Study of the calcification of bovine pericardium: analysis of the implication of lipids and proteoglycans

Generation of glycans depleted decellularized porcine pericardium, using digestive enzymatic supplements and enzymatic mixtures for food industry

BACKGROUND

Xenogeneic pericardium has been used largely for various applications in cardiovascular surgery. Nevertheless, xenogeneic pericardial patches fail mainly due to their antigenic components. The xenoantigens identified as playing a major role in recipient immune response are the Galα1-3Gal (α-Gal) epitope, the non-human sialic acid N-glycolylneuraminic acid (Neu5Gc), and the porcine SDa antigen, associated with both proteins and lipids. The reduction in glycans from porcine pericardium might hinder or reduce the immunogenicity of xenogeneic scaffolds.

METHODS

Decellularized porcine pericardia were further treated at different time points and dilutions with digestive enzymatic supplements and enzymatic mixtures applied for food industry, for the removal of potentially immunogenic carbohydrates. Carbohydrates removal was investigated using up to 8 different lectin stains for the identification of N- and O-glycosylations, as well as glycolipids. Histoarchitectural changes in the ECM were assessed using Elastica van Gieson stain, whereas changes in mechanical properties were investigated via uniaxial tensile test and burst pressure test.

RESULTS

Tissues after enzymatic treatments showed a dramatic decrease in lectin stainings in comparison to tissues which were only decellularized. Histological assessment revealed cell-nuclei removal after decellularization. Some of the enzymatic treatments induced elastic lamellae disruption. Tissue strength decreased after enzymatic treatment; however, treated tissues showed values of burst pressure higher than physiological transvalvular pressures.

CONCLUSIONS

The application of these enzymatic treatments for tissue deglycosylation is totally novel, low cost, and appears to be very efficient for glycan removal. The immunogenic potential of treated tissues will be further investigated in subsequent studies, in vitro and in vivo.

The sequential effects of a multifactorial detergent based decellularization process on bovine pericardium

Decellularization is a promising method for obtaining extracellular matrix scaffolds (ECM) to be used as replacement material in reconstructive procedures. The effectiveness of decellularization and the alterations to the ECM vary, depending on several factors, including the tissue source, composition and density. With an optimized decellularization process, decellularized scaffolds can preserve the spatial and temporal ECM microenvironment, which play an integral role in modulating cell migration, proliferation and differentiation. The exploration of a variety of decellularization protocols has led to mixed outcomes and comparisons between decellularization protocols could not attribute these differences to any single step in a multiple-step process. This study aimed to characterize the effects of each step of a multifactorial decellularization method on the scaffold structure and mechanical integrity of bovine pericardium. Each step of the decellularization process and the effect on the tissue was assessed using hematoxylin and eosin staining, electron microscopy, total protein, ECM protein and triglyceride quantification. The biomechanical properties were assessed using uniaxial tensile strength testing. Cell lysis occurred mainly during the detergent and alcohol steps. Collagen structural damage occurred during the detergent and alcohol steps, with no significant decreased in collagen concentration. No significant damage to elastin could be shown throughout the process, however glycosaminoglycans were significantly removed by detergent treatment. Triglycerides were removed mostly by the alcohol treatment. The strength of the pericardium decreased somewhat after each step of the protocol. It is important to characterize each decellularization protocol with regards to the decellularization efficiency and the effect on the ECM proteins structure and function to accurately evaluatein vivooutcomes.

Antigen removal process preserves function of small diameter venous valved conduits, whereas SDS-decellularization results in significant valvular insufficiency

Chronic venous disease (CVD) is the most common reported chronic condition in the United States, affecting more than 25 million Americans. Regardless of its high occurrence, current therapeutic options are far from ideal due to their palliative nature. For best treatment outcomes, challenging cases of chronic venous insufficiency (CVI) are treated by repair or replacement of venous valves. Regrettably, the success of venous valve transplant is dependent on the availability of autologous venous valves and hindered by the possibility of donor site complications and increased patient morbidity. Therefore, the use of alternative tissue sources to provide off-the-shelf venous valve replacements has potential to be extremely beneficial to the field of CVI. This manuscript demonstrates the capability of producing off-the-shelf fully functional venous valved extracellular matrix (ECM) scaffold conduits from bovine saphenous vein (SV), using an antigen removal (AR) method. AR ECM scaffolds maintained native SV structure-function relationships and associated venous valves function. Conversely, SDS decellularization caused significant changes to the collagen and elastin macromolecular structures, resulting in collagen fibril merging, elimination of fibril crimp, amalgaming collagen fibers and fragmentation of the inner elastic lamina. ECM changes induced by SDS decellularization resulted in significant venous valve dysfunction. Venous valved conduits generated using the AR approach have potential to serve as off-the-shelf venous valve replacements for CVI. STATEMENT OF SIGNIFICANCE: Retention of the structure and composition of extracellular matrix (ECM) proteins within xenogeneic scaffolds for tissue engineering is of crucial importance, due to the undeniable effect ECM proteins can impose on repopulating cells and function of the resultant biomaterial. This manuscript demonstrates that alteration or elimination of ECM proteins via commonly utilized decellularization approach results in complete disruption of venous valve function. Conversely, retention of the delicate ECM structure and composition of native venous tissue, using an antigen removal tissue processing method, results in preservation of native venous valve function.

