Kinetic study of collagen fixation with polyepoxy fixatives

A tough act to follow: collagen hydrogel modifications to improve mechanical and growth factor loading capabilities

Collagen hydrogels are among ​the most well-studied platforms for drug delivery and in situ tissue engineering, thanks to their low cost, low immunogenicity, versatility, biocompatibility, and similarity to the natural extracellular matrix (ECM). Despite collagen being largely responsible for the tensile properties of native connective tissues, collagen hydrogels have relatively low mechanical properties in the absence of covalent cross-linking. This is particularly problematic when attempting to regenerate stiffer and stronger native tissues such as bone. Furthermore, in contrast to hydrogels based on ECM proteins such as fibronectin, collagen hydrogels do not have any growth factor (GF)-specific binding sites and often cannot sequester physiological (small) amounts of the protein. GF binding and in situ presentation are properties that can aid significantly in the tissue regeneration process by dictating cell fate without causing adverse effects such as malignant tumorigenic tissue growth. To alleviate these issues, researchers have developed several strategies to increase the mechanical properties of collagen hydrogels using physical or chemical modifications. This can expand the applicability of collagen hydrogels to tissues subject to a continuous load. GF delivery has also been explored, mathematically and experimentally, through the development of direct loading, chemical cross-linking, electrostatic interaction, and other carrier systems. This comprehensive article explores the ways in which these parameters, mechanical properties and GF delivery, have been optimized in collagen hydrogel systems ​and examines their in vitro or in vivo biological effect. This article can, therefore, be a useful tool to streamline future studies in the field, by pointing researchers into the appropriate direction according to their collagen hydrogel design requirements.

Cross-linking method using pentaepoxide for improving bovine and porcine bioprosthetic pericardia: A multiparametric assessment study

Bioprosthetic heart valves made from bovine pericardium (BP) and porcine pericardium (PP) preserved with glutaraldehyde (GA) are commonly used in valve surgeries but prone to calcification in many patients. In this study, we compared BP and PP preserved with GA, ethylene glycol diglycidyl ether (DE), and 1,2,3,4,6-penta-O-{1-[2-(glycidyloxy)ethoxy]ethyl}-d-glucopyranose (PE). We studied the stabilities of DE and PE in preservation media along with the amino acid (AA) compositions, Fourier-transform infrared spectra, mechanical properties, surface morphologies, thermal stability, calcification, and the cytocompatibility of BP and PP treated with 0.625% GA, 5% DE, 2% PE, and alternating 5% DE and 2% PE for 3 + 11 d and 10 + 10 d, respectively. Both epoxides were stable in the water-buffer solutions (pH 7.4). DE provided high linkage densities in BP and PP owing to reactions with Hyl, Lys, His, Arg, Ser, and Tyr. PE reacted weakly with these AAs but strongly with Met. High cross-linking density obtained using the 10 d + 10 d method provided satisfactory thermal stability of biomaterials. The epoxy preservations improved cytocompatibility and resistance to calcification. PE enhanced the stress/strain properties of the xenogeneic pericardia, perhaps by forming nanostructures that were clearly visualised in BP using scanning electron microscopy. The DE + PE combination, in an alternating cross-linking manner, thus constitutes a promising option for developing bioprosthetic pericardia.

Processing of collagen based biomaterials and the resulting materials properties

Collagen, the most abundant extracellular matrix protein in animal kingdom belongs to a family of fibrous proteins, which transfer load in tissues and which provide a highly biocompatible environment for cells. This high biocompatibility makes collagen a perfect biomaterial for implantable medical products and scaffolds for in vitro testing systems. To manufacture collagen based solutions, porous sponges, membranes and threads for surgical and dental purposes or cell culture matrices, collagen rich tissues as skin and tendon of mammals are intensively processed by physical and chemical means. Other tissues such as pericardium and intestine are more gently decellularized while maintaining their complex collagenous architectures. Tissue processing technologies are organized as a series of steps, which are combined in different ways to manufacture structurally versatile materials with varying properties in strength, stability against temperature and enzymatic degradation and cellular response. Complex structures are achieved by combined technologies. Different drying techniques are performed with sterilisation steps and the preparation of porous structures simultaneously. Chemical crosslinking is combined with casting steps as spinning, moulding or additive manufacturing techniques. Important progress is expected by using collagen based bio-inks, which can be formed into 3D structures and combined with live cells. This review will give an overview of the technological principles of processing collagen rich tissues down to collagen hydrolysates and the methods to rebuild differently shaped products. The effects of the processing steps on the final materials properties are discussed especially with regard to the thermal and the physical properties and the susceptibility to enzymatic degradation. These properties are key features for biological and clinical application, handling and metabolization.

