Factors which affect the calcification of tissue-derived bioprostheses

Contact-Active Layer-by-Layer Grafted TPU/PDMS Blends as an Antiencrustation and Antibacterial Platform for Next-Generation Urological Biomaterials: Validation in Artificial and Human Urine

Urinary tract infections and urinary encrustation impede the long-term clinical performance of urological implants and medical devices. Together, biofilm formation and encrustation constitute serious complications, driving the development of next-generation urological biomaterials. The currently available bioengineered solutions have limited success during long-term usage in the urinary environment. In addressing this unmet clinical challenge, contact-active, antiencrustation surface grafting were conceived onto a dynamically cross-linked polydimethylsiloxane (PDMS) modified thermoplastic polyurethane (TPU) blend using the layer-by-layer (LbL) assembly route. To the best of the authors' knowledge, the present study is the first to investigate the LbL grafting in developing an antiencrustation platform. These multilayered assemblies strategically employed covalent cross-linking and electrostatic interaction-assisted progressive depositions of branched polyethyleneimine and poly(2-ethyl-2-oxazoline). While polyethyleneimine conferred the contact-killing bactericidal activity, the much-coveted antiencrustation properties were rendered by incorporating a partially hydrolyzed derivative of poly(2-ethyl-2-oxazoline). The performance of the resultant surface-modified TPU/PDMS blends was benchmarked against the conventional urological alloplasts, in a customized lab-scale bioreactor-based dynamic encrustation study and in human urine. After 6 weeks of exposure to an artificial urine medium, simulating urease-positive bacterial infection, the surface-modified blends exhibited a remarkable ability to suppress Ca and Mg encrustation. In addition, these blends also displayed superior grafting stability and antibacterial efficacy against common uropathogens. As high as 4-fold log reduction in the planktonic growth of Gram-negative P. mirabilis and Gram-positive MRSA was recorded with the LbL platform vis-à-vis medical-grade TPU. In conjunction, the in vitro cellular assessment with human keratinocytes (HaCaT) and human embryonic kidney cells (HEK) established the uncompromised cytocompatibility of the multilayered grafted blends. Finally, the physiologically relevant functionality of the LbL grafting has been validated using clinical samples of human urine collected from 129 patients with a broad spectrum of disease conditions. The phase-I pre-clinical study, entailing 6 week-long incubation in human urine, demonstrated significantly improved encrustation resistance of the blends. The collective findings of the present work clearly establish the success of LbL strategies in the development of stable, multifunctional new-generation urological biomaterials.

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.

When 1+1>2: Nanostructured composites for hard tissue engineering applications

Multicomponent, synergistic and multifunctional nanostructures have taken over the spotlight in the realm of biomedical nanotechnologies. The most prospective materials for bone regeneration today are almost exclusively composites comprising two or more components that compensate for the shortcomings of each one of them alone. This is quite natural in view of the fact that all hard tissues in the human body, except perhaps the tooth enamel, are composite nanostructures. This review article highlights some of the most prospective breakthroughs made in this research direction, with the hard tissues in main focus being those comprising bone, tooth cementum, dentin and enamel. The major obstacles to creating collagen/apatite composites modeled after the structure of bone are mentioned, including the immunogenicity of xenogeneic collagen and continuously failing attempts to replicate the biomineralization process in vitro. Composites comprising a polymeric component and calcium phosphate are discussed in light of their ability to emulate the soft/hard composite structure of bone. Hard tissue engineering composites created using hard material components other than calcium phosphates, including silica, metals and several types of nanotubes, are also discoursed on, alongside additional components deliverable using these materials, such as cells, growth factors, peptides, antibiotics, antiresorptive and anabolic agents, pharmacokinetic conjugates and various cell-specific targeting moieties. It is concluded that a variety of hard tissue structures in the body necessitates a similar variety of biomaterials for their regeneration. The ongoing development of nanocomposites for bone restoration will result in smart, theranostic materials, capable of acting therapeutically in direct feedback with the outcome of in situ disease monitoring at the cellular and subcellular scales. Progress in this research direction is expected to take us to the next generation of biomaterials, designed with the purpose of fulfilling Daedalus' dream - not restoring the tissues, but rather augmenting them.

