The influence of chemical treatment and suture on the elastic behavior of calf pericardium utilized in the construction of cardiac bioprostheses

Poor mechanical properties of biological tissue are known to cause wear, leading to the failure of cardiac bioprostheses made of calf pericardium. Different chemical agents such as sodium dodecyl sulfate (SDS) are presently being tested as possible inhibitors of the calcification process. The objective of this report was to determine the mechanical behavior of calf pericardium treated with SDS for 24 h and the influence of the suture on the mechanical properties of the tissue. Forty-eight samples were tested: 24 subjected to a standard treatment with glutaraldehyde (12 sewn with 4/0 silk suture thread) and 24 incubated with SDS for 24 h (12 sewn with the same suture thread). Each sutured and nonsutured sample was cut into two strips to yield paired samples. All were subjected to tensile stress to breaking point. The mean stress at breaking point in the nonsutured series treated with glutaraldehyde alone was 16.42 and 13.85 MPa, depending on the region of the pericardium, while in the sutured samples subjected to glutaraldehyde the mean stress was 7.50 and 7.63 MPa, respectively, differences which were statistically significant (p=0.03 and p=0.003, respectively) when the means for nonsutured samples from equivalent regions treated with glutaraldehyde were compared. The stress at breaking point was lower in the SDS-treated series, ranging between 2.60 and 3.56 MPa. The mathematical functions that govern the stress/strain or deformation were obtained. In the series of pericardium treated with SDS, deformations of 10% were produced with stresses of under 0.4 MPa, an outcome that is intolerable from the constructive point of view. We established a regression model that enabled us to determine the mechanical behavior of a sutured sample by testing a contiguous piece of tissue, with a high correlation coefficient (r > 0.99). We consider this finding to be of interest in the selection of pericardium for use in the construction of leaflets for cardiac bioprostheses. ©2000 Kluwer Academic Publishers     

Sutured calf pericardium: influence of the type and angle of the suture on mechanical behavior

Careful selection of the biological tissue to be used in cardiac bioprostheses and a thorough knowledge of its mechanical behavior, facilitating both the prediction of this behavior and the interactions between the tissue and the other materials employed, is the best approach to designing a durable implant. For this purpose, a study involving uniaxial tensile testing of calf pericardium was carried out. Two sets of three contiguous strips of tissue were obtained from each pericardial membrane, to perform a total of 144 trials. Two samples were sewn with one of four commercially available suture materials: Gore-Tex, nylon, Prolene and silk. In each set of three samples, the center strip remained intact and unsutured to serve as a control, while the left-hand strip was sutured at a 45° angle with respect to the longitudinal axis and the right-hand strip was sewn at a 90° angle. All the samples were tested until rupture. The results demonstrated a significant loss of mean load (p<0.01) in the sutured samples at rupture. The angle of the suture had no influence on these results, although the stress/strain curves showed that, as the tensile stress increased, the mechanical behaviors were less uniform. The rupture of the collagen fibers could explain this phenomenon. The pericardium sutured with Gore-Tex presented a greater strain, or deformation (elongation), at lower levels of stress, regardless of the angle of the suture. The tissue selection criteria, based on the use of paired samples, enabled a correct prediction of the mechanical behavior of the tissue, with excellent correlation coefficients (>0.98) and a high degree of homogeneity in the results.     

A new method for selecting calf pericardium for use in cardiac bioprostheses on the basis of morphological and mechanical criteria

The durability of existing calf pericardium bioprostheses is limited by phenomena such as mechanical stress and calcification, the factors most frequently implicated in valve failure. Varying the preferred direction of the collagen fibers influences the mechanical behavior of the pericardial membrane. Given this possible variation, a strict control of the selection of the biomaterial employed in the construction of valve leaflets is essential, but a reliable method of selection has yet to be established.This study describes the development of a new system of in vitro selection involving a hydraulic simulator that reproduces the mechanical behavior of pericardial membranes subjected to the stress of continuous flow.By combining morphological criteria such as thickness and homogeneity with those of mechanical behavior, and by selecting paired samples from different parts of the pericardium, we obtained excellent mathematical fits. Linear regression analysis provided the mode of predicting the tensile strength in a given sample when this value had been determined in its twin. The upper zones of calf pericardium, corresponding to either right or left ventricle but at a distance from ligamentous structures, showed the best mean results at rupture (60 MPa) and permitted the most reliable prediction. The expected stress for an elongation of 30% was 1.12 MPa, as was previously observed, with a 95% confidence interval of between 1.11 and 1.14 MPa.These trials, together with the careful selection of the pairs, should help to establish definitive selection criteria. © 2001 Kluwer Academic Publishers     