A Preliminary Study of the Fixation Mechanism of Collagen Reaction with a Polyepoxy Fixative

A new biomaterial has been developed by fixing native collagens with a polyepoxy compound (PC) fixative. Prior studies have shown that this biomaterial has comparable properties as compared to collagen fixed with glutaraldehyde (GA) and thus has a great promise for use as an implantable bioprosthesis. The purpose of this study was to understand the mechanism of the amino acids-PC reactions in the fixation process. Bovine arteries were fixed with PC under various pH, concentration and temperature conditions as a function of fixation time. Individual amino acid components in the fresh and the fixed arteries were assayed using a Beckman amino acid analyzer to determine the degree of tanning. The denaturation temperature (Td) was also measured on each sample. Since the denaturation temperature is a direct indication of cross-linking of individual amino acids with the fixative, the difference in the degree of tanning for the same increase in Td may be indicative of the quantity of the masked, non-cross-linked amino acids. The fixation reaction data indicated that not all amino acids were cross-linked upon contacting the PC fixative. Masking appeared to be more substantial with a fixation at higher pH values.

Evaluation of Collagen Modification and Surface Properties of a Bovine Artery via Polyepoxy Compound Fixation

Collagen of bovine internal thoracic artery (BITA) was treated with glutaraldehyde (GA) or polyepoxy compounds (PC). This study was to evaluate the surface properties as a result of tissue tanning reaction with PC. The fixation resulted in a significant reduction of available lysine, histidine, and other amino acid residues in PC fixed grafts as compared to fresh pre-fixed arteries. Among them, the lysine (Lys) content was reduced by about 80%, indicating that PC reactions mainly involve with Lys residues. Both PC and GA treatment led to crosslinking as evidenced by the increase in the denaturation temperature. The critical surface tension and the Fourier Transform Infrared Spectrum (FTIR) on a preimplant and its 96 days explant were evaluated and found to be similar. The FTIR analysis of a pre-implant and the 96 day explant indicated that there was no lipid deposition.

Review collagen-based biomaterials for wound healing

With its wide distribution in soft and hard connective tissues, collagen is the most abundant of animal proteins. In vitro, natural collagen can be formed into highly organized, three-dimensional scaffolds that are intrinsically biocompatible, biodegradable, nontoxic upon exogenous application, and endowed with high tensile strength. These attributes make collagen the material of choice for wound healing and tissue engineering applications. In this article, we review the structure and molecular interactions of collagen in vivo; the recent use of natural collagen in sponges, injectables, films and membranes, dressings, and skin grafts; and the on-going development of synthetic collagen mimetic peptides as pylons to anchor cytoactive agents in wound beds.

Kinetic Study of Modification of Collegen Peptide with Allyl Glycidyl Ether

The kinetics of reaction of collagen peptide with allyl glycidyl ether (AGE), and their temperature dependence, were studied by monitoring the reduction of the primary amine groups as a function of time. A kinetic model, with a second order with respect to the primary amine groups of collagen peptide and a first-order dependence on AGE, was developed that gave a good fit to the experimental data.

Marine polysaccharides in pharmaceutical applications: an overview

The enormous variety of polysaccharides that can be extracted from marine plants and animal organisms or produced by marine bacteria means that the field of marine polysaccharides is constantly evolving. Recent advances in biological techniques allow high levels of polysaccharides of interest to be produced in vitro. Biotechnology is a powerful tool to obtain polysaccharides from a variety of micro-organisms, by controlling the growth conditions in a bioreactor while tailoring the production of biologically active compounds. Following an overview of the current knowledge on marine polysaccharides, with special attention to potential pharmaceutical applications and to more recent progress on the discovering of new polysaccharides with biological appealing characteristics, this review will focus on possible strategies for chemical or physical modification aimed to tailor the final properties of interest.