Small Diameter Xenogeneic Extracellular Matrix Scaffolds for Vascular Applications

Currently, despite the success of percutaneous coronary intervention (PCI), coronary artery bypass graft (CABG) remains among the most commonly performed cardiac surgical procedures in the United States. Unfortunately, the use of autologous grafts in CABG presents a major clinical challenge as complications due to autologous vessel harvest and limited vessel availability pose a significant setback in the success rate of CABG surgeries. Acellular extracellular matrix (ECM) scaffolds derived from xenogeneic vascular tissues have the potential to overcome these challenges, as they offer unlimited availability and sufficient length to serve as "off-the-shelf" CABGs. Unfortunately, regardless of numerous efforts to produce a fully functional small diameter xenogeneic ECM scaffold, the combination of factors required to overcome all failure mechanisms in a single graft remains elusive. This article covers the major failure mechanisms of current xenogeneic small diameter vessel ECM scaffolds, and reviews the recent advances in the field to overcome these failure mechanisms and ultimately develop a small diameter ECM xenogeneic scaffold for CABG. Impact Statement Currently, the use of autologous vessel in coronary artery bypass graft (CABG) is common practice. However, the use of autologous tissue poses significant complications due to tissue harvest and limited availability. Developing an alternative vessel for use in CABG can potentially increase the success rate of CABG surgery by eliminating complications related to the use of autologous vessel. However, this development has been hindered by an array of failure mechanisms that currently have not been overcome. This article describes the currently identified failure mechanisms of small diameter vascular xenogeneic extracellular matrix scaffolds and reviews current research targeted to overcoming these failure mechanisms toward ensuring long-term graft patency.

Ectopic mineralization in heart valves: new insights from in vivo and in vitro procalcific models and promising perspectives on noncalcifiable bioengineered valves

Ectopic calcification of native and bioprosthetic heart valves represents a major public health problem causing severe morbidity and mortality worldwide. Valve procalcific degeneration is known to be caused mainly by calcium salt precipitation onto membranes of suffering non-scavenged cells and dead-cell-derived products acting as major hydroxyapatite nucleators. Although etiopathogenesis of calcification in native valves is still far from being exhaustively elucidated, it is well known that bioprosthesis mineralization may be primed by glutaraldehyde-mediated toxicity for xenografts, cryopreservation-related damage for allografts and graft immune rejection for both. Instead, mechanical valves, which are free from calcification, are extremely thrombogenic, requiring chronic anticoagulation therapies for transplanted patients. Since surgical substitution of failed valves is still the leading therapeutic option, progressive improvements in tissue engineering techniques are crucial to attain readily available valve implants with good biocompatibility, proper functionality and long-term durability in order to meet the considerable clinical demand for valve substitutes. Bioengineered valves obtained from acellular non-valvular scaffolds or decellularized native valves are proving to be a compelling alternative to mechanical and bioprosthetic valve implants, as they appear to permit repopulation by the host's own cells with associated tissue remodelling, growth and repair, besides showing less propensity to calcification and adequate hemodynamic performances. In this review, insights into valve calcification onset as revealed by in vivo and in vitro procalcific models are updated as well as advances in the field of valve bioengineering.

Exogenous hyaluronic acid and chondroitin sulfate crosslinking treatment for increasing the amount and stability of glycosaminoglycans in bioprosthetic heart valves

Glutaraldehyde (GLUT) crosslinked bioprosthetic heart valves (BHVs) might fail due to progressive degradation and calcification. GLUT cannot stabilize glycosaminoglycans (GAGs), which are important for BHVs' life time. In this current study we developed a new BHVs preparation strategy using exogenous hyaluronic acid (HA)/chondroitin sulfate (CS) supplement and sodium trimetaphosphate (STP) crosslinking method. Exogenous HA and CS provide additional GAGs for pericardiums. STP could link two GAGs by reacting with hydroxyl groups in GAGs' repeating polysaccharides units. The feeding ratios of HA/CS were optimized. The GAGs content and long-term stability in vitro, biocompatibility, the in vivo GAGs stability and anti-calcification potential of GLUT/HA/CS and STP treated pericardiums were characterized. We demonstrated that GLUT/HA/CS and STP treated pericardiums had sufficiently increased GAGs' amount and stability and decreased calcification. This new exogenous hyaluronic acid/chondroitin sulfate supplement and sodium trimetaphosphate crosslinking strategy would be a promising method to make BHVs with better structural stability and anti-calcification properties.

Lipoproteins in Cardiovascular Calcification: Potential Targets and Challenges

Previously considered a degenerative process, cardiovascular calcification is now established as an active process that is regulated in several ways by lipids, phospholipids, and lipoproteins. These compounds serve many of the same functions in vascular and valvular calcification as they do in skeletal bone calcification. Hyperlipidemia leads to accumulation of lipoproteins in the subendothelial space of cardiovascular tissues, which leads to formation of mildly oxidized phospholipids, which are known bioactive factors in vascular cell calcification. One lipoprotein of particular interest is Lp(a), which showed genome-wide significance for the presence of aortic valve calcification and stenosis. It carries an important enzyme, autotaxin, which produces lysophosphatidic acid (LPA), and thus has a key role in inflammation among other functions. Matrix vesicles, extruded from the plasma membrane of cells, are the sites of initiation of mineral formation. Phosphatidylserine, a phospholipid in the membranes of matrix vesicles, is believed to complex with calcium and phosphate ions, creating a nidus for hydroxyapatite crystal formation in cardiovascular as well as in skeletal bone mineralization. This review focuses on the contributions of lipids, phospholipids, lipoproteins, and autotaxin in cardiovascular calcification, and discusses possible therapeutic targets.

