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The mechanical behavior of calf pericardium, a biomaterial utilized in the manufacture of cardiac bioprostheses, in response to a short tensile cyclic test has been evaluated. The trial involved 120 samples cut longitudinally or transversely, subjected to 10 cycles until a stress of between 1 and 3 MPa was reached. Tests of hardness and tear propagation were performed, and the results were compared with a control series. The energy loss was also computed, and it was approximately 10-fold greater in the first cycle than the loss in the subsequent nine cycles. Despite this singularity, they correlated very precisely. The effect of the direction in which the tissue is cut on energy loss was not significant nor the difference between hardness prior to and after testing. The results of the tear propagation tests gave no statistical differences prior to and after testing. From the obtained results, it seems that the test carried out does not affect significantly the mechanical properties of calf pericardium., (© 2012, Copyright the Authors. Artificial Organs © 2012, International Center for Artificial Organs and Transplantation and Wiley Periodicals, Inc.)

Young ostrich pericardia (biomaterial under study for manufacturing cardiac valve leaflets), has been subjected to biaxial tension fatigue until breakage. Supraphysiological values of pressure (1 to 6 atm) have been employed to accelerate damage and, therefore, to reduce testing time but at physiological frequency in order to avoid viscoelastic behaviour changes. The lifetime fatigue curves have been obtained and large scatter has been observed in the results but this can be strongly reduced with adequate material selection. The thickness-based selection of samples has proved to be ineffective both in reducing scatter or improving strength, but the energy-based selection aided with statistical decision techniques has been shown to be very successful. The energy loss (energy under the hysteresis loop of each load and unload cycle) appears to be a very accurate predictor of the expected fatigue lifetime of the tissue., (Copyright © 2012 Elsevier B.V. All rights reserved.)

The medium-term fatigue behaviour of calf pericardium (similar to the one used to manufacture cardiac bioprostheses valve leaflets) has been studied. 96 samples were tested under fatigue subjecting them to biaxial stress at 1 Hz frequency for 5000 cycles, in 4 series of 24 samples, at several supra-physiological mean pressures and pressure amplitudes. Short-term damage parameters such as the accumulated energy consumption in 10 cycles (E10) and medium-term ones after 5000 cycles like total energy consumption (Et) and maximum displacement of the membrane (Dt) have been evaluated. E10 showed exponential growing tendency with pressure and linear tendency with pressure amplitude when only one parameter curve was plotted. Similar results were found when analysing Et and Dt. Linear correlation models were established between E10 and Et and E10 and Dt. Similar results were achieved in the four series, with excellent determination coefficients. The results confirm that the fatigue behaviour from the very first cycles of the test can predict the medium-term behaviour of the tissue by means of measurement of suitable damage markers. The tendencies observed between the parameters seem to show that the results could have been the same ones if the test had been performed at physiological pressures and amplitudes. This work opens the door to a non-destructive test of the tissue prior to employ it to manufacture valve leaflets., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

Calf pericardium, similar to that used in the manufacturing of prosthetic valve cusps, was fatigue tested. After six batches of 100 cycles of 1 MPa of loading pressure, half of the samples broke. The mean energy dissipated in the first cycle by the surviving samples was 0.16 J, which is lower than the 0.28 J dissipated by the specimens that broke (p = 0.005). The hysteresis of the first cycle was characteristic and different from the following ones and correlated superbly with fatigue resistance. Setting a threshold value for the energy of the first cycle of 0.20 J, the performance index (the percentage of true predictions) was almost 80%, and the area under the ROC curve was 0.823 (maximum value is 1). When including the mean thickness in the selection parameters, as an indirect measure of the specimen mass, the performance index grew over 95%, meaning that the error of the predictions was less than 5%. Combining both parameters in one, a high performance index is maintained at 87.5% and the area under the ROC curve increases to 0.917. This non-destructive method should help optical methods in the process of selecting the most appropriate and homogenous biological material., (© 2010 Wiley Periodicals, Inc.)

