Colorimetric method for the assay of heparin content in immobilized heparin preparations

Sepharose-bound, highly sulfated glycosaminoglycans can capture HIV-1 from culture medium

In the search for new strategies against HIV-1 and on the basis of a number of previous studies reporting on the capacity of certain polyanionic compounds to influence the replication of HIV-1, we prepared a few chemically oversulfated dermatan and chondroitin sulfates. Four of these compounds and two samples of heparin were bound to activated Sepharose through either their carboxylic groups, or their aldehydic groups, or their deacetylated primary amino groups. Some of these so-derivatised resins, packed into columns, proved able to remove HIV-1 IIIB, a laboratory adapted strain, and one clinical primary isolate from an AIDS patient, from infected cell culture medium. The resins bind the virus very tightly and could be useful for capturing the virus from infected fluids.

Modification of Collagen Matrices for Enhancing Angiogenesis

The vascularization of engineered tissues in many cases does not keep up with the ingrowth of cells. Nutrient and oxygen supply are not sufficient, which ultimately leads to the death of the invading cells. The enhancement of the angiogenic capabilities of engineered tissues therefore represents a major challenge in the field of tissue engineering. The immobilization of angiogenic growth factors may be useful for enhancing angiogenesis. The most potent angiogenic growth factor specific to endothelial cells, vascular endothelial growth factor (VEGF), occurs in several splice variants. The variant with 165 amino acids both has a high angiogenic activity and a high affinity for heparin. We therefore incorporated heparin molecules into collagen matrices by covalently cross-linking them to amino functions on the collagen. Physical binding of VEGF to the heparin may then prevent a rapid clearance from the implant, while the release rate may become coupled to the degradation of the collagen matrix. The modified matrices were characterized by determination of the extent of the heparin immobilization, the in vitro degradation rate by collagenase. For testing the angiogenic properties, non-modified and heparinized collagen specimens were--either loaded with VEGF or non-loaded--subcutaneously implanted on the back of rats. Specimens were explanted after varying periods of implantation, the dry weights and the hemoglobin contents, as well as immunostained histological sections were evaluated: heparinized collagen matrices loaded with VEGF are vascularized to a substantially higher extent as compared to non-modified matrices.

Preparation and characterization of self-assembled nanoparticles of heparin-deoxycholic acid conjugates

Various deoxycholic acid (DOCA) bearing heparin (HD) amphiphilic conjugates with different degrees of substitution (DS) with DOCA were synthesized using heparin as a hydrophilic segment and DOCA as a hydrophobic segment. Structural characteristics of these HD conjugates were investigated using 1H NMR, dynamic light scattering, zeta potential, transmission electron microscopy (TEM), and fluorescence spectroscopy. HD conjugates provided monodispersed self-aggregates in water, with mean diameters decreasing with increasing DOCA DS in the range of 120-200 nm. HD aggregates were covered with negatively charged heparin shells, exhibiting xi potentials near -56 mV. The critical aggregation concentration (cac) of the HD (0.02-0.003 mg/mL) depended upon DOCA DS. TEM images demonstrated that the shape of the self-aggregates was spherical. Partition equilibrium constants, Kv, for pyrene in the self-aggregates in water indicated that increasing DS enhanced the hydrophobicity of the self-aggregate inner core. The mean aggregation number of DOCA per hydrophobic microdomain, estimated by the fluorescence quenching methods using cetylpyridinium chloride, indicated that five to nine of HD chains comprised a hydrophobic domain in the conjugates.

Effect of plasticization on heparin release from biodegradable matrices

Heparin-loaded polymer films of poly-L-lactide (PLLA) and poly-L-lactide-co-glycolide (PLLGA) as well as poly-DL-lactide-co-glycolide (PLGA) were produced. A plasticizer, PEG, was added to the polymers. It was found that the release profile in general consisted of a burst effect, a diffusion-controlled phase and a degradation-controlled phase. The plasticizer accelerated the onset of degradation in all cases, but its effect on the release profile differed significantly depending on the polymer. The plasticizer depressed the burst effect for PLLA, and accelerated the kinetics of the diffusion-controlled phase. For the PLLGA 80/20, however, the plasticizer had no significant effect on the release profile or kinetics. We explain these observations in terms of hydrophilicity and crystallinity effects.

Silica xerogels as a delivery system for the controlled release of different molecular weight heparins

In this work, we investigated a sol-gel derived silica matrix as a delivery system for the prolonged release of different molecular weight heparins, which allows the glycosaminoglicons to retain their whole biological activity. Several xerogels were obtained by embedding different molecular weight heparins into matrices prepared by using different amount of NH4OH as a catalyst during gel formation. Gel synthesis parameters, drug release properties, and xerogels surface area were evaluated. Unfractionated, low and oligo-molecular weight heparins were embedded into xerogels and the effect of the molecular weight on the release kinetics and the retained biological activity has been investigated. The results show that the surface area of the matrix is a determinant parameter affecting drug release kinetics. This structural feature can be modified by varying the catalyst tetraethoxysilane molar ratio used during the matrix synthesis. In most cases release kinetics fitted the Higuchi diffusive model and a lower diffusion rate was observed from silica matrices characterized by a smaller surface area. In the case of matrices with lower surface area, loaded with unfractionated heparin, zero order kinetics was observed. In this paper, we have defined a heparin release silica xerogel system and we have pointed out how modulation of its synthesis parameters allows adjusting the release of heparin according to therapeutic needs.

Coils and tubes releasing heparin. Studies on a new vascular graft prototype

Coiled metallic guidewires find widespread use, for instance in interventional cardiology. It is known that release of heparin from the surface of guidewires is advantageous to prevent formation of thrombotic emboli. New coiled tubular structures, having larger inner and outer diameter as compared to guidewires, are presented. In theory these tubes can be used as interposition vascular grafts. Ten coiled tubes with an internal diameter of 690 microm were made. Five different adherent polymeric coatings with increasing hydrophilicity were used. Five tubes contained heparin in the coating and the other five were unheparinised controls. The five tubes containing heparin were studied with respect to heparin release in vitro (amount released, kinetics), and immobilised heparin that is exposed at the surface. All tubes were studied with a direct cell contact assay using 3T3 mouse fibroblast cells, a dynamic thrombin generation test, and endothelial cell growth onto the coils. It was found that the heparinised tubes lead to very little thrombin formation. It is argued that this is due to heparin that is immobilised and exposed at the inner surface of such tubes. Furthermore the coils showed to be cytocompatible and endothelial cells adhere and proliferate well onto the coils. This concept is believed to hold promise for further development of small vascular grafts.

