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

Effect of ethanol washing on porcine pulmonary artery wall decellularization using sodium dodecyl sulfate

BACKGROUND

To determine the effectiveness of ethanol (EtOH) washing on porcine pulmonary artery (PA) wall decellularization using sodium dodecyl sulfate (SDS), we compared three different washing methods (phosphate-buffered saline [PBS], pH 9 alkali, and EtOH washing).

METHODS

Fresh porcine PA walls were decellularized using 0.5% SDS and 0.5% sodium deoxycholate (SDC). The decellularized tissues were rinsed using three different washing techniques. Histological, biochemical, and mechanical analyses were conducted. Implantation into the subcutaneous tissue of rats and patch implantation into the carotid artery of dogs were performed as preliminary in vivo studies.

RESULTS

The decellularization protocol based on SDS and SDC effectively removed the cells. The major extracellular matrix (ECM) structures (collagen, elastic fiber, and glycosaminoglycan) were properly preserved with the 75% EtOH-washing method. Significantly reduced residual SDS content was identified in EtOH-washed tissues compared to that in the other methods. No significant difference in the mechanical strength test was observed between the washing methods, and the EtOH-washing method showed better results in the metabolic activity test compared to the PBS-washing method. In the rat study model, no acute rejection or massive calcification was observed. The in vivo preliminary canine study showed better cell repopulation in the EtOH-washed group.

CONCLUSION

EtOH washing of SDS-based decellularized porcine PA wall can reduce the residual SDS content and preserve ECM structures, especially the elastin content, and could also enhance cell repopulation after re-implantation.

A preclinical trial of perventricular pulmonary valve implantation: Pericardial versus aortic porcine valves mounted on self-expandable stent

Perventricular pulmonary valve implantation (PPVI) of a xenograft valve can be a less invasive technique that avoids cardiopulmonary bypass in patients who require pulmonary valve replacement. We compared the hemodynamics, durability, and histologic changes between two different xenogenic valves (pericardial vs. aortic valve porcine xenografts) implanted into the pulmonary valve position using a PPVI technique and evaluated the safety and efficacy of PPVI as a preclinical study. In 18 sheep, pericardial (group porcine pericardial [PP], n = 9) or aortic valve (group porcine aortic valve [PAV], n = 9) xenogenic porcine valves manufactured as a stented valve were implanted using a PPVI technique. The porcine tissues were decellularized, alpha-galactosidase treated, fixed with glutaraldehyde after space-filler treatment, and detoxified to improve durability. Hemodynamic and immunohistochemical studies were performed after the implantation; radiologic and histologic studies were performed after a terminal procedure. All stented valves were positioned properly after the implantation, and echocardiography and cardiac catheterization demonstrated good hemodynamic state and function of the valves. All the anti-α-Gal IgM and IgG titers were below 0.3 optical density. Computed tomography of extracted valves demonstrated no significant differences in the degree of calcification between the two groups (P = .927). Microscopic findings revealed a minimal amount of calcification and no significant infiltration of macrophage or T-cell in both groups, regardless of the implantation duration. The PPVI is a feasible technique. Both stented valves made of PP and PAV showed no significant differences in hemodynamic profile, midterm durability, and degree of degenerative dystrophic calcification.

Natural hydrogels for cartilage regeneration: Modification, preparation and application

Hydrogels, consisting of hydrophilic polymers, can be used as films, scaffolds, nanoparticles and drug carriers. They are one of the hot research topics in material science and tissue engineering and are widely used in the field of biomedical and biological sciences. Researchers are seeking for a type of material that is similar to human tissues and can partially replace human tissues or organs. The hydrogel has brought possibility to solve this problem. It has good biocompatibility and biodegradability. After entering the body, it does not cause immune and toxic reactions. The degradation time can be controlled, and the degradation products are nontoxic and nonimmunogenic; the final metabolites can be excreted outside the body. Owing to the lack of blood vessels and poor migration ability of chondrocytes, the self-healing ability of damaged cartilage is limited. Tissue engineering has brought a new direction for the regeneration of cartilage. Drug carriers and scaffolds made of hydrogels are widely used in cartilage tissue engineering. The present review introduces the natural hydrogels, which are often used for cartilage tissue engineering with respect to synthesis, modification and application methods.