The use of biological materials in the construction of bioprostheses requires the application of different chemical procedures to improve the durability of the material without producing any undesirable effects. A number of crosslinking methods have been tested in biological tissues composed mainly of collagen. The aim of this study was to evaluate the in vitro biocompatibility, the mechanical properties, and in vivo calcification of chemically modified bovine pericardium using glutaraldehyde acetals (GAAs) in comparison with glutaraldehyde (GA) treatment. Homsy's tests showed that the most cytotoxic treatment is GA whereas GAA treatments showed lower cytotoxicity. Regarding the mechanical properties of the modified materials, no significant differences in stress at rupture were detected among the different treatments. Zeta-Potential showed higher negative values for GA treatment (-4.9 +/- 0.6 mV) compared with GAA-0.625% (-2.2 +/- 0.5 mV) and GAA-1% (-2.2 +/- 0.4 mV), which presented values similar to native tissue. Similar results were obtained for calcium permeability coefficients which showed the highest values for GA treatment (0.12 +/- 0.02 mm(2)/min), being significantly lower for GAA treatments or non-crosslinked pericardium. These results confirmed the higher propensity of the GA-treated tissues for attraction of calcium cations and were in good agreement with the calcification degree obtained after 60 days implantation into young rats, which was significantly higher for the GA group (22.70 +/- 20.80 mg/g dry tissue) compared with GAA-0.625% and GAA-1% groups (0.49 +/- 0.28 mg/g dry tissue and 3.51 +/- 3.27 mg/g dry tissue, respectively; P < 0.001). In conclusion, GAA treatments can be considered a promising alternative to GA treatment.

The mechanical behavior of the young bull pericardium in a fatigue test has been studied. This material is a similar tissue to those used in valve leaflet construction for a cardiac bioprosthesis. The consumed energy on each test was evaluated and afterwards used as a predictor of the biomaterial strength. Two-hundred and nine samples were tested to cyclical fatigue. The cut-off point to determine the sample quality was whether or not they resisted at least 4500 cycles. Only 22 samples withstood over that point (10.52%). The samples were classified according to their fatigue behavior in excellent, undefined and unsuitable. By using as a reference the consumed energy in the first 25 cycles, we could distinguish correctly (between 93.2 and 96.1%) the unsuitable material and most of the excellent (between 78.1 and 95.2%). From the rejected material 77% was really detachable and from the accepted, only 50% was excellent, with an equal methodology. The receiver operating characteristics curve was employed to establish decision levels when selecting samples, being 0.85 the best area (theoretical maximum value of 1). It is concluded that the energy wasted is a good predictor of the strength of the tissue. More than 90% of the unsuitable material and 50% of the excellent material (5% of all the material) is detected with this method.

We studied the changes in energy consumption of samples of calf pericardium, when joined or not joined by sutures and adhesives, by means of hysteretic cycles. Sixty-four samples were subsequently subjected to tensile stress until rupture. An overlapping suture sewn in the form of a rectangle presented an acceptable mean resistance to rupture of over 10 MPa, although lower than the mean values in an unsutured control series where the mean resistance surpassed 15 MPa. The contribution of an acrylic adhesive to the resistance to rupture was negligible. The sutured samples that were reinforced with adhesives and had not been subjected to hysteretic cycles prior to rupture showed an anisotropic behavior. This behavior appeared to be lost in all the samples that underwent hysteretic cycles. We found an inflection point in the stress/strain curve following the stepwise increase in the load, with a value greater than and proximate to the final load applied. This inflection should be analyzed by means of microscopy. Finally, the mathematical relationship between the energy consumed and the stress applied, the strain or deformation produced and the number of cycles of hysteresis to which the samples were subjected was established as the ultimate objective of this study. The bonding systems provoked a greater consumption of energy, with the greatest consumption corresponding to the first cycle in all the series assayed. An equation relating the energy consumption in a sample to the number of hysteretic cycles to which it was subjected was obtained. Its asymptote on the x-axis indicates the energy consumption for a theoretical number of cycles, making it possible to estimate the durability of the sample.