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

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

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

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

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

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

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

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

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

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

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

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

Glutaraldehyde treatment elicits toxic response compared to decellularization in bovine pericardium

Glutaraldehyde-stabilized bovine pericardium is used for clinical application since 1970s because of its desirable features such as less immunogenicity and acceptable durability. However, a propensity for calcification is reported on account of glutaraldehyde treatment. In this study, commercially available glutaraldehyde cross-linked bovine pericardium was evaluated for its in vitro cytotoxic effect, macrophage activation, and in vivo toxic response in comparison to decellularized bovine pericardium. Glutaraldehyde-treated bovine pericardium and its extract were observed to be cytotoxic and it also caused significant inflammatory cytokine release from activated macrophages. Significant antibody response, calcification response, necrotic, and inflammatory response were noticed in glutaraldehyde-treated bovine pericardium in comparison to decellularized bovine pericardium in a rat subcutaneous implantation model. Glutaraldehyde-treated bovine pericardium also failed in acute systemic toxicity testing and intracutaneous irritation testing as per ISO 10993. With respect to healing and implant remodeling, total lack of host tissue incorporation and angiogenesis was noticed in glutaraldehyde-treated bovine pericardium compared to excellent host fibroblast incorporation and angiogenesis within the implant in decellularized bovine pericardium. In conclusion, using in vitro and in vivo techniques, this study has demonstrated that glutaraldehyde-treated bovine pericardium elicits toxic response compared to decellularized bovine pericardium which is not congenial for long-term implant performance.

Detoxification of Glutaraldehyde Treated Porcine Pericardium Using L-arginine & NABH(4)

Background

Calcification is the most frequent cause of clinical failure of bioprosthetic tissues fabricated from GA-fixed porcine valves or bovine pericardium. A multi-factorial approach using different mechanisms was recently developed to reduce the calcification of bioprosthetic tissues. The purpose of the present study was to evaluate the synchronized synergism of using L-arginine and NaBH₄, compared with ethanol and L-lysine, in glutaraldehyde treated porcine pericardium from the standpoint of calcification and tissue elasticity.

Materials and Methods

Porcine pericardium was fixed at 0.625% GA (7 days at room temperature after 2 days at 4℃). An interim step of ethanol (80%; 1 day at room temperature) or L-lysine (0.1 M; 2 days at 37℃) or L-arginine (0.1 M; 2 days at 37℃) was followed by completion of the GA fixation. A final step of NaBH₄ (0.1 M; 2 days at room temperature) was followed. Their tensile strength, thickness, and thermal stability were measured. Treated pericardia were implanted subcutaneously into three-week-old Sprague-Dawley rats for 8 weeks. Calcium content was assessed by atomic absorption spectroscopy and histology.

Results

L-arginine and NaBH₄ pretreatment (1.81±0.39 kgf/5 mm p=0.001, 0.30±0.08 mm p<0.001) significantly increased tensile strength and thickness compared with the control (0.53±0.34 kgf/5 mm, 0.10±0.02 mm). In a thermal stability test, L-arginine and NaBH₄ pretreatment (84.25±1.12℃, p=0.023) caused a significant difference from the control (86.25±0.00℃). L-lysine and NaBH₄ pretreatment (183.8±42.6 ug/mg, p=0.804), and L-arginine and NaBH₄ pretreatment (163.3±27.5 ug/mg, p=0.621) did not significantly inhibit calcification compared to the control (175.5±45.3 ug/mg), but ethanol and NaBH₄ pretreatment did (38.5±37.3 ug/mg, p=0.003).

Conclusion

The combined pretreatment using L-arginine and NaBH₄ after GA fixation seemed to increase the tensile strength and thickness of porcine pericardium, fixed with GA. Additionally, it seemed to keep thermal stability. However it could not decrease the calcification of porcine pericardium fixed with GA. NaBH₄ pretreatment seemed to decrease the calcification of porcine pericardium fixed with GA, but only with ethanol.

Preparation of ex vivo-based biomaterials using convective flow decellularization

With advantageous biomechanical properties, materials derived from ex vivo tissues are being actively investigated as scaffolds for tissue engineering applications. However, decellularization treatments are required before implantation to reduce the materials immune impact. The aim of these investigations was to assess a convective flow model as an enhanced methodology to decellularize ex vivo tissue. Isolated human umbilical veins were decellularized using two methods: rotary agitation at 100rpm on orbital shaker plates, and convective flow run at 5, 50, and 150mmHg within perfusion bioreactors. Extracted phospholipids and total soluble protein were assessed over time. Histology, SEM, and uniaxial tensile testing analysis were carried out to evaluate variation in the tissues. After 72h, samples exposed to traditional rotary agitation showed retention of whole cells and cellular components, whereas pressure-based systems showed no visual sign of cells. The convective flow method was significantly more effective at removing phospholipid and total protein than the agitation model. High transmembrane pressure (150mmHg) resulted in higher phospholipids extraction. However, a more efficient protein extraction occurred at 50mmHg. Variation in extraction rates was dependent on tissue permeability, which varied as pressure increased. Collectively, these findings show significant improvements in decellularization efficiency that may lead to more immune compliant ex vivo-derived biomaterials.

