibrils as well as in their organization on the tissue surface. The AFM images revealed areas with significantly different spatial organization of the collagen fibril bundles. We observed zones with multidirectional, stacked collagen fibrils as well as areas of thin fibrils packed regularly, densely and "in phase". The majority of the collagen fibrils reproduced the typical transverse D-banding pattern, with the band interval varying in rather wide range of 70-90 nm. Using AFM imaging, objects that correspond to some pathological states of heart valves at their early stages, i.e. some forms of mineral deposits, were observed. The FT-IR spectra allowed us to recognize main components, i.e. collagen and elastin, in di.erent layers (ventricularis, fibrosa) of the valve leaflets as well as they gave also support for the presence of mineral deposits on the valve surface. The presented results showed, that the AFM imaging and FT-IR spectroscopy can be applied as a complementary methods for structural characterization of heart valves at the molecular and supramolecular levels.     

Correlation between heart valve interstitial cell stiffness and transvalvular pressure: implications for collagen biosynthesis

It has been speculated that heart valve interstitial cells (VICs) maintain valvular tissue homeostasis through regulated extracellular matrix (primarily collagen) biosynthesis. VICs appear to be phenotypically plastic, inasmuch as they transdifferentiate into myofibroblasts during valve development, disease, and remodeling. Under normal physiological conditions, transvalvular pressures (TVPs) on the right and left side of the heart are vastly different. Hence, we hypothesize that higher left-side TVPs impose larger local tissue stress on VICs, which increases their stiffness through cytoskeletal composition, and that this relation affects collagen biosynthesis. To evaluate this hypothesis, isolated ovine VICs from the four heart valves were subjected to micropipette aspiration to assess cellular stiffness, and cytoskeletal composition and collagen biosynthesis were quantified by using the surrogates smooth muscle alpha-actin (SMA) and heat shock protein 47 (HSP47), respectively. VICs from the aortic and mitral valves were significantly stiffer (P < 0.001) than those from the pulmonary and tricuspid valves. Left-side isolated VICs contained significantly more (P < 0.001) SMA and HSP47 than right-side VICs. Mean VIC stiffness correlated well (r = 0.973) with TVP; SMA and HSP47 also correlated well (r = 0.996) with one another. Assays were repeated for VICs in situ, and, as with in vitro results, left-side VIC protein levels were significantly greater (P < 0.05). These findings suggest that VICs respond to local tissue stress by altering cellular stiffness (through SMA content) and collagen biosynthesis. This functional VIC stress-dependent biosynthetic relation may be crucial in maintaining valvular tissue homeostasis and also prove useful in understanding valvular pathologies.     

Characterization of collagen fiber architecture in the canine diaphragmatic central tendon.

The diaphragmatic central tendon (DCT), a collagenous soft tissue membrane, acts as a mechanical buffer between the costal and crural muscles. Its direction of mechanical anisotropy has been shown to correspond to the collagen fiber preferred directions. These preferred directions were determined by gross histological examination, and were thus qualitative. In this work we quantified the collagen fiber architecture throughout the DCT using small angle light scattering (SALS). Helium-Neon laser light was passed through tendon specimens and the resultant scattered light distribution, which characterized the local collagen fiber architecture, was recorded with a linear array of five photodiodes. Throughout the DCT two distinct collagen fiber populations were consistently found. For each population three parameters were determined: 1) the preferred directions of collagen fibers, 2) the volume fraction (Vf) of fibers, 3) OI, an orientation index, which ranges from 0 percent for a random network to 100 percent for a perfectly oriented network. Vector maps were used to display results from 1) and 2), and showed a primary group (G1) going from the crural to costal muscles and a secondary one (G2) running perpendicular to G1. Comparisons of Vf between G1 and G2 showed that G1 contained about three times as many fibers as G2, a ratio similar to that found for the degree of mechanical anisotropy. OI were found to be about 60 percent, indicating a high degree of orientation, with no significant regional or population differences (p less than 0.05). These quantitative results suggest that throughout the DCT the degree of mechanical anisotropy is controlled exclusively by Vf.     

Biomechanical properties of native and tissue engineered heart valve constructs

Due to the increasing number of heart valve diseases, there is an urgent clinical need for off-the-shelf tissue engineered heart valves. While significant progress has been made toward improving the design and performance of both mechanical and tissue engineered heart valves (TEHVs), a human implantable, functional, and viable TEHV has remained elusive. In animal studies so far, the implanted TEHVs have failed to survive more than a few months after transplantation due to insufficient mechanical properties. Therefore, the success of future heart valve tissue engineering approaches depends on the ability of the TEHV to mimic and maintain the functional and mechanical properties of the native heart valves. However, aside from some tensile quasistatic data and flexural or bending properties, detailed mechanical properties such as dynamic fatigue, creep behavior, and viscoelastic properties of heart valves are still poorly understood. The need for better understanding and more detailed characterization of mechanical properties of tissue engineered, as well as native heart valve constructs is thus evident. In the current review we aim to present an overview of the current understanding of the mechanical properties of human and common animal model heart valves. The relevant data on both native and tissue engineered heart valve constructs have been compiled and analyzed to help in defining the target ranges for mechanical properties of TEHV constructs, particularly for the aortic and the pulmonary valves. We conclude with a summary of perspectives on the future work on better understanding of the mechanical properties of TEHV constructs.     