The durability of prosthetic heart valve leaflets made of biological materials is limited. A tear in the biomaterial accelerates their early failure, but microtearing of the collagen fibers may be responsible for their medium-term failure. We studied the force necessary to propagate tearing in two biomaterials: ostrich and calf pericardium. One hundred twenty samples of each tissue were tested in an Elmendorf pendulum capable of measuring the force required to tear a tissue in which a predefined slit had been made. The forces required to produce tears, ranging between 2.5 and 0.25 cm in length, were determined. For ostrich pericardium, this force ranged between 67.67 and 4.80 newton, while that required to tear the same lengths of calf pericardium ranged between 70.67 and 4.70 newton. The function that relates the tearing force to the length of the tear was expressed as follows: y = 20.62x + 1.77x(2) (R(2) = 0.923) for ostrich pericardium and y = 45.57x - 7.21x(2) (R(2) = 0.936) for calf pericardium, where y is the force in newton and x is the length in centimeter. Calf pericardium was found to have a greater resistance to tearing. However, these results should be interpreted with caution owing to the fact that the thickness of the majority of the samples of ostrich pericardium was significantly less than that of calf pericardium. A more careful selection and utilization of adult ostrich pericardium would probably improve these results., ((c) 2006 Wiley Periodicals, Inc.)

We have performed a comparative analysis of glutaraldehyde-preserved ostrich pericardium, as a novel biomaterial, with bovine pericardium. The biochemical characteristics (histology, water content, amino acid composition, and collagen and elastin contents), mechanical properties, and in vivo calcification in a subcutaneous rat model were examined. Ostrich pericardium is slightly thinner and shows a higher water content (70+/-2% vs. 62+/-2%) than bovine pericardium. Additionally, ostrich pericardium presents 1.6-fold lower elastin content and a lower percentage of collagen in reference to the total protein content (68+/-2% vs. 76+/-2%). However, ostrich pericardium shows better mechanical properties, with higher tensile stress at rupture (32.4+/-7.5 vs. 11.5+/-4.6) than calf pericardium. In vivo calcification studies in a rat subcutaneous model show that ostrich pericardium is significantly less calcified than bovine pericardium (23.95+/-13.30 vs. 100.10+/-37.36 mg/g tissue) after 60 days of implantation. In conclusion, glutaraldehyde-stabilized ostrich pericardium tissue shows better mechanical properties than calf tissue. However, calcium accumulation in implanted ostrich tissue is still too high to consider it a much better alternative to bovine pericardium, and anticalcification treatments should be considered.

We studied the mechanical behavior in response to tensile stress of samples of ostrich pericardium bonded with a cyanoacrylate glue or sewn with a rectangular, overlapping suture that was subsequently sealed with the same bioadhesive. Seventy-two trials were performed in three series of 24 samples each: series AG, glued with an overlap of 1 cm2; series ASG, sewn with a rectangular, overlapping suture and sealed; and series AC, control samples that were left intact. The mean stress at rupture in series AG (glued) was 0.1 MPa, much lower than the working stress of a human valve leaflet, which is approximately 0.25 MPa. In the control series, this stress was 26.28 MPa. At rupture in series ASG (sutured/glued), the suture material was being subjected to a stress of 64.91 MPa, thus confirming the existence of an interaction between the suture and the shear stress exerted by the suture on the samples of pericardium. In series ASG, the mean value for the resistance to rupture when measured in machine kg was 8.83 kg, lower than but similar to that recorded in the control series AC (10.26 kg). The percentages of reversible deformation, or elongation, once the samples were torn were similar in series AC (19.15%) and ASG (21.93%). This phenomenon can only be explained by the damage to the collagen fibers in the area around the rupture, while other more distant regions work at a lower load within the elastic limit. We conclude that cyanocrylate adhesives alone are not suitable as bonding materials in cardiac bioprostheses. The results with the rectangular, overlapping suture, when subsequently sealed with an adhesive, can be considered good because, although this approach does not impede shear stress, it does maintain an excellent degree of resistance to rupture of the samples thus joined. We stress the need to take into account the concentration of the load in the design of bioprostheses., (Copyright 2004 Wiley Periodicals, Inc.)