Elastomeric polypeptide-based biomaterials

Elastomeric proteins are characterized by their large extensibility before rupture, reversible deformation without loss of energy, and high resilience upon stretching. Motivated by their unique mechanical properties, there has been tremendous research in understanding and manipulating elastomeric polypeptides, with most work conducted on the elastins but more recent work on an expanded set of polypeptide elastomers. Facilitated by biosynthetic strategies, it has been possible to manipulate the physical properties, conformation, and mechanical properties of these materials. Detailed understanding of the roles and organization of the natural structural proteins has permitted the design of elastomeric materials with engineered properties, and has thus expanded the scope of applications from elucidation of the mechanisms of elasticity to the development of advanced drug delivery systems and tissue engineering substrates.

Neomycin fixation followed by ethanol pretreatment leads to reduced buckling and inhibition of calcification in bioprosthetic valves

Glutaraldehyde crosslinked bioprosthetic heart valves (BHVs) have two modalities of failure: degeneration (cuspal tear due to matrix failure) and calcification. They can occur independently as well as one can lead to the other causing co-existence. Calcific failure has been extensively studied before and several anti-calcification treatments have been developed; however, little research is directed to understand mechanisms of valvular degeneration. One of the shortcomings of glutaraldehyde fixation is its inability to stabilize all extracellular matrix components in the tissue. Previous studies from our lab have demonstrated that neomycin could be used as a fixative to stabilize glycosaminoglycans (GAGs) present in the valve to improve matrix properties. But neomycin fixation did not prevent cuspal calcification. In the present study, we wanted to enhance the anti-calcification potential of neomycin fixed valves by pre-treating with ethanol or removing the free aldehydes by sodium borohydride treatment. Ethanol treatment has been previously used and found to have excellent anti-calcification properties for valve cusps. Results demonstrated in this study suggest that neomycin followed by ethanol treatment effectively preserves GAGs both in vitro as well as in vivo after subdermal implantation in rats. In vivo calcification was inhibited in neomycin fixed cusps pretreated with ethanol compared to glutaraldehyde (GLUT) control. Sodium borohydride treatment by itself did not inhibit calcification nor stabilized GAGs against enzymatic degradation. Neomycin fixation followed by ethanol treatment of BHVs could prevent both modalities of failure, thereby increasing the effective durability and lifetime of these bioprostheses several fold.

A biomaterial composed of collagen and solubilized elastin enhances angiogenesis and elastic fiber formation without calcification

Elastin is the prime protein in elastic tissues that contributes to elasticity of, for example, lung, aorta, and skin. Upon injury, elastic fibers are not readily replaced, which hampers tissue regeneration. Incorporation of solubilized elastin (hydrolyzed insoluble elastin fibers or elastin peptides) in biomaterials may improve regeneration, because solubilized elastin is able to promote proliferation as well as elastin synthesis. Porous biomaterials composed of highly purified collagen without and without elastin fibers or solubilized elastin were prepared by freezing and lyophilization. Solubilized elastin formed spherical structures that were incorporated in the collagenous part of the scaffolds and that persisted after chemical crosslinking of the scaffolds. Crosslinked scaffolds were subcutaneously implanted in young Sprague Dawley rats. Collagen-solubilized elastin and collagen scaffolds showed no calcification in this sensitive calcification model, in contrast to scaffolds containing elastin fibers. Collagen-solubilized elastin scaffolds also induced angiogenesis, as revealed by type IV collagen staining, and promoted elastic fiber synthesis, as shown with antibodies against rat elastin and fibrillin-1. It is concluded that scaffolds produced from collagen and solubilized elastin present a non-calcifying biomaterial with a capacity for soft-tissue regeneration, especially in relation to elastic fiber synthesis.

Effect of preservation methods on the performance of bovine pericardium graft in a rat model

This study investigates the effect of preservation methods on the performance of bovine parietal pericardium grafts in a rat model. Mid-ventral full thickness abdominal wall defects of 3 x 2.5 cm in size were created in 90 male Sprague-Dawley rats (300-400 g), which were divided into three groups of 30 rats each. The abdominal defects of group one and two were repaired with lyophilized and glycerolized bovine pericardium grafts, while the defects of group three were repaired with expanded polytetrafluoroethylene (ePTFE) Mycro Mesh as a positive control. Another group of 30 rats underwent sham operation and was used for comparison as negative control. Each group of rats (n = 30) was divided into five subgroups (n = 6) and killed at 1, 3, 6, 9 and 18 weeks post-surgery for gross and morphological evaluations. The rats tolerated the surgical procedure well with a total mortality of 0.05%. No serious post-operative clinical complications or signs of rejection were encountered. Adhesions between the grafts and the underlying visceral organs observed in the study were mostly results of post-surgical complications. Glycerol preservation delayed degradation and replacement of the grafts, whereas lyophilization caused early resorption and replacement of the grafts. The glycerolized grafts were replaced with thick dense fibrous tissue, and the lyophilized grafts were replaced with thin loose fibrous tissue. The healing characteristic of the bovine pericardium grafts was similar to those of the sham-operated group, and quite different from those of the ePTFE Mycro Mesh. The outcome of the present study confirmed the superiority of glycerolized bovine pericardium grafts over its lyophilized counter part.