A binding site for highly sulfated heparan sulfate is identified in the N terminus of the circumsporozoite protein: significance for malarial sporozoite attachment to hepatocytes

Circumsporozoite protein (CSP) coats the malarial sporozoite and functions to target the liver for infection, which is the first step to developing malaria. An important tissue ligand for CSP is the glycosaminoglycan heparan sulfate (HS) found on the surface of hepatocytes and in the basement membrane of the space of Disse. To better understand this efficient targeting process, we set out to identify and characterize the HS binding site(s) of CSP. We synthesized a series of peptides corresponding to five regions of Plasmodium falciparum CSP containing basic residues, a common requirement of HS binding sites, and screened them for heparin and HS binding activity. Only one of these peptides (Pf 2), which contains a motif we have named region I-plus, demonstrated both high affinity heparin/HS binding activity and the ability to block the binding of recombinant CSP to heparin-Sepharose 4B. Analysis by isothermal titration calorimetry revealed that region I-plus has a binding constant of K(d) = 5.0 microm and a stoichiometry of n = 7.8 binding sites/heparin chain. Heparin binding was dependent on the amino acid sequence of region I-plus, and the binding sites on heparin/HS are contained within a decasaccharide. Furthermore, HS oligosaccharides rich in sulfate and iduronic acid content (heparin-like) are required for efficient binding. Because liver HS is exceptionally high in both these components relative to the HS of other organs, the HS structural requirements for efficient region I-plus/HS binding are consistent with this peptide sequence functioning to target sporozoites to the liver for attachment to hepatocytes. Finally, the region I-plus heparin/HS binding site was also discovered for two other species that infect humans, Plasmodium malariae and Plasmodium vivax, further supporting the existence of a HS binding domain in the N-terminal portion of CSP.

Solid-phase enzyme modification via affinity chromatography

In the present study antileukemic enzyme L-asparaginase (ASNase) and catalase (as a model enzyme) were modified in solid-phase with activated polyethylene glycol (PEG(2)) by using ligand-immobilized affinity column systems L-asparagine-Sepharose CL-4B and Procion red-Sepharose CL-4B, respectively. Studies on change of specific activity with modification time showed negligible differences between batches of modified catalase. Modification of ASNase for 1 h resulted in 50.2% recovery of the specific activity and the attachment of 69 molecules of PEG(2) per molecule of ASNase forming 'PEGylated ASNase'. Sequential modification of ASNase by activated PEG and heparin resulted in coupling of about nine molecules of heparin per molecule of PEGylated ASNase. Intravenous (i.v.) administration of PEG(2)-modified ASNase showed prolonged presence in the blood circulation and no adverse effects or symptoms of anaphylaxis were observed in presensitized mice.

Importance of a biofouling-resistant phospholipid polymer to create a heparinized blood-compatible surface

Heparinization is believed to be one of the methods to suppress thrombus formation on blood-contacting surfaces. However, this study hypothesizes that heparinization alone might not be sufficient to provide a blood-compatible surface; that is, a surface property that resists biofouling is necessary to obtain an effective heparin-modified surface. 2-Methacryloyloxyethyl phosphorylcholine (MPC) polymers with 2-aminoethyl methacrylate (AEMA) were synthesized to immobilize heparin through ionic bonding. The primary amino groups of AEMA were considered to be the polymer surface because the zeta-potential of the surface was positive when the mole fraction of the AEMA units was above 0.2. The antithrombogenic character of the polymer surface modified with heparin was evaluated by both Lee-White and microsphere column methods. The coagulation period of human whole blood in the absence of anticoagulant in glass tubing coated with the MPC polymer was longer than that in the original glass tube. Cell adhesion was completely inhibited on the MPC polymer surface after contact with human whole blood without anticoagulant. However, many adherent blood cells were observed on poly(2-ethylhexyl methacrylate-co-AEMA) (no MPC unit) even after heparinization. These results strongly indicate that the MPC polymer is a useful substrate where the heparin works well and that the heparin-immobilized MPC polymer has superior blood compatibility to the simple MPC polymer.

Fabrication of micropored elastomeric film-covered stents and acute-phase performances

To prevent thrombus formation in the acute phase and restenosis in the subacute to chronic phase after stenting of atherosclerotic arteries, we developed a covered stent with a micropored elastomeric film, the blood-contacting surface of which was coated with a photocured gelatin layer immobilized with heparin. Segmented polyurethane (SPU) film (30 microm in wall thickness) as a cover material was multiply micropored by excimer laser-directed microprocessing (pore diameter, 30 microm; interpore distance, 125 microm). An aqueous mixed solution of benzophenone-derivatized gelatin and heparin was coated on the micropored SPU film. Upon ultraviolet light irradiation, a thin layer of a gelatin gel immobilized with heparin was formed and simultaneously fixed on the SPU film. The fully covered stents were assembled by wrapping a balloon-expandable stent with gelatin/heparin gel-layered SPU film and subsequently suturing and then gluing. To assess the validity of this covered stent in vivo, "half-covered" stents, in which half at the distal side was covered with the gel-layered SPU film, was implanted in rabbit common carotid arteries (about 3 mm in diameter). After 3 months of implantation, all the half-covered stents (n = 7) were patent. Regardless of the covered or noncovered region of the stents, the entire luminal surface of the stents was fully endothelialized and a thin neointimal tissue was formed. The potential advantages of a covered stent as designed above are discussed.

The study of the sterilization effect of gamma ray irradiation of immobilized collagen polypropylene nonwoven fabric surfaces

Exposure to gamma ray irradiation is a frequent, clean, and superior method used to prevent bacterial contamination of sterilized biomedical end products. However, the potential damage induced by gamma ray irradiation of collagen is of concern because of the decay of bioactivity, which correlates with considerable structural alterations. In this experiment, antenna-coupling microwave plasma was utilized to activate nonwoven polypropylene (PP) fabric, and then the sample was grafted to acrylic acid (AAc). Type III collagen was immobilized by using water-soluble 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide as a coupling agent. The collagen-immobilized samples, with temperatures of under 4 degrees C, were exposed to gamma ray irradiation at different dose intervals. Gamma ray irradiation was applied to evaluate the bioactivity on the collagen-immobilized nonwoven polypropylene and to determine the results of sterilization. Five kinds of sterilization index bacteria, all subject to Good Manufacturing Practice (GMP) criteria, were applied as a standard plate-count sterilization test. Our experimental results demonstrate that in human plasma incubated with various intervals of gamma ray irradiation, fibrinogen concentration decreases while platelet and red blood cell adhesion increase. However, the dose required for thrombination demonstrated a significant change in gamma ray irradiation exposure of fewer than 10 KGy (p = 0.05). The decay of bioactivity of the gamma-ray-irradiated collagen-bonded surfaces was evaluated and indicated that the decrease of R-CONHR', the degradation of amides ([broken bond]C[bond]N bonds of collagen and formation of the ROCNH(2) and O[double bond]CR' bonds), and the increase of C[bond]O, C[double bond]O bonds gradually may damage collagen by increasing the intervals of gamma ray irradiation. These effects considerably influence the bioactivity of the collagen-bonded fabric. It is clear that gamma ray irradiation exposure of approximately 10 KGy has the potential of moderating the bioactivities of collagen and therefore likely is a vital factor in the acceleration of biodegradation. The dose required for thrombination and sterilization reaches significance at 7.5 KGy.