Marine derived polysaccharides for biomedical applications: chemical modification approaches

Polysaccharide-based biomaterials are an emerging class in several biomedical fields such as tissue regeneration, particularly for cartilage, drug delivery devices and gelentrapment systems for the immobilization of cells. Important properties of the polysaccharides include controllable biological activity, biodegradability, and their ability to form hydrogels. Most of the polysaccharides used derive from natural sources; particularly, alginate and chitin, two polysaccharides which have an extensive history of use in medicine, pharmacy and basic sciences, and can be easily extracted from marine plants (algae kelp) and crab shells, respectively. The recent rediscovery of poly-saccharidebased materials is also attributable to new synthetic routes for their chemical modification, with the aim of promoting new biological activities and/or to modify the final properties of the biomaterials for specific purposes. These synthetic strategies also involve the combination of polysaccharides with other polymers. A review of the more recent research in the field of chemical modification of alginate, chitin and its derivative chitosan is presented. Moreover, we report as case studies the results of our recent work concerning various different approaches and applications of polysaccharide-based biomaterials, such as the realization of novel composites based on calcium sulphate blended with alginate and with a chemically modified chitosan, the synthesis of novel alginate-poly(ethylene glycol) copolymers and the development of a family of materials based on alginate and acrylic polymers of potential interest as drug delivery systems.

Biomedical applications of collagen

Collagen is regarded as one of the most useful biomaterials. The excellent biocompatibility and safety due to its biological characteristics, such as biodegradability and weak antigenecity, made collagen the primary resource in medical applications. The main applications of collagen as drug delivery systems are collagen shields in ophthalmology, sponges for burns/wounds, mini-pellets and tablets for protein delivery, gel formulation in combination with liposomes for sustained drug delivery, as controlling material for transdermal delivery, and nanoparticles for gene delivery and basic matrices for cell culture systems. It was also used for tissue engineering including skin replacement, bone substitutes, and artificial blood vessels and valves. This article reviews biomedical applications of collagen including the collagen film, which we have developed as a matrix system for evaluation of tissue calcification and for the embedding of a single cell suspension for tumorigenic study. The advantages and disadvantages of each system are also discussed.

The kinetics of 1,4-butanediol diglycidyl ether crosslinking of dermal sheep collagen

Dermal sheep collagen was crosslinked with 1,4-butanediol diglycidyl ether (BDDGE) or modified with glycidyl isopropyl ether (PGE). The reduction in amine groups as a function of time was followed to study the overall reaction kinetics of collagen with either BDDGE or PGE. Linearization of the experimental data resulted in a reaction order of 2 with respect to the amine groups in the PGE masking reaction, whereas a reaction order of 2.5 was obtained in the BDDGE crosslinking reaction. The reaction orders were independent of the pH in the range of 8.5-10.5 and the reagent concentration (1-4 wt %). The reaction order with respect to epoxide groups was equal to 1 for both reagents. As expected, the reaction rate was favored by a higher reagent concentration and a higher solution pH. Because the BDDGE crosslinking reaction occurs via two distinct reaction steps, the content of pendant epoxide groups in the collagen matrix was determined by treating the collagen with either O-phosphoryl ethanolamine or lysine methyl ester. The increase in either phosphor or primary amine groups was related to the content of pendant groups. Crosslinking at pH 9.0 resulted in a low reaction rate but in a high crosslink efficacy, especially after prolonged reaction times. A maximum concentration of pendant epoxide groups was detected after 50 h. Reaction at pH 10.0 was faster, but a lower crosslinking efficacy was obtained. At pH 10.0, the ratio between pendant epoxide groups and crosslinks was almost equal to 1 during the course of the crosslinking reaction.