Incorporation of intact elastin scaffolds in tissue-engineered collagen-based vascular grafts

Although collagen-based tissue-engineered blood vessels (TEBVs) have many interesting properties and have been utilized to study aspects of vascular biology, these constructs are too weak to be implanted as bypass grafts for in vivo investigations. This study presents a method to incorporate organized, intact elastin into collagen-based TEBVs to form hybrid constructs that better mimic arterial physiology and exhibit improved mechanical properties. Porcine carotids were digested with a series of autoclave and chemical treatments to elicit isolated elastin scaffolds. Elastin purity was verified via immunohistochemistry and amino acid analysis. Isolated scaffolds were combined with type I collagen and either human dermal fibroblasts (HDFs) or rat smooth muscle cells (RASMs) to form an elastin hybrid TEBV. Hybrid constructs exhibited increased tensile strengths (11-fold in HDFs; 7.5-fold in RASMs) and linear stiffness moduli (4-fold in HDFs; 1.8-fold in RASMs) compared with collagen control constructs with no exogenous elastin scaffold. Viscoelastic properties of the TEBVs also improved with the addition of an ancillary elastin scaffold as determined through stepwise stress relaxation analysis. Whereas the majority of resistance to deformation in collagen control constructs stemmed from viscous fluidlike effects, elastin hybrid constructs exhibited more ideal elastic solid mechanical behavior. Thus, elastin scaffolds can help recreate the elastic properties of native arteries. Future challenges include stimulating appropriate reorganization or synthesis of the collagen matrix to provide the necessary strength and viscoelastic properties for implantation.

Prevention of calcification in bioprosthetic heart valves: challenges and perspectives

Surgical replacement with artificial devices has revolutionised the care of patients with severe valvular diseases. Mechanical valves are very durable, but require long-term anticoagulation. Bioprosthetic heart valves (BHVs), devices manufactured from glutaraldehyde-fixed animal tissues, do not need long-term anticoagulation, but their long-term durability is limited to 15 - 20 years, mainly because of mechanical failure and tissue calcification. Although mechanisms of BHV calcification are not fully understood, major determinants are glutaraldehyde fixation, presence of devitalised cells and alteration of specific extracellular matrix components. Treatments targeted at the prevention of calcification include those that target neutralisation of the effects of glutaraldehyde, removal of cells, and modifications of matrix components. Several existing calcification-prevention treatments are in clinical use at present, and there are excellent mid-term clinical follow-up reports available. The purpose of this review is to appraise basic knowledge acquired in the field of prevention of BHV calcification, and to provide directions for future research and development.

Tissue response of defined collagen–elastin scaffolds in young and adult rats with special attention to calcification

Collagen-elastin scaffolds may be valuable biomaterials for tissue engineering because they combine tensile strength with elasticity. In this study, the tissue response to and the calcification of these scaffolds were evaluated. In particular, the hypothesis was tested that calcification, a common phenomenon in biomaterials, may be due to microfibrils within the elastic fibre, and that these microfibrils might generate a tissue response. Four scaffolds were subcutaneously implanted, viz. collagen, collagen + pure elastin, collagen+microfibril-containing, and collagen + pulverised elastic ligament (the source for elastin). Explants were evaluated at day 3, 7 and 21. In young Sprague Dawley rats, collagen + ligament calcified substantially, whereas collagen + elastin (with and without microfibrils) calcified less, and collagen did not. Calcification started at elastic fibres. In both Sprague Dawley and Wistar adult rats, however, none of the scaffolds calcified. Mononuclear cell infiltration was prominent in young and adult Sprague Dawley rats. In adult Wistar rats, this infiltration was associated with the presence of microfibrils. Degradation of scaffolds and new matrix formation were related with cellular influx and degree of vascularisation. In conclusion, absence of microfibrils from the elastic fibre does not prevent calcification in young Sprague Dawley rats, but does reduce the tissue response in adult Wistar rats. Cellular response and calcification differs with age and strain and therefore the choice of animal model is of key importance in biomaterial evaluation.