Anticoagulant activity of immobilized heparin on the polypropylene nonwoven fabric surface depending upon the pH of processing environment

Antenna coupling microwave plasma enables a highly oxidative treatment of the outmost surface of polypropylene (PP) nonwoven fabric within a short time period. Subsequently, grafting copolymerization with acrylic acid (AAc) makes the plasma-treated fabric durably hydrophilic and excellent in water absorbency. With high grafting density and strong water affinity, the pAAc-grafted support greatly becomes feasible as an intensive absorbent and as a support to promote heparin immobilization through amide bonds. For heparin immobilized in acidic condition, the carbonate groups of the molecule tend to dissolve and passive encapsulation of the molecule prevents its functional groups from bonding with the carboxylic acid of pAAc. This effect leads to inhibit the immobilization process and consequently reduces the quantity as well as the bioactivity of the immobilized heparin. In alkaline processing environment, the oxidized uronic acid residues in heparin-related glycans are presumably cleaved and the removal of some oxidized residuals before immobilization process is likely to reduce the chain length of heparin. In the latter case, anticoagulant Factors X and XII, but not thrombin, are unaffected. Anticoagulant activity test using activated partial thromboplastin time (aPTT) is more sensitive in assessing heparin-immobilized surfaces, since it corresponds to Factor X and initiates the inhibition of Factor XII and thrombin. Likewise, platelets adhesion on the surfaces decreases as the process shifted from acidic to alkaline condition, whereas the hydrophilic character of the grafted pAAc markedly contributes to extend physical insertion of platelets. The immobilized heparin has a great part of original bioactivity, depending on the pH of the processing environment and the immobilized quantity. Relative bioactivity based upon aPTT tests is partially held longer than 90 days for the sample prepared in the alkaline or neutral environment.

Minimization of protein adsorption on poly(vinylidene fluoride)

Surfaces covered with polyethylene glycol (PEG) have been shown to be biocompatible because PEG yields nonimmunogenicity, nonantigenicity and protein rejection. To produce a biocompatible surface coating, we have developed a method for grafting PEG onto modified poly(vinylidene fluoride) (PVDF) films. The first step was to create carboxy groups on the PVDF surface following covalente coupling of polyethylenimine (PEI) to achieve high density of amino groups. These surface amines were reacted with formyl-terminated PEG's with various molecular weight. The modified PVDF surface was characterized by means of static contact angle measurements, infrared (IR) spectroscopy and X-ray photoelectron spectroscopy (XPS). The influence of the chain length on lysozyme repellence was investigated by means of surface-MALDI-Tof mass spectrometry (Surface-MALDI-Tof-MS). Lysozyme adsorption was significantly suppressed on the PEG 5000 modified PVDF surface.

Effect of biologically active coating on biocompatibility of Nitinol devices designed for the closure of intra-atrial communications

Anti-thrombogenicity and rapid endothelialisation are prerequisites for the use of closure devices of intra-atrial communications in order to reduce the risk of cerebral embolism. The purpose of this study was therefore to assess the effect of bioactive coatings on biocompatibility of Nitinol coils designed for the closure of intra-atrial communications. Nitinol coils (n = 10, each) and flat Nitinol bands (n = 3, each) were treated by basic coating with poly(amino-p-xylylene-co-p-xylylene) and then coated with either heparin, r-hirudin or fibronectin. Anti-thrombogenicity was studied in vitro in a dynamic model with whole blood by partial thromboplastin time (PTT), platelet binding and thrombin generation, respectively, and cytotoxicity by hemolysis. Endothelialisation was studied on Nitinol bands with human umbilical venous endothelial cells (HUVEC) by 3-(4,5-dimethylthiazole-2yl)-2,5-triphenyl tetrazolium (MTT) assay and immnuofluorescence analysis of Ki67, vinculin, fibronectin and von Willebrand Factor. Uncoated or coated devices did not influence hemolysis and PTT. r-Hirudin (but not heparin) and fibronectin coating showed lower platelet binding than uncoated Nitinol (p < 0.005, respectively). Heparin and r-hirudin coating reduced thrombin formation (p < 0.05 versus Nitinol, respectively). HUVEC adhesion, proliferation, and matrix formation decreased in the order: fibronectin coating > uncoated Nitinol > r-hirudin coating > heparin coating > basic coating. MTT assay corroborated these findings. In conclusion, r-hirudin and fibronectin coating, by causing no acute cytotoxicity, decreasing thrombogenicity and increasing endothelialisation improve in vitro biocompatibility of Nitinol devices designed for the closure of intra-atrial communications.

Polyetherimide: a new membrane-forming polymer for biomedical applications

Membranes for biohybrid organs such as the biohybrid liver support system have to face 2 different environments, namely blood and tissue cells. Accordingly, the respective membrane surfaces must have optimal properties in terms of biocompatibility for blood or tissue. Flat membranes prepared by a phase inversion process from polyetherimide were modified by binding of tris-(hydroxymethyl)-aminomethane to obtain a surface with hydroxyl groups by binding of polyethylene imine to attach a hydrophilic macromolecule with amine groups useful as a spacer for later bonding of further ligands and by attachment of heparin. The binding of the different ligands was successful as monitored by different physicochemical methods. The blood response of plain polyetherimide was comparable to that of polyacrylonitrile, and it could be further improved by the binding of heparin. The tissue compatibility of polyetherimide and its different modifications was compared with commercial cell culture substrate membranes (Millicell) and found to be comparable for polyetherimide and even better after the modification with tris-(hydroxymethyl)-aminomethane. In conclusion, polyetherimide seems to be an interesting material for the production of membranes for application in biohybrid organ systems.