THE TRANSLATIONAL POTENTIAL OF THIS ARTICLE

This review introduces the natural hydrogels that are often used in cartilage tissue engineering with respect to synthesis, modification and application methods. Furthermore, the essential concepts and recent discoveries were demonstrated to illustrate the achievable goals and the current limitations. In addition, we propose the putative challenges and directions for the use of natural hydrogels in cartilage regeneration.

Stabilisation of Collagen Sponges by Glutaraldehyde Vapour Crosslinking

Glutaraldehyde is a well-recognised reagent for crosslinking and stabilising collagens and other protein-based materials, including gelatine. In some cases, however, the use of solutions can disrupt the structure of the material, for example, by causing rapid dispersion or distortions from surface interactions. An alternative approach that has been explored in a number of individual cases is the use of glutaraldehyde vapour. In this study, the effectiveness of a range of different glutaraldehyde concentrations in the reservoir providing vapour, from 5% to 25% (w/v), has been explored at incubation times from 5 h to 48 h at room temperature. These data show the effectiveness of the glutaraldehyde vapour approach for crosslinking collagen and show that materials with defined, intermediate stability could be obtained, for example, to control resorption rates in vivo.

Modifications on Collagen Structures Promoted by 1,4-Dioxane Improve Thermal and Biological Properties of Bovine Pericardium as a Biomaterial

Collagen is a widely used raw material for biomaterial manufacture, which generally depends on chemical modifications of this fibrillar protein with cross-linking agents to improve biocompatibility and mechanical properties. However, cross-linking reduces the natural properties of collagen, such as low immune response, low toxicity as well as the ability to promote cellular growth and attachment. In this work, the modifications promoted by 1,4-dioxane solvent on the collagen present in native bovine pericardium (NBP) matrix routinely used in bioprosthesis manufacture, with or without subsequent cross-linking by glutaraldehyde, has been studied. The structural changes of NBP evaluated by scanning electron microscopy show that 1,4-dioxane induces a more homogeneous material by increasing aggregation of collagen fibers, while transmission scanning electron microscopy shows that natural collagen fibril arrangement, integrity, and the D-periodicity pattern are maintained by solvent treatments. Measurements of thermal stability and resistance to collagenase enzymatic digestion of NBP matrices treated with 1,4-dioxane show an increase in melting temperature and decrease in biodegradability, as compared to native pericardium. Cross-linking with glutaraldehyde improves all the analyzed NBP properties, which are not impaired by previous treatment with 1,4-dioxane. Histological evaluation of NBP submitted to 1,4-dioxane treatment shows lower lipid and cell contents and improvement in other morphologic characteristics compared to native pericardium. Altogether, these results suggest the use of 1,4-dioxane organic solvent as an alternative non-cross-linking treatment for direct utilization on rich collagen matrices, resulting in materials with improved biocompatibility and physicochemical properties suitable for tissue engineering.

Development of novel combined anticalcification protocols including immunologic modification for prolonged durability of cardiac xenograft: preclinical study using large-animal long-term circulatory models

Cardiac xenografts are conventionally cross-linked with glutaraldehyde (GA) to impart tissue stability, reduce antigenicity, and maintain tissue sterility. However, GA-fixed xenografts are prone to calcification after long-term implantation in humans, because of phospholipids, free aldehyde groups, and residual antigenicity. We evaluated preclinical safety and efficacy using large-animal long-term circulatory models for our novel combined anticalcification protocol including immunological modification, which had been proven effective in small animal experiments. Bovine/porcine xenografts were treated with decellularization, immunological modification with α-galactosidase, GA fixation with organic solvent, and detoxification with glycine. Valve conduits made of these xenografts were transplanted into the pulmonary root of goats, and hemodynamic, radiological, immunohistopathological, and biochemical results were obtained for 12 months after implantation. Evaluation of echocardiography and cardiac catheterization demonstrated good hemodynamic status and function of the pulmonary xenograft valves. Durability of the xenografts was well preserved without calcification by specimen radiography and immunohistopathological examination. The calcium concentrations of the explanted xenografts were lower than the control xenografts. This preclinical study using large-animal long-term circulatory models demonstrated that our synergistic and simultaneous employment of multiple anticalcification therapies and novel tissue treatments, including immunological modifications, have promising safety and efficacy and should be examined further in future clinical studies.