The anticalcification effect of polyethylene glycol-immobilized on hexamethylene diisocyanate treated pericardium

Pathologic calcification is thought to be the main cause of failure in the present generation tissue valves fabricated from glutaraldehyde pretreated bovine pericardium (BP). The present investigation describes the in vitro calcification and enzymatic degradation of bovine pericardia after hexamethylene diisocyanate (HMDIC) crosslinking and subsequent modification with polyethylene glycol. The enzymatic degradation of these treated surfaces were monitored by scanning electron micrography and tensile strength measurements. Various proteases, such as alpha-chymotrypsin, bromelain, esterase, trypsin and collagenase were investigated for tissue stability. Incubation of these enzymes with crosslinked pericardia had variably reduced their tensile strength. Among these treated surfaces, polyethylene glycol (PEG) grafted BP via isocyanate functionalities had retained maximum strength. The PEG modified tissues had also indicated a substantial reduction in calcification, when compared to other treated tissues. Further, the biocompatibility of various pericardial tissues were established by platelet adhesion and octane contact angle measurements. It is assumed that the PEG modification of pericardium may interfere with the cellular activation of injury (platelets) to reduce tissue associated calcification. In conclusion, it seems the PEG modification of bovine pericardium via HMDIC may provide new ways of controlling tissue biodegradation and calcification. However, more in vivo studies are needed to develop applications.

Effects of double cross-linking technique on the enzymatic degradation and calcification of bovine pericardia

The strength, resorption rates, and biocompatibility of collagenous biomaterials are profoundly influenced by the method of cross-linking. The in vitro and in vivo calcification and enzymatic degradation of bovine pericardia (BP) after a series of surface modifications were studied as a function of exposure time. Collagenase degradations of modified BP were monitored by scanning electron microscopy and tensile strength measurements. Bovine pericardium was modified by a combination of different tissue fixatives such as glutaraldehyde (GA), carbodiimide (EDC), diisocyanate (HMDIC), and polyethylene glycol (PEG). GA-PEG-EDC-PEG and GA-PEG-HMDIC-PEG combination treated BP retained maximum stability in collagenase digestion compared to GATBP. In vitro calcification studies and in vivo rat subcutaneous implantations of modified pericardium have shown substantial reduction in the calcification of double cross-linked BP with PEG modification. Further, the biocompatibility aspects of pericardial tissues were established by platelet adhesion and octane contact angle. It seems that cross-links involving amino and carboxyl residues may provide new ways of controlling biodegradation and calcification.

The role of collagen in the declining mechanical properties of aging human cortical bone

The importance of the mechanical role of collagen in bone is becoming increasingly more clear as evidence mounts on the detrimental effects of altered collagen on the mechanical properties of bone. We previously examined a set of mechanical properties (material stiffness, strength, and toughness) of human femoral bone (ages 35-92) and found that a gradual deterioration in these properties occurs with age. The present study examines the collagen of the same specimens and relates the collagen properties to the mechanical ones. In the collagen we measured the concentration of stable mature crosslinks, the shrinkage temperature, and the rate of contraction during isometric heating. The changes in the concentration of mature (pyridinium and deoxypyridinium) crosslinks showed no clear relationship to age nor did they correlate with the mechanical properties. The shrinkage temperature declined with age and correlated with a bone's toughness. The maximum rate of contraction was strongly correlated with three different measures of tissue toughness, but much less to stiffness and strength. Our results reinforce speculation regarding the toughening role of collagen in bone mechanics and suggest that the fragility of aging bone may be related to collagen changes.

Characterization of a polyepoxy compound fixed porcine heart valve bioprosthesis

Concerns with the currently available bioprostheses are calcification, long-term durability, and suboptimal hemodynamic performance. It is well known that these concerns are all more or less related to the cross-linking reagent, glutaraldehyde or formaldehyde, used in fixing bioprostheses. To address these concerns, we undertook the development of a porcine bioprosthesis fixed with a polyepoxy compound. In the development of this polyepoxy compound valve, it was found that the porcine leaflets fixed with polyepoxy compound were softer and more pliable than those fixed with glutaraldehyde. In this study, a special microtoming technique was developed to section the biological tissue so that the fixation uniformity in distinct layers of porcine aortic wall could be characterized. The fixation index and the denaturation temperature measurements in distinct layers of aortic walls showed that the cross-linking density was uniform throughout the entire aortic wall for the polyepoxy compound fixed porcine valve. It was also noted that the fixation index of the polyepoxy compound fixed aortic wall (91.5 +/- 0.5, n = 3) was not significantly different from that of its valvular leaflet (90.6 +/- 0.8, n = 3). Similarly, the denaturation temperature of the polyepoxy compound fixed aortic wall (80.4 +/- 0.9 degrees C, n = 5) was statistically comparable to that of its valvular leaflet (79.0 +/- 0.5 degrees C, n = 5). The results of this study indicated that polyepoxy compound can adequately fix the entire porcine aortic wall as well as its valvular leaflets.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