We compared the mechanical resistance of 18 samples of calf pericardium bonded with a 100 mm2 overlap, by two types of glues: a cyanoacrylate (Loctite 4011) and a bioadhesive (BioGlue). Comparative tensile testing was also carried out in 40 paired samples, 20 bonded with the cyanoacrylate and 20 unbonded controls. The findings at rupture showed a greater resistance of the calf pericardium glued with cyanoacrylate, with a mean tensile strength of 0.15 MPa vs. 0.04 MPa for the biological glue (p= 0.000). They also demonstrated a loss of resistance of the samples bonded with cyanoacrylate when compared with that of the unbonded other halves of the pairs: 0.20 MPa and 0.27 MPa vs. 19.47 MPa and 24.44 MPa (p < 0.001). The method of selection by means of paired samples made it possible to establish the equations that relate the stress and strain, or deformation, with excellent coefficients of determination (R2). These equations demonstrate the marked elastic behaviour of the bonded samples. Moreover, these findings show the cyanoacrylate to be superior to the biological glue, leading to the examination of the compatibility, inalterability over time and mechanical behaviour of the cyanoacrylate in sutured samples, as well as the study of the anisotropy of the biomaterial when bonded with a bioadhesive.

Sutures are the materials presently employed to secure and give shape to the valve leaflets of cardiac bioprostheses. Their high resistance and low degree of elasticity in comparison with the calf pericardium of which the leaflets are made generates internal stresses that contribute to the failure of the bioprostheses. Biological adhesives are bonding materials that have begun to be utilized in surgery, although there is a lack of experience in their use with inert tissues or bioprostheses. We report our study of Loctite 4011, a biological glue composed of a cyanoacrylate that has been employed for medical purposes, in which samples of pericardium bonded with this adhesive were subjected to uniaxial tensile stress. The samples were glued in such a way as to leave an overlap of 1 cm2 between the surfaces of the tissue. The series included 83 samples: 12 tested 24 h after bonding, 17 after 45 days, 17 after 90 days, 19 after 106 days and 18 after 152 days. The samples subjected to deferred trials were preserved using three types of chemical substances: glutaraldehyde, glycerol or saline plus antibiotics. The mean resistance to rupture of the series tested 24 h after gluing was 0.15 MPa (1.47 machine kg). This resistance remained nearly unchanged, regardless of the preservation solution employed, for at least 152 days, the time at which the study ended. The stress-strain curves demonstrated a high degree of elasticity throughout the 152 days, a finding that was not influenced by the preservation solution. This adhesive showed a considerable resistance to tensile stress, although probably insufficient to replace sutures. However, it maintained a surprisingly high degree of elasticity in the samples. Perhaps the time has come to combine these two elements, sutures and adhesives, to improve the elasticity of the structure without a loss of resistance, and increase the durability of bioprostheses.

The purpose of this study was to compare the mechanical behavior of calf pericardium, pig pericardium and ostrich pericardium when subjected to tensile testing. Tensile stress was applied to 108 tissue samples, 36 of each type of tissue, until rupture. Groups of three adjacent strips measuring 12 x 2 cm(2) were cut longitudinally. Each group consisted of an unsutured center sample, or control, and the two contiguous samples, that on the right sutured with Gore-Tex at a 90 degrees angle with respect to the longitudinal axis and that on the left sewn with the same suture material at 45 degrees angle. The sutured samples showed a statistically significant loss of resistance (p<0.001) when compared with the corresponding unsutured tissue. The mean stresses at rupture for sutured ostrich pericardium were 21.81 and 20.81 MPa in the samples sewn at 45 degrees and 90 degrees, respectively, higher than those corresponding to unsutured calf and pig pericardium, 14.0 and 11.49 MPa, respectively, at rupture. The analysis of the stress/strain curve shows a smaller difference between sutured and unsutured ostrich pericardium than those observed in the other two biomaterials. These results demonstrate that, in addition to its greater resistance, ostrich pericardium also presents a less pronounced interaction with the suture material. Its capacity to absorb the shearing stress produced by the suture is greater. This report also confirms that the method of selection using paired samples ensures their homogeneity and makes it possible to predict the behavior of a sample by determining that of the other half of the pair.