Stem Cells on Biomaterials for Synthetic Grafts to Promote Vascular Healing

This review is divided into two interconnected parts, namely a biological and a chemical one. The focus of the first part is on the biological background for constructing tissue-engineered vascular grafts to promote vascular healing. Various cell types, such as embryonic, mesenchymal and induced pluripotent stem cells, progenitor cells and endothelial- and smooth muscle cells will be discussed with respect to their specific markers. The in vitro and in vivo models and their potential to treat vascular diseases are also introduced. The chemical part focuses on strategies using either artificial or natural polymers for scaffold fabrication, including decellularized cardiovascular tissue. An overview will be given on scaffold fabrication including conventional methods and nanotechnologies. Special attention is given to 3D network formation via different chemical and physical cross-linking methods. In particular, electron beam treatment is introduced as a method to combine 3D network formation and surface modification. The review includes recently published scientific data and patents which have been registered within the last decade.

Effect of glutaraldehyde fixation on the frictional response of immature bovine articular cartilage explants

This study examined functional properties and biocompatibility of glutaraldehyde-fixed bovine articular cartilage over several weeks of incubation at body temperature to investigate its potential use as a resurfacing material in joint arthroplasty. In the first experiment, treated cartilage disks were fixed using 0.02, 0.20 and 0.60% glutaraldehyde for 24h then incubated, along with an untreated control group, in saline for up to 28d at 37°C. Both the equilibrium compressive and tensile moduli increased nearly twofold in treated samples compared to day 0 control, and remained at that level from day 1 to 28; the equilibrium friction coefficient against glass rose nearly twofold immediately after fixation (day 1) but returned to control values after day 7. Live explants co-cultured with fixed explants showed no quantitative difference in cell viability over 28d. In general, no significant differences were observed between 0.20 and 0.60% groups, so 0.20% was deemed sufficient for complete fixation. In the second experiment, cartilage-on-cartilage frictional measurements were performed under a migrating contact configuration. In the treated group, one explant was fixed using 0.20% glutaraldehyde while the apposing explant was left untreated; in the control group both explants were left untreated. From day 1 to 28, the treated group exhibited either no significant difference or slightly lower friction coefficient than the untreated group. These results suggest that a properly titrated glutaraldehyde treatment can reproduce the desired functional properties of native articular cartilage and maintain these properties for at least 28d at body temperature.

Histologic Characteristics and Mechanical Properties of Bovine Pericardium Treated with Decellularization and α-Galactosidase: A Comparative Study

Background

Bioprostheses for cardiovascular surgery have limitations in their use following as calicification. α-galactosidase epitope is known as a stimulant of immune response and then shows a progressing calcification. The objective of this study was to evaluate histologic characteristics and mechanical properties of decellularization and treated with α-galactosidase.

Materials and Methods

Bovine pericardial tissues were allocated into three groups: fixation only with glutaraldehyde, decellularization with sodium dodesyl sulfate and decellularization plus treatment with α-galactosidase. We confirmed immunohistological characteristics and mechanical properties as fatigue test, permeability test, compliance test, tensile strength (strain) test and thermal stability test.

Results

Decellularization and elimination of α-gal were confirmed through immunohistologic findings. Decellularization had decreased mechanical properties compared to fixation only group in permeability (before fatigue test p=0.02, after fatigue test p=0.034), compliance (after fatigue test p=0.041), and tensile strength test (p=0.00). The group of decellularization plus treatment with α-galactosidase had less desirable mechanical properties than the group of decellularization in concerns of permeability (before fatigue test p=0.043) and strain test (p=0.001).

Conclusion

Favorable decellularization and elimination of α-gal were obtained in this study through immunohistologic findings. However, those treatment including decellularization and elimination of α-gal implied the decreased mechanical properties in specific ways. We need more study to complete appropriate ioprosthesis with decellularization and elimination of α-gal including favorable mechanical properties too.