Assessment and characterization of degradation effect for the varied degrees of ultra-violet radiation onto the collagen-bonded polypropylene non-woven fabric surfaces

Exposure to ultra-violet (UV)-C radiation is a frequently used method to prevent bacteria from invasion of blood-contact biomedical products. Potential damage induced by UV radiation to collagen is of concern due to the decay of bioactivity, considerably correlated with structural alterations. Our current investigation studies the collagen-bonded non-woven polypropylene (PP) fabric surface. In this experiment, antenna-coupling microwave plasma is utilized to activate PP fabric and then the sample is grafted with acrylic acid (AAc). Type III collagen is immobilized by using water soluble 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide as coupling agent. The collagen-bonded samples with sample temperature ca. 4 degrees C are then exposed to UV-254nm radiation for different time intervals. By using fourier-transformed infrared with attenuated total reflection (FTIR-ATR) and XPS (X-ray photoelectron spectroscopy), we examine the chemical structures of samples with different treatments. Coomassie brilliant blue G250 method is utilized to quantify the immobilized collagen on the PP fabric surfaces. Blood-clotting effects are evaluated by activated partial thromboplastin time, thrombin time, and fibrinogen concentration tests. By means of cell counter and scanning electron microscopy we count red blood cells and platelets adhesion in the modified porous matrix. Our experimental results have demonstrated that with pAAc-grafting of ca. 173 microg cm(-2) and immobilized collagen of 80.5+/-4.7 microg cm(-2), for human plasma incubated samples of various intervals of UV-254 nm radiation, fibrinogen concentration decreases in human plasma, while platelets and red blood cells adhesions increase before UV radiation. However, the required time for thrombination shows significant change for UV radiation exposure of less than 20 h (alpha = 0.05). The decay of bioactivity for the UV-irradiated, collagen-bonded surfaces is thus evaluated. Surface analyses indicate that the decrease of R-COOH (derivated from grafted-pAAc or de-carboxylation of collagen), amides degradation (broken-NH), and phenylalanine scission (terminated by -OH, tyrosine formation) may gradually damage collagen by increasing the intervals of UV radiation. These effects considerably influence the bioactivity of the collagen-bonded fabric. The XPS measurements of C 1s core levels at 288.4 eV (O = C-NH) and at 289.1 eV (O = C-O) illustrate significant decreases of intensity after radiation time ca. 44 h. It is clear that UV-254 nm radiation exposure for ca. 20 h has the potential impact to moderate the bioactivities of collagen and therefore act as a vital factor to accelerate biodegradation.

Sustained release of basic fibroblast growth factor and angiogenesis in a novel covalently crosslinked gel of heparin and alginate

The construction of organs by tissue engineering and regenerative engineering, using an artificial extracellular matrix, is an innovative method that is expected to replace artificial organs and organ transplantation. We have produced an artificial extracellular matrix of alginate and demonstrated that the matrix stimulated the regeneration of skin, nerve, and bone. In this report, the new matrix, which consists of heparin and alginate covalently crosslinked with ethylenediamine, was produced to stabilize and control the release of growth factors. Heparin content of the new matrix was confirmed by toluidine blue absorption, elementary analysis, and Fourier transform infrared spectrum. In vitro experiments showed that the new matrix significantly suppressed the initial burst of basic fibroblast growth factor, which is a representative member of heparin-binding growth factors, and released biologically active basic fibroblast growth factor for 1 month under physiological conditions. Obvious cellular infiltration and angiogenesis were shown to occur in the new matrix which was implanted subcutaneously in the dorsal area of rat with 1 microg of basic fibroblast growth factor for 2 weeks. This new matrix may be useful for not only the construction of transplantable blood vessels of small diameter, but also the induction of angiogenesis in regenerated skin constructed by tissue engineering.

In vitro release of heparin from silica xerogels

Heparin, a powerful anticoagulant used for the prophylaxis of both surgical and medical thrombosis, was incorporated into a silica xerogel matrix during polycondensation of organic silicate. The influence of various chemical sol-gel parameters (the properties of reaction precursors, catalyst and final moisture content of the gel and heparin concentration) was studied. The release of heparin from the gel was according to zero order during the dissolution period and the release rate of heparin was proportional to the drug load in the concentration range between 6.8 and 13.6 wt%. It was found that the catalyst used for the preparation of the gel, the final moisture content and the chemical modification of silica xerogel network have an influence on the release rate of heparin. The released heparin from all the different xerogels studied retained about 90% of its biological activity.

Development of poly(Lactic acid)/chitosan co-matrix microspheres: controlled release of taxol-heparin for preventing restenosis

Smooth muscle cell proliferation plays a major role in the genesis of restenosis after angioplasty or vascular injury. Controlled release of appropriate drugs alone and in combinations is one approach for treating coronary obstructions, balloon angioplasty, restenosis associated with thrombosis, and calcification. We demonstrated the possibility of encapsulating taxol-loaded polylactic acid (PLA) microspheres within heparin-chitosan spheres to develop a prolonged release co-matrix form. The in vitro release profile of taxol and heparin from this co-matrix system was monitored in phosphate buffered saline pH 7.4, using an ultraviolet spectrophotometer. The amount of taxol/heparin release was initially much higher, followed by a constant slow release profile for a prolonged period. The initial burst release of taxol (15.8%) and heparin (32.7%) from the co-matrix was modified with polyethylene glycol coatings (13.5% and 25.4%, respectively, for 24 hr). From scanning electron microscopy studies, it appears that these drugs diffuse out slowly to the dissolution medium through the micropores of the co-matrix. However, the surface micropores were modified with polyethylene glycol (PEG) coatings for a constant slow release profile. This PEG-coated PLA/chitosan co-matrix may target drug combinations having synergestic effects for prolonged periods to treat restenosis.

Effects of heparin immobilization on the surface characteristics of a biological tissue fixed with a naturally occurring crosslinking agent (genipin): an in vitro study

Heparinized biomaterials have been used to manufacture blood-contacting prostheses. The present study was intended to characterize the surface properties of a genipin-fixed biological tissue immobilized with heparin using the methods of ionic binding (the /h-i tissue) or covalent binding via multi-point attachment (the /h-m tissue) or end-point attachment (the /h-e tissue). The surface characteristics of test tissues evaluated were water contact angle, surface tension, protein adsorption, platelet adhesion, and cellular compatibility. Nonheparinized and the glutaraldehyde-fixed counterparts were used as controls. It was found that immobilization of heparin on the glutaraldehyde- and genipin-fixed tissues increased their hydrophilicity and surface tension and suppressed their mole ratio of adsorbed fibrinogen to adsorbed albumin and the amount of platelets adhered. Among the heparinized tissues, the /h-m tissue was more hydrophobic and had a higher mole ratio of adsorbed fibrinogen to adsorbed albumin and a greater amount of platelets adhered than the /h-i and /h-e tissues. In general, the surface characteristics of the /h-i tissue were comparable to the /h-e tissue. However, it is known that the ionically immobilized heparin may be displaced from the surface by an ion-exchange mechanism when exposed to blood. There were no significant differences in hydrophilicity, surface tension, the mole ratio of adsorbed fibrinogen to adsorbed albumin, and the amount of platelet adhesion between the glutaraldehyde- and genipin-fixed tissues in comparison with their respective counterparts. However, the cellular compatibility of the genipin-fixed tissues with or without heparinization was significantly superior to its glutaraldehyde-fixed counterparts.