Ostrich pericardium, sutured using a telescoping or overlapping technique, was studied to determine its mechanical behavior. From each of 12 pericardial sacs, four contiguous strips were cut longitudinally, from root to apex, and another four contiguous strips were cut in transverse direction. One of the strips in each set of four was used as an unsutured control and the remaining three were sutured by overlapping 0.5 cm of the tissue and sewing with Gore-tex, Prolene or Pronova. These 96 samples were then subjected to tensile testing along their major axes until rupture. The tensile stresses recorded in the suture materials at the moment tears appeared in the pericardium ranged between 55.99 MPa and 70.23 MPa for Gore-tex in samples cut in the two directions. Shear stress became ostensible at 56 MPa, with clearly evident tears. However, microfracture of the collagen fibers must be produced at much lower stress levels. The comparison of the resistance in kilograms (machine-imposed), without taking into account the sections in which the load was applied, demonstrated only a slight loss of load when the telescoping suture was employed in ostrich pericardium samples. Ostrich pericardium may continue to be an alternative biological material for the construction of heart valve leaflets.

The mechanical behavior of sutured ostrich pericardium was studied by uniaxial tensile testing. One hundred forty-four tissue specimens were assessed: 96 sutured samples (48 in which a centrally located suture was placed at an angle of 90 degrees with respect to the longitudinal axis, whereas in the remaining 48, a centrally located suture was placed at a 45 degrees angle to the longitudinal axis, in sets of 12 samples each, sewn with sutures made of Gore-Tex, nylon, Prolene, or silk), and 48 unsutured controls. Each group of 24 samples sewn at one angle or the other with the different suture materials was assayed together with a corresponding control group of 12 unsutured samples. The mean tensile strengths in the unsutured controls ranged between 30.16 MPa and 43.42 MPa, whereas those of the sutured sets ranged from 14.68 MPa to 21.91 MPa. The latter presented a statistically significant loss of resistance (p < 0.01) when compared with the unsutured tissue samples. The angle of the suture with respect to the longitudinal axis influenced the degree of shear stress produced by the suture, as well as the behavior of the different suture materials used. The set of samples sewn with Prolene appeared to be that most sensitive to changes in the angle of the suture, whereas tissue sewn at a 45 degrees angle with Gore-Tex presented lower shear stress values in comparison with samples in which the other three materials were used. A method of tissue selection based on morphological and mechanical criteria was used to ensure the homogeneity of the results in such a way that the coefficients of determination (R2) for the stress/strain curve fitting equation ranged between 0.888 and 0.995. This excellent fit made it possible, applying regression analysis, to predict the mechanical behavior of a specimen by determining that of a contiguous tissue sample. Thus, it should be possible, at least theoretically, to characterize the behavior of a specific region or zone of the biomaterial. In conclusion, ostrich pericardium exhibits strong resistance to rupture, even when sutured. The selection method used ensures the homogeneity of the samples and, thus, of the results. The angle of the suture with respect to the longitudinal axis, where the load is centered, determines the shear stress produced by the suture and the mechanical behavior of each suture material., (Copyright 2002 Wiley Periodicals, Inc.)

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

The mechanical behavior of porcine pericardium was analyzed to compare it with that of calf pericardium employed in valve leaflets for cardiac bioprostheses. Forty samples of pericardium were subjected to uniaxial tensile testing, 20 as controls and 20 exposed to loads increasing stepwise from 0.5 to 1.5 kg and to 3 kg, and thereafter to rupture, with a return to zero load between each new increment. Another 20 samples were used in biaxial tensile tests involving the application of loads increasing stepwise (to 0.5, 1.5, 3 and 5 kg) until rupture with a zero-load interval before each increment. The ultimate stresses were very similar, showing no statistically significant differences when compared in terms of type of assay, controls and study samples or region of pericardial tissue being tested. In the stepwise biaxial assays, the mean stresses at rupture were also very homogeneous. Using morphological and mechanical criteria for sample selection, it was possible to obtain mathematical fits for the stress/strain relationship, with excellent coefficients of determination. The relationship between the area under the stress/strain curve and the load applied or the strain observed was also studied in the biaxial assay as an equivalent to the cycles of hysteresis produced in the test. The increment in the area under the curve (the energy consumed) may be a good parameter for assessing the changes in the collagen fiber architecture of the pericardial tissue, changes that may help to detect early failure.