1,4-Dioxane enhances properties and biocompatibility of polyanionic collagen for tissue engineering applications

Polyanionic collagen obtained from bovine pericardial tissue submitted to alkaline hydrolysis is an acellular matrix with strong potential in tissue engineering. However, increasing the carboxyl content reduces fibril formation and thermal stability compared to the native tissues. In the present work, we propose a chemical protocol based on the association of alkaline hydrolysis with 1,4-dioxane treatment to either attenuate or revert the drastic structural modifications promoted by alkaline treatments. For the characterization of the polyanionic membranes treated with 1,4-dioxane, we found that (1) scanning electron microscopy (SEM) shows a stronger reorientation and aggregation of collagen microfibrils; (2) histological evaluation reveals recovering of the alignment of collagen fibers and reassociation with elastic fibers; (3) differential scanning calorimetry (DSC) shows an increase in thermal stability; and (4) in biocompatibility assays there is a normal attachment, morphology and proliferation associated with high survival of the mouse fibroblast cell line NIH3T3 in reconstituted membranes, which behave as native membranes. Our conclusions reinforce the ability of 1,4-dioxane to enhance the properties of negatively charged polyanionic collagen associated with its potential use as biomaterials for grafting, cationic drug- or cell-delivery systems and for the coating of cardiovascular devices.

Study on genipin: a new alternative natural crosslinking agent for fixing heterograft tissue

Background

In cardiac surgery, especially in the reconstruction of vascular structures and intracardiac defects, glutaraldehyde has usually been used as the reagent for fixing porcine or bovine pericardial tissues. But the well-known problem of calcification or cytotoxicity of glutaraldehyde motivates the search for a replacement. The aim of this study is to investigate the physical, mechanical, and biochemical characteristics of bovine pericardial tissues fixed with genipin, which is known to be a less toxic and more natural fixing reagent.

Materials and Methods

Bovine pericardial tissues were fixed with different concentrations and conditions of glutaraldehyde and genipin. To determine the physical, mechanical, and biochemical differences among different concentrations and conditions, we divided the tissue into 18 groups by concentration, the addition of organic solvents, and the timing of adding the organic solvents, and compared the characteristics of each group.

Results

Tensile strength, physical activity, and thermal stability tests revealed that the tissues fixed with glutaraldehyde were better with regard to mechanical strength and biochemical durability. However, the difference was not significant statistically.

Conclusion

Genipin can be used as an alternative crosslinking agent for pericardial tissue, considering given its physical, mechanical, biochemical characteristics and low cytotoxicity comparable to glutaraldehyde. However, further studies are needed on the immune reaction and the long term changes in genipin-fixed tissues in the human body.

Regulation of material properties in electrospun scaffolds: Role of cross-linking and fiber tertiary structure

We cross-linked scaffolds of electrospun collagen to varying degrees with glutaraldehyde using an ethanol-based solvent system and subsequently defined how the percentage of cross-linking impacts bulk and microscale material properties and fiber structure. At hydration, electrospun fibers underwent coiling; the extent of coiling was proportional to the percentage of cross-linking introduced into the samples and was largely suppressed as cross-linking approached saturation. These data suggest that electrospun collagen fibers are not deposited in a minimal energy state; fiber coiling may reflect a molecular reorganization. This result has functional/structural implications for protein-based electrospun scaffolds. Changes in fiber topology that develop during post-electrospinning processing may alter monomer organization, mask or unmask receptor binding sites, and/or change the biological properties of these nanomaterials. Hydrated scaffolds were mounted into a custom stretching device installed on a microscope stage and photographed after incremental changes in strain. Changes in fiber alignment were measured using the two-dimensional fast Fourier transform method. Fibers in all scaffolds underwent alignment in response to strain; however, the rate and extent of alignment that could be achieved varied as a function of cross-linking. We propose four distinct modes of scaffold response to strain: fiber uncoiling, fiber reorientation, fiber elongation and interfiber sliding. We conclude that bulk material properties and local microscale architecture must be simultaneously considered to optimize the performance of electrospun scaffolds.

Review: advances in vascular tissue engineering using protein-based biomaterials

The clinical need for improved blood vessel substitutes, especially in small-diameter applications, drives the field of vascular tissue engineering. The blood vessel has a well-characterized structure and function, but it is a complex tissue, and it has proven difficult to create engineered tissues that are suitable for widespread clinical use. This review is focused on approaches to vascular tissue engineering that use proteins as the primary matrix or "scaffold" material for creating fully biological blood vessel replacements. In particular, this review covers four main approaches to vascular tissue engineering: 1) cell-populated protein hydrogels, 2) cross-linked protein scaffolds, 3) decellularized native tissues, and 4) self-assembled scaffolds. Recent advances in each of these areas are discussed, along with advantages of and drawbacks to these approaches. The first fully biological engineered blood vessels have entered clinical trials, but important challenges remain before engineered vascular tissues will have a wide clinical effect. Cell sourcing and recapitulating the biological and mechanical function of the native blood vessel continue to be important outstanding hurdles. In addition, the path to commercialization for such tissues must be better defined. Continued progress in several complementary approaches to vascular tissue engineering is necessary before blood vessel substitutes can achieve their full potential in improving patient care.