Modified acellularization for successful vascular xenotransplantation

The purpose of this study was to estimate the possibilities of an acellular matrix using a modified acellularization protocol, which circumvents immunological, microbiological, and physiological barriers. We treated porcine subclavian arteries with various reagents to construct acellular grafts. Afterwards, these grafts were interposed in a mongrel dogs' abdominal aorta. Six dogs underwent interposition with fresh porcine grafts (control group), and seven had interposed acellular grafts (acellular group). The control and acellular group dogs were sacrificed at 1, 3, 5 (n=2 in each group) and 12 months (n=1 in acellular group) after the operation. Histopathological examinations were then performed, to assess the degree to which re-endothelialization, inflammation, thrombus formation, and calcification occurred. The entire acellular group, but none of the control group, exhibited re-endothelialization. The degrees to which inflammation, thrombosis, and calcification occurred were found to be lower in the acellular group. We also discovered many smooth muscle cells in the medial layer of the xenograft that had been implanted in the dog sacrificed 12 months after the operation. These results suggest that the construction of xenografts using our modified acellularization protocol may offer acceptable outcomes as a vascular xenograft.

Neomycin prevents enzyme-mediated glycosaminoglycan degradation in bioprosthetic heart valves

Bioprosthetic heart valves (BHVs) derived from glutaraldehyde crosslinked porcine aortic valves are frequently used in heart valve replacement surgeries. However, BHVs have limited durability and fail either due to degeneration or calcification. Glycosaminoglycans (GAGs), one of the integral components of heart valve cuspal tissue, are not stabilized by conventional glutaraldehyde crosslinking. Previously we have shown that valvular GAGs could be chemically fixed with GAG-targeted chemistry. However, chemically stabilized GAGs were only partially stable to enzymatic degradation. In the present study an enzyme inhibitor was incorporated in the cusps to effectively prevent enzymatic degradation. Thus, neomycin trisulfate, a known hyaluronidase inhibitor, was incorporated in cusps via 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide/N-hydroxysuccinimide (EDC/NHS) chemistry followed by glutaraldehyde crosslinking (NEG). Controls included cusps crosslinked with either EDC/NHS followed by glutaraldehyde (ENG) or only with glutaraldehyde (GLUT). NEG group showed improved resistance to in vitro enzymatic degradation as compared to GLUT and ENG groups. All groups showed similar collagen stability, measured as a thermal denaturation temperature by differential scanning calorimetry (DSC). The cusps were implanted subdermally in rats to study in vivo degradation of GAGs. NEG group preserved significantly more GAGs than ENG and GLUT. NEG and ENG groups showed reduced calcification than GLUT.

Glycosaminoglycan-targeted fixation for improved bioprosthetic heart valve stabilization

Numerous crosslinking chemistries and methodologies have been investigated as alternative fixatives to glutaraldehyde (GLUT) for the stabilization of bioprosthetic heart valves (BHVs). Particular attention has been paid to valve leaflet collagen and elastin stability following fixation. However, the stability of glycosaminoglycans (GAGs), the primary component of the spongiosa layer of the BHV, has been largely overlooked despite recent evidence provided by our group illustrating their structural and functional importance. In the present study we investigate the ability of two different crosslinking chemistries: sodium metaperiodate (NaIO(4)) followed by GLUT (PG) and 1-Ethyl-3-(3 dimethylaminopropyl) carbodiimide/N-hydroxysuccinimide (EDC/NHS) followed by GLUT (ENG) to stabilize GAGs within BHV leaflets and compare resulting leaflet characteristics with that of GLUT-treated tissue. Incubation of fixed leaflets in GAG-degrading enzymes illustrated in vitro resistance of GAGs towards degradation in PG and ENG treated tissue while GLUT fixation alone was not effective in preventing GAG loss from BHV leaflets. Following subdermal implantation, significant amounts of GAGs were retained in leaflets in the ENG group in comparison to GLUT-treated tissue, although GAG loss was evident in all groups. Utilizing GAG-targeted fixation did not alter calcification potential of the leaflets while collagen stability was maintained at levels similar to that observed in conventional GLUT-treated tissue.

Biochemical and mechanical behavior of ostrich pericardium as a new biomaterial

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.

Stability and function of glycosaminoglycans in porcine bioprosthetic heart valves

Glycosaminoglycans (GAGs) are important structural and functional components in native aortic heart valves and in glutaraldehyde (Glut)-fixed bioprosthetic heart valves (BHVs). However, very little is known about the fate of GAGs within the extracellular matrix of BHVs and their contribution to BHV longevity. BHVs used in heart valve replacement surgery have limited durability due to mechanical failure and pathologic calcification. In the present study we bring evidence for the dramatic loss of GAGs from within the BHV cusp structure during storage in saline and both short- and long-term Glut fixation. In order to gain insight into role of GAGs, we compared properties of fresh and Glut-fixed porcine heart valve cusps before and after complete GAG removal. GAG removal resulted in significant morphological and functional tissue alterations, including decreases in cuspal thickness, reduction of water content and diminution of rehydration capacity. By virtue of this diminished hydration, loss of GAGs also greatly increased the "with-curvature" flexural rigidity of cuspal tissue. However, removal of GAGs did not alter calcification potential of BHV cups when implanted in the rat subdermal model. Controlling the extent of pre-implantation GAG degradation in BHVs and development of improved GAG crosslinking techniques are expected to improve the mechanical durability of future cardiovascular bioprostheses.