The authors analyzed the mechanical behavior of the calf pericardium employed in the construction of valve leaflets for cardiac bioprostheses. Forty samples of pericardium were subjected to uniaxial tensile testing, 20 as controls and 20 exposed to loads increasing stepwise until rupture, with a return to zero load between each new increment. Another 20 samples were used similarly in biaxial tensile tests involving loads increasing stepwise until rupture, again returning to zero load between steps. The ultimate stresses in the uniaxial study were very similar and were not influenced by the region of pericardial tissue being tested or the increments in load to which the tissue was exposed. The mean stresses at rupture in the stepwise biaxial assays were significantly greater (p<0.01). Using morphological and mechanical criteria for sample selection, it was possible to obtain mathematical fits for the stress/strain relationship in both types of assays, with excellent coefficients of determination (R (2)>0.90). In uniaxial tests in which the selection criteria were not applied, the correlation improved as the load increased, a phenomenon that did not occur in the biaxial studies. The values varied throughout the different cycles, adopting exponential forms when the strain was greatest. These variations, which demonstrate that the increase in the energy consumed is a function of the stress applied and of the strain produced, should be good parameters for assessing the changes in the collagen fiber architecture of pericardial tissue subjected to cyclic stress, and may help to detect early failure.

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., (Copyright 2001 Kluwer Academic Publishers)

Using a hydraulic stress simulator, the mechanical behavior of the porcine pericardium used in the construction of cardiac valve leaflets was characterized following the same procedure employed with calf pericardium in Part 1 of this study. One hundred fifty pairs of tissue samples were subjected to tensile testing to rupture. One of the two samples from each of 120 pairs (four series of 30 pairs each) was saturated with commercially available threads made of nylon, silk, Prolene or Gore-Tex, while the other sample in each of these pairs was left unsewn. The remaining 30 pairs were employed as controls in which neither of the two samples was subjected to suturing. The sutured tissue samples showed a significant decrease in tensile strength at rupture (range: 11.61 to 21.22 MPa) when compared with unsutured samples (range: 50.80 to 89.45 MPa; p < 0.01). When these results were compared with their equivalent in calf pericardium, no significant differences were observed (the mean values at rupture in calf pericardium ranged between 211.61 MPa and 26.04 MPa). Again, the application of morphological and mechanical selection criteria to ensure the homogeneity of the samples provided excellent fit with respect to the stress/strain curves. The interaction of the different suture materials with the pericardial tissue was also assessed by comparing the mechanical behavior of the sutured samples with that of the control samples. At the working stress of a cardiac valve leaflet, 0.250 MPa, samples sewn with Gore-Tex were found to show the least difference in behavior with respect to the controls, indicating that this material presented the lowest degree of interaction with the pericardium. In conclusion, the suture clearly has deleterious effects on the resistance of both calf and porcine pericardium, which showed no statistically significant differences in terms of resistance to rupture when their respective sutured or unsutured samples were compared, except in the case of porcine pericardium sewn with silk, which presented lower resistance to rupture in all the zones studied. These findings suggest that the hypothesis that porcine pericardium is less resistant is erroneous. The Gore-Tex suture also presented a lower degree of interaction with the porcine pericardium, with values similar to the working stress of a cardiac valve leaflet. This methodology and the results should be evaluated in dynamic studies, such as fatigue testing, that not only confirm the resistance of the material but establish the durability of the samples being assayed.