Matrix alteration and not residual sodium dodecyl sulfate cytotoxicity affects the cellular repopulation of a decellularized matrix

It has been suggested that residual cytotoxic sodium dodecyl sulfate (SDS) is responsible for the low levels of cell in-growth observed in SDS decellularized tissues. To determine whether this is the case, we used 2 washing methods to remove residual SDS and extensive biochemical, mechanical, and structural analyses to determine the effects of SDS-based decellularization on porcine anterior cruciate ligament (ACL) tissue and its propensity for cellular repopulation. The level of residual SDS in decellularized tissue was reduced using 2 different washing techniques (pH = 9 buffer, 75% ethanol). After washing in pH = 9 or 75% ethanol, residual SDS concentrations in decellularized tissues were found to be approximately 8 and 23 times less than reported SDS cytotoxic levels, respectively. It was found that SDS treatment significantly reduced glycosaminoglycan levels, increased collagen crimp amplitude and periodicity, and increased susceptibility of collagen to degradation by the gelatinase enzyme trypsin. The level of repopulation and viability of autologous ACL fibroblasts in the decellularized tissue after 28 days of culture were found to be the same regardless of the washing technique and resulting level of residual SDS in the tissue. This strongly indicates that alterations in tissue matrix biochemistry or structure from SDS treatment and not residual SDS cytotoxicity are responsible for the low cell re-population observed in SDS decellularized tissues.

Delivering Octadecylphosphonic Acid Self-Assembled Monolayers on a Si Wafer and Other Oxide Surfaces

We describe a simple experimental approach for delivering self-assembled monolayers (SAMs) of octadecylphosphonic acid (OPA) on many oxide surfaces using a nonpolar medium with a dielectric constant around 4 (e.g., trichloroethylene). This approach readily results in the formation of full-coverage OPA SAMs on a wide variety of oxide surfaces including cleaved mica, Si wafer, quartz, and aluminum. Especially, the availability of delivering full-coverage OPA SAM on a Si wafer is unique, as no OPA SAMs at all could be formed on a Si wafer when using a polar OPA solution. The reason a nonpolar solvent is superior lies in the very fact that the hydrophilic OPA headgroup tends to escape from the nonpolar solution and is thus enriched at the medium-air interface. It is these OPA headgroups seeking a hydrophilic surface that make possible the well-controlled OPA monolayer on an oxide surface.

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 modification of scaffold material in building artificial dermis

Type X collagen is principal extracellular matrix (ECM) in natural dermis. To prepare artificial dermis, collagen is traditional, and most superior biomaterial. But beside collagen, the dermis also contains many other ECM. Among them, glycosaminoglycan (GAG) is another important substance. To imitate the natural dermis, and modificate the scaffold materials, two types of scaffolds were prepared: one is traditional type X collagen spongy scaffold, the other is collagen-chondroitin sulfate (CS) spongy scaffold. Collagen was blended with CS, one kind of GAG, and cross-linked by 1-ethyl-3-(3-dimethyl aminopropyl) carbodiimide (EDC). Dermis fibroblast was isolated from neonate prepuce, and dermis fibroblasts were cultured on the scaffolds. The physical and chemical properties of the scaffolds were tested, including SEM, DSC, H&E staining, immunohistochemical staining and CS content analysis and so on. The results indicated that EDC is an effective and non-cytotoxic cross-link reagent, and attaching CS into collagen scaffold could improve the stability and histocompatibility of scaffold.

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.

Diamine extension of glutaraldehyde crosslinks mitigates bioprosthetic aortic wall calcification in the sheep model