A nonlinear anisotropic model for porcine aortic heart valves

The anisotropic property of porcine aortic valve leaflet has potentially significant effects on its mechanical behaviour and the failure mechanisms. However, due to its complex nature, testing and modelling the anisotropic porcine aortic valves remains a continuing challenge to date. This study has developed a nonlinear anisotropic finite element model for porcine heart valves. The model is based on the uniaxial experimental data of porcine aortic heart valve leaflet and the properties of nonlinear composite material. A finite element code is developed to solve this problem using the 8-node super-parameter nonlinear shells and the update Lagrangian method. The stress distribution and deformation of the porcine aortic valves with either uniform and non-uniform thicknesses in closed phase and loaded condition are calculated. The results showed significant changes in the stress distributions due to the anisotropic property of the leaflets. Compared with the isotropic valve at the same loading condition, it is found that the site of the peak stress of the anisotropic leaflet is different; the maximum longitudinal normal stress is increased, but the maximum transversal normal stress and in-plane shear stress are reduced. We conclude that it is very important to consider the anisotropic property of the porcine heart valves in order to understand the failure mechanism of such valves in vivo.     

Remodelling of the angular collagen fiber distribution in cardiovascular tissues

Understanding collagen fiber remodelling is desired to optimize the mechanical conditioning protocols in tissue-engineering of load-bearing cardiovascular structures. Mathematical models offer strong possibilities to gain insight into the mechanisms and mechanical stimuli involved in these remodelling processes. In this study, a framework is proposed to investigate remodelling of angular collagen fiber distribution in cardiovascular tissues. A structurally based model for collagenous cardiovascular tissues is extended with remodelling laws for the collagen architecture, and the model is subsequently applied to the arterial wall and aortic valve. For the arterial wall, the model predicts the presence of two helically arranged families of collagen fibers. A branching, diverging hammock-type fiber architecture is predicted for the aortic valve. It is expected that the proposed model may be of great potential for the design of improved tissue engineering protocols and may give further insight into the pathophysiology of cardiovascular diseases.     

Biaxial mechanical properties of bovine jugular venous valve leaflet tissues

Venous valve incompetence has been implicated in diseases ranging from chronic venous insufficiency (CVI) to intracranial venous hypertension. However, while the mechanical properties of venous valve leaflet tissues are central to CVI biomechanics and mechanobiology, neither stress–strain curves nor tangent moduli have been reported. Here, equibiaxial tensile mechanical tests were conducted to assess the tangent modulus, strength and anisotropy of venous valve leaflet tissues from bovine jugular veins. Valvular tissues were stretched to 60% strain in both the circumferential and radial directions, and leaflet tissue stress–strain curves were generated for proximal and distal valves (i.e., valves closest and furthest from the right heart, respectively). Toward linking mechanical properties to leaflet microstructure and composition, Masson’s trichrome and Verhoeff–Van Gieson staining and collagen assays were conducted. Results showed: (1) Proximal bovine jugular vein venous valves tended to be bicuspid (i.e., have two leaflets), while distal valves tended to be tricuspid; (2) leaflet tissues from proximal valves exhibited approximately threefold higher peak tangent moduli in the circumferential direction than in the orthogonal radial direction (i.e., proximal valve leaflet tissues were anisotropic; \(p<0.01\)); (3) individual leaflets excised from the same valve apparatus appeared to exhibit different mechanical properties (i.e., intra-valve variability); and (4) leaflets from distal valves exhibited a trend of higher soluble collagen concentrations than proximal ones (i.e., inter-valve variability). To the best of the authors’ knowledge, this is the first study reporting biaxial mechanical properties of venous valve leaflet tissues. These results provide a baseline for studying venous valve incompetence at the tissue level and a quantitative basis for prosthetic venous valve design.     