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.

Effect of alternative crosslinking techniques on the enzymatic degradation of bovine pericardia and their calcification

The in vitro calcification and enzymatic degradation of bovine pericardia (BP) after a series of surface treatments were studied as a function of exposure time. The degradation of these treated surfaces was monitored by scanning electron micrography and tensile strength measurements. Polyethylene glycol-(PEG) grafted BP and glutaraldehyde-(GA) treated BPs retained maximum stability in collagenase digestion compared with SDS-treated BP. The ability of alpha chymotrypsin, bromelain, esterase, trypsin, and collagenase to modulate the degradation of SDS-, GA-, PEG-, Carbodiimide-, and glycidylether-treated BPs also was investigated. Incubation of various enzymes to these crosslinked pericardia variably reduced the tensile strength of these tissues. It is conceivable that chemical treatments of pericardial tissues might have altered their physical and chemical configuration and the subsequent degradation properties. In vitro calcification studies showed a substantial reduction in the calcification profile of PEG-grafted bovine pericardia compared to other treated tissues. Furthermore, the biocompatibility aspects of pericardial tissues were established by platelet adhesion and octane contact angle. In conclusion, it seems that the surface modification of bovine pericardia via GA-PEG grafting may provide new ways of controlling biodegradation and calcification.

Influence of polyethylene glycol graftings on the in vitro degradation and calcification of bovine pericardium

Calcification is a frequent cause of the clinical failure of bio-prosthetic heart valves fabricated from glutaraldehyde pretreated bovine pericardium (GATBP). This article reports on various chemical techniques for grafting polyethylene glycol (PEG) on bovine pericardium, their biostability, and calcification. The process of calcification profile was studied by in vitro experiments via the incubation of pericardial samples in a metastable solution of calcium phosphate. The calcification profile of PEG-modified bovine pericardium through glutaraldehyde linkages was significantly reduced compared to other methods of grafting. The mechanical property of these PEG-modified tissues after enzyme (collagenase) digestion and calcification were also investigated. PEG grafting of BP via glutaraldehyde or hexamethylene diisocyanate had shown better mechanical stability compared to other grafting methods used. In conclusion, it seems that the surface modification of bovine pericardium through high molecular weight PEGs via glutaraldehyde linkages may provide new ways of controlling tissue biodegradation and calcification.

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.

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.

Shrinkage temperature versus protein extraction as a measure of stabilization of photooxidized tissue

A rise in thermal denaturation temperature has been utilized as an indication of stabilization of collagen-containing materials such as pericardial tissue and porcine heart-valve leaflets following treatment with glutaraldehyde, Denacol, or other chemical agents. In contrast, stabilization of bovine pericardial tissue by dye-mediated photooxidation does not result in a significant rise in shrinkage temperature comparable with these treated materials. It was therefore hypothesized that a rise in shrinkage temperature is not a necessary indication for tissue stabilization. A sensitive protein extraction assay has been developed which can be used to monitor the stabilization of pericardial tissue by a variety of treatment methods, including photooxidation. A reduction in extractable protein, as analyzed by polyacrylamide gel electrophoresis, is noted for pericardial tissue treated with photooxidation, glutaraldehyde, or Denacol. Loss of extractable protein, as a function of treatment time, correlates well with a significant rise in shrinkage temperature for pericardium treated with glutaraldehyde or Denacol but not with photooxidation. This difference is attributed to the stabilization processes of glutaraldehyde and Denacol, which involve extensive crosslinking and polymer formation within and in addition to the native pericardial matrix, leading to a rise in matrix complexity and thermal stability. In contrast, photooxidation is a catalytic process involving modification and crosslink formation within existing matrix components, resulting in a material with little added matrix complexity.