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.

Glutaraldehyde-fixed kangaroo aortic wall tissue: histology, crosslink stability and calcification potential

Stentless aortic heart valve substitutes, manufactured from biological tissues, are fixed with glutaraldehyde to cross-link collagen, reduce antigenicity, and sterilize the tissue. Despite improved cross linking, reduced antigenicity, and various anticalcification measures, the aortic wall tissue present in these prostheses tends to calcify. The aim of this study was to assess the morphology, collagen cross-link stability, and calcification potential of glutaraldehyde-preserved kangaroo aortic wall tissue as opposed to porcine aortic wall tissue. Porcine and kangaroo aortic wall tissues were fixed in 0.625% buffered glutaraldehyde. Histology and cross-link stability were examined. Calcification potential was determined in the subcutaneous rat model. Kangaroo aortic wall tissue was significantly (p < 0.01) less calcified than porcine aortic wall tissue (26.67 +/- 6.53 versus 41.959 +/- 2.75 microg/mg tissue) at 8 weeks. In conclusion, the histological differences between kangaroo and porcine aortic wall tissue correlate well with the reduced calcification potential of kangaroo aortic wall tissue. The reduced calcification potential could result in improved long-term durability of stentless kangaroo heart valves as bioprostheses.

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.

Virus safety of a porcine-derived medical device: evaluation of a viral inactivation method

The goal of this study was to evaluate the efficacy of a virus-inactivating process for use during the preparation of porcine-derived extracellular matrix biomaterials for human clinical implantation. Porcine small intestine, the source material for the tissue-engineered, small intestinal submucosa (SIS) biomaterial, was evaluated. Relevant enveloped, non-enveloped, and model viruses representative of different virus families were included in the investigation: porcine parvovirus (PPV), porcine reovirus, murine leukemia retrovirus (LRV), and porcine pseudorabies (herpes) virus (PRV). Samples of small intestine were deliberately inoculated with approximately 1 x 10(7) plaque-forming units (PFU) of virus which were thereafter exposed to a 0.18% peracetic acid/4.8% aqueous ethanol mixture for time periods ranging from 5 minutes to 2 hours. Enveloped viruses were more easily inactivated than non-enveloped viruses, but material processed for 30 minutes or longer inactivated all of the viruses. D(10) values were calculated and used to extrapolate the extent of inactivation after 2 hours. Viral titers were reduced by more than 14.0 log(10) PPV, 21.0 log(10) reovirus, 40.0 log(10) PRV, and 27.0 log(10) LRV, meeting international standards for viral sterility. These results demonstrate that treatment of porcine small intestine with a peracetic acid/ethanol solution leads to a virus-free, non-crosslinked biomaterial safe for xenotransplantation into humans.

Periodate-mediated glycosaminoglycan stabilization in bioprosthetic heart valves

Bioprosthetic heart valves (BPHVs) derived from glutaraldehyde-crosslinked porcine aortic valves are frequently used in heart valve replacement surgeries. However, the majority of bioprostheses fail clinically because of calcification and degeneration. We have recently shown that glycosaminoglycan (GAG) loss may be in part responsible for degeneration of glutaraldehyde-crosslinked bioprostheses. In the present studies, we used a mild reaction of periodate-mediated crosslinking to stabilize glycosaminoglycans in the bioprosthetic tissue. We demonstrate the feasibility of periodate reaction by crosslinking major components of extracellular matrix of bioprosthetic heart valve tissue, namely type I collagen and hyaluronic acid (HA). Uronic acid assay of periodate-fixed HA-collagen matrices showed 48% of HA disaccharides were bound to collagen. Furthermore, we show that such reactions are also feasible to fix glycosaminoglycans present in the middle spongiosa layer of bioprosthetic heart valves. The periodate reactions were compatible with conventional glutaraldehyde crosslinking and showed adequate stabilization of extracellular matrix as demonstrated by thermal denaturation temperature and collagenase assays. Moreover, uronic acid assays of periodate-fixed BPHV cusps showed 36% reduction in the amount of unbound GAG disaccharides as compared with glutaraldehyde-crosslinked cusps. We also demonstrate that calcification of BPHV cusps was significantly reduced in the periodate-fixed group as compared with the glutaraldehyde-fixed group in 21-day rat subdermal calcification studies (periodate-fixed tissue Ca 72.01 +/- 5.97 microg/mg, glutaraldehyde-fixed tissue Ca 107.25 +/- 6.56 microg/mg). We conclude that periodate-mediated GAG fixation could reduce structural degeneration of BPHVs and may therefore increase the useful lifetime of these devices.