Synthesis, chemical and rheological characterization of new hyaluronic acid-based hydrogels

New hyaluronic acid-based hydrogels have been synthesized. The carboxylate groups of hyaluronan were activated in order to bind the amino terminal groups of the di-amine cross-linking reagent. Different hydrogels were obtained according to the different di-amine cross-linking agents (1,3-diaminepropane, 1.6-diaminohexane, PEG500 di-amine. and PEG800 di-amine). The cross-linked polymer (C.L.Hyal) was then sulphated (C.L.HyalS) by a heterogeneous reaction using sulphur trioxide pyridine complex (SO3-Py). The thermo-mechanical properties and swelling degree were evaluated and are discussed in relation to the chemical structure and the hydrophilic character of the gels. The different behaviours of C.L.Hyal and C.L.HyalS indicate the important role of sulphated groups.

A simple, nondestructive assay for bound hyaluronan

A simple, convenient, nondestructive method is described for the quantitative determination of bound hyaluronan. The method is based on the binding of the cationic dye Toluidine Blue O to the D-glucuronate component of the hyaluronan repeat disaccharide. Quantification is accomplished without interference by the dye's metachromatic properties. The method is easily adapted to hyaluronan coated medical devices and should be useful to developers and manufacturers of such devices and coatings.

Maintenance of CD34 expression during proliferation of CD34+ cord blood cells on glycosaminoglycan surfaces

Recent studies have indicated that glycosaminoglycan (GAG) interactions with hematopoietic progenitors play a significant role in the regulation of hematopoiesis. However, the details of these interactions are not clear. In this study, we examined the role of soluble and immobilized GAGs in the proliferation of CD34+ cells. Chitosan, a cationic polysaccharide, was used to immobilize GAGs in ionic complex membranes. The GAGs studied were heparin, hyaluronate, and chondroitin sulfates A, B, and C. CD34-enriched umbilical cord blood cells were seeded onto tissue culture plates coated with the GAG-chitosan complex membranes. Cultures were maintained in medium supplemented with stem cell factor and interleukin 3 for up to six weeks, during which total and CD34+ cell numbers were determined by flow cytometry. Total cell number expansion ranged from 25-fold to 40-fold after six weeks. However, only heparin and chondroitin sulfate B (CSB) surfaces retained a significant CD34+ fraction. All other surfaces exhibited declines in CD34 expression, with negligible CD34+ percentages remaining after four weeks. In contrast, heparin and CSB surfaces exhibited CD34+ fractions as high as 90% after four weeks. GAG desorption studies indicated that the observed effects were partly mediated by desorbed GAGs in a concentration dependent manner. Subsequent studies showed that sustained high (160 microg/ml) heparin levels had toxic effects, while the same concentration of CSB exhibited more rapid early proliferation of CD34+ cells. In conclusion, this culture system has demonstrated the ability to produce simultaneous proliferation and CD34+ cell enrichment of a partially purified cord blood population by controlling the nature and levels of GAG moieties to which the cells are exposed. The results indicate that specific GAGs can significantly influence the growth and differentiation characteristics of cultured CD34+ cells.

The heparin/heparan sulfate-binding site on apo-serum amyloid A. Implications for the therapeutic intervention of amyloidosis

Serum amyloid A isoforms, apoSAA1 and apoSAA2, are apolipoproteins of unknown function that become major components of high density lipoprotein (HDL) during the acute phase of an inflammatory response. ApoSAA is also the precursor of inflammation-associated amyloid, and there is strong evidence that the formation of inflammation-associated and other types of amyloid is promoted by heparan sulfate (HS). Data presented herein demonstrate that both mouse and human apoSAA contain binding sites that are specific for heparin and HS, with no binding for the other major glycosaminoglycans detected. Cyanogen bromide-generated peptides of mouse apoSAA1 and apoSAA2 were screened for heparin binding activity. Two peptides, an apoSAA1-derived 80-mer (residues 24-103) and a smaller carboxyl-terminal 27-mer peptide of apoSAA2 (residues 77-103), were retained by a heparin column. A synthetic peptide corresponding to the CNBr-generated 27-mer also bound heparin, and by substituting or deleting one or more of its six basic residues (Arg-83, His-84, Arg-86, Lys-89, Arg-95, and Lys-102), their relative importance for heparin and HS binding was determined. The Lys-102 residue appeared to be required only for HS binding. The residues Arg-86, Lys-89, Arg-95, and Lys-102 are phylogenetically conserved suggesting that the heparin/HS binding activity may be an important aspect of the function of apoSAA. HS linked by its carboxyl groups to an Affi-Gel column or treated with carbodiimide to block its carboxyl groups lost the ability to bind apoSAA. HDL-apoSAA did not bind to heparin; however, it did bind to HS, an interaction to which apoA-I contributed. Results from binding experiments with Congo Red-Sepharose 4B columns support the conclusions of a recent structural study which found that heparin binding domains have a common spatial distance of about 20 A between their two outer basic residues. Our present work provides direct evidence that apoSAA can associate with HS (and heparin) and that the occupation of its binding site by HS, and HS analogs, likely caused the previously reported increase in amyloidogenic conformation (beta-sheet) of apoSAA2 (McCubbin, W. D., Kay, C. M., Narindrasorasak, S., and Kisilevsky, R. (1988) Biochem. J. 256, 775-783) and their amyloid-suppressing effects in vivo (Kisilevsky, R., Lemieux, L. J., Fraser, P. E., Kong, X., Hultin, P. G., and Szarek, W. A. (1995) Nat. Med. 1, 143-147), respectively.

Polypyrrole-heparin composites as stimulus-responsive substrates for endothelial cell growth

Heparin is a potent anticoagulant which can be immobilized on biomaterial surfaces to increase their hemocompatability. In the present work, we have electrochemically synthesized composites comprising heparin and the electrically conducting polymer polypyrrole. The incorporation and exposure of heparin were controlled by varying key conditions of polymer synthesis (i.e., applied current and synthesis time). The resulting composite polymers were electroactive after synthesis and the amount of heparin exposed in the polymer could be increased (up to threefold) by switching the polymers from their oxidized to reduced states. Polymer reduction was achieved by either application of negative potentials (-0.4 to -0.7 V for 90 s) or exposure to aqueous reductant (0.1M sodium dithionite for 30 min). Heparin-polypyrrole composites remained stable after autoclaving, displaying no significant loss of electroactivity, and had a shelf life of at least 2 years postautoclaving. Finally, the composites were found to be excellent substrates for the growth of human endothelial cells.