Evaluation of an epoxy-fixed biological patch with ionically bound heparin as a pericardial substitute

In an attempt to develop an improved pericardial substitute, we undertook the development of an epoxy-fixed biological patch with ionically bound heparin. The study was to evaluate the cross-linking characteristics of this newly developed biological patch using its glutaraldehyde-fixed counterpart as a control. In addition, the feasibility of using this newly developed biological patch as a pericardial substitute was assessed in a canine model. In the study, it was observed that the epoxy-fixed biological patch appeared more similar to the native pericardium in colour and was more pliable than its glutaraldehyde-fixed counterpart. Also, both the epoxy- and glutaraldehyde-fixed biological patches had significant increases in fixation index and denaturation temperature as compared to the fresh one (p < 0.05). In the canine study, the epoxy-fixed biological patch with ionically bound heparin was found to have significantly less adhesion formation than those currently used clinically (p < 0.05).

Crosslinking characteristics of an epoxy-fixed porcine tendon: effects of pH, temperature, and fixative concentration

Injury to the anterior cruciate ligament (ACL) often results in functional instability within the knee joint and may induce a severe articular deterioration. Clinically, these patients may require an ACL reconstruction. In an attempt to develop an improved ligament prosthesis, a prototype xenograft--an epoxy-fixed porcine Achilles tendon--was developed. The study was intended to investigate the crosslinking characteristics of the epoxy-fixed porcine tendons fixed at different pHs (4.0, 7.4, 9.0, and 10.5), temperatures (4, 25, and 37 degrees C), and fixative concentrations (1, 2, or 4%). Samples of each group were taken out at various elapsed fixation periods. The crosslinking characteristics-fixation index, denaturation temperature, and moisture content-of each sample were determined. Additionally, the stiffness of the epoxy-fixed tendons under various fixation conditions was qualitatively compared. In the study it was noted that the color of porcine Achilles tendons remained natural after epoxy fixation. With increasing pH, temperature, or fixative concentration, the fixation indices and denaturation temperatures of the epoxy-fixed tendons increased. However, the fixed porcine tendons tended to be stiffer at a higher pH, temperature, or fixative concentration. The moisture contents of the epoxy-fixed tendons were relatively lower than the fresh ones, while they were approximately the same at different pHs, temperatures, and fixative concentrations. The implications of these findings for the epoxy-fixed porcine tendons in ACL reconstruction require further investigation.

Crosslinking characteristics of porcine tendons: effects of fixation with glutaraldehyde or epoxy

Anterior cruciate ligament (ACL) injuries, if left untreated, often produce significant disability in the athletically active population. Currently, autogenous tissue is the most commonly used substitute for ACL reconstruction because its immunogenicity is virtually nonexistent. However, the functional amount of autogenous tissue available for transplantation is limited. Additionally, this transplantation procedure may create a defect at the donor site, which can result in functional disability. To address these concerns, a prototype xenograft ligament prosthesis, epoxy-fixed porcine Achilles tendon, was developed. This study was intended to investigate the crosslinking characteristics of the epoxy-fixed porcine tendon. The fresh and glutaraldehyde-fixed porcine Achilles tendons were used as controls. Fresh porcine Achilles tendons procured from a slaughterhouse were used to fabricate the ligament prostheses. A 4% epoxy (ethylene glycol diglycidyl ether) solution or a 0.625% glutaraldehyde solution was employed to fix the porcine tendons. Samples of each group were taken out at various elapsed fixation periods. The crosslinking characteristics- denaturation temperature, moisture content, and fixation index-of each sample were then determined. In the study, it was learned that the crosslinking rate for the glutaraldehyde fixation was faster than that for the epoxy fixation. While the denaturation temperatures and the fixation indices for both studied groups were higher than for the fresh one, the denaturation temperature of the glutaraldehyde-fixed tendon was relatively higher than its epoxy-fixed counterpart. However, the fixation index and the moisture content for both studied groups were comparable. Also, it was noted that the epoxy-fixed tendon appeared more natural as compared to its glutaraldehyde-fixed counterpart. The implications of these findings for the epoxy-fixed tendon in the clinical ACL reconstruction require further investigation.