Bovine pericardium for heart valve bioprostheses:in vitro andin vivo characterization of new chemical treatments

This study is aimed at investigating the bovine pericardium treated with different chemical procedures applied to prevent dystrophic calcification; decellularization of the fresh pericardium (samples B and C); fixation of the pericardium with glutaraldehyde (samples A, B, C, D and E); detoxification with aminoacids (samples A and B) and storage in a solution of benzoic acid esters. Pericardial sacs were harvested and delivered to the laboratories to be submitted to the chemical treatments. The samples E have been treated as the samples D but before the implantation they were exposed to the surgical lamp in order to promote some drying. The samples were tested for their mechanical properties and shrinkage temperature (at 1 week and after 36 months). Thein vivo tests were performed by means of implantation in a paravertebral subcutaneous position in rats. At the explant (2, 4 and 8 weeks) the samples were submitted to histological assay and the calcium content determined by spectroscopic atomic absorption. All the samples showed loss of tensile strength and elongation at 36 months (except for the sample A), as well as a moderate diminution of the shrinkage temperature. The histology showed that the decellularized samples (B, C) were thicker than the others and the collagen fibres were extensively homogenated. The cell colonization was macrophagic for the samples A and D while it was also composed of giant cells in the samples B, C and E at 8 weeks. The von Kossa's staining was positive only for the samples D and E after 4 weeks of implantation. The calcium content of the samples D was 285.3 mg/g at 8 weeks while in E it was 44.4 mg/g dry tissue at 4 weeks; for the remaining samples the calcium content did not increase with the time (2.1–2.3 mg/g at 8 weeks). In conclusion, the pericardium decellularization and detoxification associated with its storage in a glutaraldehyde-free solution is a promising method in order to realize more durable pericardial bioprostheses. The investigated tissue treatments applied to the bovine pericardium permit removal of the calcium nucleation sites, and hence the avoidance of the severe drawback of the aldehyde crosslink, but at the same time maintain the necessary and well known tissue stability.     

The influence of elastic anisotropy on the propagation of fracture

The model of a propagating crack introduced by Craggs for an isotropic solid is extended to the case of general elastic anisotropy. The general theory for propagation under tensile or shear stresses is derived. As for most two dimensional problems in anisotropic elasticity, the solution involves the roots of a sixth degree polynomial so that it is necessary to proceed numerically at some stages. A computer program has been written to do this. This is used to show that the shape of the square cracks which are produced in the interior of silicon-iron by the internal pressure of electrolytic hydrogen may be due to elastic anisotropy. On this basis, predictions can be made as to the shape of cracks in other metals, in particular molybdenum, vanadium and tantalum. These metals are representative of the four combinations of fracture plane and deviation from isotropy. Explicit formulae are given for the special orientations where the sixth degree polynomial can be explicitly factorized.     

The influence of orientation on the photoluminescence behavior of ZnO thin films obtained by chemical solution deposition

A new type of ZnO thin films synthesized from chemical solution deposition at low temperature has been presented. X-ray powder diffraction and field emission scanning electron microscopy investigation reveal that the novel structured ZnO film is uniform and its [0001] direction is parallel to the substrate. The photoluminescence spectrum of this film shows strong ultraviolet band-gap emission and weak defect-related visible emission comparing to that of [0001]-oriented film, indicating high crystal quality of the non-[0001]-oriented ZnO film.     

Porcine pericardial membrane subjected to tensile testing: preliminary study of the process of selecting tissue for use in the construction of cardiac bioprostheses

The durability of cardiac bioprostheses is limited fundamentally by structural failure due to mechanical fatigue and calcification. In the present report, we analyze, using an in vitro hydraulic simulator to test tensile strength, the mechanical behavior of porcine pericardium for the purpose of establishing the criteria for selecting the biomaterial, taking into account both morphological criteria (thickness and homogeneity of the specimens) and mechanical criteria (stress at breaking point), using the epidemiological model of paired samples. The stress at breakage was found to range widely from 24.07 MPa to 100.29 MPa, although we observed no statistically significant differences when comparing the mean results in the different regions and zones of the pericardium being studies. The application of the selection criteria in the present series resulted in an excellent mathematical fit in terms of the stress/elongation (R ² > 0.95), making it possible to establish, by means of linear regression, the prediction of the tensile strength in one zone on the basis of the values observed in its paired specimen. © 2001 Kluwer Academic Publishers     

The influence of micro-formation on the in-plane elastic behaviour of paper

The in-plane stiffness of a paper sheet decreases as the amount of interfibre bonding is reduced. Part of this reduction has been explained in the past as due to the absence of stress in the constituent fibrous material in directions transverse to the fibre axis in a lightly bonded sheet. A model is offered to explain how the remaining reduction of stiffness for lightly bonded sheets is dependent on the uniformity of mass distribution, i.e., the formation, of the sheet. The theory is shown to be consistent with reported experimental results.     