A novel method to immobilize bioactive substances on hydrophobic surfaces using a polymerizable cationic lipid

We have successfully developed a novel method to stably immobilize bioactive substances that have anionic groups, such as heparin and succinylated collagen (SC), on hydrophobic surfaces through ionic complexation using a polymerizable cationic lipid, diallyl(dioleyl)ammonium bromide (DADOA). It is composed of a hydrophobic part consisting of long hydrocarbon chains and a hydrophilic head with double bonds which render it polymerizable. Analysis of the modification with DADOA and heparin suggested that the modification formed a thin layer, roughly 60 nm in thickness, as a result of the spontaneous deposition of DADOA and heparin dissolved in water, through the hydrophobic interaction between DADOA and the surface and the ionic complexation between DADOA and heparin. The heparin deposition and its rate of release in plasma were 1.5 microg/cm2 and 0.0017 U/cm2/min, respectively. Cytotoxicity test results showed that the polymerization of the deposited DADOA rendered the modified surface stable and noncytotoxic. Further, antithrombogenicity and cell attachability test results demonstrated that heparin and SC were effectively immobilized on hydrophobic surfaces through ionic complexation. This method has proved useful for the modification of the hydrophobic surfaces of medical devices because the modification process can be performed under aqueous conditions without the use of organic solvents which induce crazing/cracking of plastic casings.

Heparin immobilized on proteins usable for arterial prosthesis coating: growth inhibition of smooth-muscle cells

Gelatin or a mixture of albumin and gelatin has been proposed for the coating of vascular grafts according to their surface thrombogenicity and biocompatibility, and the possibility of biodegradation. Heparin treatment of hemocompatible surfaces improved the patency of prostheses. In this study, different amounts of heparin were immobilized on these protein gels using a water-soluble carbodiimide [1-ethyl-3-(3-dimethylaminopropyl) carbodiimide]. The results showed a coupling of heparin with gelatin and/or albumin at the surface of the gels, stable for as long as 1 month. From 0.20 to 3.60 microg x cm(-2), heparin could be immobilized. The antiproliferative activity of immobilized heparin was controlled toward bovine smooth-muscle cells grown on these gels. Cell growth inhibition was dose dependent, but the percentages of inhibition were lower at day 8 than at day 4 at any heparin concentration used under experimental conditions. Referring to heparin in solution, immobilized heparin displayed an antiproliferative activity that improved the potential interest for coating.

Antibodies From Patients With Heparin-Induced Thrombocytopenia/Thrombosis Recognize Different Epitopes on Heparin: Platelet Factor 4

Antibodies associated with heparin-induced thrombocytopenia/thrombosis (HITT) are now thought to be specific for complexes formed between heparin and platelet factor 4 (PF4), a basic protein found normally in platelet alpha granules. How these antibodies cause thrombocytopenia and, in some patients, thrombosis, is not fully understood, in part because purified antibodies that could be labeled and used as probes to characterize target epitopes have not been available. We developed a novel method for antibody purification involving binding to and elution from PF4 complexed to heparin immobilized by end-linkage (EL) to a solid phase. Isolated antibodies were functional and after biotinylation, reacted with heparin: PF4 complexes in the same manner as unlabeled antibodies. Using these probes, we found that antibodies from 11 patients with HITT recognized two, and probably three, distinct sites on heparin: PF4 complexes. The antibodies did not bind to PF4 complexed with heparin immobilized by multiple chemical cross-linkages, suggesting that the heparin molecule must be in a flexible, relatively unconstrained state to react with PF4 in such a way as to create sites for HITT antibody binding.

Antibodies From Patients With Heparin-Induced Thrombocytopenia/Thrombosis Recognize Different Epitopes on Heparin: Platelet Factor 4

Antibodies associated with heparin-induced thrombocytopenia/thrombosis (HITT) are now thought to be specific for complexes formed between heparin and platelet factor 4 (PF4), a basic protein found normally in platelet alpha granules. How these antibodies cause thrombocytopenia and, in some patients, thrombosis, is not fully understood, in part because purified antibodies that could be labeled and used as probes to characterize target epitopes have not been available. We developed a novel method for antibody purification involving binding to and elution from PF4 complexed to heparin immobilized by end-linkage (EL) to a solid phase. Isolated antibodies were functional and after biotinylation, reacted with heparin: PF4 complexes in the same manner as unlabeled antibodies. Using these probes, we found that antibodies from 11 patients with HITT recognized two, and probably three, distinct sites on heparin: PF4 complexes. The antibodies did not bind to PF4 complexed with heparin immobilized by multiple chemical cross-linkages, suggesting that the heparin molecule must be in a flexible, relatively unconstrained state to react with PF4 in such a way as to create sites for HITT antibody binding.

Preparation and characterization of heparin-containing SBS-g-DMAEMA copolymer membrane

The grafting of dimethyl amino ethyl methacrylate (DMAEMA) onto styrene-butadiene-styrene triblock copolymer (SBS) membrane was subsequently conducted by UV-radiation induced graft copolymerization without degassing to obtain the SBS-g-DMAEMA copolymer membrane. The substituted amino groups on the SBS-g-DMAEMA graft copolymer membrane were quaternized with iodomethane, and then the membrane was treated with heparin to prepare the heparin-containing SBS-g-DMAEMA copolymer membrane (SBS-g-DMAEMA-HEP). The graft copolymer membrane (SBS-g-DMAEMA) and the heparin-containing SBS-g-DMAEMA copolymer membrane (SBS-g-DMAEMA-HEP) were characterized by FTIR spectroscopy. The heparin content was determined by toluidine blue heparin assay. Contact angle, water content, and protein adsorption of fibrinogen and albumin experiments were also performed to evaluate the effect of graft amount and heparin content on the biocompatibility of SBS-g-DMAEMA and SBS-g-DMAEMA-HEP graft copolymer membranes. By using Kaelble's equation, the surface tension of SBS-g-DMAEMA and SBS-g-DMAEMA-HEP were determined. It was found that with increasing grafting amount and the heparin content, the surface tension and water content of SBS-g-DMAEMA membrane increased, whereas the contact angle decreased. The amount of the adsorption of albumin and fibrinogen decreased with increasing graft amount and heparin content. However, there was a minimum for adsorption of proteins in the SBS-g-DMAEMA and SBS-g-DMAEMA-HEP membranes.