Treatment of bioprosthetic heart valve tissue with long chain alcohol solution to lower calcification potential

The use of glutaraldehyde-treated biological tissue in heart valve substitutes is an important option in the treatment of heart valve disease. These devices have limited durability, in part, because of tissue calcification and subsequent tearing of the valve leaflets. Components thought to induce calcification include lipids, cell remnants, and residual glutaraldehyde. We hypothesized that treatment of glutaraldehyde-treated bioprosthetic heart valve material using a short and long chain alcohol (LCA) combination, composed of 5% 1,2-octanediol in an ethanolic buffered solution, would reduce phospholipid content and subsequently lower the propensity of these tissues to calcify in vivo. Phospholipid content of glutaraldehyde-treated porcine valve leaflets and bovine pericardium was found to be 10.1 +/- 4.3 (n = 7) and 3.9 +/- 0.48 (n = 2) microg/mg dry tissue, respectively, which was reduced to 0.041 +/- 0.06 (n = 7) and 0.21 +/- 0.05 (n = 4) microg/mg dry tissue, respectively, after LCA treatment. Calcification potential of the treated tissues was assessed using a rat subcutaneous implant model. After 60 days of implantation, calcium levels were found to be 171 +/- 32 (n = 11) and 83 +/- 70 (n = 12) mg/g dry weight for glutaraldehyde-treated porcine leaflets and bovine pericardium, respectively, whereas prior LCA treatment resulted in reduced calcium levels of 1.1 +/- 0.6 (n = 12) and 0.82 +/- 0.1 (n = 12) mg/g dry weight, respectively. These data, taken together, support the notion that treatment of glutaraldehyde-treated tissue with a short and long chain alcohol combination will reduce both extractable phospholipids and the propensity for in vivo calcification.

Effects of periodate and chondroitin 4-sulfate on proteoglycan stabilization of ostrich pericardium. Inhibition of calcification in subcutaneous implants in rats

Chemical modification of biological materials used in the manufacture of cardiac valves tends to reduce the relatively high degree of biodegradation and calcification of the implanted bioprostheses. The most widely used treatment to reduce biodegradability of the valves is glutaraldehyde fixation. However, this treatment is potentially toxic and induces tissue calcification. In order to minimize these undesirable effects, we have analyzed the effect of a pre-fixation of endogenous proteoglycans and exogenous glycosaminoglycans, as well as the borohydride reduction influence on the different modified ostrich pericardium implants after subcutaneous implantation in rats. The presence of calcific deposits was detected in all implanted GA-fixed samples; however, calcification was highly reduced in both groups of periodate-prefixed materials, which showed also a very low Ca/P molar ratio. Borohydride post-treatment of these biomaterials resulted in a significant increase in calcium phosphate precipitation, with the appearance of calcium deposits mainly in an amorphous form even though X-ray diffraction allowed the detection of brushite- and apatite-like crystals. Regarding tissue stability, no significant differences were found among the borohydride-untreated implants but higher levels of matrix metalloproteinases were observed by gelatin zymography in the periodate pre-fixed materials. This increase was partially reduced by pre-fixation of exogenous chondroitin 4-sulfate. On the other hand, borohydride post-treatment not only increased calcification, but also reduced tissue stability and increased the presence of matrix-degrading activities.

A novel chemical modification of bioprosthetic tissues using L-arginine

A novel chemical modification of biological tissues was developed by the direct coupling of bioactive molecule, L-arginine to bovine pericardium (BP). The modification involves pretreatment of BP using GA and followed by grafting arginine to BP by the reaction of residual aldehyde and amine group of L-arginine. BP was modified by direct coupling of bioactive molecules and the effect of L-arginine coupling on calcification and biocompatibility was evaluated in vitro and in vivo. Modified BPs were characterized by measuring shrinkage temperature, mechanical properties, digestion resistance to collagenase enzyme, in vitro plasma protein adsorption and platelet adhesion, and in vivo calcification. Thermal and mechanical properties showed that the durability of arginine treated tissue increased as compared with fresh tissue and GA treated tissue. Resistance to collagenase digestion revealed that modified tissues have greater resistance to enzyme digestion than did fresh tissue and GA treated tissue. Lower protein adsorption and platelet adhesion were observed on modified tissue than non-modified tissue. In vivo calcification study demonstrated much less calcium deposition on arginine treated BP than GA treated one. Obtained results attest to the usefulness of L-arginine treated BP for cardiovascular bioprostheses.

Comparison of the mechanical behaviors of biological tissues subjected to uniaxial tensile testing: pig, calf and ostrich pericardium sutured with Gore-Tex

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.

Mechanical behavior of chemically treated ostrich pericardium subjected to uniaxial tensile testing: influence of the suture

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.

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

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

Gelatinases in soft tissue biomaterials. Analysis of different crosslinking agents

Chemical modification of pericardium-based cardiac valves tends to reduce the relatively high degree of biodegradation and calcification of the implanted bioprostheses. We analysed the tissue properties of pericardium from young calves and pigs after crosslinking with different agents (glutaraldehyde. diphenylphosphorylazide (DDPA), 1-ethyl-3,3-dimethyl-aminopropyl-carbodiimide (EDAC)) and when exposed to anticalcification treatments (chloroform/methanol or ethanol) prior to glutaraldehyde (GA) crosslinking. Protein extraction after tissue homogenisation in the presence of detergents showed that crosslinking using GA or DPPA was much more effective. The amounts of protein extracted from these two groups of chemically modified pericardium were significantly lower: the other modified tissues presented only a slight reduction when compared with untreated tissue. Matrix metalloproteinases- (MMP) 2 and 9 were detected in native pericardium from calf and pig by zymography. While the MMP-9/MMP-2 activity ratio was close to 1 in pig pericardium, it was 8.5-fold higher in bovine tissue. Crosslinking with GA and with DPPA almost completely abolished gelatinase activities, even when equal amounts of solubilised protein were loaded onto the zymograms. Anticalcification treatments followed by GA crosslinking or treatment with EDAC were not as effective in reducing gelatinase activities; but, interestingly, a relative reduction of MMP-9 versus MMP-2 was detected. The presence of these gelatinase activities in pericardium may contribute to the in vivo degradability of pericardium-based cardiac valves.