Evolution of microstructure and mechanical behavior of concretes utilized in marine environments

Concrete in marine environments is exposed to chemical mechanisms of deterioration, most of them involving chloride and sulfate ions. The principal motivation of this study is to try to minimize the expansive reactions between the aggressive ions and the cement matrix. However, the most effective protection will undoubtedly be the one that prevents the penetration of aggressive substances. The ingress of any aggressive substance in concrete is determined by concrete’s porous structure, especially the accessible connected porosity. This porosity is defined by the composition of the concrete and the chemical characteristics of the cement. The results clearly show that concrete that includes silica fume is significantly less porous and less permeable. In the rest of the mixtures studied, the porosity is higher, and the pore radius is the most decisive factor in defining the permeability.     

The influence of wavelength in the X-ray measurement of elastic stress

The X-ray diffractometer method applied to the measurement of elastic stresses in mild steel is described. Chromium and cobalt radiation have been used to investigate the effect of X-ray wavelength on the accuracy of the technique. Effects of elastic anisotropy and beam penetration have been considered.     

The influence of the elastic modulus on computed craze surface stress distributions

The surface stresses along the contour of a crack-tip craze in a glassy thermoplastic can be computed from measured craze displacements by applying finite element methods. It is shown that calculated crack-tip craze surface stress distributions are highly dependent on the accuracy of the evaluation of the elastic modulus of the bulk polymer. Methods of estimating the modulus are considered. A method based on fitting the Dugdale model to the measured craze profile gives rate- and time-dependent moduli which are consistent with measured moduli at strain levels comparable with those in the compact tension specimen near the interface with the craze. The major part of the sample is at a much lower strain level and a new method, based on interference optical measurements and finite element computations of crack displacements, is developed to estimate the appropriate modulus. Craze surface stresses are computed for two cases in which the estimated modulus for the two parts of the sample significantly differ. In a creep test, the modulus near the craze is lower than that in the remainder of the specimen and it is seen that any stress concentration at the crack tip is suppressed. This explains why the crack remains stationary while the craze continues to grow. In the case of crack propagation, the modulus in the region around the craze has the higher value and the stress at the crack tip increases. The crack therefore continues to propagate. This revised version was published online in November 2006 with corrections to the Cover Date.     

The influence of elastic mismatch between indenter and substrate on Hertzian fracture

This paper is concerned with indentation testing of brittle materials. It is shown that a mismatch of elastic constants between the indenter and component being tested has a profound influence on the stress state induced. It is shown that the development of surface flaws into cracks is severely impeded if the indenter is more rigid than the substrate and vice versa. A quantitative assessment of this phenomenon, in terms of both the contact stress field generated and the stress intensity factor experienced by defects, is given. This permits a more precise determination of either the fracture toughness or the surface flaw distribution to be made.     

The influence of chemical composition and thermo-mechanical treatment on Ti–Nb–Ta–Zr alloys

Ti–Nb–Ta–Zr quaternary alloying system is very promising for biomedical alloys. It is due to good mechanical properties and corrosion resistance of titanium alloys. Moreover no potentially harmful elements are contained in this system.Mechanical properties were influenced by changing the chemical composition and by various heat-treatment operations. The alloys were prepared by arc melting and then they were hot forged (900–1000 °C). After solution treatment 850 °C/0.5 h/water quenched, cold swaging was carried out with section reduction about 85%. As final heat treatment aging at 450 °C/8 h/furnace cooling was used.Mechanical properties were measured from tensile tests results at cold swaged and aged specimens. The microstructure was observed by using light microscopy and transmission electron microscopy (TEM)-thin foils method. X-ray diffraction analysis reveals the phase composition. By using these techniques the changes in microstructure caused by precipitation during aging treatment were clarified. After aging, the presence of ω or α phases may occur. Influence of changing Zr and Ta contents on mechanical properties and also on precipitation of secondary phases during aging treatment was observed.     

The influence of tranverse fibre properties on the in-plane elastic behaviour of paper

The present work considers the importance of the transverse or non-axial fibre properties in determining the in-plane elastic behaviour of a paper sheet. Network models have long been used in considering the relationship between the mechanical behaviour od paper on the one hand,a nd the properties of the fibres which constitute the sheet and sheet structure on the other hand. It is shown that discrepancies between experimental results and the predictions of netweork models may be explanined by incorporation of tranverse fibre properties while maintaining the basic assumption of uniform strain throughout the sheet.     