Comparison of five procedures for the purification of insoluble elastin

Elastin is an insoluble, highly cross-linked protein, providing elasticity to organs like lung. aorta, and ligaments. Despite its remarkable mechanical properties. elastin has found little use as a biomaterial. Purification of intact elastin from elastic fibres presents a major challenge, among others for the intimate interwoveness of elastin and microfibrils. Insoluble elastin preparations tend to calcify, which may be due to calcium-binding microfibrillar (e.g. fibrillin). In this study, elastin was purified from horse ligamentum nuchae using five different procedures. One procedure is based on treatment with 0.1 M NaOH, another on autoclaving and treatment with cyanogen bromide. Three other procedures are based on combinations of extraction steps and enzyme digestions. Purity of preparations was assessed by sodium dodecyl sulphate polyacrylamide gel electrophoresis, amino acid analysis, bright field immunofluorescence and transmission electron microscopy. The procedure involving extractions/enzymes combined with an early application of 2-mercaptoethanol and cyanogen bromide gives a highly pure elastin preparation. Electron microscopic analysis showed that this preparation is devoid of microfibrillar components. This procedure is therefore the method of choice for preparation of insoluble elastin as a biomaterial for tissue engineering.

Homocysteine decreases chondrocyte-mediated matrix mineralization in differentiating chick limb-bud mesenchymal cell micro-mass cultures

The differentiating chick limb-bud mesenchymal cell micro-mass culture system has been used as a model for monitoring the effects of matrix modification on cell-mediated calcification. In this study, we show that treating these micro-mass cultures with homocysteine (Hcys) impairs cartilage calcification. Cultures were treated from day 2 to day 7 with two nonphysiological concentrations of Hcys equivalent to 100x and 1000x avian serum levels (0.36 and 3.6 mmol/L), and from days 9-13 with one tenth the concentration. Mineralization assays were done at days 16, 19, and 21, and matrix and cell properties were examined between days 5 and 21. Mineral accretion, based on differential (45)Ca uptake (mineralizing minus control cultures), was significantly reduced in the high-Hcys-concentration group, and slightly reduced in the low-Hcys-concentration group. Electron microscopy at culture day 21 showed that the collagen matrix was less abundant and its banding pattern less obvious in the Hcys-treated groups than in the untreated cultures. Pyridinoline (Pyr) and deoxypyridinoline (d-Pyr) contents were not detectable in day 21 cultures with either 0.36 or 3.6 mmol/L homocysteine, whereas values in mineralizing and nonmineralizing controls ranged from 0.06 to 0.08 and 0.03 to 0.06 (moles/mole collagen) for Pyr and d-Pyr, respectively. Fourier transform infrared (FTIR) imaging also indicated a decreased content of pyridinoline cross-links. Hcys caused other matrix changes as well. Whereas at culture day 5 there was no significant difference in the number of chondrocyte nodules formed, by day 11 the proteoglycan content (measured by Alcian blue dye binding at 595 nm) was significantly reduced in both mineralizing and control cultures in the high- and low-Hcys groups. In contrast, there were no detectable differences in type X collagen and alkaline phosphatase staining in the mineralizing cultures with or without Hcys supplements. Because vital dye stains and electron microscopy studies indicated that cells in the control and experimental groups did not differ in terms of viability, the observed differences cannot be attributed to toxicity. Thus, Hcys treatment, which causes matrix disorganization, decreases the ability of the matrix to support mineralization.

High glutaraldehyde concentrations mitigate bioprosthetic root calcification in the sheep model

BACKGROUND

Fixation at high glutaraldehyde (GA) concentrations mitigated bioprosthetic calcification in the rat model. The present study intended to verify this observation in the circulatory sheep model.

METHODS

Porcine aortic roots were either fixed in 0.2%, 1.0%, or 3.0% GA. Eight roots per group were implanted in the distal aortic arch of sheep. After six weeks and six months calcification and inflammation were quantitatively and qualitatively assessed.