Proteoglycans in dentinogenesis

The predominant proteoglycans present in predentin and dentin are the chondroitin-sulphate-rich decorin and biglycan and the keratan-sulphate-rich lumican and fibromodulin. These are small, interstitial, leucine-rich proteoglycans which have recently been shown to exist in gradients across the predentin. Antibodies recognizing chondroitin sulphate show a decreasing gradient from the pulpal aspect toward the mineralizing front, the converse being true for keratan sulphate. Antidecorin shows an increase toward the mineralization front. Evidence from biochemical, autoradiographic, and immunohistochemical studies implies that such changes may be brought about by gradients of metalloproteinases. This offers the possibility that the proteoglycans organize the collagen network for receipt of phosphoproteins and phospholipids, the former being evident only at the onset of dentin formation. The suggestion is raised that glycosaminoglycan-depleted leucine-rich protein cores act as sequester points for receipt of phosphoproteins in particular. The rigid, spatially oriented glycosaminoglycan chains on decorin and biglycan are known to bind calcium and may feature directly in mineral initiation.

Copper retention, calcium release and ultrastructural evidence indicate specific Cuprolinic Blue uptake and peculiar modifications in mineralizing aortic valves

Previously, reactions with copper phthalocyanines at 0.05 M critical electrolyte concentration were found to cause demineralization in calcifying porcine aortic valves after subdermal implantation in rat, as well as simultaneous visualization of peculiar phthalocyanine-positive layers around cells and cell-derived matrix vesicles. In the present investigation, an appraisal was made of the mechanism and specificity of reactions with Cuprolinic Blue by comparing quantitatively calcium release and copper retention by calcified aortic valves reacted with this phthalocyanine under different critical electrolyte concentration conditions, and the corresponding ultrastructural patterns. It was found that (i) decalcifying properties are inversely proportional to salt molarity; (ii) reactivity to Cuprolinic Blue is critical electrolyte concentration-dependent, since the greatest copper retention occurred in 0.05 M critical electrolyte concentration Cuprolinic Blue-reacted samples, the only ones that also exhibited phthalocyanine-positive layers; (iii) the appearance of phthalocyanine-positive layers depends on Cuprolinic Blue uptake, revealing pericellular clustering of calcium-binding, anionic molecules; and (iv) minor Cuprolinic Blue uptake occurs by residual proteoglycans which still remain in the extracellular matrix after 6-week-long subdermal implantation. The present results indicate that this method is appropriate for the study of mineralized tissues and illustrate peculiar tissue modifications occurring at least in the experimental conditions used here.

Chemical treatment and tissue selection: factors that influence the mechanical behaviour of porcine pericardium

Calcification and mechanical failure are the major causes of the loss of cardiac bioprostheses. The chemical treatments used to stabilize the tissue employed are considered to play a fundamental role in the development of these two phenomena, although the problem is multifactorial and the underlying causes are yet to be fully identified. Currently, there is an ongoing search for chemical treatments capable of reducing or eliminating the process of calcification while preserving the mechanoelastic characteristics of the tissue. One of the approaches to this effort is the elimination of the phospholipid component from the biological tissue employed in prosthesis construction. There is evidence that this component may be responsible for the precipitation of calcium salts. The present study compares two delipidating chemical treatments involving chloroform/methanol and sodium dodecyl sulfate (SDS) with the use of glutaraldehyde (GA) alone. For this purpose, porcine pericardial tissue was subjected to tensile strength testing employing a hydraulic simulator. A total of 234 samples were studied 90 treated with GA, 72 treated with chloroform/methanol and 72 treated with SDS. The mean breaking strength was significantly higher in the samples treated with GA (between 43.29 and 63.01 MPa) when compared with those of tissue treated with chloroform/methanol (29.92-42.30 MPa) or with SDS (13.49-19.06 MPa). In a second phase of the study, selection criteria based on morphological and mechanical factors were applied to the pericardial membranes employing a system of paired samples. The mathematical analysis of the findings in one fragment will aid in determining the mechanical behavior of its adjacent twin sample. In conclusion, the anticalcification chemical treatments tested in the experimental model conferred a lesser mechanical resistance than that obtained with GA. On the other hand, the utilization of paired samples was found to be useful in the prediction of the mechanical behavior of porcine pericardial tissue. Nevertheless, in order for our method of selection to be considered the most adequate approach, it will be necessary to validate these findings in dynamic studies involving a real, functional model.