A hydraulic stress simulator was employed to study the mechanical behavior of the calf pericardium used in the construction of cardiac valve leaflets. One hundred eighty pairs of tissue samples were subjected to tensile testing to rupture. One of the two samples from each of 144 pairs (four series of 36 pairs each) was sutured with commercially available threads made of nylon, silk, Prolene or Gore-Tex, while the other sample in each of these pairs was left unsewn. The remaining 36 pairs were employed as controls in which neither of the two samples was subjected to suturing. The sutured tissue samples showed a significant decrease in tensile strength at rupture (range: 11.81 to 26.04 MPa) when compared with unsutured samples (range: 39.38 to 87.96 MPa; p < 0.01). The application of morphological and mechanical selection criteria to maximize the homogeneity of the samples provided excellent fit with respect to the stress/strain curves. This method made it possible to carry out a predictive study of the mechanical behavior of a sutured sample, based on that observed in the corresponding unsutured fragment. The interaction of the different suture materials with the pericardial tissue was also assessed by comparing the mechanical behavior of the sutured samples with that of the control samples. At stresses of less than 0.8 MPa, samples sewn with Gore-Tex were found to show the least difference with respect to the controls, indicating that this material presented the lowest degree of interaction with the pericardium. In conclusion, the degree of the loss of resistance to tearing of the sutured samples is of no value in the selection of the optimal suture material. The selection process applied makes it possible to predict the mechanical behavior in response to suturing of a given unsewn tissue specimen by determining that of its sutured mate. The similarity between the findings in samples sewn with Gore-Tex and in the unsutured controls indicates a lesser degree of interaction between the suture material and the pericardium employed in the construction of cardiac valve leaflets.

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 non-sutured 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 non-sutured 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 non-sutured 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 \gt 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., (Copyright 2000 Kluwer Academic Publishers)

Using morphological and mechanical criteria and applying a method involving paired samples that is widely employed in epidemiology, we obtained an excellent prediction of the mechanical behavior of the calf pericardium used in the construction of cardiac bioprostheses. The method of selection employed in this study may be a highly useful tool for guaranteeing the mechanical resistance of calf pericardium, with a very low level of error.

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\gt 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., (Copyright 2000 Kluwer Academic Publishers)

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.

The purpose of this study was to assess the elastic behaviour of calf pericardium used in the construction of cardiac bioprosthesis valve leaflets, sutured with different types of commercially available sutures: silk, Gore-Tex, Surgilene and nylon. Thirty-two samples (four series of eight samples each) were subjected to tensile strength testing to breakage. The breaking stress (MPa) ranged between 4.89 MPa for samples sutured with Gore-Tex and 5.22 MPa for those sewn with nylon. Three samples from each series were subjected to a stepwise stress test, involving increasing levels of stress followed by return to zero, to define the elastic limit (the cut-off point beyond which strain is no longer reversible). Analysis of the results provided the mathematical functions that govern the elastic behaviour (stress/strain) within the elastic range for each type of sutured sample. The series sutured with Surgilene presented the highest mean value (1.649 MPa). Finally, a statistical study was carried out to determine which series showed the greatest probability of having the least interaction between the thread and the pericardium. Allowing an interval of +/- 10%, Gore-Tex showed the best probability in this respect. However, real fatigue testing is necessary to definitively determine which is the best suture to use.

Since the development of cardiac prostheses, numerous chemical treatments have been assayed to prevent the process of their mineralization. The effect of chemical treatment with amino acids is assessed in a subcutaneous implantation model in rats. Pericardial tissue from young calves was treated with L-lysine, L-glutamine, L-arginine or L-glutamic acid, each at a concentration of 0.5 M, following treatment with 0.625% glutaraldehyde. Then, the tissue was implanted into young rats for periods of 21 and 60 d, after which the calcium accumulated was quantified by atomic absorption spectroscopy. Values similar to or higher than those found in control samples indicated a lack of effectiveness of these treatments. Only in the 21-d implantation samples treated with L-lysine and L-arginine was less calcium accumulated than in the control tissue. After 60 d of implantation, all groups showed high levels of calcium deposition. The values obtained after 60 d of subcutaneous implantation were 87.5 +/- 52.4 mg Ca2+ per g dry weight of tissue for L-lysine, 108.7 +/- 43.5 mg Ca2+ per g dry weight of tissue for L-glutamine, 130.4 +/- 22.4 mg Ca2+ per g dry weight of tissue for L-glutamic acid, 119.3 +/- 27.6 mg Ca2+ per g dry weight of tissue for L-arginine and 100.0 +/- 38.3 mg Ca2+ per g dry weight of tissue for the control group. Treatment with amino acids does not appear to prevent the calcification of cardiac bioprostheses or of collagen-based biomaterials when assayed in a model of subcutaneous implantation.