We previously have been able to show that fixation at increasing concentrations of glutaraldehyde (GA) leads to mitigated rather than facilitated tissue calcification. The purpose of the present study was to introduce additional crosslinks and provide evidence that crosslink density may be an underlying inhibitory principle. Entire aortic roots were chosen to verify the concept on the challenging aortic wall tissue. Porcine aortic roots were crosslinked with 0.2% GA, 3%GA, and 3% GA containing an interim step that introduced diamine bridges. Crosslink efficiency was determined on the basis of shrinkage temperature (SrT degrees ), resistance to protease digestion (RPD), residual amine analysis (RA), and tensile modulus (E(10)). Calcium levels, calcification patterns, and inflammation were assessed after 6 and 24 weeks of implantation in a sheep circulatory model. Crosslink efficiency in aortic wall tissue was moderately affected by increasing the fixative concentration from 0.2% GA to 3% GA (SrT degrees from 85.7 degrees +/- 0.3 degrees to 87.5 degrees +/- 0.3 degrees C, p < 0.002; RPD from 24.2 +/- 1.2 to 29.1 +/- 0.7%, p < 0.003; RA from 0.069 +/- 0.004 to 0.058 +/- 0.003 micromol/mg, p < 0.03, and E(10) from 1.9 +/- 0.11 to 2.94 +/- 0.34 MPa, p < 0.01), but it was distinctly enhanced when diamine bridges were introduced (SrT degrees from 87.5 degrees +/- 0.3 degrees to 93.4 degrees +/- 0.3 degrees C, p << 0.0001; RPD from 29.1 +/- 0.7 to 68.4 +/- 1.8%, p << 0.0001; and E(10) from 2.94 +/- 0.34 to 6.80 +/- 0.61 MPa, p < 0.0003). Aortic wall calcification was reduced significantly by increasing the GA concentration from 0.2 to 3% [37.8%, p = 0.076 (6 weeks) and 34.0%, p = 0.008 (24 weeks)] and further reduced by the introduction of additional diamine [84.0%, p = 0.006 (6 weeks) and 29.8%, p = 0.037 (24 weeks)]. The combined effect of increased GA concentration plus an interim diamine step on aortic wall tissue resulted in a 90% and 53.7% reduction of calcification after 6 weeks and 24 weeks, respectively. The correlation coefficients between calcification and SrT degrees, RDP, and E(10) was -0.9767, -0.9460, and -0.9740, respectively (6 weeks). The inflammatory host reaction regularly found in 0.2% fixed tissue was practically abolished through the introduction of diamine bridges. Our study demonstrated a distinct correlation between the mitigation of aortic wall calcification and three parameters used to assess crosslink density.

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.

Metabolic stability of glutaraldehyde cross-linked peptide DNA condensates

The stability of peptide DNA condensates was examined after introducing glutaraldehyde to cross-link surface amine groups. A 20 amino acid peptide (CWK(18)) was used to condense DNA into small (70 nm) condensates. The reaction between glutaraldehyde and peptide DNA condensates was indirectly monitored using a fluorescence-based assay to establish reaction completion in 4-5 h when using glutaraldehyde-to-peptide ratios of 1 to 4 mol equiv. Higher levels of glutaraldehyde cross-linking led to significant increases in particle size. The improved stability imparted by glutaraldehyde cross-linking was demonstrated by the increased resistance of DNA condensates to shear stress induced fragmentation. The cross-linked condensates were also significantly more resistant to in vitro metabolism by serum endonucleases. A decrease in the magnitude of transient gene expression was determined for cross-linked DNA condensates which also resulted in a 10-day steady-state expression when cross-linking with 4 mol equiv of glutaraldehyde. The results suggest that cross-linking DNA condensates may provide a means to alter the time course of transient gene expression by inhibiting DNA metabolism.

Collagen--biomaterial for drug delivery

The use of collagen as a biomaterial is currently undergoing a renaissance in the tissue engineering field. The biotechnological applications focus on the aspects of cellular growth or delivery of proteins capable of stimulating cellular response. However, basic knowledge about collagen biochemistry and the processing technology in combination with understanding of the physico-chemical properties is necessary for an adequate application of collagen for carrier systems. The purpose of this review article is to summarize information available on collagen dosage forms for drug delivery as well as to impart an overview of the chemical structures and the galenical properties including detailed description of the processing steps - extraction, purification, chemical crosslinking and sterilization. The most successful and stimulating applications are shields in ophthalmology, injectable dispersions for local tumor treatment, sponges carrying antibiotics and minipellets loaded with protein drugs. However, the scientific information about manipulating release properties or mechanistic studies is not as abundant as for some synthetic polymers.