Ostrich pericardium, a biomaterial for the construction of valve leaflets for cardiac bioprostheses: mechanical behaviour, selection and interaction with suture materials

The mechanical behavior of ostrich pericardium was studied for the purpose of assessing its utility in the construction of bioprosthetic cardiac valve leaflets. The tissue was tested biaxially using a hydraulic simulator that subjected it to increasing stress until rupture. One hundred eighty trials were performed, 36 with unsutured pericardium and four series of 36 trials each with pericardium sutured with silk, Prolene, nylon or Gore-Tex. The samples were tested in pairs from three different pericardial regions. One sample from each pair (the predictive specimen) was assessed according to morphological and mechanical criteria, while the other (the predicted or selectable specimen) was subjected only to morphological analysis. The findings show that ostrich pericardium treated with glutaraldehyde according to standard methods has an excellent resistance to rupture in biaxial testing, withstanding stresses of up to 100 MPa, and never lower than 30 MPa. Its resistance to rupture is lowered by suturing, a loss that is less pronounced when silk sutures are used. The results with Gore-Tex are very homogeneous and the elastic behavior of the pericardium/suture unit appears to be similar to that of unsutured tissue, suggesting that the interaction between the two biomaterials is minor. Similar results were observed in the series sutured with Prolene and nylon. The use of paired samples makes it possible to closely estimate the mechanical behavior of the tissue in a given zone by determining that of its mate. The statistical study shows that this estimation is not conditioned by the suture employed, thus validating this approach and providing more precise criteria for tissue selection.

Influence of the selection of the suture material on the mechanical behavior of a biomaterial to be employed in the construction of implants. Part 2: Porcine pericardium

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.

Acellular vascular tissues: natural biomaterials for tissue repair and tissue engineering

Various research groups around the world are actively investigating cardiovascular prostheses of biological origin. This review article discusses the need for such bioprosthetics and the potential role for natural tissues in cardiovascular applications such as cardiac valves and vascular grafts. Upon implantation, unmodified natural materials are subject to chemical and enzymatic degradation, seriously decreasing the life of the prosthesis. Therefore, methods such as glutaraldehyde and polyepoxide crosslinking treatments and dye-mediated photooxidation have been developed to stabilize the tissue while attempting to maintain its natural mechanical properties. Also, residual cellular components in a bioprosthetic material have been associated with undesired effects, such as calcification and immunological recognition, and thus have been the motivation for various decellularization processes. The effects of these stabilization and decellularization treatments on mechanical, biological and chemical properties of treated tissues have been investigated, specifically with regard to calcification, immunogenicity, and cytotoxicity concerns. Despite significant advances in the area of cardiovascular prostheses, there has yet to be developed a completely biocompatible, long-lasting implant. However, with the recent advent of tissue engineering, the possibility of applying selective cell seeding to naturally derived bioprosthetics moves us closer to a living tissue replacement.

Uniaxial and biaxial tensile strength of calf pericardium used in the construction of bioprostheses: biomaterial selection criteria

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.

Assessment of pericardium in cardiac bioprostheses. A review

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

Reduction of calcification by various treatments in cardiac valves

The importance of glutaraldehyde pretreated bioprosthetic heart valves fabricated from bovine pericardium or porcine aortic valves is well realized in the management of valvular heart diseases. But, calcification limits the durability and is the most frequent cause of failure of these bioprosthetic heart valves. Various research groups in the world are actively involved in describing, understanding, and preventing calcification of bioprosthetic heart valves. Since there is no satisfactory clinical means for preventing or treating this disorder, attempts are made to improve the anticalcification properties of the replacement valves in the preparation stage itself. Research in this area is very active, and many newer approaches are made to mitigate the problem. An attempt has been made in the present article to review various theories put forward to explain the causative factors involved and mechanistic aspects of biocalcification and to present various strategies attempted for the prevention of calcification with the special feature on the work done in the area in our laboratory.

Tissue response to biomaterials used for staple-line reinforcement in lung resection: a comparison between expanded polytetrafluoroethylene and bovine pericardium

OBJECTIVE

A study in a canine model of lung-reduction surgery evaluated the tissue response to polytetrafluoroethylene (ePTFE) and bovine pericardium (BP) used for staple-line reinforcement.

METHODS

In each of ten dogs, BP was placed in one lung and ePTFE in the other. The implants were retrieved at 30, 95, or 167 days after implantation and studied histologically. The connective tissue covering the implants was measured and analysis of variance was used to compare results with the two materials.

RESULTS

At 30 days, the BP specimens showed focal chronic inflammation and thin tissue coverage, whereas the ePTFE specimens had no focal inflammation and thick tissue coverage. At 95 and 167 days, the inflammation in the BP specimens had resolved, but tissue coverage remained minimal, and there was no resorption of the BP. In the ePTFE specimens, tissue coverage had increased. Analysis of variance comparing representative tissue specimens showed that the tissue encapsulating the ePTFE was significantly thicker than that surrounding the BP (P < 0.0001). No air leaks, staple-line disruptions, or infections occurred in the study.

CONCLUSIONS

Neither ePTFE nor BP is resorbable. Both materials have been used successfully, without resultant infections, for clinical staple-line reinforcement. The more favorable tissue response to ePTFE observed in this study may have clinical ramifications. Comparative clinical studies of the two materials are needed.

Methods for the treatment of collagenous tissues for bioprostheses

Collagenous tissue as a biomaterial possesses many favourable characteristics and advantages over synthetic materials. The resemblance to human tissue suggests that it has a performance advantage over alternative materials. This advantage has been exploited to produce clinical devices that have been implanted in patients for more than a quarter of a century. The method of treating collagenous tissue for bioprostheses has developed from crude exposure of tissue to chemicals to a sophisticated level of considering the biochemical, chemical, engineering and clinical aspects of the process. This review focuses on the various chemical and physical treatments that have made the bioprostheses possible, highlighting the chemical agents and the cross-linking mechanism involved.