In vitro evaluation of heparinized Cuprophan hemodialysis membranes

Cuprophan hemodialysis membranes can be heparinized using N,N'-carbonyldiimidazole (CDI) as a coupling agent. In this study, the characteristics of heparinized Cuprophan membranes have been evaluated. After immobilization, heparin partially retained its biologic activity. An anticoagulant activity of 12.4 +/- 4.2 mU/cm2 was measured using a thrombin inactivation assay. Immobilized heparin also displayed an anti-complement activity. After contact with human serum; heparinized Cuprophan induced no generation of significant amounts of fluid phase terminal complement complex (TCC), whereas untreated Cuprophan induced the generation of substantial amounts of TCC. Heparinization did not affect the permeability of Cuprophan for model solutes with molecular weights up to 12,000 g/mol except for sulfobromophthalein sodium salt. The permeability of Cuprophan for sulfobromophthalein sodium salt was slightly decreased after heparinization. The ultrafiltration rate of Cuprophan increased by about 30% after heparinization, probably owing to an increased swelling of the membrane in water. Heparinized Cuprophan incubated in phosphate-buffered saline at 37 degrees C showed some release of heparin. These amounts of released heparin, however, were very low as compared to the amounts of heparin which are systemically administered during clinical hemodialysis treatment. It is concluded that Cuprophan membranes heparinized by means of the CDI-activation procedure are highly promising for application in hemodialyzers to be used for the treatment of patients with reduced or without systemic administration of heparin.

Development of chitosan/polyethylene vinyl acetate co-matrix: controlled release of aspirin-heparin for preventing cardiovascular thrombosis

Aspirin and heparin were embedded in chitosan/polyethylene vinyl acetate co-matrix to develop a prolonged release form. The in vitro release profiles of these drugs from the co-matrix system were monitored in Tris HCl buffer pH 7.4, using a UV spectrophotometer. The amount of drug release was initially much higher. followed by a constant slow release profile for a prolonged period. The initial burst release was substantially modified with styrenebutadiene coatings. From scanning electron microscopy studies it appears that the drugs diffuse out slowly to the dissolution medium through the micropores of the co-matrix. The released aspirin-heparin from the co-matrix system had shown their antiplatelet and anticoagulant functions. The results propose the possibility of delivering drug combinations, having synergestic effects for therapeutic applications.

Bioactive heparin surfaces from derivatization of polyacrylamide-grafted LLDPE

Primary amine groups were introduced into polyacrylamide-LLDPE films, using the Hofmann degradation synthesis. The Hofmann degradation was studied at room temperature using sodium hypochlorite and sodium hydroxide at different concentrations. Diazotized heparin was covalently bound to the grafted LLDPE film via the primary amine groups. Surfaces were analysed with ESCA, ATR-IR, chloride titration and Toluidine Blue. Evaluation of the biological activity of the heparinized surfaces was made by measuring the capacity for binding antithrombin (AT) and inhibition of the activated coagulation factor XII (FXIIa). The heparinized surfaces were able to bind up to 3 pmol cm-2 of AT in solution with ionic strengths of I = 0.15 and I = 0.40. No activation of the adsorbed FXII was detected.

New polyurethane compositions able to bond high amounts of both albumin and heparin. II: Copolymers and polymer blends

Haemocompatible new urethane copolymers and polymer blends containing, in the chain extender, a long chain alkyl group (able to bond albumin) or a tertiary ammonium group able, after suitable quaternization reaction, to bind ionically significant amounts of heparin, were prepared. The copolymers were characterized by differential scanning calorimetry, intrinsic viscosity determinations, infrared spectroscopy and nuclear magnetic resonance (1H and 13C). Biological in vitro evaluation has shown that the adsorption sequence for albumin and heparin, respectively, onto films of the various copolymers and blends, exerts a great influence. From scanning electron microscopy measurements it was seen that the bonding type of albumin to the polymer films plays a determining role on the platelet activation. A phase segregation occurring on the polymer blends surface was demonstrated by X-ray photoelectron spectroscopy measurements.

Preparation and surface characterization of functional group-grafted and heparin-immobilized polyurethanes by plasma glow discharge

Functional group-grafted polyurethanes were prepared by oxygen plasma discharge treatment, followed by graft polymerization of 1-acryloylbenzotriazole (AB) and a subsequent substitution reaction of AB with sodium hydroxide and ethylene diamine. The primary amine or carboxylic acid groups grafted on the surfaces were coupled with heparin using water-soluble carbodiimide. The modified surfaces were characterized by measuring the water contact angle, electron spectroscopy for chemical analysis and attenuated total reflection Fourier-transform infrared spectroscopy. The amount of heparin covalently immobilized on the primary amine- and carboxylic acid group-grafted polyurethanes were 2.0 and 1.4 micrograms cm-2, respectively, as determined by the toluidine blue method. The water contact angle of the polyurethanes was decreased by AB grafting, and further decreased by the introduction of functional groups such as carboxylic acid and primary amine and immobilization of heparin, showing increased hydrophilicity of the modified surfaces. Heparin was almost not released from the immobilized surfaces in the physiological solution for 100 h, indicating good stability of immobilized heparin.

Importance of specific amino acids in protein binding sites for heparin and heparan sulfate

Heparin and heparan sulfate bind a variety of proteins and peptides to regulate many biological activities. Past studies have examined a limited number of established heparin binding sites and have focused on basic amino acids when modeling binding site structural motifs. This study examines the prevalence of individual amino acids in peptides binding to heparin or heparan sulfate. A 7-mer random peptide library was synthesized using the 20 common amino acids. This 7-mer library was affinity separated using both heparin and heparan sulfate-Sepharose. Bound peptide populations were eluted with a salt step gradient (pH 7) and analysed for amino acid composition. Peptides released from heparin-Sepharose by 0.3 M NaCl were enriched in arginine, lysine, glycine and serine; and depleted in methionine and phenylalanine. In contrast, peptides released from heparan sulfate-Sepharose were enriched in arginine, glycine, serine, and proline (at 0.15 M NaCl). These peptides were depleted in histidine, isoleucine, methionine (not detectable) and phenylalanine. In the heparin binding sites of proteins, which have been published, the enriched amino acids were arginine, lysine and tyrosine. Depleted amino acids include aspartic acid, glutamic acid, glutamine, alanine, glycine, phenylalanine, serine, threonine and valine. This study demonstrates that heparin and heparan sulfate bind different populations of peptide sequences. The differences in amino acid composition indicate that the positive charge density and spacing requirements differ for peptides binding these two glycosaminoglycans.