The chemical protecting group concept applied in crosslinking of natural tissues with glutaraldehyde acetals

This work describes the results of the controlled crosslinking of collagen matrices by glutaraldehyde based on a double protection strategy, glutaraldehyde acetals and lysine protonation due to the acidic conditions of acetal formation. Materials crosslinked by this approach were characterized by thermal stability comparable to those obtained by conventional procedures with mechanical properties expected for bioprosthesis manufacture and with a higher stability toward collagenase hydrolysis. The integrity of the microfibrillar structure was confirmed by optical and scanning electronic microscopy. The results indicate that the glutaraldehyde acetals procedure may be of potential use for the crosslinking of bovine pericardium used in the manufacture of bioprosthetic devices. Advantages may be related to the production of materials with homogeneous crosslinking distributions, associated with better definition in the nature of the chemical link that they introduce, due to a better distribution of glutaraldehyde within the tissue matrix before the crosslinking reaction is allowed to occur. As a result, materials with improved biological and mechanical properties are expected.

Altered mechanical properties in aortic elastic tissue using glutaraldehyde/solvent solutions of various dielectric constant

The extent to which elastic tissue can be crosslinked in aldehydes and the mechanism of such action is unresolved in the literature. We have used glutaraldehyde/solvent solutions of decreasing dielectric constant (phosphate buffer, methanol, 95% ethanol, n-propanol, n-butanol) to alter the mechanical properties of aortic elastic tissue obtained from autoclaved and CNBr-purified bovine aortae. Treated and untreated hoop samples were examined for stress-strain and stress relaxation behavior and for residual stress using opening angle experiments as per Fung. The extent of exogenous crosslinking was analyzed through amino acid analysis. Mechanical properties of autoclaved elastic tissue varied with dielectric constant in glutaraldehyde/solvent treatments; however, solvent treatment alone produced no effect. Extensibility decreased with decreasing dielectric constant while tensile modulus changed over a range from -2.4% (-0.86 kPa) for glutaraldehyde/buffer to +35.3% (+14.3 kPa) for glutaraldehyde/n-propanol (untreated-treated). Residual stress experiments similarly showed a systematic decrease in opening angle with decreasing dielectric constant. Differences ranged from 10.5 degrees for glutaraldehyde/buffer to 22.2 degrees for glutaraldehyde/n-butanol. Interestingly, purification of aortae with CNBr reduced the effects of glutaraldehyde/n-butanol treatment. We hypothesize that CNBr differentially degraded the elastin-associated microfibrillar proteins in aortic elastic tissue, thus producing the observed differences in mechanical behavior. The observed phenomena in this study may be attributed to the composite structure of elastic tissue: elastin and microfibrillar protein. During treatment, conformational changes in elastin facilitated by polar/nonpolar interactions occurred which then were "locked" in by glutaraldehyde crosslinking of the microfibrillar proteins. By this mechanism the increases in both stiffness and time-dependent behavior observed after treatment may be explained.

Effect of applied uniaxial stress on rate and mechanical effects of cross-linking in tissue-derived biomaterials

Conformational changes in collagen fibrils, and indeed the triple helix, can be produced by application of mechanical stress or strain. We have demonstrated that the rate of cross-linking in glutaraldehyde and epoxide homobifunctional reagents can be modulated by uniaxial stress (strain). Two poly(glycidyl ether) epoxides were used: Denacol EX-810 (a small bifunctional reagent), and Denacol EX-512 (a large polyfunctional reagent). To prevent any possible effect from being masked by saturation of cross-linking sites, bovine pericardium was cross-linked to such an extent that the increase in collagen denaturation temperature, Td, was one-half of the maximal rise achievable with each reagent. Uniaxial tensile stress of 0, 15, 124 or 233 kPa was applied during cross-linking. Cross-linking rate (as observed by increase in Td) increased with increasing stress to a maximum at 124 kPa in glutaraldehyde at pH 7 but decreased in EX-810 at pH 7. In each case, the effect was small but statistically significant. No effect was observed with the larger EX-512. Cross-linking under increasing stress also showed systematic effects on mechanical properties: decreasing extensibility and plastic strain while increasing tensile strength. In each case, the effects of the epoxides were slightly different from those of glutaraldehyde. In preparation for the above experiments, studies of the effect of pH, temperature, and exposure time were carried out for each epoxide and (to a lesser extent) for glutaraldehyde. Again, systematic changes in mechanical properties were observed with increasing Td. Conformational changes in collagen produced by mechanical stress (strain) modulate the rate of cross-linking and the resulting mechanical properties; however, the effects are sensitive to the reagent employed.