Since the development of cardiac bioprostheses, numerous chemical treatments have been assayed to prevent mineralization. The effectiveness of chemical treatments that eliminate lipids from the tissue was tested by combining two models. First, handmade bovine pericardial bioprostheses, subjected to chemical treatment with chloroform/ methanol and glutaraldehyde or treated with glutaraldehyde alone for use as controls, were subjected to mechanical stress in a heart valve, accelerated wear tester (100 x 10(6) consecutive cycles). Then, the bioprostheses were unstitched and tissue samples were taken from the portion subjected to maximal stress (P1) and from that surrounding the sewing ring, which had not been subjected to mechanical stress (P2), for subcutaneous implantation. After 21 and 60 days of implantation, we observed calcification of the samples subjected to mechanical stress, even after delipidating treatment, with no significant differences with respect to the control group. However, the treated samples from the portion not subjected to mechanical stress presented a slighter accumulation of calcium after 60-day implantation (5.60 +/- 3.09 mg Ca2 +/g dry weight of tissue) versus the control group (47.17 +/- 20.4 mg Ca2+/g dry weight of tissue), the difference of which was statistically significant (p < 0.01). At the time of these medium-term studies, marked calcification was observed in tissue subjected to delipidating treatment in the zones that underwent mechanical stress.

Since the development of cardiac prostheses, numerous chemical treatments have been assayed to prevent the process of their mineralization, causing 60% of the failures. The effect of the extraction of lipids from the tissue employed in porcine valves is assessed in a model of subcutaneous implantation in rats. Tissue from aortic and pulmonary porcine valves was treated with chloroform-methanol and 0.625% glutaraldehyde and was implanted into young rats for periods of 21 and 60 d. The calcium accumulated was then quantified by atomic absorption. The effectiveness of this treatment is demonstrated by the detection of much lower calcium values than in the control group. For aortic valve tissue, the values obtained were 40.5 and 188.1 micrograms Ca2+/mg dry weight of tissue for implantation times of 21 and 60 d, respectively, versus 5.48 and 1.4 micrograms Ca2+/mg dry weight of tissue for the same tissue treated with chloroform-methanol. The values obtained with pulmonary valve tissue were very similar: 72.46 and 108.06 micrograms Ca2+/mg dry weight tissue versus 0.67 and 0.80 micrograms Ca2+/mg dry weight tissue for implantation periods of 21 and 60 d, respectively. Thus, phospholipids may be totally or partially responsible for the calcification of the porcine valve tissue employed in the construction of cardiac bioprostheses.

A retrospective clinical, epidemiological study was carried out with the 53 cases of tuberculosis diagnosed in our center during 1991. The incidence rate was 4.4 per 1,000 clinical histories. Fifteen patients (28.3%) were HIV+, and five were marginalized illegal immigrants (9.4%). Seventy-three percent of the HIV+ patients and 42% of the HIV- patients presented extrapulmonary involvement. In-hospital mortality was 20% in HIV+ patients and 2.6% among the remainder.

Assessment of relaxation (loss of load within a given time) without apparent deformation is a necessary step before durability assay of biomaterials. From results obtained using calf pericardium, the following conclusions were drawn: (a) there is no limit to relaxation for this biomaterials; (b) the lesser the load applied, the greater the relaxation; and (c) the relaxation curve follows the logarithmic function y = K1 - K2 - Int. These findings demonstrate the poor behaviour of the biomaterial at supposedly low loads and suggest that rupture-point load is not a reliable reference to determine the safety coefficient of calf pericardium used in cardiac bioprostheses.