Immobilization of heparin on polylactide for application to degradable biomaterials in contact with blood

The poly-(D, L-lactide) RESOMER R208 (Boehringer-Ingelheim, Germany) was modified with heparin to improve the blood contacting properties of the material. The immobilization of herapin was carried out by covalent binding with glutaraldehyde as the coupling agent. The reaction conditions, such as temperature and time, were varied to optimize the binding of heparin. The efficiency of the immobilization was monitored with respect to the total amount of coupled herapin with a toluidine blue assay and the anticoagulant activity of immobilized heparin with a factor Xa assay. The hemocompatibility of the modified polylactide was estimated after blood-material contact by the activation of platelets measured with an enzyme immuno assay for GMP140. Immobilization at ambient temperature and a reaction time of 2 h resulted in maximal heparin binding, high anticoagulant activity, and low thrombogenicity. Since the remaining unsaturated aldehyde groups of the coupling agent may cause a low hemocompatibility of the material, washing of the heparinized polylactide was carried out with ethanol. However, it was shown that washing diminished the anticoagulant activity of heparin and increased the thrombogenicity. The prolonged storage of heparinized polylactide in phosphate buffered saline for 8 days demonstrated that small quantities of heparin were released but the hemocompatibility was further improved, indicated by an increasing anticoagulant potential and a decrease in platelet activation with incubation time. A comparison of polylactide, heparinized polylactide, polypropylene, and Pellethane with respect to platelet activation by GMP140 assay and scanning electron microscopy, revealed that the heparinization of polylactide substantially improved the hemocompatibility of RESOMER R208, making the material comparable to Pellethane.

Interaction of antithrombin III with surface-immobilized albumin-heparin conjugates

The interaction between antithrombin III (ATIII) and albumin-heparin conjugates covalently coupled onto carboxylated polystyrene beads either in buffer containing albumin or in plasma was studied using 14C-labeled ATIII. Binding isotherms of ATIII were modeled using a summation of two Langmuir equations. These equations describe the binding of ATIII to two different sets of binding sites, one with a high, the other with a low affinity of ATIII to these sites are 9 x 10(6) L/mol and 0.3 x 10(6) L/mol, respectively. The binding of ATIII to surface binding sites with a high affinity for ATIII was correlated with the presence of specific ATIII binding sites in the immobilized heparin. Binding of ATIII from albumin solutions to binding sites with a low affinity for ATIII was dominated by nonspecific binding of ATIII to the immobilized heparin. A third small fraction of the surface bound. ATIII is probably adsorbed to sites on the surface not covered with heparin. In the case of the binding of ATIII to the heparinized surface from plasma solutions, a fraction of initially adsorbed ATIII was desorbed by other plasma proteins. This desorption in combination with direct competition between ATIII and other plasma proteins resulted in lower ATIII surface concentrations using plasma as compared to the ATIII surface concentrations obtained using albumin solutions. The binding of ATIII to nonspecific binding sites was almost completely inhibited in the presence of plasma proteins. The amount of ATIII bound to immobilized heparin via specific ATIII binding sites was 30% lower in plasma solutions as compared to the specific binding of ATIII using albumin solutions. It is concluded that the accessibility of immobilized heparin for ATIII in plasma decreases by binding of heparin-binding proteins onto the immobilized heparin and/or adsorption of other plasma proteins on the heparinized surface.

Water-soluble poly(acrylamide-allylamine) derivatives of saccharides for protein-saccharide binding studies

Water-soluble poly(acrylamide-allylamine) copolymers containing covalently bound amino groups, prepared by copolymerization of acrylamide and allylamine, can be used as general carriers for coupling of different types of saccharides or saccharide derivatives. The water-soluble macromolecular carbohydrate derivatives can be easily labelled and used in various solid-phase techniques to study protein-saccharide interaction. Two types of coupling reaction were used to prepare polyacrylamide derivatives of saccharides: reductive amination was applied to couple the reducing disaccharides and a carbodiimide reaction was used to couple heparin via its carboxyl groups to the amino groups of the poly(acrylamide-allylamine) derivative. Peroxidase labelled or biotinylated derivatives were shown to be useful in studies on the binding properties of lectins and proteins from boar seminal plasma.

Heparin immobilized chitosan--poly ethylene glycol interpenetrating network: antithrombogenicity

This work deals with the synthesis and blood compatibility studies of Heparin immobilized chitosan--polyethyleneglycol (Chit-PEG) hydrogels for various biomedical applications. Chit-PEG interpenetrating net work (IPN) had been synthesised by crosslinking different ratios of chitosan with glutaraldehyde using schiffs base reaction mechanism and interpenetrating polyethyleneglycol (PEG) to form hydrogen bonding between the amino hydrogen in chitosan and polyether oxygen. An optimum gel combination was selected from the IPN of Chit-PEG and used for bonding heparin. This modified gel had dramatically improved its blood compatibility. The antithrombotic function of this gel and the release profile of heparin had been investigated using coagulation assays, and spectrophotometric quantitation. Recalcification times of plasma exposed to heparin immobilized Chit-PEG hydrogel were markedly increased as compared to heparin free gels. The anticoagulant function of this gel matrix may be due to partially released heparin and bonded heparin.

Heparin surface immobilization through hydrophilic spacers: thrombin and antithrombin III binding kinetics

The immobilization of heparin onto polymeric surfaces using hydrophilic spacer groups has been effective in curtailing surface induced thrombus formation. In this study, the effect of hydrophilic spacers (PEO) on the binding kinetics of immobilized heparin with antithrombin III (ATIII) and thrombin was investigated. Monodispersed, low molecular weight heparin was fractionated on an ATIII affinity column to isolate high-ATIII affinity heparin. This high-ATIII affinity fraction was immobilized onto a styrene/p-amino styrene random copolymer surface using hydrophilic poly(ethylene oxide) (PEO) spacer groups. Styrene/p-amino styrene random copolymer was chosen as the model surface to provide quantitative and reproducible surface concentrations of available amine groups, grafted PEO spacers, and immobilized heparin. The polymer substrate was coated onto glass beads, tolylene diisocyanate modified PEO was covalently coupled to the surface, followed by heparin immobilization. The bioactivity of immobilized heparin was 16.2%, relative to free heparin, and a 1:1 binding ratio between heparin and PEO was achieved. The binding of ATIII and thrombin to control surfaces (no heparin), soluble heparin, heparin immobilized directly onto the surface, and heparin immobilized via spacer groups, were compared. Soluble heparin bound both thrombin and ATIII, while heparin immobilized directly onto the surface bound only thrombin. Spacer-immobilized heparin bound both ATIII and thrombin, although to a lesser extent than soluble heparin. Thus, the enhanced bioactivity of spacer-immobilized heparin, compared to direct-immobilization, may be attributed to the retention of ATIII binding.