On the Presence of Affine Fibril and Fiber Kinematics in the Mitral Valve Anterior Leaflet

In this study, we evaluated the hypothesis that the constituent fibers follow an affine deformation kinematic model for planar collagenous tissues. Results from two experimental datasets were utilized, taken at two scales (nanometer and micrometer), using mitral valve anterior leaflet (MVAL) tissues as the representative tissue. We simulated MVAL collagen fiber network as an ensemble of undulated fibers under a generalized two-dimensional deformation state, by representing the collagen fibrils based on a planar sinusoidally shaped geometric model. The proposed approach accounted for collagen fibril amplitude, crimp period, and rotation with applied macroscopic tissue-level deformation. When compared to the small angle x-ray scattering measurements, the model fit the data well, with an r² = 0.976. This important finding suggests that, at the homogenized tissue-level scale of ∼1 mm, the collagen fiber network in the MVAL deforms according to an affine kinematics model. Moreover, with respect to understanding its function, affine kinematics suggests that the constituent fibers are largely noninteracting and deform in accordance with the bulk tissue. It also suggests that the collagen fibrils are tightly bounded and deform as a single fiber-level unit. This greatly simplifies the modeling efforts at the tissue and organ levels, because affine kinematics allows a straightforward connection between the macroscopic and local fiber strains. It also suggests that the collagen and elastin fiber networks act independently of each other, with the collagen and elastin forming long fiber networks that allow for free rotations. Such freedom of rotation can greatly facilitate the observed high degree of mechanical anisotropy in the MVAL and other heart valves, which is essential to heart valve function. These apparently novel findings support modeling efforts directed toward improving our fundamental understanding of tissue biomechanics in healthy and diseased conditions.     

Straightening of curved pattern of collagen fibers under load controls aortic valve shape

The network of collagen fibers in the aortic valve leaflet is believed to play an important role in the strength and durability of the valve. However, in addition to its stress-bearing role, such a fiber network has the potential to produce functionally important shape changes in the closed valve under pressure load. We measured the average pattern of the collagen network in porcine aortic valve leaflets after staining for collagen. We then used finite element simulation to explore how this collagen pattern influences the shape of the closed valve. We observed a curved or bent pattern, with collagen fibers angled downward from the commissures toward the center of the leaflet to form a pattern that is concave toward the leaflet free edge. Simulations showed that these curved fiber trajectories straighten under pressure load, leading to functionally important changes in closed valve shape. Relative to a pattern of straight collagen fibers running parallel to the leaflet free edge, the concave pattern of curved fibers produces a closed valve with a 40% increase in central leaflet coaptation height and with decreased leaflet billow, resulting in a more physiological closed valve shape. Furthermore, simulations show that these changes in loaded leaflet shape reflect changes in leaflet curvature due to modulation of in-plane membrane stress resulting from straightening of the curved fibers. This effect appears to play an important role in normal valve function and may have important implications for the design of prosthetic and tissue engineered replacement valves.     

Collagen type I and hyaluronic acid based hybrid scaffolds for heart valve tissue engineering

Tissue engineers have achieved limited success so far in designing an ideal scaffold for aortic valve; scaffolds lack in mechanical compatibility, appropriate degradation rate, and microstructural similarity. This paper, therefore, has demonstrated a carbodiimide-based sequential crosslinking technique to prepare aortic valve extracellular matrix mimicking (ECM) hybrid scaffolds from collagen type I and hyaluronic acid (HA), the building blocks of heart valve ECM, with tailorable crosslinking densities. Swelling studies revealed that crosslinking densities of parent networks increased with increasing the concentration of the crosslinking agents whereas crosslinking densities of hybrid scaffolds averaged from those of parent collagen and HA networks. Hybrid scaffolds also offered a wide range of pore size (66-126 μm) which fulfilled the criteria for valvular tissue regeneration. Scanning electron microscopy and images of Alcian blue-Periodic acid Schiff stained samples suggested that our crosslinking technique yielded an ECM mimicking microstructure with interlaced bands of collagen and HA in the hybrid scaffolds. The mutually reinforcing networks of collagen and HA also resulted in increased bending moduli up to 1660 kPa which spanned the range of natural aortic valves. Cardio sphere-derived cells (CDCs) from rat hearts showed that crosslinking density affected the available cell attachment sites on the surface of the scaffold. Increased bending moduli of CDCs seeded scaffolds up to two folds (2-6 kPa) as compared to the non-seeded scaffolds (1 kPa) suggested that an increase in crosslinking density of the scaffolds could not only increase the in vitro bending modulus but also prevented its disintegration in the cell culture medium.     

Functional analysis of bioprosthetic heart valves

Glutaraldehyde-treated bovine pericardium is used successfully as bioprosthetic material in the manufacturing of heart valves leaflets. The mechanical properties of bovine pericardial aortic valve leaflets seem to influence its mechanical behaviour and the failure mechanisms. In this study the effect of orthotropy on tricuspid bioprosthetic aortic valve was analysed, using a three-dimensional finite element model, during the entire cardiac cycle. Multiaxial tensile tests were also performed to determine the anisotropy of pericardium. Seven different models of the same valve were analysed using different values of mechanical characteristics from one leaflet to another, considering pericardium as an orthotropic material. The results showed that even a small difference between values along the two axes of orthotropy can negatively influence leaflets performance as regard both displacement and stress distribution. Leaflets of bovine pericardium bioprostheses could be manufactured to be similar to natural human heart valves reproducing their well-known anisotropy. In this way it could be possible to improve the manufacturing process, durability and function of pericardial bioprosthetic valves.