RESULTS

By increasing the GA concentration from 0.2% to 3.0%, aortic wall calcification could be reduced by 38% after 6 weeks and 34% after 6 months of implantation (p < 0.01). Mineralization coincided with the presence of elastin although calcium was predominantly found in cell nuclei and membranes. Leaflet calcification was absent in all groups after 6 weeks but in a few leaflets presented as heterogeneous, nodular spongiosa deposits after 6 months. Overall, differences between 0.2%-, 1.0%-, and 3.0%-fixed tissue were quantitative but not qualitative regarding distribution patterns. There was no significant difference in inflammatory host reaction between all groups.

CONCLUSIONS

We have shown in the circulatory sheep model that the anticalcific effect of better cross-linking seems to outweigh the intrinsic pro-calcific effect of GA accumulation in bioprosthetic aortic wall tissue.

In vivo behavior of epoxy-crosslinked porcine heart valve cusps and walls

Calcification limits the long-term durability of xenograft glutaraldehyde-crosslinked heart valves. In this study, epoxy-crosslinked porcine aortic valve tissue was evaluated after subcutaneous implantation in weanling rats. Non-crosslinked valves and valves crosslinked with glutaraldehyde or carbodiimide functioned as control. Epoxy-crosslinked valves had somewhat lower shrinkage temperatures than the crosslinked controls, and within the series also some macroscopic and microscopic differences were obvious. After 8 weeks implantation, cusps from non-crosslinked valves were not retrieved. The matching walls were more degraded than the epoxy- and control-crosslinked walls. This was observed from the higher cellular ingrowth with fibroblasts, macrophages, and giant cells. Furthermore, non-crosslinked walls showed highest numbers of lymphocytes, which were most obvious in the capsules. Epoxy- and control-crosslinked cusps and walls induced lower reactions. Calcification, measured by von Kossa-staining and by Ca-analysis, was always observed. Crosslinked cusps calcified more than walls. Of all wall samples, the non-crosslinked walls showed the highest calcification. It is concluded that epoxy-crosslinked valve tissue induced a foreign body and calcification reaction similar to the two crosslinked controls. Therefore, epoxy-crosslinking does not represent a solution for the calcification problem of heart valve bioprostheses.

Characterization and biocompatibility of epoxy-crosslinked dermal sheep collagens

Dermal sheep collagen (DSC), which was crosslinked with 1, 4-butanediol diglycidyl ether (BD) by using four different conditions, was characterized and its biocompatibility was evaluated after subcutaneous implantation in rats. Crosslinking at pH 9.0 (BD90) or with successive epoxy and carbodiimide steps (BD45EN) resulted in a large increase in the shrinkage temperature (T(s)) in combination with a clear reduction in amines. Crosslinking at pH 4.5 (BD45) increased the T(s) of the material but hardly reduced the number of amines. Acylation (BD45HAc) showed the largest reduction in amines in combination with the lowest T(s). An evaluation of the implants showed that BD45, BD90, and BD45EN were biocompatible. A high influx of polymorphonuclear cells and macrophages was observed for BD45HAc, but this subsided at day 5. At week 6 the BD45 had completely degraded and BD45HAc was remarkably reduced in size, while BD45EN showed a clear size reduction of the outer DSC bundles; BD90 showed none of these features. This agreed with the observed degree of macrophage accumulation and giant cell formation. None of the materials calcified. For the purpose of soft tissue replacement, BD90 was defined as the material of choice because it combined biocompatibility, low cellular ingrowth, low biodegradation, and the absence of calcification with fibroblast ingrowth and new collagen formation.

Reticulação progressiva de pericárdio bovino com glutaraldeído para confecção de válvulas cardíacas biológicas

Este trabalho descreve um novo processo para a reticulação de matrizes de colágeno com glutaraldeído para a preparação de materiais para a confecção de válvulas cardíacas biológicas, e consiste no tratamento do pericárdio bovino com concentrações progressivamente crescentes com este reagente no intervalo de concentração entre 0,005 e 0,5% em tampão fosfato, pH 7,4. O perfil da reação com glutaraldeído, quando comparado com o procedimento convencional, foi mais homogêneo e os materiais obtidos apresentaram propriedades térmicas e mecânicas similares. Entretanto, mostraram-se significativamente mais estáveis à degradação enzimática, principalmente em relação à tripsina (17 x), que é devido provavelmente à formação de ligações de reticulação mais homogêneas e/ou em maior densidade. Os resultados mostram que metodologia seqüencial estudada, associada ao tratamento convencional, pode constituir-se em uma técnica de pré-tratamento promissora para a preparação de materiais mais eficientes para confecção de biopróteses.