The influence of the shape and elastic strains of quantum dots on diffuse X-ray scattering

The influence of the shape and elastic-strain fields of buried quantum dots (QDs) on the angular distribution of diffuse X-ray scattering is studied. Elastic lattice displacements were calculated using the Green function method. Diffuse scattering from a semiconductor matrix with QDs having the shape of a cylinder, truncated cone, and spheroid was numerically simulated. It is shown that angular distributions of diffuse scattering for QDs with different shapes have significant distinctions at a small volume density of nanostructures in the crystal. At a large QD density, which is typical for superlattices, these distinctions become weak.     

Tensile behavior contrast of basalt and glass fibers after chemical treatment

Basalt and glass fibers were treated with sodium hydroxide and hydrochloric acid solutions respectively for different periods of time. Both the mass loss ratio and the strength maintenance ratio of the fibers were examined after the treatment. The morphologies of the fiber surfaces were characterized using scanning electron microscopy and their compositions were analyzed using energy dispersive X-ray spectroscopy. For the basalt fibers, the acid resistance was much better than the alkali resistance. Nevertheless, for the glass fibers, the acid resistance was nearly the same as the alkali resistance. Based on the experimental results, possible corrosion mechanisms are addressed.     

The influence of cryostructure on the creep behavior of ice-rich permafrost

Unconfined constant stress creep (CSC) tests were performed in order to look at the influence of cryostructure on the creep behavior of ice-rich undisturbed permafrost soils and remolded frozen soils within the temperature range from − 1 °C to − 2 °C. Undisturbed ice-rich permafrost soils were sampled from a Pleistocene age yedoma or “ice-complex” permafrost deposit in Interior Alaska. Cryostructure or the pattern of ice inclusions within a frozen soil is a direct indicator of the geologic and cryogenic genesis of permafrost soils. The data indicate that cryostructure influences the creep behavior of permafrost soils. Undisturbed soils with massive cryostructure showed higher induced creep strains and minimum strain rates than the more ice-rich undisturbed soils. Remolded soils with massive cryostructure experienced significantly lower creep strains and lower strain rates than the undisturbed soils. Deformation rates increase rapidly above a threshold stress value for remolded soils. From an engineering viewpoint, use of creep rates from remolded soils is non conservative and under predicts the creep rates of undisturbed soils. The orientation of ice lenses can facilitate motion along the ice lens–soil contact. Similarly, folding of ice lenses may occur, thus inducing anisotropic lateral strains. The ice facies tested indicate that for the temperature and stress range tested, ice creeps at a slower rate than frozen soils.     

The influence of the degree of heterogeneity on the elastic properties of random sphere packings

The macroscopic mechanical properties of colloidal particle gels strongly depend on the local arrangement of the powder particles. Experiments have shown that more heterogeneous microstructures exhibit up to one order of magnitude higher elastic properties than their more homogeneous counterparts at equal volume fraction. In this paper, packings of spherical particles are used as model structures to computationally investigate the elastic properties of coagulated particle gels as a function of their degree of heterogeneity. The discrete element model comprises a linear elastic contact law, particle bonding and damping. The simulation parameters were calibrated using a homogeneous and a heterogeneous microstructure originating from earlier Brownian dynamics simulations. A systematic study of the elastic properties as a function of the degree of heterogeneity was performed using two sets of microstructures obtained from Brownian dynamics simulation and from the void expansion method. Both sets cover a broad and to a large extent overlapping range of degrees of heterogeneity. The simulations have shown that the elastic properties as a function of the degree of heterogeneity are independent of the structure generation algorithm and that the relation between the shear modulus and the degree of heterogeneity can be well described by a power law. This suggests the presence of a critical degree of heterogeneity and, therefore, a phase transition between a phase with finite and one with zero elastic properties.     

The influence of the elastic properties and dimensions of coatings on the stiffnes of encapsulated fiber composites

The effect of coating around fibers embedded in an isotropic matrix on the stiffness of the composite, due to the variation of the coating thickness and rigidity, is examined in this paper. The model of the representative volume element (RVE) of the composite consisted of a cylindrical fiber coated by an annular layer, the encapsulated fiber being embedded in a matrix annulus enveloped by an infinite region of an equivalent composite, which constitutes the rest of the composite. This model constitutes an extension of the Christensen and Kerner models. By varying the rigidity and the thickness of the encapsulation, the longitudinal, transverse and shear elastic moduli of the composite were determined. Interesting results concerning the composite stiffness were established by selecting types of composite having different rigidities for the constituent phases. Furthermore, specific examples of encapsulated fiber composites usually encountered in applications were considered where the properties of the composite are changed by the sizing effect due to encapsulations.