Cytocompatibility of calf pericardium treated by glutaraldehyde and by the acyl azide methods in an organotypic culture model

Glutaraldehyde (GTA) is used to cross-link collagen-based biomaterials, but these materials are often cytotoxic. In order to overcome this problem, we have proposed the use of the acyl azide methods with either hydrazine or diphenylphosphoryl azide (DPPA) as reagents. In this paper we determine the cytocompatibility of acyl azide- and GTA-treated pericardium in vitro, by an organotypic chick aorta culture technique developed for the evaluation of the propensity of vascular cells (both endothelial and smooth muscle cells) to migrate and grow on the surface of biomaterials. We first examined pericardium stabilization as a function of GTA concentration and time, so that we could minimize residual GTA molecules in the material. Treatment for 72 h with 0.05% GTA was optimal for thermal stabilization of the pericardium with a denaturation temperature (Td) of 86.8 degrees C, providing similar results to treatment with 0.6% GTA for 4 h (Td = 85.1 degrees C). Pericardium treated in this way was, however, poorly cytocompatible with little vascular cell migration and growth when compared with tissues treated by the acyl azide methods. The best results were obtained with 0.5% DPPA; treated tissues showed a high level of cross-linking (Td = 82.4 degrees C) and three-fold increases in cell growth and migration over those in a non-toxic control.

In vitro evaluation of cytotoxicity of diepoxy compounds used for biomaterial modification

The toxicity of various diepoxy compounds used for biomaterials crosslinking was investigated with a cell culture method and compared with an in vivo method. The neutral red uptake by cells was used to count the number of cells still alive after contact with the diepoxy compounds, because this method was more sensitive in cell counting than the other four methods studied in this work. The amount of neutral red taken up by cells depended strongly on the activity of cells in comparison with other methods; only small amounts of neutral red were taken up when cells were in a low activity state even if they were still alive. The in vitro toxicity of diepoxy compounds evaluated by the neutral red method revealed a good correlation with that found by the in vivo Draize test. The in vitro cytotoxicity to a cell line of L929 was closely related to that of primary culture cells of the normal rabbit cornea epidermal cell. The toxicity of diepoxy compounds was lower as their chain was longer, probably because of the lower chemical reactivity. All the diepoxy compounds investigated in this study exhibited lower cytotoxicity than formaldehyde, glutaraldehyde, and a water-soluble carbodiimide.

Influence of ethylene oxide gas treatment on the in vitro degradation behavior of dermal sheep collagen

The influence of ethylene oxide gas treatment on the in vitro degradation behavior of noncrosslinked, glutaraldehyde crosslinked or hexamethylene diisocyanate crosslinked dermal sheep collagen (DSC) using bacterial collagenase is described. The results obtained were compared with the degradation behavior of either nonsterilized or gamma-sterilized DSC. Upon ethylene oxide sterilization, reaction of ethylene oxide with the free amine groups of DSC occurred, which resulted in a decreased helix stability, as indicated by a lowering of the shrinkage temperature of all three types of DSC. Except for the low strain modulus the mechanical properties of the ethylene oxide sterilized materials were not significantly altered. gamma-Sterilization induced chain scission in all three types of DSC, resulting in a decrease of both the tensile strength and the high strain modulus of noncrosslinked and crosslinked DSC. When exposed to a solution of bacterial collagenase, ethylene oxide sterilized materials had a lower rate of degradation compared with nonsterilized DSC. This has been explained by a reduced adsorption of the collagenase onto the collagen matrix as a result of the introduction of pendant N-2-hydroxy ethyl groups.