Bovine pericardium for dural grafts: clinical results in 35 patients

OBJECTIVE

The United States Food and Drug Administration has recently approved the marketing of bovine pericardium as a dural graft material, but literature reports of this use are limited. Bovine pericardium has been widely used for grafts in cardiac surgery and seems to have suitable properties for use as a dural graft. We report the use of glutaraldehyde-processed bovine pericardium for dural grafts in 35 patients undergoing cranial and craniospinal operations with the objective of providing a clinical assessment of this material and technique.

METHODS

This report is a retrospective analysis of 35 patients. All available records were reviewed and information regarding the indication for grafting, graft size, complications, and outcome were collected and analyzed for all patients.

RESULTS

Indications for grafting included meningioma resection, posterior fossa craniotomy, Chiari decompression, dural-based metastases, and trauma. Outcomes were good or excellent in 32 patients; the three fair or poor outcomes were not related to surgical closure. In no patient was the dural graft a significant factor in outcome. Bovine pericardium was found to be easily sutured to be watertight using standard suture material. The material is relatively inexpensive and requires no additional incision. It has low antigenicity and toxicity, good strength, and minimal elasticity.

CONCLUSION

In this clinical assessment, bovine pericardium was found to be an excellent dural graft material.

Solvent environment modulates effects of glutaraldehyde crosslinking on tissue-derived biomaterials

Bioprosthetic materials utilized in the construction of heart valves and vascular grafts possess limited performance and viability in vivo. This is due (in part) to the failure of these materials to mimic the mechanical properties of the host tissue they replace. If bioprosthetic materials could be engineered to meet the mechanical performance required in vivo, the functional lifetime of implants would be increased. In this study, glutaraldehyde/solvent solutions of decreasing dielectric constant (polarity) were utilized to modify the properties of crosslinked collagen in whole bovine pericardial tissue. Solvents included phosphate buffer, methanol, 95% (w/w) ethanol, n-propanol, and n-butanol. Exogenous crosslinking was verified in collagen by thermal denaturation tests and amino acid analyses. Tensile mechanical behavior of collagenous pericardial samples was found to depend upon the dielectric constant (polarity) of the glutaraldehyde/solvent solutions employed; however, treatment in the solvents alone had little, if any, effect. As the dielectric constant of the solvents decreased, three mechanical properties were systematically altered: plastic strain fell from a mean of 8.9 +/- 1.5% (buffer) to 1.6 +/- 0.4% (n-butanol); strain at fracture increased from 32.2 +/- 2.6% (buffer) to 55.6 +/- 4.6% (n-butanol); and percent stress remaining after 1000-s stress relaxation from an 80-g initial load fell from 86.3 +/- 1.1% (buffer) to 76.9 +/- 1.0% (n-butanol). Crosslinking using a glutaraldehyde/n-butanol solution produced materials with tensile mechanical behavior that was very close to that of fresh tissue; however, the flexural properties of the treated tissue were different from those of fresh tissue. This decoupling of the flexural and tensile mechanical behaviors of crosslinked bioprosthetic materials is unique to this form of treatment. The observed phenomena may be the results of conformational changes in collagen facilitated by polar/nonpolar interactions with the solvent that are "locked in" by the action of glutaraldehyde. This technique may aid in the "customized" design of mechanical properties in tissue-derived biomaterials.

Influence of stress on calcification of delipidated bovine pericardial tissue employed in construction of cardiac valves

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.

Calcification of pericardial tissue pretreated with different amino acids

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.

Lipids in predentine and dentine

Using two histochemical methods, malachite green-aldehyde and iodoplatinate, phospholipids were visualized in the predentine of rat incisors in the spaces located between collagen fibers and in dentine as needle-like structures located along individual or groups of mineralizing collagen fibers. The same staining pattern was seen with phospholipase A2-gold. Autoradiographic investigation using 3H choline as labelled precursor, visualized the incorporation of membrane-associated and extracellular choline-containing phosphatidyl choline and sphingomyelin. The cell and membrane-associated labelling decreased gradually between 24 and 4 days, whereas incorporation of the labelled precursor as stable extracellular matrix component was seen in dentine. In addition to these investigations, pharmacologically induced (suramine) and genetically (Krabbe's disease) lysosomal storage pathology was investigated. Defects due to lipid metabolism alterations were seen in predentine and/or in dentine. The major differences visualized here between the non-mineralized and mineralized compartments and interactions between phospholipids and proteoglycans, support the view that phospholipids as matrix components play an important role in the mechanisms of dentine formation and mineralization.

Inhibition of the calcification of porcine valve tissue by selective lipid removal

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.

Development of a pericardial acellular matrix biomaterial: biochemical and mechanical effects of cell extraction

There is evidence to suggest that the cellular components of homografts and bioprosthetic xenografts may contribute to calcification or immunogenic reactions. A four-step detergent and enzymatic extraction process has been developed to remove cellular components from bovine pericardial tissue. The process results in an acellular matrix material consisting primarily of elastin, insoluble collagen, and tightly bound glycosaminoglycans. Light and electron microscopy confirmed that nearly all cellular constituents are removed without ultrastructural evidence of damage to fibrous components. Collagen denaturation temperatures remained unaltered. Biochemical analysis confirmed the retention of collagen and elastin and some differential extraction of glycosaminoglycans. Low strain rate fracture testing and high strain rate viscoelastic characterization showed that, with the exception of slightly increased stress relaxation, the mechanical properties of the fresh tissue were preserved in the pericardial acellular matrix. Crosslinking of the material in glutaraldehyde or poly(glycidyl ether) produced mechanical changes consistent with the same treatments of fresh tissue. The pericardial acellular matrix is a promising approach to the production of biomaterials for heart valve or cardiovascular patching applications.