Biological materials fixed with an epoxy compound: comparison of the effects with or without ionically bound heparin

Biological materials have been used as prosthetic devices such as heart valves, vascular grafts, and pericardial patches. These biological materials have to be fixed with crosslinking reagents and sterilized subsequently before they can be implanted in humans. Recently, a new crosslinking reagent, epoxy compound, has been used to fix bioprostheses. In this fixation technique, heparin may be ionically bound on the tissue surface. It has been shown that the amount of heparin bound to the tissue surface is proportional to the quantity of protamine impregnated in the biological tissues. However, it is not known if the impregnation of protamine will affect the crosslinking density of the biological tissues. This study was designed to compare the crosslinking densities of the epoxy compound fixed biological tissues with or without heparinization. Fresh porcine aortic valves procured from a slaughter house were first impregnated in various concentrations of protamine sulfate (0, 0.5, 1.0, or 1.5%) for about 30 min. The porcine aortic valves were then crosslinked in a 4% epoxy compound solution (Denacol EX-313). The porcine samples were taken out at various elapsed fixation periods: 18, 25, 48, 72, 96, and 120 h. Finally, the crosslinked porcine aortic valves were heparinized in a 0.5% sodium heparin solution for about 1 h. The crosslinking densities of the porcine leaflet and the aortic wall of each sample were determined by measuring their shrinkage temperatures. It was revealed that the impregnation of various concentrations of protamine did not seem to significantly alter the shrinkage temperatures of the porcine leaflet and the aortic wall throughout the entire fixation process (p > 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of molecular structure of poly(glycidyl ether) reagents on crosslinking and mechanical properties of bovine pericardial xenograft materials

With the identification of the exacerbating effect of glutaraldehyde on calcification of heart valve materials, there exists a renewed interest in both alternative reagents and the effects of crosslinking on connective tissues. One potentially useful class of reagents are poly(glycidyl ether) compounds. We have examined 5 of these reagents with different molecular sizes and functionalities for their effects on mechanical properties and collagen denaturation (shrinkage) temperature. Samples of bovine pericardium were tested fresh or after 48 h fixation in one of the five compounds for denaturation temperature, stress-strain response, stress relaxation, plastic deformation, and fracture properties. Of the compounds tested, those with intermediate length backbones and 4 or 5 epoxide groups were most effective in producing intrahelical crosslinking and increased denaturation temperature over 48 h. However, in samples examined after 17 months of fixation, all reagents had equivalently increased the denaturation temperature. Examination of mechanical results revealed two distinct mechanisms for mechanical change. Observed shifting of the stress-strain curve to the right (due to shrinkage), increased plastic deformation, and some reduction of stress relaxation are all unrelated to denaturation temperature (and hence to changes in intrahelical crosslinking). An alternate mechanism, perhaps formation of intermolecular crosslinks may be responsible. Intrahelical crosslinking produces only lesser reductions in stress relaxation. Cross-comparison of reagents of differing molecular structure provides a useful tool toward increased understanding of the mechanical consequences of tissue crosslinking.

Fixation of bioprosthetic tissues with monofunctional and multifunctional polyepoxy compounds

Collagen from a native tissue is fixed with a polyepoxy compound (PC) for use as a new biologic prosthetic material. Prior studies have shown that this biomaterial has comparable properties with collagen fixed with glutaraldehyde (GA), and thus has great promise for biomedical applications. A prior kinetic study indicated that the reaction between the functional groups of collagen and the multifunctional epoxy EX-313 is a 2.5th-order reaction. The purpose of this study was to understand the mechanism of the amino acid-PC reactions in a fixation process. Bovine arteries were fixed with a monofunctional PC (EX-131) and a multifunctional PC (EX-313) as a function of fixation time. A sequential fixation with a second fixative was used to identify the available remaining reactive sites from a prior fixation. The denaturation temperature (Td) was measured on each sample. Because the denaturation temperature is a direct indication of crosslinking of individual amino acids with the fixative, the increase in Td of a subsequent fixation may be indicative of the available remaining amino acids. The fixation index was measured on each sample to reflect the increase of fixation completion in a sequential fixation process. The fixation index and crosslink data also revealed that the reactive amino acids for EX-131 and EX-313 may not be exactly the same. The data in this study suggest that a monofunctional fixative can pre-react with the amino acids of collagen to effectively block further fixation of collagen with a second fixative. This amino acid masking may be associated with collagen branching. Collagen branching and its effect on denaturation temperature are described.